U.S. patent application number 10/678128 was filed with the patent office on 2004-04-15 for adaptive electric field modulation of neural systems.
Invention is credited to Gluckman, Bruce J., Schiff, Steven J..
Application Number | 20040073273 10/678128 |
Document ID | / |
Family ID | 22614990 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040073273 |
Kind Code |
A1 |
Gluckman, Bruce J. ; et
al. |
April 15, 2004 |
Adaptive electric field modulation of neural systems
Abstract
The present invention relates to devices and methods of
modifying the neuronal activity of a neural system comprising
neurons, comprising, one or more of the following steps, measuring
the neuronal activity of a neural system; and applying an oriented
electric field to said neural system effective to modify the
neuronal activity of the neural system, wherein the magnitude and
polarity of said applied electric field is changed in response to
the measured neuronal activity. The present invention also relates
to devices and methods for treating brain disorders, such as
epilepsy and Parkinson's disease, comprising, one or more of the
following steps, applying a sub-threshold and oriented electric
field in situ to the brain of a patient having such a disorder in
an amount effective to reduce the abnormal activity of the brain,
wherein the electric field is applied through field electrodes in
contact with the brain. The present invention also relates to
methods and devices for restoring or repairing a brain function,
such as sensation (e.g., taste, or smell), somatic activity,
auditory activity, visual activity, or motor activity. It can also
be used for testing drugs, pharmacological agents, and other
modulators of neuronal function.
Inventors: |
Gluckman, Bruce J.;
(Arlington, VA) ; Schiff, Steven J.; (Bethesda,
MD) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
22614990 |
Appl. No.: |
10/678128 |
Filed: |
October 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10678128 |
Oct 6, 2003 |
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09729929 |
Dec 6, 2000 |
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6665562 |
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60169280 |
Dec 7, 1999 |
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Current U.S.
Class: |
607/48 |
Current CPC
Class: |
A61N 1/36034 20170801;
A61N 1/40 20130101; A61N 1/36067 20130101; A61N 1/36025 20130101;
A61N 1/36031 20170801; A61N 1/36064 20130101 |
Class at
Publication: |
607/048 |
International
Class: |
A61N 001/18 |
Claims
1. A method of modifying the neuronal activity of a neural system
comprising neurons, comprising: a) measuring the neuronal activity
of a neural system; b) applying an oriented electric field to said
neural system effective to modify the neuronal activity of the
neural system, wherein the magnitude and polarity of said applied
electric field is changed in response to the measured neuronal
activity.
2. A method of claim 1, wherein the applied electric field is
sub-threshold.
3. A method of claim 1, wherein the measuring is performed
simultaneously with the applying an oriented electric field.
4. A method of claim 1, wherein the applied field is proportional
to the neuronal activity.
5. A method of claim 1, wherein the applied field is proportional
to the difference between the root-mean-square of the measured
activity and a predetermined threshold activity value in a
predetermined frequency band.
6. A method of claim 1, wherein the applied electric field is under
full-wave control.
7. A method of claim 1, wherein the applied electric field is
half-wave rectified.
8. A method of claim 1, further comprising, representing the
measured electric activity as the root-mean-square within a
frequency band of 100-500 Hz, averaged over a time y.
9. A method of claim 1, wherein the electric field is produced
using two field electrodes positioned external to the neurons in
the neural system.
10. A method of claim 1, wherein the electric field is oriented
parallel to the somatic-dendritic axis of the neurons in the neural
system.
11. A method of claim 1, wherein the oriented electric field is
applied adaptively in response to changes in measured neuronal
activity of the neural system.
12. A method of claim 11, wherein the neuronal activity is
electrical activity.
13. A method of claim 1, wherein the measuring electrodes are
positioned on an isopotential to the applied electric field.
14. A method of claim 1, wherein the neuronal activity is
spontaneous epileptiform electrical activity and the applied
electric field suppresses it.
15. A method of claim 1, wherein the neuronal activity is
spontaneous epileptiform electrical activity and the applied
electric field induces or augments it.
16. A method of claim 1, wherein the applied electric field
modifies the firing rate of neurons in the neural system.
17. A method of claim 1, wherein the applied electric field is
created by injecting current into the system.
18. A method of claim 17, wherein the current is injected until a
predetermined field potential is reached.
19. A method of claim 1, with the proviso that the applied electric
field is not a continuous stationary field.
20. A method of claim 1, wherein the neuronal activity is modified
to restore sensation, somatic activity, auditory activity, visual
activity, or motor activity.
21. A method of treating epilepsy in a patient in need thereof,
comprising: applying a sub-threshold and oriented electric field in
situ to the brain of a patient having epilepsy in an amount
effective to reduce epileptiform activity of the brain, wherein the
electric field is applied through field electrodes located in a
position effective to produce a field in said brain.
22. A method of claim 21, wherein the electric field is applied to
the brain when epileptiform activity is detected through measuring
electrodes.
23. A method of claim 21, wherein the applied electric field is
under full-wave control.
24. A method of claim 21, wherein the applied electric is half-wave
rectified.
25. A method of treating Parkinson's disease in a patient in need
thereof, comprising: applying a sub-threshold and oriented electric
field in situ to the brain of a patient having Parkinson's disease
in an amount effective to reduce tremors, rigidity, or difficulty
in initiating movement, wherein the electric field is applied
through field electrodes in contact with the brain.
26. A method of claim 25, wherein the electric field is applied to
the brain adaptively.
27. A method of identifying agents which modulate the neuronal
activity of a neural system comprising neurons, comprising: a)
measuring the neuronal activity of a neural system; b) applying an
oriented electric field to said neural system effective to modify
the neuronal activity of the neural system, wherein the magnitude
and polarity of said applied electric field is changed in response
to the measured neuronal activity; and c) administering an agent in
an effective amount which modulates the neuronal activity of the
neural system.
28. A field-producing device for modifying the neuronal activity of
a neural system comprising neurons, comprising: (a) field electrode
means for applying an external electric field to a neural system;
(b) field application electronic means for generating an external
field to a neural system, which is operably connected to (a) field
electrode means; (c) measuring means for monitoring the neural
activity of the neural system; (d) measurement electronics means
for recording neural activity, which is operably connected to (d)
measuring electronic means; (e) feedback controller means for
determining the amount of external field to apply to the neural
system, which is operably connected to (b) field application means
and (c) measuring means.
29. A field-producing device of claim 28, further comprising: (f)
sensing means for detecting the external field produced by the
field electrode means; (g) sensing electronic means for recording
the field produced by the field electrode means, which is operably
connected to (f) sensing electrode means and (b) field application
means.
Description
[0001] This application claims the benefit of provisional
application Serial No. 60/169,280, filed Dec. 7, 1999, which is
hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] Numerous attempts have been made to suppress epileptic
seizures in human patients with indirect electrical stimulation at
sites remote from the epileptic focus, including cerebellum (Cooper
et al., 1976; Van Buren et al., 1978), thalamus (Cooper et al.,
1985; Fisher et al., 1992), and vagal nerve (Murphy et al., 1995;
McLachlin, 1997). Surprisingly, there has been far less
investigation of the technology required to directly control an
epileptic focus electrically. It has been shown that direct current
injection into tissue could suppress evoked (Kayyali and Durand,
1991) or spontaneous (Nakagawa and Durand, 1991; Warren and Durand,
1998) epileptiform activity in brain slices. Even simple periodic
pacing of a neuronal network with direct electrical stimulation
(Kerger and Schiff, 1995) can reduce seizure-like events. In
addition, there is some evidence that nonlinear control schemes
might be useful in manipulating epileptiform activity (Schiff et
al., 1994). In each of these cases, the stimulation was applied in
the form of short current pulses directly into the tissue that
evoke neuronal firing. Recently, it was demonstrated that steady
state (DC) electric fields oriented parallel to pyramidal cells
were capable of suppressing epileptic seizure activity in in vitro
hippocampal brain slices (Gluckman et al., 1996a). Such field
application led to nearly complete suppression of neuronal
activity, yet due to a combination of polarization effects
(electrode and tissue) and neuronal adaptation, this effect was
transient.
DESCRIPTION OF THE DRAWINGS
[0003] FIG. 1. (A) is a top view schematic drawing of a perfusion
chamber used to adaptively modulate the neuronal activity of an
isolated neural system. (B) is a side view schematic of the same
chamber. The brain slices rest on a nylon mesh just below the upper
surface of the perfusate of artificial cerebrospinal fluid (ACSF),
and the atmosphere above the perfusate is warmed to the bath
temperature of 35.degree. C. and saturated with 95% O.sub.2-5%
CO.sub.2. An electric field is imposed on the slice by a set of
Ag--AgCl electrodes embedded in the floor of the chamber. The
potential difference applied between parallel plate electrodes F1
and F2 is feedback controlled so that the average field measured at
sensing electrodes S1 and S2 is proportional to a program voltage.
An additional pair of electrodes, G, are used as recording
ground.
[0004] FIG. 2. Power spectral density (PSD) for recorded activity
and applied field stimulus in the case for which the stimulus was a
low frequency random signal (A) and for which the stimulus was a
typical feedback control signal (B). For display purposes, the
stimulus PSD was vertically scaled such that its amplitude matched
that of the recorded activity PSD at low frequencies. In both
cases, the stimulus PSD falls off quickly (.about.f.sup.2) for
frequencies, f above about 4 Hz, in contrast to the neuronal
activity PSDs, which have significant spectral power up to
approximately 350 Hz. Also shown are the PSDs of the recorded
neuronal activity after removal of an estimate of the stimulus
artifact. These signals are indistinguishable from the original
recording for frequencies above .about.2 Hz. In (B) the raw signal
lies slightly below the processed signal for low frequencies. These
results indicate that the applied field during control is not
simply masking the neuronal activity in the recording process
during control. The stimulus artifact accounts for less than 5% of
the RMS recorded signal amplitude.
[0005] FIG. 3. Adaptive control of seizure activity using applied
electric fields. In each panel, the main trace is the raw
extracellular potential recording. Insets are tracings of activity,
filtered to illustrate the high frequency activity, shown at
expanded scales. In each case, a dashed line is used to demarcate
when control is turned on. A,B: Examples of seizure suppression
from separate experiments using electric fields applied as a
negative feedback parameter. Electrographic seizures are observed
as an increase in high frequency activity atop large low-frequency
deflections (Traynelis and Dingledine, 1988). In B, seizures occur
interspersed among frequent short network bursts (Rutecki et al.,
1985). C: Example of seizure induction achieved using positive
feedback.
[0006] FIG. 4. Event detection results for a single 90 minute
recording, with different electric field stimuli applied. The lower
trace indicates feedback gain (G, left axis) or amplitude (A, right
axis) of the applied stimulus. Greek letters indicate type of
stimulus: baseline (no letter); full-wave feedback control
(.alpha.); half-wave rectified feedback control (.beta.); constant
amplitude suppressive field (.gamma.); low frequency noise
(.delta.); suppressive half-wave rectified low frequency noise
(.epsilon.); positive feedback control (.mu.). Two types of event
detection were used to identify synchronous neuronal activity from
the recorded field potentials. "RMS events" were detected from
variations in the RMS power in the frequency band 100-350 Hz. "DC
events" were detected by threshold detection after low pass
filtering the recordings at 10 Hz. The character of both types of
events, as quantified by their average and maximal amplitudes as
well as their duration, was visibly changed from baseline when
control was applied. No events of either type were observed during
the final and longest (16 minutes) application (.alpha..sub.3) of
full-wave control.
[0007] FIG. 5. Traces and spectrograms of activity with and without
control for same experiment as FIG. 4. (A) Activity (lower trace)
and applied field (upper trace) from the final application of
full-wave control (.alpha..sub.3) from FIG. 4 and the baseline
preceding it. (B,C) A 15 second long trace and spectrogram of a
seizure-like event (B) and of activity during control (C) from A.
The upper traces in B and C are the activity, high-pass filtered at
100 Hz. The spectrograms (B, C, D) are calculated in overlapping
vertical frequency bins 50 Hz tall from 25-350 Hz, and in
overlapping horizontal time windows 0.05 s wide. (D) Spectrogram
for longer period illustrating contrast between baseline and
controlled activity.
[0008] FIG. 6. Examples of activity during non-feedback electric
field stimulus for the same recording as FIG. 4. For each set, the
upper trace is of the recorded activity, while the applied field is
shown in the lower trace. (A) Application of constant-amplitude
(DC) suppressive field (4.gamma.). (B) Application of full-wave low
frequency noise field (4.delta.). (C) Application of half-wave
rectified low frequency noise field (4.epsilon.). In each case,
large neuronal events are observed, though the full-wave noise
field did have the effect of breaking up the seizure-like events
into shorter durations.
[0009] FIG. 7. Comparison of power spectral density (PSD) of
recorded activity during control (lines with symbols) as compared
to baseline (lines without symbols). The control corresponds to the
final control application in FIG. 4, and the baseline corresponds
to the final baseline application. PSDs were calculated in
overlapping 1.64 s (214 point) windows. The power averaged over the
windows is shown in A, while the window to window variance of power
is shown in B. For both measures, the controlled activity falls
well below that of the baseline activity.
[0010] FIG. 8. Statistics of the RMS power of recorded activity in
the frequency band 100-350 Hz, calculated in 1.64 s windows, for
baseline (squares), full-wave control (circles) and half-wave
rectified control (triangles). Statistics correspond to all
applications independent of gain for the recording of FIG. 4. The
normalized histogram and cumulative probability are shown in (A)
and (B). It is clear that the baseline activity has many windows
with much higher power than either type of control. These windows
correspond to the first phase of the seizures. The inset in A is
the normalized histogram of power calculated with logarithmically
spaced bins (power, abscissa; frequency, ordinate) for baseline
(boxes) and full-wave control (circles). From this plot, it is
observed that deviations to both high and low power are eliminated
during full-wave control. The windows with extremely low power
correspond to the latter phase of the seizures and the recovery
times following them. The power variance vs. average power is
plotted in (C) for these three conditions. The two types of control
are statistically well distinguished from that of the baseline
activity.
[0011] FIG. 9. Examples of network activity when control is
released. In each panel, the inset is the activity for the full
control period, indicated in gray, plus the baseline periods before
and after. The trace in (A) corresponds to the same experiment as
FIG. 3A, with half-wave rectified control. The network oscillates
between excitation similar to seizure onset and being suppressed by
the controller. When control is released, this activity proceeds
immediately into a full seizure-like event. B,C Traces from another
experiment in which half-wave rectified control (B) was compared to
non-rectified control (C). For half-wave rectification, seizures
were observed very soon (0-3 s) after control was released, as
compared to 12-18 s for non-rectified control. The time base for
the insets is the same, and indicated in (A). The inset vertical
scale is half that of the main traces.
DESCRIPTION OF INVENTION
[0012] The present invention relates to devices and methods for
modulating the neuronal activity of a neural system comprising
neurons, such as a brain, brain regions, or any in vivo or in vitro
collection of neurons. In particular, the present invention
involves the use of applied electric fields to modulate the
behavior of a target neural system. In preferred embodiments, the
polarity and magnitude of the applied electric field is varied
according to information gathered from the modulated neural system,
or any other desired source chosen to provide feedback, to modulate
the strength of the applied electric field. In such embodiments,
preferably a sub-threshold stimulus is administered to modulate to
the neural system. The methods and devices of the present invention
can be used to treat diseases of the nervous system, to restore
neuronal function, paralysis, and motor and sensory deficits, to
produce prosthetic devices that interact and modulate neuronal
activity, to enhance or suppress neuronal activity and associated
phenotypes, and the like.
[0013] A preferred method of the present invention relates to
modifying the neuronal activity of a neural system comprising
neurons, comprising one of more of the following steps, in any
order: measuring the neuronal activity, or other behavior, of a
neural system; and applying an oriented electric field to said
neural system effective to modify the neuronal activity of the
neural system, wherein the magnitude and polarity of said applied
electric field is changed in response to the measured neuronal
activity.
[0014] A neural system in accordance with the present invention can
be any ensemble of one or more neurons, and/or other excitable
cells, such as muscle, heart, retinal, cochlear, tissue culture
cells, stem or progenitor cells, including cell-electrode interface
devices and the like. Cells can be coupled electrically,
chemically, or combinations thereof. The neural system can be an
entire brain, ganglia, nerve, etc., or it can be a region or
portion of it. Any animal source of material is suitable, including
neural systems of invertebrates, such as mollusks, arthropods,
insects, etc., vertebrates, such as mammals, humans, non-human
mammals, great apes, monkeys, chimpanzees, dogs, cats, rats, mice,
etc. In the examples, a specific region of a mammalian brain is
dissected out and placed in a chamber where its activity is
modified. However, physical isolation of a target brain region is
unnecessary; the activity modulation can be performed in situ, as
well. Preferred target regions include, but are not limited to,
neocortex, sensory cortex, motor cortex, frontal lobe, parietal
lobe, occipital lobe, temporal lobe, thalamus, hypothalamus, limbic
system, amygdala, septum, hippocampus, formix, cerebellum, brain
stem, medulla, pons, basal ganglia, globus pallidum, striatum,
spinal cord, ganglion, cranial nerves, peripheral nerves, retina,
cochlea, etc.
[0015] In one step of a preferred method, the neuronal activity of
the neural system is measured. By the term "neuronal activity," it
is meant any measurable physical behavior, output, or phenotype of
the system. For example, neurons typically display variations in
their membrane potential, such as action potentials,
depolarizations, and hyperpolarizations. These changes in the
membrane potential can be utilized as a measure of neuronal
activity, e.g., by monitoring intracellularly in a single neuron,
or extracellularly, the electrical activity of a single neuron or
the activity of an ensemble of neurons. Behaviors, or other
products of a neural system (e.g., hormones, growth factors,
neurotransmitters, ions, etc.) can also be detected, and used as a
feedback signal to determine the magnitude and strength of the
modulating applied field. For instance, if a purpose is to elicit
movement of a limb, then the neuronal activity can be limb motion.
The neuronal activity which is measured or assessed can be a subset
of the total activity observed in the system, e.g., a particular
frequency band of the full neural signal. In the examples,
hippocampus slices were monitored for neuronal activity. Although
the measuring electrode detected various types of activity,
including spontaneous neuronal firing, slow burst activity, and
background noise, as well as fast frequency epileptic seizures, it
was desired to modulate only the latter. Thus, for these purposes
the neuronal activity can be considered to be only the events of
interest, e.g., the epileptic seizures.
[0016] Methods for measuring and recording neuronal activity can be
accomplished according to any suitable method. In preferred
embodiments of the invention, the neuronal activity is monitored
extracellularly by measuring the extracellular electrical potential
of a target population of neurons. Such measurements can reveal
complex spikes or burst activity, sharp or slow waves, epileptiform
spikes or seizures, arising from one or more neurons in the neural
system.
[0017] The neuronal activity can be measured by recording the
neural system's electrical potential in the extracellular space.
The electrodes used to measure the field potential produced by the
neural system are referred to as "measuring electrodes" or
"recording electrodes." One or more electrodes can be used to
measure the field potential. In preferred embodiments, two or more
electrodes are utilized. The field potentials recorded at a given
extracellular site will depend on a variety of factors, including
the location of the electrode(s) with respect to the soma and
dendritic layers, the architecture of the neural system, the
perfusion solution, etc.
[0018] The measuring electrodes can detect the field potential from
the applied field as well as the activity generated by the neural
system. There are a number of methods that can be used to
distinguish the neuronal activity from the applied fields. For
example, in in vitro hippocampal slices, a pair of differential
electrodes, aligned as closely as possible to the isopotential of
the applied field, were used as measuring electrodes. They are
"differential" in the sense that an active electrode is placed in
the tissue, preferably near the cell body layer of the target
neurons, while the reference electrode is placed preferably in the
bath external to the tissue. The values obtained from each
electrode can be electronically subtracted from each other,
reducing background noise. For in vivo use, the differential
measuring electrodes can be placed at the same isopotential with
respect to the applied field. The electrodes can be as close to the
target population as possible, without damaging it. Other methods
to reduce noise and the artifact from the applied field can be used
as well, either alone, or in combination with the differential
electrodes, including filtering and post-processing of the measured
signal.
[0019] Recording from the electrodes can be performed routinely.
For instance, measurements can be made with an AC amplifier if the
frequency and number of extracellular bursts are of interest. It
can be equipped with filters to cut off frequencies below and above
a particular range (band-pass filter) and amplify the signal in
preferred ranges, e.g., 50-1000 Hz, preferably, 100-500 Hz. A DC
amplifier can also be used, if slower potential changes are of
interest.
[0020] A method in accordance with the present invention also
involves applying an oriented electric field to the neural system
effective to modify the neuronal activity of the neural system,
preferably where the magnitude and polarity of said applied
electric field is changed in response to the measured neuronal
activity. Preferably, the applied field is oriented in a particular
direction with respect to the somatic-dendritic axis of the neurons
in the neural system. Most preferably, the field is parallel to the
somatic-dendritic axis. Changing the strength of the applied field
in response to a measured activity of the neural system can also be
referred to as "adaptive modulation" since the strength of the
applied field is adjusted based on an activity value of the neural
system (e.g., electrical activity, motor activity, such as limb
motion, etc.). A function of the applied electric field is to
modify the neuronal activity of the neural system. The electric
field is thus applied to the neural system in an amount adequate to
change the neuronal behavior of the neural system. Any amount of
field which changes the neural system's behavior is an effective
applied field. It is believed that a mechanism that underlies
adaptive modulation is the ability of the applied field to alter
the neuron's excitability by changing its threshold; however, the
invention is not bound nor limited to any theory, explanation, or
mechanism of how it works.
[0021] In preferred methods of the present invention for in vitro
applications, two pairs of electrodes can be used in the field
application step. A pair of "field electrodes" can be used to
produce the applied field. A second pair of electrodes, "sensing
electrodes," can be used to measure or sense the field generated by
the "field electrodes." The sensing and field electrodes can
comprise the same materials described above for the measuring
electrodes. In certain applications, however, such as in vivo
applications, a field can be applied without sensing
electrodes.
[0022] In preferred embodiments of the invention, the effective
amount of applied field is sub-threshold with respect to the field
potential experienced by the neural system. By the term
"sub-threshold," it is meant that the amount of applied field or
current does not reliably, with 100% probability, initiate new
action potentials within the neural system. In contrast, the
application of a supra-threshold stimulus reliably, with a high
degree of probability, results in neuronal firing. A sub-threshold
potential is, for example, less than 100 mV/mm, preferably 50 mV/mm
and less, more preferably, 25 mV/mm and less, such as 20 mV/mm, 15
mV/mm, or 10 mV/mm. The sub-threshold potential refers to the
potential generated at the level of the target neurons. The amount
of potential actually produced by the field electrodes is less
important that the field perceived by the target neurons. It is the
generated field sensed by the neurons that determines whether a
stimulus is sub- or supra-threshold.
[0023] In response to the applied electric field, the activity of
the neural system can be modified in any desired manner, e.g., the
activity can be suppressed, reduced, decreased, diminished,
eliminated, counteracted, canceled out, etc., or it can be
enhanced, increased, augmented, facilitated, etc. To determine
whether the activity of the system has been modified, preferably
the same neuronal activity measured in the measurement step is
re-measured. Most preferably, the measurement of the neuronal
activity is performed simultaneously and continuously with the
applied field.
[0024] Any effective electrodes can be used for the recording,
sensing, and field electrodes, including, e.g., metal, steel,
activated iridium, platinum, platinum-iridium, iridium oxide,
titanium oxide, silver chloride, gold chloride, etc., where the
electrode can be insulated by glass or lacquer, as well as silicon
microelectronics, including tetrode or other multielectrode arrays
or bundles, multichannel and ribbon devices. Typically, the
electrodes can have relatively large tips with low resistance to
detect activity from a number of neuronal elements within the
neural system. Smaller tipped electrodes can be used for monitoring
activity from single neurons or smaller populations. Activity can
be measured from one or more electrodes, preferably two or more. In
some cases, it may be desired to record from several regions of the
neural system in order to characterize its activity. Recordings of
intracellular, extracellular, or a combination thereof, can be
analyzed separately, or together. The electrodes can be AC- or
DC-coupled.
[0025] For certain purposes, iridium oxide type electrodes may be
preferred since they are relatively nontoxic to cells, as well as
being effective carriers of high current and charge densities. An
activated iridium or iridium alloy wire can be used, or a metal
substrate, such as noble metal (e.g., Au, Pt, or PtIr), ferrous
steel alloy, stainless steel, tungsten, titanium, Si microprobe,
etc., or other suitable substrate, can be coated with a film of
iridium oxide to produce an effective electrode. Any suitable
method to prepare the coating can be used, including, but not
limited to, an activation process (e.g., Loeb et al., J. Neuro.
Sci. Methods, 63:175-183, 1995; Anderson et al., IEEE Trans.
Biomed. Eng., 36:693-704, 1989) to form activated iridium oxide
films (AIROFs), thermal decomposition (Robblea et al., Mat. Res.
Soc. Symp. Proc., 55:303-310, 1986) to form thermal iridium oxide
films (TIROFs), reactive sputtering (15) to form sputtered iridium
oxide films (SIROFs), electrodepositing (Kreider et al., Sensors
and Actuators, B28:167-172, 1995) to form electrodeposited iridium
oxide films (EIROFs), etc.
[0026] As described herein, it has been found that adaptive
modulation of a neural system can be used to modify its neuronal
activity. In preferred embodiments, this is achieved by
characterizing the neuronal activity and then using a feedback
algorithm to determine the field magnitude necessary to modulate
its activity. Neuronal activity can be characterized by various
measurements, depending upon the particular activity that is being
assessed. When electrical activity is a determinant, then
measurements can include, e.g., local field polarity and magnitude
(e.g., -10 mV), burst activity, burst amplitude, burst frequency,
power in a predetermined frequency band of activity, non-burst
activity, single or small population firing rate, amplitude or
phase of periodic activity, such as theta rhythm, root-mean-square
(RMS), variance, etc. In general, any suitable measure of neuronal
activity can be used as the feedback stimulus for the applied
field. The feedback stimulus can also be determined by multiple
measurements, e.g., electrical activity, limb motion, cochlear
activity, etc.
[0027] In the examples, the neuronal activity, after appropriate
filtering, was characterized by the RMS fluctuations of the
measured signal, serving as the feedback stimulus. An electric
field was subsequently applied in proportion to the RMS.
Specifically, the instantaneous RMS activity (e.g., the last 0.25
sec of activity) was low pass filtered with a time constant .tau.
to yield A.sub..tau.. This value was compared with a threshold
value, as determined by the long time average of the RMS (e.g., the
last 30 seconds of activity). The magnitude of the applied field
was then derived by calculating the difference between the
A.sub..tau. and the threshold multiplied by a gain factor. Any
suitable methods and/or algorithm for determining field strength
and polarity can be used, e.g., linear and nonlinear proportional
feedback, proportional-integral-differential feedback, etc.
[0028] The values for instantaneous activity and threshold can be
selected empirically, e.g., based on the activity characteristics
of the system and the neuronal activity that is to be controlled.
The goal is to choose a time scale that distinguishes the activity
of interest from the baseline activity of the system. When a
timescale for the threshold (e.g., the last 30 seconds of total
activity) and instantaneous (e.g., last 0.25 sec of total activity)
activity determinations are selected, the difference between such
values should permit detection of the onset of the activity of
interest.
[0029] A gain factor can be chosen such that the output of the
applied field is adequate to modulate the neuronal activity that is
being monitored. It can be empirically derived, based on previous
performance of the neural system and various considerations,
including, e.g., magnitude of the onset of the event which is being
assessed, magnitude of the applied field necessary to modulate the
neural system, characteristics of the field electrodes,
characteristics of the neural system environment, etc. In the
experiments described herein, a gain was chosen such that a typical
difference between A.sub..tau. and the threshold yielded a field in
the range of order of 10 mV/mm. Successful control was achieved for
the same experiment with gains differing by an order of magnitude
indicating that the choice of gain was not critical.
[0030] The applied field can utilize the full feedback signal
("full-wave control"), or, it can be half-wave rectified. When
half-wave rectification is used, a field is applied only when the
instantaneous activity (or the calculated A.sub..tau.) is above (or
below) the threshold value. In the examples described below, a
field was applied only when there was a positive difference between
the instantaneous activity and the threshold. Thus, half-wave
rectification indicates that the field is applied in only one
direction. For full-wave control, a field is applied continuously
when there is any difference between the instantaneous activity (or
calculated A.sub.-.tau.) and the threshold value. The outcome of
half-wave rectification is the application of a field in only one
direction, while full-wave control results in both negative and
positive applied fields, depending upon the sign of the difference
between instantaneous activity and threshold. As a result,
full-wave control can involve the administration of both excitatory
and suppressive signals, while half-wave rectification involves
only one kind of signal, either excitatory or suppressive,
depending upon the direction of the applied field. The experiments
described below show that full-wave control was generally superior
to half-wave rectification for seizure suppression, for reducing
withdrawal seizures, and for obtaining a more regular baseline of
neuronal activity.
[0031] Full-wave control may also be desirable to avoid substantial
electrode and tissue polarization which occurs when half-wave
rectification is used. In the latter case, the electrodes may need
to repolarized between field applications, e.g., by applying bias
currents to the electrodes.
[0032] In general, the duration and intensity of the applied field
can be determined by the measured activity. If the purpose is to
eliminate neuronal activity, then preferably a field potential, or
current, is applied until the activity level is reduced below a
threshold level. At this point, the field can be discontinued until
activity is observed again. The applied field is preferably not a
stationary field, such as the fields described in Gluckman et al.,
J. Neurophys., 76:4202-4205, 1996; U.S. Pat. No. 5,800,459. See,
also, U.S. Pat. Nos. 5,797,965 and 5,522,863.
[0033] Activity can also be augmented, induced, or initiated. In
the examples, reversing the field potential converted sporadic
bursts into a full-blown seizure. In this case, the feedback
stimulus is positive feedback, where the applied field is used to
enhance activity, e.g., by producing depolarization toward
threshold and/or recruiting more neurons into the activity. Here
the sign of the gain factor is switched so that a negative field is
applied when the RMS activity goes above threshold, forcing the
network to become more excitable. The ability to create activity in
vitro and in vivo is useful in variety of ways. It can be used to
create animal models for epilepsy or electroconvulsive therapy
(ECT) and for testing agents which modulate these brain behaviors
for therapeutic, prophylactic, and research purposes. It can also
be used to induce ECT in humans for therapeutic purposes.
[0034] In some instances, a neural system will exhibit ongoing
neuronal activity, such as spike activity varying in amplitude and
frequency. This information can be processed in any suitable way to
serve as a threshold stimulus for the applied field. For instance,
the activity in a certain frequency band can be of particular
interest because it indicates that certain state of the neural
system has been reached, such as epilepsy. It therefore may be
desired to apply the electric field only when the system becomes
epileptic. This can be accomplished by processing the measured
neuronal activity, and applying the field when a predetermined
threshold of activity is reached. For example, the long-term
average of spontaneous or non-epileptic activity can be determined
and used as the stimulus threshold, where no field is applied
unless the long-term average, or a function of the average, is
exceeded. A particular characteristic of neural activity can also
be compared to a matched filter using a temporal, spectral, or
wavelet filter, or a nonlinear filter, and its output compared with
a threshold.
[0035] The methods and devices of the present invention are useful
in any endeavor in which it is desired to modify the behavior of a
neural system. In general, an applied field in accordance with the
present invention can be utilized to modulate any neural activity,
including, e.g., synchronized firing, oscillatory firing, pulsating
activity, and any in-phase activity of a neural system. Because of
such ability to augment or reduce neuronal activity of a neural
system, the invention is useful for modulating many kinds of output
which arise from neural systems, including motor, sensory,
emotional, behavioral, etc.
[0036] For example, the methods and devices of the present
invention are useful for treating brain diseases characterized by
aberrant neuronal activity. Epilepsy, for instance, is a brain
disorder characterized by recurrent seizures, affecting 1-2% of the
population. In this disease, the pattern of neuronal discharge
becomes transiently abnormal. In the examples, an in vitro slice
preparation is utilized to illustrate how epilepsy can be treated
in accordance with the present invention. When perfused in a high
potassium concentration, these networks show a broad range of
interictal-like and epileptiform activity, from network wide
synchronous events to local and propagating events. Application of
the adaptive electric field can be used to suppress the
epileptiform activity, effectively treating and controlling the
brain disorder.
[0037] A modulatory effect can be achieved analogously in situ. For
instance, to treat a patient having epilepsy, a device can be
utilized which simulates the pair of field electrodes used in the
in vitro method. The field electrodes can be positioned in any
arrangement which is effective to produce a modulatory field. They
can be in contact with brain tissue or associated meninges, e.g.,
by inserting, through an occipital entrance hole, one, or more,
long flat electrode strips that contacts the long axis of the
hippocampus surface in the temporal horn of the lateral ventricle.
A round electrode (e.g., a single depth electrode with one or 10
more suitable high current contacts) can also be utilized, e.g., by
placing it within the long axis of the hippocampus in order to
produce a radial electric field. Electrodes can also be external to
the brain, e.g., on the scalp. The electrode strip preferably
produces an effective electric field. Useful electrode strips
include non-polarizing biocompatible electrodes embedded in
silastic sheets with sealed electrode-lead connections, similar to
those used for cochlear implants, e.g., a Clarion Cochlear Implant,
comprising iridium oxide electrodes sealed within a curved silastic
silicone elastomer sheath. In another embodiment, a sheet
comprising multiple electrodes can be placed over the neocortex in
the subdural, subarachnoid, or epidural spaces, or within the sulci
of the brain. Thin electrodes can also be inserted into brain
tissue. In general, any types or combinations of electrodes, such
as those mentioned above, can be used.
[0038] In addition to epilepsy, any brain disorder that displays
abnormal activity, such as oscillatory or pulsating activity, can
be treated analogously. Such diseases, include, schizophrenia,
depression (unipolar or bipolar), Parkinson's disease, anxiety,
obsessive-compulsive disorder (OCD), etc., where the electric field
is applied to the particular brain region exhibiting the abnormal
activity, e.g., cortex, hippocampus, thalamus, etc. Parkinson's
disease is characterized by decreased activity in cells that
produce dopamine. Patients with the disease experience tremors,
rigidity, and difficulty in movement. Patients with Parkinson's
disease can be treated by applying an electric field in an amount
effective to ameliorate one or more symptoms of the disease.
Preferably, the applied field is sub-threshold. The field
electrodes can be placed in any suitable region of the brain, such
as the thalamus or basal ganglia. The electrodes can be of the same
in situ type described above for treating epilepsy. The amount of
applied field can be changed in response to an electrical activity
in the brain, or in response to a manifestation of such electrical
activity. For instance, the field can be applied until one or more
symptoms are eliminated, such as tremors or difficulty in
initiating movement. In such case, the field can be operated
manually by the patient, or the behavior can be monitored
automatically by feedback sensors either within the brain or placed
strategically along the body to sense the behavioral output.
[0039] A method of the present invention also relates to restoring
or repairing a brain function. These functions include, e.g.,
sensory functions, such as vision, hearing, smell, touch, and
taste, motor activity and function, somatic activity and function,
etc. For instance, the method can be useful to treat a condition
where an animal (e.g., a human) has lost its vision due to a
peripheral defect, such as the loss of an eye, but the visual
cortex is largely intact. The present invention can be used to
restore vision by creating patterned activity in the brain using an
applied field. For example, devices can be used to capture images
(e.g., light intensity, wavelength, etc.), process the information,
and use the information as a feedback stimulus to the visual
cortex, or a subservient pathway, modulating the on-going cortical
activity analogously to how epileptic activity was induced from
non-epileptic activity as described above and below. Similar
strategies can be applied to restoring other lost functions, e.g.,
hearing or touch to the auditory or somatosensory cortex,
respectively.
[0040] The present invention also relates to a field-producing
device for modifying the neuronal activity of a neural system
comprising neurons. Such device is not a voltage-clamp device, or a
patch-clamp, as used to clamp the activity of single neurons, or
parts thereof. A field-producing device can comprise one or more of
the following components: (a) field electrode means for applying an
external electric field to a neural system; (b) field application
electronic means for generating an external field to a neural
system, which is operably connected to (a) field electrode means;
(c) measuring means for monitoring the neural activity of the
neural system; (d) measurement electronics means for recording
neural activity, which is operably connected to (d) measuring
electronic means; (e) feedback controller means for determining the
amount of external field to apply to the neural system, which is
operably connected to (b) field application means and (c) measuring
means; (f) sensing means for detecting the external field produced
by the field electrode means; (g) sensing electronic means for
recording the field produced by the field electrode means, which is
operably connected to (f) sensing electrode means and (b) field
application means. The device can be used for in vitro
applications, or as as in vivo prosthetic devices for treating
brain disorders, such as epilepsy and Parkinson's disease, and
restoring brain function. In the latter case, the (f) sensing
electrodes and (g) electronics are optional.
[0041] FIG. 1 illustrates an in vitro field-producing device. In
this example, the (b) field application electronic means and (g)
sensing electronic means are bundled together, along with an
isolation stage. The (d) measuring electronic means is an amplifier
of the type typically used to record extracellular and
intracellular neuronal activity. The (e) feedback controller means
in the example is a computer loaded with the appropriate software
for taking data in from the recording electronics and outputting a
signal, derived from feedback algorithm, to the field electronics.
FIG. 1 also contains a computer ("user interface 7) for recording
and displaying information from the various components of the
device The device preferably is for applying a sub-threshold field.
It can further comprise a power source for generating the applied
field (e.g., a direct or inductive source); external feedback
sensors for detecting behavioral output, etc.
[0042] For in vivo applications, various methods can be used to
place the electrodes the in target tissue, including, visually,
stereotactically, endoscopically, ultrasonically, x-rays (such as
CT scan), nuclear magnetic resonance, electrical activity, etc.
[0043] In addition to identifying characteristics to be used in
calculating a feedback stimulus, an additional parameter that can
be varied is the choice of the activity that is being measured.
Thus, for instance, the feedback stimulus activity can be measured
intracellularly from one or more neurons, or extracellularly,
capturing field potential from single neurons or a neuronal
population. Additionally, the feedback stimulus can be remote or
external to the neural system. Thus, the feedback stimulus can be
recorded at the site of field application (e.g., using measuring
electrodes placed in the tissue), at site remote from the field
application, or using a behavioral feedback stimulus, such as
movement of a limb when motor activity is modulated, or the ability
to experience a sensation when sensory activity is modulated.
[0044] The present invention also relates to methods of identifying
pharmacological agents which modulate the neuronal activity of a
neural system comprising neurons, comprising one or more of the
following steps in any order, e.g., measuring the neuronal activity
of a neural system; applying an oriented electric field to said
neural system effective to modify the neuronal activity of the
neural system, wherein the magnitude and polarity of said applied
electric field is changed in response to the measured neuronal
activity; and administering an agent which modulates the neuronal
activity of the neural system. Such a method is especially useful
for identifying agents that can be used therapeutically and/or
prophylactically in brain disease. Any agent can be administered to
the neural system, including, e.g., neurotransmitter agonists and
antagonists (such as, serotonin, dopamine, GABA, glutamate),
sympathomimetics, cholinergics, adrenergics, muscarinics,
antispasmodics, hormones, peptides, genes (sense and antisense,
including genetic therapy), metabolites, cells (e.g., where neural
grafting is being used as a modulatory therapy), sedatives,
hypnotics, anti-epileptics (e.g., acetazolamide, amphetamine,
carbamazepine, chloropromazine, clorazepate, dextroamphetamine,
dimenhydrinate, ephedrine, divalproex, ethosuximide, magnesium
sulfate, mephenyloin, metharbital, methsuximide, oxazepam,
paraldehyde, pamethadione, phenacemide, phenobarbital,
phensuximide, phenyloin, primidone, trimethadione, valproate,
etc.), hormones, peptides, etc.
[0045] In an in vitro method and device of the present invention, a
slice of rat brain tissue obtained from the hippocampus of the
temporal lobe is perfused with an oxygenated physiological
perfusate fluid ("ACSF" or artificial cerebrospinal fluid) in an
interface-type perfusion chamber (e.g., Hass-style) comprising an
inlet 9 and outlet 10 for continuously replacing the perfusate. A
heated oxygen/carbon dioxide gas (95% oxygen, 5% carbon dioxide at
35.degree. C.) is provided through inlet 11. The top of the chamber
can be open, or covered.
[0046] The anatomy of the brain tissue includes layers of pyramidal
neurons of the Cornu Ammonis (CA) regions. In order to induce
seizures, the ACSF perfusate is replaced through the inlet 9 with a
high potassium solution, comprising 8.5 mM potassium and 141 mM
chloride. The elevated potassium produces epileptic activity
characterized by events in the form of spontaneous burst firings
and seizure-like events within the two regions (CA3 and CA1
respectively) at opposite ends of the Cornu Ammonis. Seizure-like
activity can also be produced by other treatments, including,
penicillin, low magnesium, kainic acid lesions, or any one of the
epileptogenic compounds. Additionally, naturally-occurring and
induced mutants which result in aberrant brain activity, including
mutants produced by genetic-engineering, e.g., in channel genes and
receptor genes, can be used as a source of brain tissue.
[0047] The brain tissue slice labeled by reference numeral 1 in
FIG. 1 is supported on a nylon mesh 2 submerged in artificial
cerebrospinal fluid the perfusate within a chamber formed by an
annular wall 3. A pair of parallel spaced Ag--AgCl field electrode
plates 4 (F1, F2) are placed on the floor of the chamber,
positioned in such a manner to produce an electric field parallel
to the soma-dendritic axis. The field electrodes 4 are spaced apart
from each other, for example by 1.8 cm. An electric field is
established between the electrodes 4 in the perfusion chamber
within which the tissue slice 1 is submerged in the perfusate
fluid. A pair of ground electrodes 10 (G) are positioned on the
floor of the chamber. A pair of Ag--AgCl sensing electrodes 5 (S1,
S2), placed 12 mm apart, are shown in FIG. 1 for sensing the field
produced by the electrodes 4 and to feedback control the field in
the chamber. Micropipette measuring electrodes 12 (above the
chamber) are used to measure neuronal activity extracellularly. The
electronics are set up so that the potential between S1 and S2 is
equal to a gain (of 1 or 0.1) times the program potential (from the
computer or a waveform generator).
[0048] The measuring electrodes 12 are adjacent to the pyramidal
cell layer of the brain tissue slice 1 at a position along a field
isopotential to minimize recording artifact by means of
differential amplification. Such positional arrangement of the
electrodes 12 allows for continuous recording of neuronal activity
in the brain tissue slice 1 despite relatively substantial changes
in the electric field established between the electrodes 4.
[0049] The potential measured through the measuring electrodes 12
are filtered through the recording amplifier 6 and directed to the
user interface for monitoring and parameter control 7 and the
feedback controller 8. The monitoring and parameter control 7 can
accept input from the recording electrode 6 and the feedback
controller 8, and display and record such input. Based on the
measured activity from the recording electrodes 12, an electric
field is externally imposed on the brain tissue slice 1 by applying
a potential difference to the electrodes 4 through the field
application electronics 9. The amount of generated field is
determined by the feedback controller 8 which accepts information
from the recording (measuring) electrode electronics 6 about the
activity of the neural system, and using a selected algorithm
(either as software, hardware, or a combination), generates a
signal to the field electronics 9. This signal to the field
electronics 9 results in the application of a field by the field
electrode means 4. The field application electronics 9 comprises an
amplifier circuit through a 4-probe feedback technique which
applies a potential (or current) between the field electrodes 4 in
order to set the field between the sensing electrodes 5 equal to
the amplifier's program voltage times a gain (gain=1 or 0.1). Built
into this circuit is a layer of ground isolation stage that allow
its potentials to float from those of the recording system.
[0050] The electronics used to control the field can comprise an
input stage A, a standard summing amplifier with a switchable gain
of either 1.0 or 0.1 and a low pass frequency of 10 kHz. The output
of A is sent both to a monitoring stage B, and to an isolated
output stage C. The monitoring stage B can be composed of a unity
gain non-inverting amplifier which acts as a buffer to a monitoring
channel for recording the summed input. The output stage C can be a
circuit utilizing the Analog Devices AMP01 instrumentation
amplifier and a OP37 op-amp which provides the feedback stabilized
field via the Ag--AgCl electrode plates in a chamber D. This stage
can be separately powered by rechargeable batteries in order to
isolate this circuit from measurement ground. Unity gain buffers
(e.g., from an AD712 op-amp) used to minimize the current through
sensing plates S1 and S2.
EXAMPLES
[0051] Materials and Methods
[0052] Tissue preparations. Sprague-Dawley rats weighing 125-150 gm
were anesthetized with diethyl-ether and decapitated in a
accordance with a George Mason University Animal Use Review Board
approved protocol. Hippocampal slices 400 .mu.m thick were prepared
with a tissue chopper, cut either transversely or longitudinally
with respect to the long axis of the hippocampus, and placed in an
interface type perfusion chamber at 35.degree. C. After 90 min of
incubation in normal artificial cerebrospinal fluid (ACSF: 155 mM
Na.sup.+, 136 mM Cl.sup.-, 3.5 mM K.sup.+, 1.2 mM Ca.sup.2+, 1.2 mM
Mg.sup.2+, 1.25 mM PO.sub.4.sup.2-, 24 mM HCO.sub.3.sup.-, 1.2 mM
SO.sub.4.sup.2-, and 10 mM dextrose), the perfusate was replaced
with elevated potassium ACSF (8.5 mM [K.sup.+] and 141 mM
[Cl.sup.-]) and the slices were allowed another 30 min incubation
time. In some experiments, transverse slices were further cut so as
to isolate just the CA1 region, and then allowed to incubate longer
until seizures were observed.
[0053] Experimental apparatus and electronics. A schematic of the
experimental system is shown in FIG. 1. A uniform electric field
was introduced by passing current between a pair of large Ag--AgCl
plates embedded in the chamber floor relatively far from the slice
(17 mm plate separation). A 4 electrode technique was employed,
where a separate pair of electrodes was used to sense the field in
addition to the pair of field producing electrodes (Cole, 1972).
This eliminated effects from the slow polarization known to occur
even in "nonpolarizing" Ag--AgCl electrodes. Field application
electronics were used that control the current between the field
plates such that the potential difference between the sensing
electronics equals an input voltage signal and such that the
potential of the plates float with respect to signal ground
(defined by a pair of Ag--AgCl plates near the chamber midline).
The input voltage signal to the field electronics was
computer-generated, and low pass filtered (<30 kHz) in order to
eliminate artifacts from the digital to analog conversion.
[0054] Electrophysiological recordings: Synchronous neuronal
population activity was monitored by measuring the extracellular
potential in the cell body layer of the CA1 region. Extracellular
recordings were made with paired saline filled micropipette
electrodes (1-4 M.OMEGA.) and a differential DC coupled amplifier
(Grass Model P16). In order to produce a feedback system,
measurement of neuronal activity must be performed simultaneously
with the applied field. Two approaches to minimizing artifact from
the field in the recordings were used. First, the micropipette
electrodes were aligned as close as possible to an isopotential of
the applied field. Alignment was achieved by applying a sinusoidal
field and adjusting the position of the reference electrode so as
to minimize the field artifact. This allowed us to measure neuronal
activity in the presence of relatively large (50-100 mV/mm) fields
with high resolution and without saturating the recording
amplifiers. Second, since some stimulus artifact persists in our
measurements, we additionally restricted the frequency content of
the applied field to be distinct from that of the measured activity
of primary interest.
[0055] Feedback algorithm. For feedback purposes we characterized
the neuronal activity associated with seizures as the RMS of the
recorded activity measured within a frequency band of 100-500 Hz,
averaged over a time which varied from 0.1-1.5 s. The applied field
was proportional to the positive difference between this RMS
activity and a threshold value. The threshold was set by an average
(.about.30-3000 s) of the measured RMS power. The frequency content
of the applied field was restricted to less than 10 Hertz. For
practical purposes, a maximal (saturation) field amplitude was
enforced. In some applications, the output field was half-wave
rectified (i.e. when the RMS was below threshold, no field was
applied). Both the gain and the threshold were set empirically. In
general, optimal control was found with a moderate gain which could
be estimated by .about.(50 millivolts/mm)/(peak recorded power of a
seizure).
[0056] Field strengths are presented in units of mV/mm, with
positive field correspondingly aligned with the primary
dendrite-soma axis to produce a suppressive effect, as illustrated
at the bottom of FIG. 1. Gains are presented in arbitrary units,
with positive gain corresponding to negative feedback mode.
[0057] Analysis Methods
[0058] Seizure-like events in these slices are characterized from
extracellular field potential recordings by an extended burst of
high frequency (100-350 Hz) activity accompanied by a relatively
large (0.2-5 mV) low frequency (0.01-1 Hz) negative potential shift
which typically lasts many seconds. Three methods were used to
characterize neuronal activity from the field potential recordings.
First, events were detected from the high frequency activity in the
field potentials. The RMS power in the frequency band 100-300 Hz
was calculated from the field potential recordings with a time
constant of 0.1-0.5 s, then analyzed with a simple threshold
crossing event detection scheme. These "RMS events" were then
characterized by their average and maximum power and duration.
Second, events were detected from the low frequency deflection in
the field potentials. The field potential recordings were low-pass
filtered with a cutoff at 10 Hz, and threshold crossing again
applied. These "DC events" were characterized by their average and
maximum potential shift, as well as duration. We note that because
these analyses are based on distinct or separate frequency bands,
they are independent measures. Finally, spectral methods were used
to characterize average frequency content of the neuronal activity
during different types of stimuli.
[0059] Prior to each of the above-mentioned analyses, the linear
component of the stimulus artifact was calculated from the
cross-correlation coefficient between the field-potential
recordings and the stimulus. The stimulus artifact accounted for
less than 5% of the RMS deviations in the field-potential
recordings.
[0060] Results
[0061] Electric fields are known to modulate neuronal activity and
even transiently suppress seizure-like activity (Gluckman, et. al.,
1996a). Our objective in this work was to demonstrate that, when
applied in a feedback fashion, that control of seizure-like network
behavior could be achieved for extended periods of time.
[0062] Field Characteristics
[0063] Critical to performing these experiments was our ability to
record neuronal activity independent of the applied time-varying
electric field stimulus with minimal field stimulation artifact in
the recording. We achieve this with the use of DC differential
recordings from paired electrodes aligned to be nearly on the same
isopotential of the applied field. We further restricted our
applied field to have frequency content in a band distinct from
that of the signal in which we were interested. This distinction is
illustrated in FIG. 2. Power spectra for recorded activity and
applied field are shown for both the case where the applied field
is noise (2A) and the case where the field is a typical feedback
signal (2B). In addition, we have post-processed our recording to
eliminate the residual artifact, which typically constitutes less
than 5% of the RMS field-potential variations. The power spectra
for the processed signals is also shown in these plots, and is
indistinguishable from the unprocessed signals except at low (<3
Hz) frequencies. These results indicate that the applied field
during control is not simply masking the neuronal activity in the
recording process during control. Since the applied field was
restricted to have frequency content below 10 Hz, it only changes
the character of the field potential recordings at the lowest
frequencies.
[0064] Overview of Control Phenomena
[0065] There is a characteristic low frequency negative potential
shift of the tissue associated with these seizure-like events in
vitro (Traynelis and Dingledine, 1988) that is quite similar to the
slow low frequency potential shifts observed during in vitro
seizures (Wadman et al., 1992). Typical seizure-like events in
these slices exhibited durations of order 5-25 seconds and
inter-event intervals of order 40 seconds, and low frequency
(0.01-1 Hz) potential shifts of order 0.2-5 mV. Recording to
recording variations in the morphology and amplitude of DC
deflection can be attributed to the details of the measurement
electrode location with respect to both the origin of the seizure
and to the position of the reference electrode.
[0066] Seizure Suppression. In FIGS. 3A and 3B we show examples
that illustrate how an electric field can be used to adaptively
suppress seizure-like activity within the CA1. Suppression is
achieved by using negative feedback. In both cases the high
frequency activity, towards which the suppression algorithm is
directed, is significantly attenuated. The DC shift was completely
eliminated (3A) during suppression for some slices, while it was
partially retained (3B) for others. During control, some non-zero
level of network activity is still observed from the field
potentials (third inset in each). We have documented successful
suppression in 20 of 30 seizing slices with which we applied
adaptive control.
[0067] Control can often be maintained for prolonged periods of
time. To date, the longest we have maintained control is 16 minutes
in a slice otherwise exhibiting seizures approximately every 40
seconds. Since the amplitude, duration and interval between of the
events slowly change over the course of an hour (see FIG. 4), 16
minutes is near the limit for reliable suppression testing in this
system.
[0068] Seizure Enhancement. Positive feedback, set by changing the
sign on the gain which reverses the applied field polarity, can be
used to either enhance seizures or even create seizures where none
were observed beforehand. In FIG. 3C, we show an example of the
characteristic population burst-firing events seen in high
[K.sup.+] hippocampal slices (Rutecki et al., 1985) in the
uncontrolled state. With positive feedback control, the adaptively
applied field now enhances the brief network bursts into large
seizure-like events with the substantial low frequency potential
shifts characteristic of seizures. We have documented seizure
generation in all 4 non-sizing slices with which we applied
positive feedback control.
[0069] Comparison of Parameters: a Single Experiment
[0070] Detailed event extraction results for a 90-minute recording
from a single experiment is shown in FIG. 4. In this experiment, we
compared the application of negative feedback both with and without
half-wave rectification of the applied field at various gains,
application of a constant amplitude suppressive field and random
waveform fields, as well as positive feedback control. From this
experiment, we extracted events both from the RMS power in the
frequency band 100<f<350 Hz, which we term "RMS events," and
events from the low frequency (f<10 Hz) potential shifts, which
we term "DC events."
[0071] The type of stimulus applied is indicated in the lower
trace, where the height of the blocks indicate either the gain (G,
left axis) used in the proportional feedback routine, or the
amplitude (A, right axis) of the waveform applied. The Greek
letters indicate the type of stimulus applied, as indicated in the
figure caption. Baseline recordings of 1-4 minutes were made
between stimuli. In the upper plots are shown the duration, maximum
and average deflections (DC or RMS power) of all events extracted
either from the RMS power ("RMS events", upper trace for each pair)
or low frequency deflections ("DC events") as a function of time.
Values for all extracted events are plotted. For the maximum and
average deflections, the horizontal lines correspond to the trigger
threshold for defining an event. As expected, the maximum
deflections are always greater than or equal to the trigger
threshold. In contrast, the average deflection need not be larger
than the trigger threshold. Therefore, the trigger threshold
provides a logical dividing line between large and small events in
the average deflection plots. In the duration plots, a horizontal
line at 3 seconds is plotted as a rough threshold for
distinguishing seizure-like episodes from smaller burst-like
events.
[0072] Feedback Suppression. Negative (i.e. suppressive) feedback,
indicated by a negative gain, was applied with both full-wave
(.alpha.) and half-wave (.beta.) rectification. Even at the
smallest gain used (.alpha..sub.1, .beta..sub.1), all six types of
event characteristics are distinct from the baseline activity
(black) for both detection schemes. At the intermediate gain used,
no DC events were observed during the non-rectified control
(.alpha..sub.2), while only short, low power RMS events were
observed. For half-wave rectified control at comparable gain
(.beta..sub.2), short, small events were observed from both the DC
and the RMS event extraction. At the highest gain used for
non-rectified control (.alpha..sub.3, starting at time 3960 s), no
DC or RMS events were detected throughout the 16 minutes of control
application.
[0073] Examples of activity for this experiment with and without
control are shown in FIG. 5. The upper pair of traces (A)
correspond to the measured field potential (lower) and applied
field (upper) starting 2 minutes prior to the last application of
non-rectified control (.alpha..sub.3). The baseline activity,
without control, is characterized by large seizure-like events that
start with a burst of high frequency activity, which are
accompanied by a large low frequency potential shifts. Details of
one of these events are shown in the trace of B at an expanded
scale (15 s), high-pass filtered at 100 Hz, along with a
spectrogram of the activity covering frequencies from 25-350 Hz.
The power associated with these seizures can be observed in the
spectrogram to start at high frequencies (near 120 Hz) and progress
toward lower frequencies, a characteristic known as a `spectral
chirp`. Similar spectral chirps have been observed to be spectral
signature of human seizures (Schiff, et. al., 2000). The neuronal
activity following the seizure-like events in our experiments, as
measured by the RMS power, is depressed across all frequencies.
[0074] Expanded views for recorded neuronal activity during control
are shown in FIG. 5C with the same scales as B. Although the RMS
power fluctuates during control (C), it never approaches the level
observed in baseline (B). Note that the color scale is logarithmic.
This behavior continues throughout the 16-minute of this control
application (FIG. 4, .alpha..sub.3), where the fluctuation are
never large enough to trigger the RMS event detection. A
spectrogram corresponding to a longer period (150 s) crossing from
baseline to control is shown in D. Throughout the control period,
the RMS power activity lacks both the characteristic highs and lows
observed during non-controlled activity. We note that this power
reduction/stabilization occurs across all frequencies displayed
(25-350 Hz), whereas the applied field was constrained to have
frequency content only below .about.10 Hz. The RMS amplitude of the
applied field averaged over the full control period was 4.8 mV/mm,
and typically much smaller than the allowed maximum of 17.5
mV/mm.
[0075] Suppression with constant field: A relatively large
suppressive constant (DC) field (16.7 mV/mm) was applied starting
at time 900 s (FIG. 4, .gamma.). As was observed in earlier work
(Gluckman, et. al. 1996a), this had the effect of suppressing the
large seizure like events observed with no field. However, the
effect had limited duration, as a large seizure-like event was
observed 276 seconds after initiation of the field, as shown in
FIG. 6A. This is in contrast to the 600 s period of control
initiated at tine t=1400 s, during which no large events were
observed (FIG. 4, .alpha..sub.2).
[0076] Stimulation with low frequency noise: One hypothesis might
be that any low frequency field might elicit a similar suppressive
effect on the neuronal activity. We have tested various
non-adaptive periodic and random signals. Although such signals do
tend to modulate neuronal activity, we have observed little
effective suppressive effect on seizures. Examples of a random
signals were used in the experiment of FIG. 4. Application 6
corresponds to a full-wave (suppressive and enhancing) random
field, while .epsilon. corresponds to a half-wave rectified (only
suppressive) random field. Each was restricted to have frequency
content below 1 Hz. Examples of activity from each of these
applications are shown in FIGS. 6B,C. The full-wave random field
(6B) did have the overall effect of breaking up the seizures in
time and decreasing their duration as measured by the RMS event
extraction (Top of 4). However, the maximum amplitude of those
events as measured in the RMS was typically larger than baseline,
and comparable findings were reflected in the low frequency
deflections (DC events). The half-wave rectified field (6C) had
little effect at either amplitude used.
[0077] Positive Feedback control. We applied a positive feedback
for a short duration during this experiment. During this time, two
events were observed, both of which were relatively large as
measured from the average and maximum deflection for both RMS and
DC detection methods (FIG. 4, .mu.), as compared to the baseline
events nearby in time.
[0078] Statistics using power spectra: The character of the neural
activity during control can be further quantified from the average
power spectra. Spectra from the last control application in FIG. 4
and the baseline recording following it are shown in FIG. 7A. These
averages were calculated by averaging the spectra of 1.63 s
(2.sup.14=16384 points, recorded at 10 kHz) half-overlapping
windows. The standard deviation of power as a function of
frequency, which represents window to window power variations, is
shown in 7B. For both of these measures, the curve for the
controlled activity (line with symbols) lies well below that of the
baseline activity.
[0079] Although our objective was to suppress the seizure-like
events, the control law we used (the algorithm) was designed to
limit the RMS power of recorded neural activity in a frequency band
from 100-500 Hz. We can therefore quantify the success of this
controller by investigating the statistics of the RMS power
integrated over the frequency band 100-350 Hz, again for
overlapping 1.63 s windows. The power above about 250 Hz is
negligible (FIG. 5). This measure should be independent of stimulus
artifact, since the power associated with the stimulus is confined
to frequencies below 10 Hz (FIG. 2). Normalized histograms of this
integrated power are shown in FIG. 8A, for the baseline recordings
(squares), during full-wave feedback control (.alpha., circles) and
half-wave rectified control (.beta., triangles) for the whole
recording of FIG. 4. The distributions for all three conditions are
populated primarily with windows of low power. The windows with
high power are of great interest, since we associate high power in
this frequency band with the first portion of the seizure-like
events. To highlight the tails of these distributions, we compute
the cumulative probability, shown in FIG. 8B. This distribution,
C(p), can be understood to be the fraction of windows with power
greater than p. From it, we observe that the maximum power observed
during baseline is roughly 4 times higher than observed during
control. In addition, roughly 3% of the windows during baseline
activity have higher power than the maximum observed during either
type of control.
[0080] The high-frequency burst of activity in the uncontrolled
seizure-like events is usually followed by a quiet, refractory-like
period. During full-wave control, the objective of the control
algorithm was to maintain a target level of activity by either
suppressing or exciting the network. In order to further illustrate
the controller's efficacy, we show in the inset of 8A the
normalized histogram of power for baseline (squares) and full-wave
feedback (circles, thick line) control computed with logarithmic
bins (power, abscissa; frequency, ordinate). From this graph, it is
clear that such excursions to low power are also curtailed during
full-wave control. Half-wave rectified control (not shown) also
decreased these excursions, but to a lesser extent.
[0081] The window-to-window variance of the integrated power is
plotted vs. the average power in FIG. 8C for each of these
conditions (baseline, control, and rectified control). We use the
variance as a measure of the width of the distribution. The
baseline activity is clearly differentiated statistically from both
types of controlled activity using either the mean or variance as
measures.
[0082] Release Phenomena
[0083] The character of the activity during control varied from
experiment to experiment. It depended both on variations in the
network activity as well as our choice of parameters for the
controller. In some cases, (FIG. 3A), during control, the
network-controller system would be in a cyclic state. The network
would begin to become more excited and then the controller would
apply a field, causing the neural activity to become quiet. The
field would then decrease, and the cycle would repeat. In these
cases, large seizure-like events were observed nearly immediately
when the controller was turned off. An example of such a seizure
following release is illustrated in FIG. 9A, for the same control
run as FIG. 3A. The upper trace is the recorded field potentials,
while the lower trace is the applied field. In other cases, the
amount of intervention by the controller cycled on a longer time
scale (of order a minute), often reaching a point at which no field
would be applied for a few seconds. In those cases, the activity
when control was released depended on the phase of this cycle. If
the controller was actively suppressing when shut off, then a
seizure would progress (FIG. 9B). Otherwise, one would appear
later, but within a few seconds of release.
[0084] In the majority of these experiments only half-wave
rectified control was used. This has the effect of only suppressing
activity when it is above the threshold. If we use the full
proportional feedback control signal (full-wave control), the
effect is not only to suppress when the activity level is too high,
but to also excite when the activity level is too low. In the two
longer experiments (2 slices from 2 rats) in which we compared
full-wave to half-wave rectified control with similar parameters,
upon release the network was consistently quiet for a period
comparable to roughly half the baseline inter-event interval. An
example of full-wave release is shown in FIG. 9C for comparison
with half-wave release of 9B in the same network. During this
experiment, designed to contrast the network responses to these
different control algorithms, we alternated solely between
rectified and non-rectified control (with baseline in-between) at
constant gain. The intervals between turning off control and the
next event were 0.1-6 s for rectified control (3 applications) and
14-17 s (4 applications) for full signal control. Application of a
Student's t test estimates these distributions to be different with
greater than 95% significance. Similar results were observed for
the experiment of FIG. 4.
[0085] Results Summary
[0086] Clear suppression of the seizure-like activity compared to
the baseline activity during was achieved using feedback control
through electric field stimulation in 20 of 30 seizing slices (4
whole transverse slices, 21 cut transverse slices, and 5 CA1
longitudinal slices; prepared from 21 rats). Half-wave control was
applied in all, and full wave control was applied in 5, of the
successful suppression applications. We analyzed 5 experiments in
detail as described for the experiment in FIGS. 4-8. In each of
those experiments, the RMS power and power fluctuation in the
frequency band 100-350 Hz during control was significantly lower
than during baseline recordings, as in FIG. 8C. In each, there were
clear differences in the character (duration, average and maximum
power) of the events as extracted from the RMS power, and 4 out of
5 revealed clear differences from events extracted from the DC
deflections. In 6 experiments (6 slices from 6 rats), we maintained
control for periods of at least 5 minutes without breakthrough
seizures before parameters were changed. In addition, we generated
seizures in non-seizing slices by applying positive feedback in 4
experiments (4 slices from 4 rats).
[0087] Control Failure
[0088] We were not always successful in controlling seizures, and
the reasons for failure appear multifactorial. Procedural and
equipment problems often played a role. Specifically, failure to
closely align the reference electrode on the same isopotential of
the applied field as the measurement electrode played a role in at
least 3 of the outright failures, and prevented detailed analysis
from at least another 3 experiments. The formation of large air
bubbles deformed the electric field in one experiment. In three
other cases, control parameters (especially the filter settings)
were not found which would suppress the seizures and not respond to
the background activity. This would occur for example when the
events had very little of the high frequency signature at seizure
initiation, so suppressive field was not applied until too
late.
[0089] More interesting are some of the dynamical failures to
control. In some cases of half-wave control, the activity level
would be modulated by the field, but would continue to increase
until the controller would saturate at the maximum allowed field
amplitude. The seizure would then be free to break through, as
observed with constant field application (FIG. 6A). After these
`breakthrough` seizures, the RMS activity would then decrease, and
the field would return to zero. Breakthrough seizures could often
be eliminated by increasing the maximum field amplitude. In four of
the complete failures, breakthrough seizures were observed within
one typical seizure interval of initiation of control. In four of
the successful experiments, breakthrough seizures either were only
observed after 3-7 minutes (3-10 seizure intervals) of control, or
appeared as relatively small events compared to the uncontrolled
activity. In at least three of the cases for which we failed to
control the activity, subsequent multiprobe measurements of
activity indicated that the seizures were initiating at points
distant from where we were controlling, and were propagating toward
the microelectrode.
[0090] For further aspects of neurophysiology, reference is made to
Kandel and Schwartz, 4.sup.th Edition, and, Fundamentals of
Neuroscience, Zigmond et al.
[0091] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The preceding preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0092] The entire disclosure of all patents and publications, cited
above and in the figures are hereby incorporated in their entirety
by reference.
[0093] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
REFERENCES
[0094] Adair, R (1991) Constraints on biological effects of weak
extremely-low-frequency electromagnetic fields. Phys. Rev. A
43:1039-1048.
[0095] Arieli A, Sterkin A, Grinvald A, Aertsen A (1996) Dynamics
of ongoing activity: explanation of the large variability in evoked
cortical responses. Science 273: 1868-1871 Barbarosie M, Avoli M
(1997) CA3-driven hippocampal-entorhinal loop controls rather than
sustains In Vitro limbic seizures, J. Neurosci. 17: 9308-9314.
[0096] Bawin S M, Abu-Assal M L, Sheppard A R, Mahoney M D, Adey W
R (1986a) Long-term effects of sinusoidal extracellular electric
fields in penicillin-treated rat hippocampal slices. Brain Res.
399: 194-199.
[0097] Bawin S M, Sheppard A R, Mahoney M D, Abu-Assal M, Adey W R
(1986b) Comparison between the effects of extracellular direct and
sinusoidal currents on excitability in hippocampal slices. Brain
Res. 362: 350-354
[0098] Belair J, Glass L, an der Heiden U, Milton, J. (eds) (1995).
Dynamical Disease: Mathematical Analysis of Human Illness.
(Woodbury: AIP Press).
[0099] Bragdon A C, Kojima H, Wilson, W A (1992). Suppression of
interictal bursting in hippocampus unleashes seizures in entorhinal
cortex: a proepileptic effect of lowering [K.sup.+].sub.o and
raising [Ca.sub.2.sup.+].sub.o. Brain Research 590: 128-135
[0100] Borck C, Jefferys, J G R (1999). Seizure-Like Events in
Diinhibited Ventral Slices of Adult rat hippocampus. J.
Neurophysiol. 82:2130-2142
[0101] Chan C Y, Nicholson C (1986) Modulation by applied electric
fields of Purkinje and stellate cell activity in the isolated
turtle cerebellum. J. Physiol. (Lond.) 371: 89-114
[0102] Chan CY, Houndsgaard J, Nicholson C (1988) Effects of
electric fields on transmembrane potential and excitability of
turtle cerebellar Purkinje cells In Vitro. J. Physiol. (Lond.) 402:
751-771
[0103] Cole K S (1972) Membranes ions and impulses (Berkeley: Univ.
California Press).
[0104] Cooper I S, Amin I, Riklan M, Waltz J M, Poon T P (1976)
Chronic cerebellar stimulation in epilepsy. Arch. Neurol. 33:
559-570.
[0105] Cooper I S, Upton A R (1985) Therapeutic implications of
modulation of metabolism and functional activity of cerebral cortex
by chronic stimulation of cerebellum and thalamus. Biol Psychiatry
20: 811-813.
[0106] Fisher R S, Uematsu S, Krauss G L, Cysyk B J, McPherson R,
Lesser R P, Gordon B, Schwerdt P, Rise M (1992) Placebo-Controlled
pilot study of centromedian thalamic stimulation in treatment of
intractable seizures. Epilepsia 33, 841-851.
[0107] Ghai R S, Bikson M, Durand, D M (2000) Effects of applied
electric fields on low-calcium epileptiform activity in the CA1
region of rat hippocampal slices. J. Neurophysiol 84, 274-80.
[0108] Gluckman B J, Neel E J, Netoff T I, Ditto W L, Spano M L,
Schiff S J (1996a) Electric field suppression of epileptiform
activity in hippocampal slices. J. Neurophysiol. 76: 4202-4205.
[0109] Gluckman B J, Netoff T I, Neel E J, Ditto W L, Spano M L,
Schiff S J (1996b) Stochastic resonance in a neuronal network from
mammalian brain. Phys. Rev. Lett., 77: 4098-4101.
[0110] Jefferys J G R (1981) Influence of electric fields on the
excitability of granule cells in guinea-pig hippocampal slices. J.
Physiol. (Lond.) 319: 143-152.
[0111] Jerger K, Schiff S J (1995) Periodic pacing an In Vitro
epileptic focus, J. Neurophysiol. 73: 876-879.
[0112] Kayyali H, Durand D (1991) Effects of applied currents on
epileptiform bursts In Vitro. Exper. Neurol. 113: 249-254.
[0113] Lesser R P, Kim S. H., Beyderman L., Miglioreti D. L.,
Webber W. R. S., Bare M., Cysyk B., Krauss G., Gordon B (1999).
Brief bursts of pulse stimulation terminate after discharges caused
by cortical stimulation. Neurology 53: 2073-2081.
[0114] Llins R R, Ribary U, Jeanmonod D, Kronberg E, Mitra P P
(1999) Thalamocortical dysrhythmia: A neurological and
neuropsychiatric syndrome characterized by magnetoencephalography.
Proc. Nat. Acad. Sci. 96: 15222-15227.
[0115] McLachlan R S (1997) Vagus nerve stimulation for intractable
epilepsy: A review. Journal of Clinical Neurophysiology 14:
358-368.
[0116] Murphy J V, Hornig G, Schallert G (1995) Left vagal nerve
stimulation in children with refractory epilepsy. Arch. Neurol. 52:
886-889.
[0117] Nakagawa M, Durand D (1991) Suppression of spontaneous
epileptiform activity with applied currents. Brain Res. 567:
241-247.
[0118] Rushton W A H (1927) The effect upon the threshold for
nervous excitation of the length of nerve exposed, and the angle
between current and nerve. J. Physiol. (Lond.) 63: 357-377.
[0119] Rutecki P A, Lebeda F J, Johnston D (1985). Epileptiform
activity induced by changes in extracellular potassium in
hippocampus, J. Neurophysiol. 54: 1363-1374
[0120] Schiff S J, Jerger K, Duong DH Chang, T., Spano, M L, Ditto
W L (1994). Controlling chaos in the brain. Nature 370:
615-620.
[0121] Schiff S J, Colella D, Hughes E, Conry J, Creekmore J W.,
Marshall A., Bozek-Kuzmicki M., Weinstein S L, Benke G., Gaillard
WD, and Jacyna G M. (2000) Brain Chirps: Spectrographic Signatures
of Epileptic Seizures. Clinical Neurophysiology 111, 953-958.
[0122] Staley K J, Longacher M, Bairns J S, Yee A (1998)
Presynaptic modulation of CA3 network activity. Nat. Neurosci. 1:
201-209.
[0123] Terzoulo C A, Bullock T H (1956) Measurement of imposed
voltage gradient adequate to modulate neuronal firing. Proc. Nat.
Acad. Sci. 42: 687-694.
[0124] Tranchina D, Nicholson C A (1986) Model for the polarization
of neurons by extrinsically applied electric fields. Biophys. J.
50: 1139-1159.
[0125] Traynelis S F, Dingledine R (1988) Potassium-induced
spontaneous electrographic seizures in the rat hippocampal slice.
J. Neurophysiol. 59: 259-276.
[0126] Van Buren J M, Wood J H, Oakley J, Hambrecht F (1978)
Preliminary evaluation of cerebellar stimulation by double-blind
stimulation and biological criteria in the treatment of epilepsy.
J. Neurosurgery 48: 407-416.
[0127] Velasco M, Velasco F, Velasco A L, Boleaga B, Jimenez F,
Brito F, Marquez I (2000) Subacute electrical stimulation of the
hippocampus blocks intractable temporal lobe seizures and
paroxysmal EEG activities. Epilepsia 41: 158-169.
[0128] Wadman W J, Juta A J, Kamphuis W, Somjen G G (1992) Current
source density of sustained potential shifts associated with
electrographic seizures and with spreading depression in rat
hippocampus. Brain Res. 570: 85-91.
[0129] Warren R J, Durand D (1998) Effects of applied currents on
spontaneous epileptiform activity induced by low calcium in the rat
hippocampus. Brain Research 806: 186-195.
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