U.S. patent number 3,799,146 [Application Number 05/187,003] was granted by the patent office on 1974-03-26 for hearing testing method and audiometer.
This patent grant is currently assigned to Neuro-Data, Inc.. Invention is credited to Erwin Roy John, Robert Laupheimer.
United States Patent |
3,799,146 |
John , et al. |
March 26, 1974 |
HEARING TESTING METHOD AND AUDIOMETER
Abstract
A hearing testing method permits the testing of infants and
others who cannot respond and provides an accurate and rapid test
for others. A group of pure tones at selected amplitudes and
frequencies is played, through earphones, to a subject. Electrodes
connected to the subject's head detect the subject's brainwaves
which are evoked responses to those tones. The brain waves are
amplified, averaged according to the tone frequencies,
automatically analyzed on a statistical basis using the "t" test,
and used to display a profile of the subject's hearing ability.
Inventors: |
John; Erwin Roy (Riverdale,
NY), Laupheimer; Robert (Westbury, NY) |
Assignee: |
Neuro-Data, Inc. (Cliffside
Park, NJ)
|
Family
ID: |
22687216 |
Appl.
No.: |
05/187,003 |
Filed: |
October 6, 1971 |
Current U.S.
Class: |
600/544;
600/559 |
Current CPC
Class: |
A61B
5/377 (20210101); A61B 5/121 (20130101) |
Current International
Class: |
A61B
5/0476 (20060101); A61B 5/0484 (20060101); A61B
5/12 (20060101); A61b 005/04 () |
Field of
Search: |
;128/2,2R,2Z,2.1B,2.1R
;235/150.53 ;179/1N |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Marsoner et al., "Medical & Biological Engineering," Vol. 8,
1970, pp. 415-418..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Gerber; Eliot S.
Claims
We claim:
1. An instrument for the testing of the hearing of a subject,
including means to make audible to the subject a series of tones at
selected frequencies and loudness levels; a contact adapted to be
worn by the subject to detect his brain waves; an
electroencephalograph amplifier having an input and output whose
input is connected to said contact; a signal detector having an
input and an output and whose input is connected to said amplifier
output, said signal detector comprising an absolute value circuit
having its input connected to the input of said signal detector; a
polarity shifting circuit whose input is connected to the output of
said absolute value circuit, an integrator whose input is connected
to the output of said shifting circuit, and a comparator whose
input is connected to said integrator and whose output is connected
to the output of said signal detector; a single-input multi-output
switch whose input is connected to said signal detector output; a
plurality of memory section counters connected to said switch
outputs; a display device connected to said memory sections; and a
programmer connected to said tone means and to said switch to
direct the evoked responses at each tone to one of the memory
sections.
2. An instrument as in claim 1 wherein said tone means includes a
variable frequency oscillator whose frequency is controlled by said
programmer and a variable attenuator controlled by said programmer,
said oscillator and said attenuator both being connected to said
programmer.
3. An instrument as in claim 1 wherein said memory sections are
up-down counters.
4. The method of testing the ability of a subject to hear tones,
consisting of playing for the subject a recording of a musical
composition which includes the tones selected to be tested at
selected levels of loudness, obtaining and amplifying the brain
waves of the subject during said playing, averaging the subject's
amplified brain waves with the brain wave response at each said
tone being averaged with responses at the same tone and loudness,
and displaying the subject's averaged response at the said tones
and loudness levels.
5. An instrument for the testing of the hearing of a subject, said
instrument including means to make audible a series of tones at
selected frequencies and at selected loudness levels; a contact
adapted to be worn by the subject to detect his brain waves; an
electroencephalograph amplifier having an input and an output whose
input is connected to said contact; a "t" test computer whose input
is connected to the output of said amplifier, said computer having,
connected in sequence, a first set of squaring circuits, an
averaging circuit, a second set of squaring circuits, differential
circuits, a first set of divider circuits, a summing circuit and a
square root circuit and also having a differential circuit whose
input is connected to the averaging circuit, a second dividing
circuit whose input is connected to the outputs of the differential
circuit and the square root circuit, and an absolute value circuit
whose input is connected to said dividing circuit; and display
means connected in series with said computer to display the hearing
test results.
6. The instrument of claim 5 and also including a programmer
connected to and controlling said tone means and a significance
detector whose input is connected to the output of said absolute
value circuit, said programmer being connected to the output of
said significance detector to receive control signals controlling
its program.
7. The instrument of claim 6 wherein said tone means includes a
variable frequency oscillator and a variable attenuator and wherein
said programmer is connected to and controls said oscillator and
said attenuator.
8. The instrument of claim 5 wherein said display device is an X-Y
pen recorder adapted to produce a profile of the subject's
hearing.
9. The instrument of claim 5 and including a significance detector
to set the "t" test standard value connected to said computer and
said programmer, upon attainment of which standard "t" value a
control signal is provided by said significance detector to said
programmer.
10. An instrument as in claim 5 wherein the said averaging circuit
of said "t" test computer is a four-channel average response
computer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the testing of hearing and more
particularly to an audiometer which produces a profile of the
subject's hearing ability.
At the present time the usual hearing test involves the services of
skilled personnel. A pair of earphones is placed on the subject,
i.e., the person to be tested, and eight or more tones at different
degrees of loudness are played through the earphones. The
technician asks the subject, "Can you hear the tone?" and the
subject answers "yes" or "no". A profile is constructed showing the
least loudness, at each tone, at which the subject said that he
heard the tones.
This conventional procedure has a number of drawbacks. It is
relatively expensive as it requires the attention of a skilled
technician. It is not an objective test because it depends upon the
skill and care of the technician giving the test and the attention
and truthfulness of the subject. The test leads to errors as the
subject may become bored or fatigued by the repetition of tones.
And the test cannot be administered to infants, animals and others
unable or unwilling to verbally respond.
OBJECTIVES OF THE INVENTION
It is the objective of the present invention to provide an
audiometer and testing method which:
A. IS AUTOMATIC AND DOES NOT REQUIRE THE SERVICES OF SKILLED
PERSONNEL TO ADMINISTER THE TEST;
B. MAY BE ADMINISTERED TO INFANTS, ANIMALS AND OTHERS UNABLE TO
GIVE A VERBAL REPLY;
C. DOES NOT DEPEND UPON THE JUDGMENT OF THE SUBJECT TO WHOM THE
TEST IS GIVEN; AND
D. IS NOT BORING OR FATIGUING TO THE SUBJECT.
SUMMARY OF THE INVENTION
A series of preferably eight or more pure tones, i.e., frequencies,
are produced, either by a variable frequency oscillator or from a
recording on the track of a magnetic tape cassette. Each of the
tones is at different levels of loudness or an attenuator is used
to vary the loudness. In one embodiment, the tones constitute, or
are part of, a musical composition, so that the subject hears a
pleasant song. The tones and their amplitude are controlled by a
programmer, for example, a digital logic circuit or a track on a
tape having control signals in digital form. The tones are played
through earphones to the subject and the subject's evoked
responsive brainwaves are picked-up by electrodes removably
attached to his head.
The brainwaves are amplified by an electroencephalograph (EEG)
amplifier and averaged in a signal averaging device. The averaged
signals are automatically compared, on a statistical basis, with
background noise in a special purpose "t" test computer. The output
of the statistical computer, which automatically computes the "t"
test, is to a display device. Preferably the display device is a
single pen X-Y recorder which produces a visual graph corresponding
to a profile of the subject's hearing ability at each of the
selected tone frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a block circuit diagram of the instrument of the
embodiment of the present invention;
FIG. 2 is a block circuit diagram of the "t" test computer;
FIGS. 3A-6 are circuit diagrams of portions of the "t" test
computer;
FIG. 7A is a block circuit diagram of the instrument of FIG. 1
showing the programmer (logic circuit) in greater detail;
FIG. 7B is a timing diagram showing the timing of the various steps
of the instrument of FIG. 1;
FIG. 7C is a circuit diagram of the discrete attenuator;
FIG. 7D shows the wave shapes of the tone generated by the
instrument of FIG. 1;
FIG. 8 is a top plan view of a suitable pen recorder;
FIG. 9 is a block circuit diagram of the second embodiment of the
present invention;
FIG. 10 is a block circuit diagram of the signal detection computer
and "up-down" counters used in the embodiment of FIG. 9; and
FIGS. 11 and 12 are diagrams of electrical wave shapes in the
circuit of FIG. 10.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1, the first preferred embodiment of the present
invention consists of a combination of devices constituting a
testing instrument. The subject 10 being tested sits before the
testing instrument and wears a pair of earphones 19 and a headband.
The headband has a contact electrode 11 which may be wetted with
glycerine and touches the subject's forehead. A reference electrode
12 is clipped to the subject's ear lobe.
The electrodes 11 and 12 pick-up the subject's brain waves and
communicate them to electroencephalograph (EEG) amplifier 13. The
output of the amplifier 13, over line 14, is to the "t" test
computer. The output of the computer 15 is to the significance
detector 22 (control signals being shown in dot-dash line). The
significance detector 22 is connected to the programmer and clock
(logic circuit 17) and to the counter 23. The counter is connected
to X-Y recorder 16. The programmer and clock 17 controls the
frequency of the variable frequency oscillator 18, which supplies
pure tones (oscillations) in the audio range. Such tones may be
derived from a crystal controlled oscillator. The tones from
oscillator 18 are amplified in audio amplifier 20. The output of
amplifier 20 is to variable attenuator 21, which variably
attenuates the loudness of the tones under the control of
programmer 17. The output of attenuator 21 is connected to one
earphone of the set of earphones 19. The programmer controls the
tones (for example 8-11 selected frequencies generally in the range
of 50-10,000 Hz) and the loudness (for example 23 loudness-sound
pressure levels).
A suitable circuit for the programmer is shown in FIG. 7A, which
shows a digital logic circuit. An alternative to the variable
frequency oscillator and digital programmer is to use a magnetic
tape read-out, preferably a cassette magnetic tape player having
two reading heads. One reading head would read one track of the
tape having a musical composition and play it.
Preferably the musical composition is either (a) a song containing
400 notes, each note being a tone of the selected eight frequencies
so that each frequency has 50 notes, or (b) a musical composition
in which 400 tones (eight frequencies 50 times each) appears
between instrumental portions. The second reading head of the
read-out would read a second parallel track on the tape having code
signals in digital form. Preferably the second track has, in binary
code, the numbers 1-8 corresponding to the eight frequencies, with
the numbers preceding its tone by teh switching time. The code
signals indicate the particular tone and, in addition, may indicate
its loudness level.
It should be noted that the instrument tests physiological reaction
and does not take account of emotional factors which might cause a
person to think that he did not hear a sound, although his brain
waves indicate his response to that sound.
The testing method is used as a search for the threshold of the
subject's hearing. Each tone is first played at its least loudness
and then the loudness level increased. When the threshold level, at
each frequency, is reached, i.e., the predetermined "t" test value
for that frequency is met, the pen recorder records that event.
After each threshold is reached, the test recommences, at the least
loud level, at the next frequency.
In operation, for example, the audiometer may look for responses at
11 frequencies in the band 30-20,000 Hz and 23 levels of loudness.
The subject's evoked responses to the tones which he hears through
the earphones are the X inputs. The subject's background ongoing
brain activity is the Y input. The Y input may be obtained either
before the test is started or, alternatively, in the intervals
between tones, i.e., between evoked responses.
The "t" test is a statistical test for a measure of the
significance of the difference between two sample populations. For
example, for a sample size of N = 10, corresponding to 10 sweeps
(10 repetitions of each tone at each loudness level), to obtain a
level of significance P of 0.001 (the result occurring by random
chance 1 in 1,000) the "t" result must be 4.587. With 25 sweeps and
P = 0.001 the "t" result is 3.725.
Preferably both the number of sweeps N and the level of
significance may be varied by dials on the programmer to set a
predetermined "t" test standard. For example, the tester may set
the maximum number of sweeps N at 25 and the level of significance
P at 0.001. For each tone and loudness level either the "t" test of
the evoked response (X values), compared to brain wave ongoing
activity background (Y values), will exceed 3.725 (the
predetermined "t" standard) or be less than 3.725. If the "t" test
evoked response result is larger than the standard, then there is
only 1 in 1,000 chances that the result was by accident and
consequently the test shows that the subject very likely responded
to the tone. Upon such "t" test result, the pen recorder will
record the tone and loudness level. As soon as a tone evokes a
response, meeting the "t" test predetermined standard, the "t" test
computer sends a control signal, over line 15', to programmer 17.
The programmer will then automatically start to test for the next
frequency, omitting further loudness levels at the frequency at
which the response was evoked.
Preferably the "t" test computer will send out its control signal
as soon as the predetermined "t" test standard is reached, even
though the standard is reached before the maximum set number of
sweeps. For example, if N is set at 25 and the P set at 3.725 and
the P value is exceeded on the 11th sweep, a control signal on line
15' will be sent and the and the remaining 14 sweeps omitted, since
a satisfactory set of evoked responses has been obtained.
Alternatively, at less cost, the audiometer may be set at the
factory to perform a fixed number of sweeps P to obtain a fixed
value of "t", thereby setting the level of significance. In this
alternative, at each tone and loudness level a "go-no go" signal
would be shown, for example, by the pen recorder or by a light.
The preferred embodiment of the T-test computer is shown in FIG. 2.
As shown, the computer has two inputs -- an X input on line 100 and
a Y input on line 101. The inputs 100 and 101 are to a two-channel
sample and hold circuit 102. The purpose of the sample and hold
circuit 102 is to sample the two signals X and Y and to hold them
so that they become in phase. A suitable sample and hold circuit is
shown in FIG. 3A. The output lines 103 and 104 of the sample and
hold circuit 102 are each directly connected to one channel of a
four-channel average response computer 105. In addition, the
outputs 103 and 104 are connected to respective squaring circuits
106 and 107, the details of the squaring circuit being given in
connection with FIG. 5. The average response computer 105 gives a
value of samples taken periodically in time divided by the number
of samples, thereby providing a running average, that is, an
average which changes with the additional samples. A suitable
average response computer is described in Clynes U.S. Pat. No.
3,087,487. The number of samples N is determined by the operator.
The output of the first channel 108 is the average of the sum of
the values of X, i.e., the sum of the voltages of each of the
samples divided by the number of samples N, which is the mean and
may be expressed by the formula: .SIGMA.X/N.sub.x = M.sub.1. The
output of the channel 109 of the average response computer 105 is
the sum of the X values squared over the number of samples and may
be expressed by the formula: .SIGMA.X.sup.2 /N.sub.x. The output of
channel 110 is the sum of the Y values over the number of samples
and may be expressed by the formula: .SIGMA.Y/N.sub.y = M.sub.2 and
the output of channel 111 is the sum of the Y values squared over
the number of samples and may be expressed by the formula:
.SIGMA.Y.sup.2 /N.sub.y.
Each of the channels is connected to a four-channel sample and hold
circuit 111'. The only purpose of the sample and hold circuit 111'
is to eliminate time skewing errors. An alternative is to have a
separate memory for each of the channels, in which case the sample
and hold circuit 111' would not be necessary. The circuits of each
of the four channels of the sample and hold circuit 111' are the
same as the sample and hold circuit shown in FIG. 3A.
The output of channel 108, which is the mean, is then squared in a
squaring circuit 112 and similarly the output of channel 110 is
squared in a squaring circuit 113. Each of the squaring circuits is
the same as shown in FIG. 5. The output of the squaring circuit and
the output of channel 109 are then combined in a differential
amplifier 114. Similarly the outputs of the squaring circuit 113
and channel 110 are combined in differential amplifier 115. The
detailed circuit of a suitable differential amplifier is shown in
FIG. 4A. The formula for the computation which occurs in the
differential amplifier 114 is:
(.SIGMA.X.sup.2 /N.sub.x) - (.SIGMA.X/N.sub.x).sup.2 =
.sigma..sub.x.sup.2
and the formula for the mathematical computation which occurs in
the differential amplifier 115 is
(.SIGMA.Y.sup.2 /N.sub.y) - (.SIGMA.Y/N.sub.y).sup.2 =
.sigma..sub.y.sup.2
The outputs of the differential amplifiers are connected to the
respective divide circuits 116 and 117, the details of which are
shown in FIG. 5. The divide circuit 116 divides the variance
.sigma..sub.x.sup.2 by the number of samples. The output of the
divide circuits 116 and 117 are connected to summing amplifier
(adder) 118 which performs the following mathematical computation:
(.sigma..sub.x.sup.2 /N.sub.x) + (.sigma..sub.y.sup.2 /N.sub.y), a
suitable circuit being shown in FIG. 4B. The output of the summing
amplifier 118 is to a square root circuit 119, the details of which
are given in FIG. 5. The output of the square root circuit is to
the divide circuit 120, a suitable divide circuit being shown in
FIG. 5. The second input to the divide circuit is from a
differential amplifier 121 which may be of the type shown in FIG.
4A. The differential amplifier 121 provides the difference between
the two means, that is, it accomplishes the mathematical
computation as follows:
(.SIGMA.x/Nx) - (.SIGMA.y/Ny)
The output of the divide circuit 120 is to the absolute value
circuit 122, shown in FIG. 6, which provides the final result of
the "t" test.
All of the computations necessary for the "t" test have been
provided by the circuit of FIG. 2 and the "t" test result is taken
at the output 123. The "t" test computation performed by the
circuit of FIG. 2 is as follows: ##SPC1##
A suitable squaring circuit, as shown in FIG. 3B, uses three
integrated circuits. The integrated circuits 150 and 151 are
operational amplifiers and may be of the type Motorola No. MC
1556-G. That integrated circuit is a compensated and monolithic
operational amplifier. The integrated circuit 152 is a multiplier
which, suitably, may be Motorola Type 1594-L. The multiplier, as
its two inputs 153 and 154 derived from a common line 155 which is
the output of the operational amplifier 150, and acts to square the
input from line 155; that is, its inputs are tied together. A
suitable integrated circuit is a monolithic four-quadrant
multiplier where the output voltages are a linear product of two
input voltages. The Motorola 1594-L is a variable transconductance
multiplier with internal level shift circuitry and voltage
regulation. The scale factor is adjustable and preferably is set to
be 1/10 of input. An operational amplifier 151 is used to complete
the multiplier connections from the integrated circuit 152. Its
output 156 provides a square of the input at 157. This type of
multiplier connection is described in further detail in the
specification sheet dated October 1970 DS-9163 from Motorola of
Phoenix, Arizona, of their 1594-L integrated circuit.
A suitable sample and hold circuit is shown in FIG. 3A. It uses an
operational amplifier 140. Preferably operational amplifier 140 is
an integrated circuit, for example, of the type Motorola No. 1456G,
described above.
A suitable differential amplifier circuit is shown in FIG. 4A. It
uses an operational amplifier 160 having two inputs 161 and 162.
Preferably the operational amplifier 160 is an integrated circuit.
A suitable integrated circuit is Motorola No. MC 1456G described in
the specification sheet DS9147R1 dated April 1970 as being
epitaxial passivated and monolithic. It has a power supply voltage
of +18V dc and -18V dc, a power bandwidth of 40K Hz and power
consumption of 45m W max.
The summing amplifier of FIG. 4B also uses an operational amplifier
165. The two inputs to be added are connected to one input of the
amplifier 165. A suitable operational amplifier is the integrated
circuit Motorola No. 1456G described above.
A suitable divider circuit is shown in FIG. 5. It uses a linear
multiplier 170 and an operational amplifier 171. Preferably the
multiplier 170 and the amplifier 171 are integrated circuits. A
suitable integrated circuit for the multiplier 170 is Motorola No.
1594, described above, and for the amplifier Motorola No. 1456G,
also described above. The inputs are 172 and 173 and the output at
174.
A suitable square root circuit is shown in FIG. 5. The square root
circuit is a special case of a divider in which the two inputs to
the multiplier are connected together. Consequently the input line
173 and the input line 172 are connected together to form a common
input line 175, shown in dashed line and the ground.
A suitable absolute value circuit is shown in FIG. 6. It uses two
operational amplifiers 176 and 177. Preferably they are integrated
circuits and may be of the type Motorola No. 1456G described above.
The input 178 is to the minus input of amplifier 176 and the output
179 is from amplifier 177. The purpose of the circuit of FIG. 6 is
to provide a positive quantity if the X or the Y terms are larger,
the absolute value being the value regardless of the plus or minus
sign of the quantity.
FIG. 7A is similar to FIG. 1 but shows the programmer 17 in greater
detail. As shown in FIG. 7A the electrodes from the subject 10 are
connected to the EEG amplifier 14 and the amplifier 14 is connected
to the "t" test computer 15. The results of the "t" test
computation go to the significance detector 22. The probability
level is manually set in the significance detector by means of dial
50. When the "t" test is determined to be significant, in
accordance with the selected and dialed probability level, a signal
is communicated, over line 51, to the sound pressure level (SPL)
logic circuit 52. The logic circuit 52 will indicate to the sound
pressure level counter 53 whether the sound level is to be
increased or decreased. The counter 53 is connected, by five lines
(for 5 bits) to the level decoder 54. The decoder over six lines 55
(only one of which is shown) provides a control signal to the
variable attenuator 21.
The audiometer is started, by means of switch 56, which is
connected to the state counter 57. The state counter 57 is
connected, by line 57a, to the EEG amplifier 14. If an artifact
occurs, for example, an eye movement, the overly large voltage
amplitude of the artifact will re-start the state counter. The
state counter 57 is connected to the timing and clock circuit 58.
The timing and clock circuit, in turn, is connected to the
frequency counter 59. The frequency counter 59 is controlled by 11
push-buttons 60 which manually set the frequency at which the test
is to start. For example, the frequencies may range from 125 Hz to
8,000 Hz. However, it may be desired to start the test not at 125
Hz, but at 2,000 Hz. There are preferably 11 push-buttons provided
for the 11 frequencies.
The frequency counter 59 is connected to the decoder 60a by means
of four lines, i.e., a four-bit counter. The decoder is connected
by 8 lines, only one of which is shown, to the variable frequency
oscillator 18. The variable frequency oscillator 18 receives a
digital control signal and produces the desired frequency.
Preferably the digitally controlled variable frequency oscillator
18 may consist of eight input lines each one of which is connected
to a field effect transistor. The transistors are connected to a
common line which is one input of an operational amplifier
integrator. The output from the integrator is connected to an input
comparator whose output is connected to a voltage reference clamp.
The output of the integrator is in the form of a triangular wave
and its frequency depends upon which of the field effect
transistors are turned on. The output from the integrator is
connected to a sine converter which converts the triangular shaped
waves into sine waves.
An output from the frequency counter 59 is to the digital-to-analog
converter 61 whose output provides the information of the X axis of
the X-Y pen recorder 16. Similarly, the sound pressure level
counter 53 is connected to the digital-to-analog converter 62 which
provides the information for the Y axis.
FIG. 7A also shows a masking source circuit 63 which is controlled
from the decoder 60a and whose output is to one of the earphones
19. The masking source is operated only when a tone is produced. It
provides random noise to the ear which is not being tested. Such
masking noise helps prevent an evoked response due to the ear which
is not being tested picking up the test tone.
As shown in FIG. 7C a digital control attenuator consists of a
series of resistances. The switches 70, 71, 72 of which, for
example, there may be 10 or more, are relay closure contacts. When
those contacts are closed, the particular resistances are placed in
the circuit between V.sub.in and V.sub.out to attenuate the tones.
It has been found that a rapid rise and fall of the tone sounds
like a click noise and should be avoided. To avoid such "click
sounds" preferably the variable frequency oscillator 18 is not
directly connected to the attenuator but rather the tones shaped
first through a wave-shaping circuit 75. The wave-shaping circuit
75 is preferably a trapezoidal generator which is connected to a
modulator. The tones from oscillator 18 are connected to the input
of the modulator and are modulated by the trapezoidal generator.
Consequently, at the output of the modulator the tones are a sine
wave in the form of a trapezoidal envelope, as shown in FIG. 7D.
The period D, which is the duration of each tone, may be set by the
front panel of the device and, in the case of a trapezoidal shape
envelope, is at the half points of the trapezoid. The time T, i.e.,
the time from one tone to another, is set by the front panel, as is
the duration D of each tone. Alternatively, the time D may be fixed
within the device and the time T may be random to avoid
habituation.
FIG. 7B illustrates the operation of the circuit of FIG. 7A. When
the start button 56 is pushed, it provides a control signal 80
which starts the operation. At first the "t" test computer 15 takes
data for the null reference, that is, it takes data for the X input
of the "t" test. This data is gathered before tones are sent to the
earphones 19 and is the subject's background ongoing brain
activity. At the conclusion of the null reference, the audio
stimulator (oscillator 18) is enabled and will start to provide a
sequence of sounds. The desired frequency, which may be initially
set to one of 11 frequencies in the example shown in FIG. 7B, is
started at 125 Hz, as shown by the line 83. The sound pressure
level, which is digitally set at the variable attenuator 21 is
first set for 10 db at 84. It is then raised to 20 db at 85. It
will be noted at 20 db that there is a "t" test significance
response 86. Consequently, the SPL logic 52 provides a control
signal, which is a decreasing control signal, to the counter 53 and
the signal is then attenuated at 87 to 15 db. As shown on line 88
the circuit is set to give two sound pressure level increments when
there is not a significance according to the "t" test. If there is
such a "t" test significant response, however, as shown by the
signal 89, there is only a one-level change. At 90 there is a "t"
test significant response (at 125 Hz, 20 db) and a print-out signal
91 occurs.
A suitable pen recorder is shown in FIG. 8. The display pen 50a
consists of a single stylus 51a which is positioned over a movable
broad band of recording paper 52a. The stylus 51a is supported on a
slide 53a transverse to the direction of movement of the paper
52a.
The display pen marks the paper 52 at the threshold of auditory
evoked response at each frequency F1-F8 a profile of the subject's
hearing. This series of eight dots F1-F8 may be connected later
manually, as shown by the dashed lines, to produce a line display
showing.
FIG. 9 shows the second preferred embodiment of the present
invention. The subject 210 being tested sits before the testing
instrument and wears a pair of earphones 219 and a headband. The
headband has a contact electrode 211 which may be wetted with an
electrolyte and touches the subject's forehead. A reference
electrode 212 is clipped to the subject's ear lobe. The electrodes
211 and 212 pick-up the subject's brain waves and communicate them
to electroencephalograph (EEG) amplifier 213. The output of the
amplifier 213, over line 214, is to the signal detector 215
(described below). The detector 215 is connected to switch 216, an
electro-mechanical switch, or preferably a solid state electronic
switch, which connects the input from detector 215 to alternatively
any one of the output lines 220-227 the selection of the lines
220-227 being under control of control signals received over line
218 from the programmer 217. A suitable circuit for the programmer
is shown in FIG. 7A, which shows a digital logic circuit.
The programmer and clock 217 provides control signals, over line
217', to a variable frequency oscillator 233, of the type described
above. The oscillator 233 is connected to audio amplifier 231,
which is connected to variable attenuator 232, of the type
described above. The attenuator 232 is connected to provide tones
to the earphones 219. The programmer, over line 218, controls
switch 216 so that the brain wave response at each tone appears on
only one output line 220-227.
The brain wave responses on each line 220-227 go to up-down
counters 220'-227' which are averaging memory sections. The peak
amplitude of brain wave responses at each tone and at each tone
level of loudness are averaged. For example, the oscillator
provides each of eight tones, at their least loudness level, 25
times, i.e., 25 notes of each tone. The peak amplitudes of the
brain wave responses are averaged as a technique to eliminate
noise, such as muscle artifact. If the averaged amplitudes are zero
at each loudness level at each frequency, then the subject did not
respond and therefore did not hear the tones. If there is a
positive average, i.e., an up or a down count, at one tone, then
the subject heard that tone.
Each of the up-down counters 220'-227' is connected by a line
240-247 to a recording display 250 of the type described above.
The testing method is used as a search for the threshold of the
subject's hearing. Each tone is first played at its least loudness
and then the loudness level increased. When the threshold level, at
each frequency, is reached, i.e., the predetermined and pre-set
average value for that frequency is met, the pen recorder records
that event. After each threshold is reached, the test recommences,
at the least loud level, at the next frequency.
The circuit for the signal detector 215 is shown in FIG. 10. It
includes an absolute value circuit 271, for example a bridge
rectifier having four diodes, which first produces an absolute
value, that is, it produces a unipolar output voltage which is
independent of the incoming polarity. The outgoing polarity,
positive or negative, is then controlled by a polarity reversing
switch circuit 272. The polarity reversing circuit 272 is connected
to an integrator 273, such as an operational amplifier. The
integrator 273 is connected to a comparator 274.
The polarity reversing circuit 272 and the integrator 273 are
connected to the programmer 217" whose clock provides time pulses.
Every pulse from the clock reverses the polarity switch 272. In
addition, every other pulse erases and resets the integrator 273.
In addition, the time pulses are simultaneous with the tones, the
moment of occurrence of such tones being designated by the term
"tone stimulus pulse" in FIGS. 11 and 12.
The computing cycle is initiated by pulse 280 from the programmer
217. Pulse 280 resets the polarity switch 272, erases the
integrator 273 and begins the baseline sampling interval 281
lasting 250 milliseconds. Pulse 282 from programmer 217 reverses
the polarity switch. A note is heard by the subject as a stimulus
and briefly interrupts the computing cycle to protect against
artefact, after which the evoked response sampling interval 283
begins and lasts for 250 seconds. Thus, the integrated activity
accumulated during the period of evoked response to the note
stimulus is subtracted by the comparator 274, from the integrated
baseline activity preceding delivery of the note stimulus.
For example, if the integrator output during interval 281 is a
rising positive waveform, the polarity shift occasioned by pulse
282 will cause a descending negative waveform of equal duration
283. The signals shown in FIG. 11 are taken at the output of the
integrator 273. If there is only noise and no signal, as seen in
baseline interval 280 of FIG. 11 then the rising level during
baseline interval 280 will go to a certain level 285 of FIG. 11. On
the other hand, if there is both noise and signal present (as shown
by response interval 291 of FIG. 12), then the descending voltage
during interval 283 will cross the zero value of the integrator and
to the level 292.
If the signal plus the noise is greater than the noise, the lower
level 292 of the descending voltage slope will be below a
predetermined voltage level and there will be a plus-one count
registered in one of the up-down counters 220'-227'. Conversely, if
the signal plus the noise is less than the noise, then the lower
end will be above the predetermined voltage level and a minus-one
will be registered in one of the up-down counters 220'-227'. If
only noise is present, then the run-up and the run-down should
average to zero, or about zero, in the counter corresponding to the
tone. If there is a signal, in addition to the noise, then there
will be a positive average or a negative average, the negative
average corresponding to systematic inhibition of the noise. The
up-down counters 220'-227', consequently, only determine the
presence, or absence, of a signal (a brain wave response) at each
of the tone frequencies, but does not give any information
regarding its wave shape or amplitude.
Modifications may be made in the present invention within the scope
of the subjoined claims. For example, (a) the variable frequency
oscillator may be replaced by separate tuned oscillators; (b) the
programmer may be other than a logic circuit or a program track on
the magnetic tape, for example, it may be a mini-computer or a
punched paper tape reader; and (c) the recording pen may be
replaced by a bank array of signal lights, for example, 56 lights
for 8 frequencies and seven loudness levels.
* * * * *