U.S. patent number 3,753,433 [Application Number 05/107,215] was granted by the patent office on 1973-08-21 for electroencephalophone and feedback system.
This patent grant is currently assigned to Aquarius Electronics. Invention is credited to Frank Bakerich, Robert T. Scully.
United States Patent |
3,753,433 |
Bakerich , et al. |
August 21, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
ELECTROENCEPHALOPHONE AND FEEDBACK SYSTEM
Abstract
An electroencephalophone which is battery operated and
completely self-contained in a headset to be worn by the user and
having a plurality of scalp electrodes carried on flexible leads
permitting placement of the electrodes on the user's head when the
headset is mounted thereon and having an electronic circuit and
transducer for converting brain wave signals picked up by the
electrodes into recognizable audio tones enabling the user to
listen to his own brain wave generation. The scalp electrode
signals are amplified and chopped by an audible oscillator signal
the frequency of which is controlled by the scalp electrode
signals. The resulting combined amplitude and frequency modulation
of the audible generator tone provides a distinctive sound for
brain wave recognition.
Inventors: |
Bakerich; Frank (Mendocino,
CA), Scully; Robert T. (Mendocino, CA) |
Assignee: |
Aquarius Electronics
(Mendocino, CA)
|
Family
ID: |
22315472 |
Appl.
No.: |
05/107,215 |
Filed: |
January 18, 1971 |
Current U.S.
Class: |
600/545; 600/390;
600/28 |
Current CPC
Class: |
A61B
5/7415 (20130101); A61B 5/374 (20210101); A61B
5/332 (20210101); A61B 5/301 (20210101); A61B
5/369 (20210101) |
Current International
Class: |
A61B
5/0476 (20060101); A61B 5/0404 (20060101); A61B
5/0402 (20060101); A61B 5/048 (20060101); A61B
5/0428 (20060101); A61b 005/05 () |
Field of
Search: |
;128/2.1A,2.1B,2.1R,2.6R,1C,422 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
We claim:
1. An electroencephalophone feedback system comprising:
scalp electrodes;
an amplifier having an input connected to said electrodes and
having an output;
an output circuit connected to said amplifier output and including
a voltage actuated chopper device and a series load impedance
connected thereto;
a transducer connected to said load impedance;
a relaxation oscillator generating an audible frequency connected
to and driving said chopper device, said oscillator having a timing
capacitor controlling its said frequency; and
means connected to said amplifier output and capacitor for charging
said capacitor at a rate dependent upon the potential at said
amplifier output.
2. A system as defined in claim 1:
a voltage supply and common;
said oscillator comprising a uni-junction transistor having its
emitter connected to said capacitor and its bases connected to said
voltage supply and common to fire when its emitter is raised to a
predetermined voltage, said means being connected to said emitter
to periodically raise the voltage thereat to said predetermined
value.
3. A system as defined in claim 2, said means comprising a field
effect transistor connected to said output, emitter, capacitor and
voltage supply.
4. A system as defined in claim 3, said field effect transistor
having its drain terminal connected to said output, its gate
terminal connected to said voltage supply and its source terminal
connected to said emitter and capacitor.
5. A system as defined in claim 2;
an impedance connected in series in the connection between common
and said uni-junction base connected to common; and
output terminals connected to said impedance for providing a
frequency modulated brain wave signal.
6. A system as defined in claim 1 and having a common, and said
chopper device comprising a bipolar transistor having its emitter
connected to said common, its base connected to said oscillator,
and its collector connected through said load impedance to said
output.
7. A system as defined in claim 1 wherein said transducer is
connected across said load impedance; and
a pair of parallel reversely connected diodes connected in series
with said transducer and impedance.
8. A system as defined in claim 1, and manually controlled means
for adjusting the magnitude of said output potential thereby
setting the range of amplitude and frequency modulation.
9. A system as defined in claim 8 said transducer being connected
across said load impedance; and
a pair of parallel reversely connected diodes connected in series
with said transducer and impedance providing a squelch action below
a predetermined signal level, said signal level being controlled by
said manually controlled means.
10. A system as defined in claim 1, said amplifier comprising a
narrow band-pass amplifier having peak sensitivity in the range of
about 4 to 13 cycles per second and a sharp roll-off outside of
said range.
11. A system as defined in claim 10, said amplifier comprising a
dual operational amplifier.
12. A system as defined in claim 1, said amplifier having an input
impedance substantially higher than the scalp contact resistance of
said electrodes to minimize generation of artifactual signals.
13. A system as defined in claim 12 wherein said amplifier
comprises a pair of source follower field effect transistor
pre-amplifiers connected to said electrodes; and
a dual operational amplifier having a balanced input connected to
the outputs of said pre-amplifiers.
14. A system as defined in claim 13 wherein said operational
amplifier has common mode rejection thereby eliminating common mode
noise originating at said electrodes and the requirement for system
operation in a shielded room.
15. A self-contained electroencephalophone comprising:
a headset having an earphone housing and a headband connected
thereto for mounting on the head of a user;
a plurality of scalp electrodes adapted for mounting on the user's
head to provide an electric signal pickup of brain waves;
flexible leads carried by said housing and connected to said
electrodes for permitting placement of the latter on the user's
head when the housing is mounted thereon; and
an electronic circuit carried by said housing and comprising an
amplifier having an input connected to said electrodes and having
an output; an output circuit connected to said amplifier output and
including a voltage actuated chopper device and a series load
impedance connected thereto; an oscillator generating an audible
frequency connected to and driving said chopper device; and means
connected to said amplifier output and said oscillator for
controlling said oscillator frequency as a function of the
potential at said amplifier output; and
a transducer carried by said housing and connected to said load
impedance.
16. An electroencephalophone as defined in claim 15, carried by
said housing said electronic circuit including a voltage supply and
common;
said oscillator having a timing capacitor controlling its frequency
and comprising a uni-junction transistor having its emitter
connected to said capacitor and its bases connected to said voltage
supply and common to fire when its emitter is raised to a
predetermined voltage; and
said means comprising a field effect transistor having its drain
terminal connected to said amplifier output, its gate terminal
connected to said voltage supply and its source terminal connected
to said emitter and capacitor.
17. An electroencephalophone as defined in claim 16:
said chopper device comprising a bipolar transistor having its
emitter connected to said common, its base connected to said
oscillator, and its collector connected through said load impedance
to said output.
18. An electroencephalophone as defined in claim 17, and a pair of
parallel reversely connected diodes carried by said housing
connected in series with said transducer and impedance.
19. An electroencephalophone as defined in claim 15;
a resistor connected to said output;
an EEG output terminal;
a voltage supply carried by said housing;
a transistor having its base connected to said resistor, its
collector connected to said voltage supply, and its emitter
connected to said terminal;
a light source connected to said terminal; and
a light detector electrically insulated from said headset and
optically coupled to said light source for providing an electrical
brain wave signal.
Description
The invention relates to electroencephalographs in which the EEG
output is converted into an audio signal enabling audio study and
analysis of the type and character of brain waves being
generated.
Researchers and investigators have found that there is a
relationship between subjective states of mind and observable brain
wave activity as seen with the aid of an electroencephalograph
(EEG). Anxiety, focused attention, relaxed inattention, the various
stages of sleep, and meditation appear to have correlating brain
wave patterns. Anxiety, focused attention and active thought tend
to produce beta waves considered as those having a frequency
greater than about 13 Hz. Relaxed, alert inattention, with the eyes
closed or at least defocused with no patterned visual input, will
tend to produce alpha waves considered as those having frequencies
from about 8 Hz to about 13 Hz. Deeper states of relaxation,
meditation, sleep will involve lower frequencies in the theta and
delta ranges considered as between about 4 Hz to about 8 Hz, and
below 4 Hz respectively. The electroencephalograph thus provides an
important research, educational, and training tool to aid doctors,
investigators and patients in better understanding and achieving
various states of mind. For example, there has been a continuing
search for a means of achieving a peaceful state of mind. Various
meditative techniques, some very ancient and some quite new, have
been and are being used. However, these techniques have a common
drawback in that they do not provide an immediate feedback as to
the progress that the individual is making. It is known that a
peaceful, meditative state of mind is accompanied by the production
of alpha waves and experimental studies have shown that a person,
given appropriate feedback information as to the brain waves he is
generating, can learn to turn on the alpha rhythm at will.
Various forms of feedback information have been proposed. It has
been found that the sense of hearing is far superior to the sense
of sight as a harmonic analyzer, and is therefore more adept at
detecting and analyzing patterns of activity. Stewart, Belcher and
Morris, Auditory Analysis of Electroencephalogram:
Electroencephalophony, Encephalography and Clinical
Neurophysiology, Volume II, pages 161-164 (May, 1959 ed.). The
practical difficulty with listening to brain wave patterns directly
is that the clinically interesting frequencies are in the
sub-audible range. The brain's electrical output must therefore be
transformed before the analytical merits of the human auditory
system can be utilzed. The mentioned publication describes several
methods by which this can be accomplished and suggests a combined
amplitude and frequency modulation of an audio carrier by the brain
waves as providing a highly satisfactory method.
One of the disadvantages of EEG equipment heretofore available for
research, study and training has been the high cost of such
equipment and accordingly the investigations have so far been
conducted in the research settings of major medical laboratories.
Circuitry and components heretofore developed and used have been
complex and relatively large and cumbersome. An object of the
present invention is to provide an electroencephalograph feedback
system of the character described in which the very best of earlier
techniques are used, e.g., amplitude and frequency modulation,
while at the same time circuitry and components have been modified
and evolved to provide a small inexpensive yet completely effective
instrument.
Another object of the present invention is to provide an
electroencephalophone of the character described which is battery
operated and completely self-contained whereby all of the apparatus
is exclusively and comfortably worn on the head of the user without
any trailing wires and thus affording the user mobility,
convenience and safety from electric shock.
A further object of the present invention is to provide an
electroencephalophone of the character above which may be mounted
on the head of the user with ease and facility, is easy to operate,
and in which the desired brain waves are amplified and raised in
pitch to a pleasant comfortable audio frequency, substantially free
of background noise.
Still another object of the present invention is to provide an
extremely small, battery operated EEG circuit combined with an
electroencephalophone and output circuit providing FM telemetry
data and a direct EEG signal, the total system being sufficiently
small and light in weight to be comfortably worn on the head of the
user.
Still a further object of the present invention is to provide an
electroencephalophone of the character above which will provide
dependable substantially carefree service over a long period of
use.
The invention possesses other objects and features of advantage,
some of which of the foregoing will be set forth in the following
description of the preferred form of the invention which is
illustrated in the drawings accompanying and forming part of this
specification. It is to be understood, however, that variations in
the showing made by the said drawings and description may be
adopted within the scope of the invention as set forth in the
claims.
Referring to said drawings:
FIG. 1 is a perspective view of an electroencephalophone
constructed in accordance with the present invention.
FIG. 2 is a cross sectional view of a portion of the
electroencephalophone.
FIG. 3 is a bottom view of the electroencephalophone.
FIG. 4 is a graph of the electronic amplifier output.
FIG. 5 is a schematic wiring diagram of the electronic circuit.
FIG. 6 is a schematic wiring diagram of the battery circuit.
FIG. 7 is a schematic wiring diagram of a photo coupler used in
connection with the present system.
The electroencephalophone of the present invention has as one of
its principal features the fact that it is entirely self-contained
in association with a headset 11 having at least one earphone
housing 12 and a headband 13 connected thereto for mounting on the
head of the user; a plurality of scalp electrodes 14, 15 and 16
adapted for mounting on the user's head to provide an electric
signal pickup of brain waves; electronic circuit means, see FIGS. 5
and 6, for converting brain wave signals into an audio output
signal and being mounted in housing 12; flexible leads 17, 18 and
19 carried by the headset and connected to electrodes 14, 15 and 16
permitting placement of the latter on the user's head when the
headset is mounted in place and connecting the electrodes to the
input of the electronic circuit means; a transducer 21 in housing
12 connected to the output of the electronic circuit means; and a
battery 22 mounted in housing 12 for powering the electronic
circuit.
Preferably a binaural headset is used having a pair of earphone
housings 12 and 24 each fitted with a transducer 21 and 26 (see
FIG. 5); and advantage is here taken of the second earphone housing
24 to include therein a second battery 23 connected in the circuit
as seen in FIG. 6. Electric cables 27 and 28 extend between the
housings 12 and 24 through a conduit provided in the headband 13;
and leads 17, 18 and 19 are here conveniently mounted through an
opening 29 in the headband conduit. Housings 12 and 24 are here
mounted for reciprocation on headband rods 31 for adjusting the
headset to the user's head; and the housings 12 and 24 are
preferably provided with the usual ear pads 32 and 33. The
electronic circuit as seen in FIGS. 5 and 6 is miniaturized and
constructed on a circuit board 34 in housing 12.
With reference to FIGS. 5 and 6, the electroencephalograph feedback
system of the present invention comprises briefly a balanced,
differential input, high impedance amplifier which accomplishes the
task of initially picking up the weak brain wave signals at
electrodes 14, 15 and 16 and amplifying these signals to a
relatively high voltage which can be used to drive the amplitude
and frequency modulation portions of the circuit. The amplifier is
here composed of a pair of source follower field effect transistors
Q1 and Q2 connected to a pair of operational amplifiers Q3A and
Q3B. Q1 and Q2 comprise a preamplifier whose purpose is to provide
high input impedance. Field effect transistors offer the advantages
of lower equivalent input noise and lower costs. Electrode 14 is
here connected through capacitor C1 to the gate of transistor Q1;
electrode 15 is connected to ground and through capacitor C5 to the
midpoint of voltage divider made up of R5, R7, R8 and R6 connected
in series between the drain and source of Q1 and Q2. Electrode 16
is connected to the gate of Q2 through capacitor C2. Similar input
RC impedance networks R1, C3 and R2, C4 are connected in the input
circuits to Q1 and Q2.
The input amplifier Q1 and Q2 functions to provide a high input
impedance for the electrodes 14-16 so as to make less critical the
quality of the contact of the electrodes with the scalp. Preferably
R6 is made adjustable and may be set to a balanced condition at the
factory. With such an input amplifier, the electrodes 14-16 see a
very high impedance, as compared to the input of operational
amplifier Q3A. Input condenser C1 is connected to the base of Q1.
The addition of C3 in the input impedance provides protection
against radio frequency noise. Q1 and Q2 may be either N or P
channel. Components R3 and R4, C5 and C6 form a decoupling network
for transistors Q1 and Q2. The output leads 36 and 37 of the input
amplifier Q1 and Q2 are connected through C7 and C8 across an
output impedance composed of R9 and R10, and to input pins 8 and 9
of operational amplifier Q3A. The input impedance of the present
circuit is about 1 million ohms which is many times larger than the
scalp contact resistance of the electrodes and which accordingly
minimizes the generation of artifactual signals.
Q3A and Q3B here comprises a dual externally frequency compensated
operational amplifier such as Fairchild Semiconductor Model .mu.
A739C dual integrated operational amplifier. The pin marking on the
drawing corresponds with the pin identification on the Fairchild
amplifier. Q3A is an operational amplifier so constructed as to
eliminate common mode signals, such as power line noise, radio
frequency pickup, etc., thus presenting at its output only clear
brain wave signals.
The band width as well as gain of amplifier Q3A is closely
controlled by components C9, C10, R9, R10, R11, R12 as well as C7,
C8, C1, C2, C3 and C4.
C1 and C2 isolate dc potentials developed by contact between the
electrodes and skin, while allowing low frequency brain wave
signals to be conducted to transistors Q1 and Q2.
C3 and C4 provide a low impedance path for short circuiting all
radio signals to ground thereby eliminating annoying radio station
interference.
C7 and C8 also provide increasingly better conduction for high
frequencies between Q1 and Q2 and Q3A. This effect is compensated
for by C9 and C10 both of which tend to reduce the gain of
amplifier 3A for high frequencies.
R9 and R10 serve the function of controlling the gain and dc
stability of the operational amplifiers.
R13, R14 and C11 form a high frequency filter, and C12 forms an
additional low frequency filter to drive operational amplifier Q3B.
The latter's response is further modified by C15, C16, C14 and C18.
C14, C15 and C16 all tend to reduce the amplifier high frequency
response while C13 tends to set the low frequency response roll off
point. The result of the combination of all of these frequency
response determining elements is a relatively sharp band pass
amplifier whose output is delivered through R17 to an output
circuit point 38.
The dc stability of operational amplifier Q3B is maintained by
components R15, R16, R28, R17 and R18. The amplified EEG signal at
point 38 is applied to the base of bi-polar transistor Q4 which is
here connected as an emitter follower so that it has only unity
voltage gain and much lower output impedance than input impedance.
The output of amplifier Q4 is connected to one end of potentiometer
R19, the moving contact 39 of which here functions as a sensitivity
control. A manually engagable knob, here identified by the same
numeral 39, may be seen on housing 12 for use by the operator in
adjusting the sensitivity of the apparatus.
The effect of the frequency response determining elements above
described is to produce a narrow band amplifier having an output
substantially as shown in FIG. 4 where the voltage amplitude is
plotted as the ordinate against frequency in Hz as the abscissa. It
will be noted that the amplifier has a peak sensitivity in the
range of about 4-13 cycles per second and a sharp roll off outside
of this range. As hereinabove noted the apparatus is particularly
designed for enabling the user to sense alpha wave generation and
it will be noted that the amplifier peaks out in the alpha range of
8-12 cycles per second. As training progresses, and alpha
generation becomes easier, the user may begin to listen for the
deeper rhythmic theta train leading to a deeper meditative state.
It will be noted that the amplifier output is still high in the
theta range of about 4-8 cycles per second. On the other hand the
amplifier output falls off sharply in the beta range of frequencies
greater than 13 Hz.
As hereinabove noted the practical difficulty with listening to
brain wave patterns directly is that the clinically interesting
frequencies as herein noted are in the sub-audible range.
Accordingly the balance of the electronic circuit converts the low
frequency brain wave signal into a clearly recognizable audio
signal by using the low frequency signal to modulate an oscillator
operating in the audio range whereby a combination amplitude and
frequency modulation is obtained. The balance of the electronic
circuit for accomplishing this result comprises briefly a
relaxation oscillator Q6 having a timing capacitor C20 and C19;
means, Q5, connected to output 39 and the timing capacitor,
C20-C19, for charging the capacitor at a rate dependent upon the
potential at 39; an output circuit including a voltage actuated
chopper device, Q7, and a series load impedance R25 connected to
output 39 and being connected to oscillator Q6 for controlling the
operation of the chopper device and current flow in the load
impedance 25 synchronously with the operation of the oscillator;
and an electronic audio read-out device, transducers 21 and 26,
connected to load impedance 25.
As a feature of the present construction, oscillator Q6 comprises a
uni-junction transistor having its bases B1 and B2 connected to
supply voltage, battery 23, and common (ground) 41 through battery
22, R23 and R24 so as to fire when its emitter e is raised to a
predetermined voltage, here about 4 to 8 volts. Emitter e is
connected via conductor 42 to capacitors C19 and C20. C20 is
connected to common through battery 22 and C19 is connected to
common through battery 22 through the base and emitter of
transistor Q7 and resistor 27.
As a further feature of the present construction, the means for
controlling the charging rate of capacitors C19 and C20 as a
function of EEG potential at output 39 here comprises a field
effect transistor Q5 having its drain terminal connected to output
39 through capacitor C17 and resistor R21, its gate terminal
connected to supply voltage 23 and its source terminal connected
via conductor 42 to the emitter of Q6 and capacitors C19 and C20.
The relaxation oscillator thus provided furnishes a pleasing
triangular shaped output tone having a frequency of about 1,000 Hz.
Capacitor C19 couples the oscillator signal to Q7 thus turning the
chopper on and off with the oscillations from Q6. Additionally,
resistor R26 acts to provide dc bias for transistor Q7.
As another feature of the present circuit, the chopper device here
comprises a bipolar transistor Q7 having its emitter connected
through R27 to supply voltage 22, its base connected through
capacitor C19 to the emitter of oscillator Q6, and its collector
connected through load impedance R25, capacitor C18 and resistor
R29 to output 39. The primary 43 of output transformer T1 is
connected by conductors 44 and 45 across impedance R25; and the
secondary winding 46 of transformer T1 is connected by conductors
47 and 48 to transducers 21 and 26. The result of the foregoing is
that the current passing through the earphones becomes a replica of
the brain wave signal chopped by the oscillator signal produced by
oscillator Q6 and thus comprises an amplitude and frequency
modulated presentation of the brain waves.
The use of the field effect transistor Q5, and more particularly
its connections in the present circuit, are unorthodox in that the
normal connections for the FET in a voltage dependent resistor mode
are to connect its drain to the supply voltage and its gate to the
control voltage. The reversal of these two connections in the
present case enables a substantial simplification in the circuitry.
The relaxation oscillator circuit Q6 differs from other relaxation
oscillators in that its frequency is adjusted by changing the rate
of charging of its timing capacitance rather than shifting the
firing voltage of the triggering device.
To summarize, the voltage fluctuation at C19 due to firing of Q6
turns on and off chopper Q7. Accordingly, brain wave signals at R25
are chopped to ground by Q7 at the running frequency of oscillator
Q6. When the EEG voltage at point 39 goes positive, the running
frequency of oscillator Q6 will rise due to decreasing resistance
of FET Q5 causing more rapid charging of C19 and C20. At the same
time that the frequency increases, voltage at output 39 is going
up, increasing current flow through R29, C18, R25, Q7 and R27 to
ground. As a result the voltage drop across R25 increases thereby
providing amplitude modulation of the signal. On decreasing voltage
at point 39, the reverse effect takes place, e.g., reduction in
frequency of the oscillator, the chopping action of Q7 and
amplitude drop across R25. The overall variation in frequency may
be in the order of about 500 Hz to about 2,000 Hz. The combination
of these two actions, viz, frequency and amplitude modulation,
provide a most distinctive sound for brain wave recognition which
is at the same time most pleasant to listen to for long periods of
time. When the user is not producing a desired brain wave pattern,
the output at the transducer is as close to silence as possible.
The sound produced by alpha wave generation is similar to the
vibrato of an organ at about middle C13 the frequency of the
vibrato being in the range of about 8- 10 cycles per second.
Three electrodes are preferably used although it would be possible
to have a balanced input with only two electrodes. The third
electrode, usually black, shown as electrode 15, is desired to
provide an actual ground or electrical midpoint connection between
the other two electrodes 14 and 16, commonly red electrodes. A wide
variety of electrode placements may be used on the head depending
on the part of the brain to be monitored. It is usually desirable
to employ a minimum amount of electrode paste on the electrodes and
particularly in the hair, and accordingly, a common placement of
the electrodes is to locate the black electrode on the skin of the
forehead over one eye, one red electrode on the skin of the
forehead over the other eye, and the other red electrode at the
back of the head generally behind the first electrode. These may be
held conveniently in place by an elastic headband 20.
Another important feature of the present invention is the
incorporation of a squelch circuit which eliminates output signals
below a predetermined level. This circuit here comprises a pair of
reversely connected germanium diodes D1 and D2 which are connected
in series with load impedance R25 and the primary 23 of transformer
T1. Reversely connected diodes D1 and D2 connected in this manner
are effective in eliminating annoying background noise and
cooperate with variable resistor R19 for adjusting not only the
magnitude and frequency of the output signal but also the threshold
of squelch action. Signals not of interest are suppressed by the
diodes by reason of the fact that when point 39 is at a relatively
constant voltage, almost no output will appear at transformer T1
due to the fact that capacitor C18 conducts almost no current down
through R25 and this small current produces a voltage across R25
insufficient to cause the diodes to conduct. On the other hand, a
significant signal at 39 will be emphasized not only in the
amplitude of output at T1, but also by shift in frequency of the
oscillator. Thus the combination of the sensitivity control R19,
the squelch circuit, frequency and amplitude modulation cooperate
to emphasize as much as possible the signals which are of interest
and to de-emphasize as much as possible background noise which is
not of interest. Also placing the squelch circuit at the "back end"
of the circuit has the very significant advantage of applying the
squelch only where it is needed-- thus not affecting the FM or EEG
output signals which may be taken from the circuit ahead of the
squelch circuit. Also, by reason of the placement of the squelch
circuit, the signal level is such that no biasing is needed to set
the proper squelch level. The threshold level depends upon the
component values in the output circuit. These are chosen to
eliminate signals below a minimum value. The squelch circuit is
thus automatically set to eliminate all audible signals of less
than the preset level which is the threshold of comfortable
listening. The sensitivity control R19 also adjusts the squelch
level since it changes the peak-to-peak signal voltage across R25
as well as adjusting the FM deviation by changing the amount of EEG
signal fed to Q5 through R21 and C17. It will also be noted that
the present circuit is limited in low frequency response in that it
is AC coupled by capacitor C18. The presence of capacitor C18
ensures that there will be no carrier audible unless there is a
brain wave signal present above a minimum frequency and amplitude,
the circuit thus serving to achieve carrier suppression.
Other features of advantage of the present circuit is the provision
for use of auxiliary equipment. As will be seen from FIG. 3, four
output jacks 51, 52, 53 and 54 are provided on the headset housings
12 and 24 for monitoring the operation of the unit by a user, and
obtaining an FM output and an EEG output therefrom. Jack 51 is
connected across transducers 21 and 26 and is coupled to a normally
closed switch 56. An observer may plug in a headphone into jack 51
and listen to the feedback signal generated by the patient or user
while cutting off the feedback to the patient. Jack 52 is similarly
connected across transducers 21 and 26 but without a switch
incorporated therewith so that the observer plugging his headset
into jack 52 will listen to the feedback along with the
patient.
Jack 53 is here connected across resistor R24 in the oscillator
circuit and will accordingly provide a low impedance positive pulse
signal of several volts amplitude corresponding in frequency to the
oscillator formed by Q6, R25, R24 and C20. Since the signal derived
is pure FM, it is possible to obtain a highly satisfactory
recording qualitywise using even an inexpensive consumer-type audio
tape recorder. The signals can later be de-modulated by a simple
diode pump circuit to recover the original brain wave signal. An
interesting and important advantage is obtained in using a stereo
casette recorder having one channel connected to the heatset and
the other channel connected to a microphone so that as significant
brain wave patterns are experienced by the subject he may make an
appropriate note of them on the second channel of the recorder.
Thus after a relatively long session the particularly significant
portions of the recording may be quickly isolated from the balance
of the tape. This output may also be used to drive an FM radio
transmitter thus providing FM-FM telemetry data.
Jack 54 provides an EEG output and is connected at the output of a
network formed by R30, transistor Q8, and resistor R20, the latter
being connected to point 38 in the circuit. More specifically, Q8
has its base connected to R20, its collector connected to battery
23, and its emitter connected through R30 to jack 54. This network
provides a high current output EEG signal in amplified form but
without audible tone. This signal may be fed to chart recorders,
oscilloscopes, or tuned-computer systems which will sort out
particular spectra or groups of brain waves. In all instances where
auxiliary equipment is power line operated, it is mandatory that
the auxiliary equipment positively prevent an accidental feedback
of high voltage to the headset. One means of accomplishing this
safety feature is to use the signal to drive a gallium arsenide
photo diode to provide a light signal replica of the EEG signal.
This light signal may be picked up and detected by a Darlington
photo transistor thus providing a means of extracting the brain
wave signal from the present battery operated electroencephalophone
and making it available for use with power line operated equipment
without electrical shock hazard to the user, the connection between
the power line operated equipment and the present headset being
light transmission. The circuit formed by resistor R20, transistor
Q8, and resistor R30 draws no current from the battery unless and
until the gallium arsenide photo diode is plugged into the EEG
output jack.
Earphone housings 12 and 24 are provided with demountable end
covers 57 and 58 which are removed to open the interior battery
compartments for insertion and removal of batteries 22 and 23. The
present circuit is designed for use with, and the compartments are
dimensioned to receive, standard size 9 volt transistor radio
batteries. Manually operable switches 61 and 62 are carried by the
earphone housings and are connected to the batteries as shown in
FIG. 6. The dual operational amplifier, relaxation oscillator and
chopper circuit are preferably microminiaturized by means of hybrid
integration techniques for compact mounting on circuit board 34 in
headset housing 12.
The light emitting portion of the photon coupler is a gallium
arsenide light emitting diode D3 (see FIG. 7) whose anode is
connected by conductor 66 to the common side of plug 67 adapted for
connection to output jack 54. The cathode of the diode is connected
by conductor 68 through a current limiting resistor R36 to the
center pin of plug 67. An ordinary silicon diode D4 is reverse
connected across the photon coupler's light emitting diode D3 to
protect it from excessive reverse transients.
The phototransistor Q9 portion here shown is a Darlington
phototransistor. A feedback resistor R31 is connected between its
base and collector to provide dc stability. The emitter is
"grounded," that is, it is connected to the negative power supply
lead 69 which is also the shield 71 of the cable conductor 72
leading to the photon coupler. The light emitting
diode-phototransistor combination, together with D4, R36, and R31
are all mounted as close as possible to the plug 67 which is used
to connect the photon coupler to the EEG output jack 54. The output
from this assembly is a shielded cable whose shield is connected to
the emitter of Q9 and whose center conductor 72 is connected to the
collector of Q9. The remainder of the circuitry may be mounted
fairly remotely (several yards) from this assembly.
The collector of Q9 is connected via the cable through a load
resistor R33 to the plus supply of battery 73. The EEG signal
appears at this point. From the collector the signal is conducted
by way of a current limiting resistor R32 to the base of transistor
Q10, an emitter follower. The base of Q10 is also connected to one
side of a switch 74 through filter condenser C21, the opposite side
of the switch being connected to common. This switch allows a
filter capacitor C21 to be inserted or removed from base to ground.
This filter capacitor is inserted when the photon coupler is used
with the EEG output to filter out high frequency oscillator noise
which may be present in the output from the headset. When the
photon coupler is used with the FM output, this switch is opened to
allow the higher frequency FM signal to be amplified.
The output of the emitter follower Q10 is coupled by a dc blocking
capacitor C22 to the center pin 76 of output jack 77 having its
outside contact connected to common 69. Q10's emitter load resistor
R34 and output dc restoring resistor R35 are connected on opposite
sides of capacitor C22 to common. This signal taken from jack 77
and appearing across resistor R35 may be used to drive chart
recorders, oscilloscopes, and other power line operated equipment
without electrical shock hazard to the subject wearing the
headset.
The following is a schedule of suggested component values:
C1 1mf/35 v R10 470 K ohms C2 1mf/35v R11 10 meg ohms C3 0.001mf
R12 150 K ohms C4 0.001mf R13 470 K ohms C5 1000mf/6v R14 470 K
ohms C6 1000mf/6v R15 270 K ohms C7 10mf/35v R16 1 meg ohm C8
10mf/35v R17 4.7 K ohms C9 0.001mf R18 47 K ohms C10 0.33mf/35v R19
5 K ohms C11 0.2mf/20v R20 47 K ohms C12 1mf/35v R21 8.2 K ohms C13
10mf/35v R22 4.7 K ohms C14 0.01mf/20v R23 100 ohms C15 0.47mf/3v
R24 100 ohms C16 0.22mf/35v R25 4.7 K ohms C17 22mf/16v R26 100 K
ohms C18 22mf/16v R27 100 ohms -- 1 K ohm C19 0.47mf/35v R28 150 K
ohms C20 0.05mf/20v R29 1 K -- 100 ohms C21 .22mf/35v C22 250mf/12v
R1 1meg ohm R30 2.2 K ohms R2 1 meg ohm R31 22 meg ohms R3 10 K
ohms R32 100 K ohms R4 10 K ohms R33 2.2 K ohms R5 100 ohms R34 4.7
K ohms R6 200 ohms R35 10 K ohms R7 22 K ohms R36 220 ohms R8 22 K
ohms C21 0.02 mf R9 470 K ohms C22 250 mf/10v
D1, D2 Transition S3443G germanium signal diodes D3, Q9
Commercially available in a combined package as a General Electric
PC4-73 photon coupler D4 Silicon signal diode Q1, Q2 Fairchild
Semiconductor S34134, N-channel field effect transistor Q3
Fairchild Semiconductor .mu. MA739C dual integrated operational
amplifier Q4, Q7, Q8 General Electric 2N5825 transistor (NPN
silicon) Q5 Fairchild Semiconductor 2N4360, P-channel field effect
transistor Q6 Motorola Semiconductor 2N4871 uni-junction transistor
Q9 Monsanto MCT-2 Q10 Fairchild S34134 T1 5,000 ohms to 16 ohms
output transformer
All capacitors are .mu. f/dc working volts.
All electrolytics have polarity indicated.
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