- Email: [email protected]
A microcomputer-based binaural measurement system
A MICROCOMPUTER-BASED MEASUREMENT SYSTEM F.H.Y. Chan, P.W.F.
Poon, K.H. Cheng, H.F. Li, and J.C.C. Hwang
ABSTRACT
A microcomputer is used to control a duul-channel, sine-wave generator as the excitution to both the left and right ears in experimental subjects, and fw on-line measurement of the neuron& spike counts. llre effects of Keywords:
varying the waking pattern and phase angle between the two channels can be investigated. After normulisation of spike counts, the result can be displayed as a 3-D plot on a CRT or an x-y plotter. The system is usejisl for general steady state and transient binaural measurement
Ear, signal analysis, microcomputer
INTRODUCTION Earlier studies on the anatomy of the auditory nervous system have revealed a complicated picture iv*. In particular, most cells were thought to receive converging inputs from origins that could be traced to both ears. Recordings of neuronal activities with microelectrodes, showed that most cells along the auditory pathway could be affected by binaural sounds in a somewhat complicated fashion3. Behavioural investigation on binaural hearing in man, both clinical and experimental, further demonstrated various kinds of perceptual phenomena that could be better understood with proper knowledge concerning binaural responses of the auditory cells’. However, electrophysiological studies on binaural responses of auditory neurons have been relatively few5*6. One dilficulty has been that a powerful and usually expensive computer has been required to obtain precise control over the stimulating sounds. The present experiment attempts to overcome some of the major problems encountered by those interested in binaural studies of the auditory system by constructing an inexpensive microcomputer-based system that could make quick assessment of the binaural properties of auditory neurons. It may be more meaningful to treat the animal as a whole since acoustic properties of the external ear were shown to play an important role in sound perception’. We therefore present sounds directly to the animal, instead of via hollow ear-bars through headphones, and measure the neuronal spike discharge as a function of the binaural stimulation controlled at the speaker. Certain sound parameters were implicated or shown in the literature to be important elements for altering auditory cell responses”“. The cell responses are for instance, sensitive to (a) dichotic tone stimulation, (b) inter-aural masking, (c) interaural phase and intensity differences, (d) direction of change in tonal frequency and (e) the presence of a second tone (two-tone suppression). The above responsive characteristics can be studied with the present microcomputer system. Deparmwnt of Electrical Engineering and Physiology, University of Hong Kong, Haking Wong Bldg., Pokfulam Road, Hong Kong. G?J1984 Butteworth & Co (Publishers) 0141~5425/84/030212-07 $03.00 212
BINAURAL
Ltd
J. Biomed. Eng. 1984, Vol. 6, July
In view of the fact that most acoustic signals we hear are complex binaural sounds, consisting of more than one frequency and usually involve characteristic frequency changes, the importance of studying such binaural responsiveness in the auditory system, using an inexpensive and convenient tool, is obvious for the understanding of many sychophysical phenomena that could lead to use K, 1 neurological applications.
SYSTEM
DESCRIPTION
Hardware System The hardware system is constructed around a microcomputer (Motorola-ADS) which includes: (a) 16K-byte RAM for program and data storage (b) 2K ROM for system monitor (c) VDU and keyboard for data/program entry (d) Cassette interface for program/data back-up storage. The same basic hardware has been used for online analysis of neuronal spike trains’* and characteristic frequency and tuning curves of auditory neurons l3 . Besides these basic facilities, input and output modules must be designed to suit this experiment. The output module consists of two crucial sections: (a) dual-channel pure-tone, generator, and (b) plotter drivers/control. The dual-channel pure-tone generator circuitry is shown in Figure I. Basically, each channel is driven from a peripheral interface adapter (PIA) (M6821) under the control of the microprocessor. As a high precision sine-wave up to at least 10 kHz is desired, direct digital signals to form the sine-wave from the microprocessor will be impracticable. It is obvious that a 12%step sine-wave of 10 kHz needs an output rate in excess of 1 MHz, which is not practical if it is directly supported under software control. Moreover, while the sine-wave is generated, the microprocessor must be sufficiently free to monitor spike activities and extract the necessary statistics. Thus, each channel is implemented using externally programmable counters and EPROMs. One of the counters is for frequency control, and is presettable from port B of the PIA. This S-bit counter is used for the actual selection of one of the 128-steps (or 256-steps) of a
Binaural measurement:
Ao-
F.H. Y. Chun et al.
I
A,
filter
u
CONTROL
BUS
channel
I
f,
J
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I
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x31
Figure
1
Block diagram of the dual channel, pure tone generator.
sine-wave. By varying the preset-value of the first counter, the highest frequency attainable is 4 MHz/(2 x 128) 5 120 kHz. To allow for phasedifference between the two channels to be software selectable, one of the channels is additionally phase-selected via port A. This is accomplished by initialising the second counter (for step selection) of one channel to zero while that of the other is initialised to a pre-selected value under software control through port A. The initialisation is performed simultaneously, being timed by one of the control outputs from the PIA. The outputs from the EPROM’s are supplied to a high precision dual-channel &bit digital-toanalogue converter (Burr-Brown MP-11) whose block diagram is shown in Figure 2~. To smooth the resulting waveform, a second order low ass active Butterworth filter (Figure 2b) is employe x to remove the higher frequency harmonics of the digital-to-analogue converter output. The filtered outputs are then passed through an integrated audio amplifier (Project/one mark XX) that provides the optional analogue mixing mode in the experiment. The head of the animal was futed by means of a screw cemented to the exposed dorsum of the skull. Presentation of sound is through a pair of matched miniature stereo headphones (Pioneer SE-L4) coupled snugly onto the external ears. The harmonic distortion for a sine wave at 1 kHz monitored at the speaker with a calibrated condenser microphone (Bruel and Kjaer 4149) and a harmonic distortion meter (Heath-kit lM-58) is l%, and is less than 2% from 100 Hz to 10 kHz.
Overall frequency response of the system is 4 dB from 100 Hz to 10 kHz. Sound intensity is fared at 40 dB SPL at the speaker. The other output section is for plotting the mean spike count (firing rate) measured from inferior collicular cells under varying left- and right-ear excitations from the two channels just described. A convenient medium for this output is obtained by producing a S-dimensional plot on a storage oscilloscope, as illustrated in Figure 3. The X- and Yaxes represent the frequencies of the sine-wave presented to the left- and right-ear respectively, whereas the Z-axis is the normalised mean spike count (normalised with respect to the maximum of these counts). Thus the sensitivity of either ear under different combinations of pure-tone excitation is depicted pictorially. To support this threedimensional plot, the X- and Y-inputs of the oscilloscope are supplied from one separate dualchannel digital-to-analogue converter directly driven b a PIA. Therefore, the microprocessor is responsi g le for sending out suitable values to the digital-to-analogue converter while the plotting is carried out. For the recording of neural activities, a tungsten microelectrode was used. It was driven stereotaxically into the brain with a hydraulic stepping microdrive (David Kopf Instruments). Exuacellular spike activity was first amplified (Grass P16B and Princeton Applied Research 113 preamplifiers, in cascade) and then conditioned (Frederick Haer
J. Biomed Eng. 1984, Vol. 6, July
213
Binaural measurement: F.H. Y. Chun et al.
step ( + 60 Hz) and then the frequency of the former channel is sweeped again. In so doing, it is expected that the ear connected to the former channel may be less sensitive to the pure-tone, as the sweeping rate is faster than that supplied to the other ear. In this walking pattern, there is always a frequency change to one ear, only while the input to t’he other is being held constant. In the software design there is provision for starting the sweep at any of the four corners. The sweeping rate is also presettable from the keyboard.
B2 ADDRESS DETERMINATION I c Al3
ADDRESS DECODER I
AZ
OUTPUT I
a 22K
V in 0.01 pF
T
I
I
b
Figure 2 (a) Dual channel Butterworth lowpass fdter.
D/A converter;
(b) 2nd order
Amplitude Analyser) to give +5 volt rectangular pulses for the spikes selected for study. The input to the microcomputer is obtained by timing the interval of neuronal spikes. Since we are only interested in extracting the spike count, the external circuitry is quite simple and composed of a threshold detector (Figure4). The output of the detector is used to interrupt the microprocessor informing the latter of the arrival of a spike.
The second pattern is a nested-square pattern starting from the centre of the frequency range. The frequencies of the pure-tones are increased/ decreased step-wise in the form of a maze until the entire frequency range is covered (Figure 5b). Even though the starting point is at the centre, both the sweeping rate and direction is controllable. Because of the symmetry of this pattern, it is expected that interchange of channel output to either ear should not cause significant change to the result obtained. The third pattern is a triangular traversal where the frequency range is swept diagonally, as indicated in F&we 56. The effect of changing the frequency of both channels simultaneously can be investigated using this pattern. Of course, both the starting point and the direction of the sweep are selectable. Due to the asymmetry of the pattern, interchange of input to either ear is likely to generate different results. Finally, the fourth pattern is a square pattern that grows from one of the corners, as shown in Figure 3d. Except possibly due to the effect of reversal of the travel direction, interchange of input to either ear does not seem to introduce topological differences that may effect the result obtained. Voiinrliscd 5p?C:oymt
Software system To facilitate this binaural experiment, the software must allow for the study of the effects of varying some parameters in the generation of sine-waves to either ear. Of particular interest to us are: (i)
the effects of varying the walking pattern of the combination of pure-tones sent to either ear; (ii) the effects of phase-shifts artificially controlled between the tones supplied to both ears; (iii) the effects of feeding two time-varying pure-tones simultaneously to each ear or to both ears. There are four representative walking patterns of ure-tones generated, as depicted in Figure 5. The f!irst pattern allows the frequency of one channel to vary while the other is held constant (rectangular sweep). Afterwards, the latter is increased by one
214
J. Biomed. Eng. 1984, Vol. 6, July
IOkD
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Figure
4
Impulse
threshold
detection.
Binaural measurement: F.H. Y. Chan et al.
Even though there are only four basic walking patterns investigated, by choosing the starting points and the initial direction, other topolgoical variations are obtainable with sofmare control only. Indeed, there are many other possible walking patterns which may generate interesting results. However, they will be left to future investigations. The effects of phase-shifts between the two puretones may be easily studied as the phase angle between the two channels is directly presettable from port A of the PIA as previously explained. There is provision in the program to preset this PIA output before starting the sine-wave generator and measurement. Apart from the obvious application of feeding the two channels to the two ears, there is additional circuitry to combine the signals of the two channels together to form a two-tone stimulus for each or both ears. The effect of varying the frequency, rate of change of frequency and the phase angle between the two sinusoidal waves, can also be investigated using the same procedure. The The complex relationship between the measured spike counts and excitation can be clearly shown using a suitable three-dimensional plot. In this project, the plotting is performed using isometric projection of the mean spike count as the X-axis is varied, keeping the Y-axis constant (constant-ysweep) and vice versa. The use of isometric projection involves the transformation of data from a three dimensional structure for display onto a two dimensional plane. Figure 6a shows the relationship for point P at (x, y) and point R&c, y) in the new
X
Figure
6(a)
Coordinate
transformation
for isometric projection
coordinate system (X, Y, Z) in relation to the original coordinate system (X’, Y, Z’). If X, is the projection of P on X’, and Z, is the projection of R, on Z’, then
3
= (y - Jc) cos 3o” =
2,
= R, (x, y) - (x + y) sin 30’ = R&Y)
(x 6”
-
To produce different angles of display, the plot can be rotated so that four different angular plots may be obtained. This rotation can be achieved quite easily by re-shuffling the mean spike count values, as indicated in the flowchart drawn in Figure 6. Let R,(x, y) be the mean spike count under pure-tones of x Hz and y Hz supplied to the left and right ear respectively, then the y-input
(to
OSdOSCOpe)
=
R,
(x,
y)
-
y
and x-input
(to oscilloscope)
= 5@?
fi
when constant-y-sweep is performed. Similarly the constant-x-sweep, the inputs will be: y-input
= J!+?
x-input
= R,
for
fi
fx, y)
- y
There is provision for the microprocessor to drive a x-y plotter for producing hard-copies of the isometric projection.
b Figure
5
Walking patterns.
The entire software system is actually structured hierachically into three layers. The innermost layer (level 0) houses the system monitor. The intermediate layer (level 1) contains the interactive monitor, and the outermost layer (level 2) contains the application programes. Any program in the outer layer can call any routine in an inner layer, as illustrated in Figure 7. The decomposition of these layers reveals:
J. Biomed Eng. 1984, Vol. 6, July
215
Binaural measurement:
Level 0:
1.
F.H. Y. Ghan et al
Level 1:
Basic Utilities
Interactive Monitor 1. Entry to editor/monitor 2. Interactive entry of parameters 3. Activation of application programs
1.1 assembler 1.2 editor 1.3 VDU handler 2.
Level 2:
System monitor
Application Program 1. Dual-channel stimulus production and data acquisition 2. Three-dimensional plot
2.1 dump/load file 2.2 debug 2.3 task activator
‘-i 3-D
R,(X,-X,Y,-Y)
PLOT
R (Y,X,-XI
R (Y,-Y,X)
= Ro(X,Y)
= Ro(X,Y) I
I
ISOMETRIC PROJECTION
\
Z
,
Figure’ 6 (b)
218
J. Biomcd.
Flowchart of output
program.
Eng. 1984, Vol. 6, July
CONSTANT
Y PLOT
Binaural measurenent: EH. Y. C/tan et al.
The overall flowchart of the dual-channel stimulus generator is drawn in FtgUre 8. First, a suitable walking pattern is chosen. The mode bits then identify which comer of the frequency region should be used as the starting point as well as whether the x-and y-channels should be exchanged. Subsequently, the external sine-wave generators are initialized properly with phase-shift inserted if so specified, and activated. Simultaneously, the interrupt handler is enabled to count the occurrence of spikes which is the parameter of interest. The program terminates when the entire walking pattern is traversed. EXPERIMENTAL
DPROG = ? I
ry, WALKING -D PATTERN GENERATOR , 1
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RESULTS
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The system has been employed to test the response characteristic of auditory neurons in the inferior colliculus, the largest brainstem nucleus, of adult cats under a-chloralose anaesthesia (80 mg/kg). Typical examples of the neural responses are shown in Figures 9-10. Each diagram represents the result of about 4 minutes of data acquisition, including display on the CRT. The following observations are noted:
I
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x-y
cali freauency synthesizer
_I
driving orders in a table list, value Indexed by X’ . Y’
driving subroutine 1 activate interrupt handling
1. The results are generally reproducible. The preparation usually produced stable results over 4 hours of continuous recordings. 2. Neurons showed differential responses to binaural inputs. For instance, cell response dependend on binaural inputs but was active only monaurally (Figure 9a) or could be driven by both ears differentially (Figure 9b). 3. Neurons were found to respond to the binaural stimulation in a complex manner not shown previously (Figwe IOU).
4
RETURN TO LEVEL I MONITOR
6
b
Overall flowchart of dual channel generator: Parameters: 1. DPROG, DPHASE, DMODE; 2. subroutines
Figure 8
MULT, DMDE,
BASIC
UTILITIES
ARAD; 3. INTERRUPT HANDLER.
4. The response was found to depend on the stimulus walking pattern. In general, cells in the inferior colliculus were most sensitive to binaural tones of rapidly changing frequencies (Figure 1 Ou, walk pattern_ C) than to tones of more steady frequencies (Figure lob, walk pattern A). Furthermore, it was found that many cells consistently showed differential sensitivity to the direction of frequency change. For example, response is greater when the tone changed from a high towards low frequency but less so in the reverse direction (note the staggered peaks in Figure 9a).
5. There is no observable effect caused by the Figure 7
JI)E
6:3-D
Software organization.
change of phase angle introduced between the two channels. For the population of neurons examined,
J. BiomedEng. 1984, Vol. 6, July 217
Binaural measurement:
Figure9 I..\ -^-
^.._^
F.H. Y. Ghan et al
1
the characteristic frequencies are much higher than 500 Hz, and according to previous studies on the neurons sensitivity to binaural phase difference, our results are consistent with the existing literature that interaural phase difference has little effect on the responsiveness of cells in the frequency range above 500 Hz. However, the effects of phase shift between the two sides may become significant when cells having characteristic frequencies lower than 500 Hz are being studied. REFERENCES Harrison, J.M. and Howe, M.E. anatomy of the afferent auditory nervous system of mammals in Handbook of Sensory Physiology Vol. V/l: (Eds W.D. Keidel and W.D. Nell) Springer-Verlag, N.Y. 1974, 283-336. Morest, D.K. The collateral system of the medial nucleus of the trapezoid body of the cat, its neuronal architecture and relation to the olivo-cochlear bundle. Brain Bes. 1968, 9: 288-311. Semple, M.N. and Aitkin, L.M. Representation of sound frequency and laterality by units in the central interaction in low-frequency neurons in inferior colliculus of the cat II. Effects of chanaina rate and direction of interaural phase.J. NeurOphys%. y979, 50: 1006-1019. Celfand, S.A. Heating an introduction to psychological and physiological acoustics. (Ed S.A. Celfand) Marcel Dekker, N.Y. 1981. Kuwada, S. and T.C.T. Binaural interaction in lowfrequency neurons in inferior colliculus of the cat I. Effects of long interaural delays, intensity and repetition rate on interaural delay function.J. Neurophysiol. 1983, 50: 981-999. Yin, T.C.T. and Kuwada, S. Binaural interaction in low frequency neurons in inferior colliculus of the cat III. Effects of changing frequency J. Neurophysiol. 1983, 50: 1020-1042.
218
J. Biomed. Eng. 1984, Vol. 6, July
Figure 16 Effect of walking pattern on response (a) walk pattern C; (b) walk pattern A.
of one cell:
CCNCLUSIONS A practical microcomputer-based binaural measurement system has been successfully developed and tested. It enables accurate, repeatable and controllable input stimulus to be generated, and for the results to be quickly measured. Some interesting results are obtained and the 3-D plot proves to be very useful in showing the inter-relationship between various parameters. The same system can also be used for general auditory experiments in measuring frequency response and the centre-frequency of auditory neurons. 7
8
9 10
11
12
13
Butler, R.A. The influence of the external and middle ear on auditory discriminations in Handbook of sensory physiology Vol. V/2: (Eds W.d. Keidel and W.D. Neff), SpringerVerlag, N.Y. 1975, 247-260 Nuetzel, J.M. and Hafter, E.R. Discrimination of interaural delays in complex waveforms: Spectral effects. J. Acoust. Sot. Am. 1981 69: 1112-1118. Moore, B.C.J. Introduction to thepsychology of hearing (Ed. B.C.J. Moore) University Park Press, Baltimore, 1977 Aitkin, L.M., Anderson, D.J. and Brugge, J.F. Tonotopic organization and discharge characteristics of single neurons in the nuclei of the lateral lemniscus of the cat. J. Neur@tysiol. 1970, 33, 421-440. Arthur, R.M., Puffer, R.R. and Suga, N. Properties of “Two-tone inhibition” in primary auditory neurons. J. Physiol. 197 1, 212: 593-609. Li, H.F., Chan, F.H.Y., Hwang, J.C. Microcomputer-based spike train analysis. Computer Programs in Biomedtine: 198 1, 13, 61-72. Chan, F.H.Y., Li, H.F., Poon, P.W.F., and Hwang, J.C. On-line micro-computer analysis of response characteristics of auditory neurons to synthetic tones. Rot. IEEE Conf Frontiers of Computers in Medtiinz, 1981, 87-89.
Poon, K.H. Cheng, H.F. Li, and J.C.C. Hwang
ABSTRACT
A microcomputer is used to control a duul-channel, sine-wave generator as the excitution to both the left and right ears in experimental subjects, and fw on-line measurement of the neuron& spike counts. llre effects of Keywords:
varying the waking pattern and phase angle between the two channels can be investigated. After normulisation of spike counts, the result can be displayed as a 3-D plot on a CRT or an x-y plotter. The system is usejisl for general steady state and transient binaural measurement
Ear, signal analysis, microcomputer
INTRODUCTION Earlier studies on the anatomy of the auditory nervous system have revealed a complicated picture iv*. In particular, most cells were thought to receive converging inputs from origins that could be traced to both ears. Recordings of neuronal activities with microelectrodes, showed that most cells along the auditory pathway could be affected by binaural sounds in a somewhat complicated fashion3. Behavioural investigation on binaural hearing in man, both clinical and experimental, further demonstrated various kinds of perceptual phenomena that could be better understood with proper knowledge concerning binaural responses of the auditory cells’. However, electrophysiological studies on binaural responses of auditory neurons have been relatively few5*6. One dilficulty has been that a powerful and usually expensive computer has been required to obtain precise control over the stimulating sounds. The present experiment attempts to overcome some of the major problems encountered by those interested in binaural studies of the auditory system by constructing an inexpensive microcomputer-based system that could make quick assessment of the binaural properties of auditory neurons. It may be more meaningful to treat the animal as a whole since acoustic properties of the external ear were shown to play an important role in sound perception’. We therefore present sounds directly to the animal, instead of via hollow ear-bars through headphones, and measure the neuronal spike discharge as a function of the binaural stimulation controlled at the speaker. Certain sound parameters were implicated or shown in the literature to be important elements for altering auditory cell responses”“. The cell responses are for instance, sensitive to (a) dichotic tone stimulation, (b) inter-aural masking, (c) interaural phase and intensity differences, (d) direction of change in tonal frequency and (e) the presence of a second tone (two-tone suppression). The above responsive characteristics can be studied with the present microcomputer system. Deparmwnt of Electrical Engineering and Physiology, University of Hong Kong, Haking Wong Bldg., Pokfulam Road, Hong Kong. G?J1984 Butteworth & Co (Publishers) 0141~5425/84/030212-07 $03.00 212
BINAURAL
Ltd
J. Biomed. Eng. 1984, Vol. 6, July
In view of the fact that most acoustic signals we hear are complex binaural sounds, consisting of more than one frequency and usually involve characteristic frequency changes, the importance of studying such binaural responsiveness in the auditory system, using an inexpensive and convenient tool, is obvious for the understanding of many sychophysical phenomena that could lead to use K, 1 neurological applications.
SYSTEM
DESCRIPTION
Hardware System The hardware system is constructed around a microcomputer (Motorola-ADS) which includes: (a) 16K-byte RAM for program and data storage (b) 2K ROM for system monitor (c) VDU and keyboard for data/program entry (d) Cassette interface for program/data back-up storage. The same basic hardware has been used for online analysis of neuronal spike trains’* and characteristic frequency and tuning curves of auditory neurons l3 . Besides these basic facilities, input and output modules must be designed to suit this experiment. The output module consists of two crucial sections: (a) dual-channel pure-tone, generator, and (b) plotter drivers/control. The dual-channel pure-tone generator circuitry is shown in Figure I. Basically, each channel is driven from a peripheral interface adapter (PIA) (M6821) under the control of the microprocessor. As a high precision sine-wave up to at least 10 kHz is desired, direct digital signals to form the sine-wave from the microprocessor will be impracticable. It is obvious that a 12%step sine-wave of 10 kHz needs an output rate in excess of 1 MHz, which is not practical if it is directly supported under software control. Moreover, while the sine-wave is generated, the microprocessor must be sufficiently free to monitor spike activities and extract the necessary statistics. Thus, each channel is implemented using externally programmable counters and EPROMs. One of the counters is for frequency control, and is presettable from port B of the PIA. This S-bit counter is used for the actual selection of one of the 128-steps (or 256-steps) of a
Binaural measurement:
Ao-
F.H. Y. Chun et al.
I
A,
filter
u
CONTROL
BUS
channel
I
f,
J
#I
I
D/A Xwerter
x31
Figure
1
Block diagram of the dual channel, pure tone generator.
sine-wave. By varying the preset-value of the first counter, the highest frequency attainable is 4 MHz/(2 x 128) 5 120 kHz. To allow for phasedifference between the two channels to be software selectable, one of the channels is additionally phase-selected via port A. This is accomplished by initialising the second counter (for step selection) of one channel to zero while that of the other is initialised to a pre-selected value under software control through port A. The initialisation is performed simultaneously, being timed by one of the control outputs from the PIA. The outputs from the EPROM’s are supplied to a high precision dual-channel &bit digital-toanalogue converter (Burr-Brown MP-11) whose block diagram is shown in Figure 2~. To smooth the resulting waveform, a second order low ass active Butterworth filter (Figure 2b) is employe x to remove the higher frequency harmonics of the digital-to-analogue converter output. The filtered outputs are then passed through an integrated audio amplifier (Project/one mark XX) that provides the optional analogue mixing mode in the experiment. The head of the animal was futed by means of a screw cemented to the exposed dorsum of the skull. Presentation of sound is through a pair of matched miniature stereo headphones (Pioneer SE-L4) coupled snugly onto the external ears. The harmonic distortion for a sine wave at 1 kHz monitored at the speaker with a calibrated condenser microphone (Bruel and Kjaer 4149) and a harmonic distortion meter (Heath-kit lM-58) is l%, and is less than 2% from 100 Hz to 10 kHz.
Overall frequency response of the system is 4 dB from 100 Hz to 10 kHz. Sound intensity is fared at 40 dB SPL at the speaker. The other output section is for plotting the mean spike count (firing rate) measured from inferior collicular cells under varying left- and right-ear excitations from the two channels just described. A convenient medium for this output is obtained by producing a S-dimensional plot on a storage oscilloscope, as illustrated in Figure 3. The X- and Yaxes represent the frequencies of the sine-wave presented to the left- and right-ear respectively, whereas the Z-axis is the normalised mean spike count (normalised with respect to the maximum of these counts). Thus the sensitivity of either ear under different combinations of pure-tone excitation is depicted pictorially. To support this threedimensional plot, the X- and Y-inputs of the oscilloscope are supplied from one separate dualchannel digital-to-analogue converter directly driven b a PIA. Therefore, the microprocessor is responsi g le for sending out suitable values to the digital-to-analogue converter while the plotting is carried out. For the recording of neural activities, a tungsten microelectrode was used. It was driven stereotaxically into the brain with a hydraulic stepping microdrive (David Kopf Instruments). Exuacellular spike activity was first amplified (Grass P16B and Princeton Applied Research 113 preamplifiers, in cascade) and then conditioned (Frederick Haer
J. Biomed Eng. 1984, Vol. 6, July
213
Binaural measurement: F.H. Y. Chun et al.
step ( + 60 Hz) and then the frequency of the former channel is sweeped again. In so doing, it is expected that the ear connected to the former channel may be less sensitive to the pure-tone, as the sweeping rate is faster than that supplied to the other ear. In this walking pattern, there is always a frequency change to one ear, only while the input to t’he other is being held constant. In the software design there is provision for starting the sweep at any of the four corners. The sweeping rate is also presettable from the keyboard.
B2 ADDRESS DETERMINATION I c Al3
ADDRESS DECODER I
AZ
OUTPUT I
a 22K
V in 0.01 pF
T
I
I
b
Figure 2 (a) Dual channel Butterworth lowpass fdter.
D/A converter;
(b) 2nd order
Amplitude Analyser) to give +5 volt rectangular pulses for the spikes selected for study. The input to the microcomputer is obtained by timing the interval of neuronal spikes. Since we are only interested in extracting the spike count, the external circuitry is quite simple and composed of a threshold detector (Figure4). The output of the detector is used to interrupt the microprocessor informing the latter of the arrival of a spike.
The second pattern is a nested-square pattern starting from the centre of the frequency range. The frequencies of the pure-tones are increased/ decreased step-wise in the form of a maze until the entire frequency range is covered (Figure 5b). Even though the starting point is at the centre, both the sweeping rate and direction is controllable. Because of the symmetry of this pattern, it is expected that interchange of channel output to either ear should not cause significant change to the result obtained. The third pattern is a triangular traversal where the frequency range is swept diagonally, as indicated in F&we 56. The effect of changing the frequency of both channels simultaneously can be investigated using this pattern. Of course, both the starting point and the direction of the sweep are selectable. Due to the asymmetry of the pattern, interchange of input to either ear is likely to generate different results. Finally, the fourth pattern is a square pattern that grows from one of the corners, as shown in Figure 3d. Except possibly due to the effect of reversal of the travel direction, interchange of input to either ear does not seem to introduce topological differences that may effect the result obtained. Voiinrliscd 5p?C:oymt
Software system To facilitate this binaural experiment, the software must allow for the study of the effects of varying some parameters in the generation of sine-waves to either ear. Of particular interest to us are: (i)
the effects of varying the walking pattern of the combination of pure-tones sent to either ear; (ii) the effects of phase-shifts artificially controlled between the tones supplied to both ears; (iii) the effects of feeding two time-varying pure-tones simultaneously to each ear or to both ears. There are four representative walking patterns of ure-tones generated, as depicted in Figure 5. The f!irst pattern allows the frequency of one channel to vary while the other is held constant (rectangular sweep). Afterwards, the latter is increased by one
214
J. Biomed. Eng. 1984, Vol. 6, July
IOkD
4 LM339
P IOkSl
-5v
Figure
4
Impulse
threshold
detection.
Binaural measurement: F.H. Y. Chan et al.
Even though there are only four basic walking patterns investigated, by choosing the starting points and the initial direction, other topolgoical variations are obtainable with sofmare control only. Indeed, there are many other possible walking patterns which may generate interesting results. However, they will be left to future investigations. The effects of phase-shifts between the two puretones may be easily studied as the phase angle between the two channels is directly presettable from port A of the PIA as previously explained. There is provision in the program to preset this PIA output before starting the sine-wave generator and measurement. Apart from the obvious application of feeding the two channels to the two ears, there is additional circuitry to combine the signals of the two channels together to form a two-tone stimulus for each or both ears. The effect of varying the frequency, rate of change of frequency and the phase angle between the two sinusoidal waves, can also be investigated using the same procedure. The The complex relationship between the measured spike counts and excitation can be clearly shown using a suitable three-dimensional plot. In this project, the plotting is performed using isometric projection of the mean spike count as the X-axis is varied, keeping the Y-axis constant (constant-ysweep) and vice versa. The use of isometric projection involves the transformation of data from a three dimensional structure for display onto a two dimensional plane. Figure 6a shows the relationship for point P at (x, y) and point R&c, y) in the new
X
Figure
6(a)
Coordinate
transformation
for isometric projection
coordinate system (X, Y, Z) in relation to the original coordinate system (X’, Y, Z’). If X, is the projection of P on X’, and Z, is the projection of R, on Z’, then
3
= (y - Jc) cos 3o” =
2,
= R, (x, y) - (x + y) sin 30’ = R&Y)
(x 6”
-
To produce different angles of display, the plot can be rotated so that four different angular plots may be obtained. This rotation can be achieved quite easily by re-shuffling the mean spike count values, as indicated in the flowchart drawn in Figure 6. Let R,(x, y) be the mean spike count under pure-tones of x Hz and y Hz supplied to the left and right ear respectively, then the y-input
(to
OSdOSCOpe)
=
R,
(x,
y)
-
y
and x-input
(to oscilloscope)
= 5@?
fi
when constant-y-sweep is performed. Similarly the constant-x-sweep, the inputs will be: y-input
= J!+?
x-input
= R,
for
fi
fx, y)
- y
There is provision for the microprocessor to drive a x-y plotter for producing hard-copies of the isometric projection.
b Figure
5
Walking patterns.
The entire software system is actually structured hierachically into three layers. The innermost layer (level 0) houses the system monitor. The intermediate layer (level 1) contains the interactive monitor, and the outermost layer (level 2) contains the application programes. Any program in the outer layer can call any routine in an inner layer, as illustrated in Figure 7. The decomposition of these layers reveals:
J. Biomed Eng. 1984, Vol. 6, July
215
Binaural measurement:
Level 0:
1.
F.H. Y. Ghan et al
Level 1:
Basic Utilities
Interactive Monitor 1. Entry to editor/monitor 2. Interactive entry of parameters 3. Activation of application programs
1.1 assembler 1.2 editor 1.3 VDU handler 2.
Level 2:
System monitor
Application Program 1. Dual-channel stimulus production and data acquisition 2. Three-dimensional plot
2.1 dump/load file 2.2 debug 2.3 task activator
‘-i 3-D
R,(X,-X,Y,-Y)
PLOT
R (Y,X,-XI
R (Y,-Y,X)
= Ro(X,Y)
= Ro(X,Y) I
I
ISOMETRIC PROJECTION
\
Z
,
Figure’ 6 (b)
218
J. Biomcd.
Flowchart of output
program.
Eng. 1984, Vol. 6, July
CONSTANT
Y PLOT
Binaural measurenent: EH. Y. C/tan et al.
The overall flowchart of the dual-channel stimulus generator is drawn in FtgUre 8. First, a suitable walking pattern is chosen. The mode bits then identify which comer of the frequency region should be used as the starting point as well as whether the x-and y-channels should be exchanged. Subsequently, the external sine-wave generators are initialized properly with phase-shift inserted if so specified, and activated. Simultaneously, the interrupt handler is enabled to count the occurrence of spikes which is the parameter of interest. The program terminates when the entire walking pattern is traversed. EXPERIMENTAL
DPROG = ? I
ry, WALKING -D PATTERN GENERATOR , 1
#3
m -c; I
c
RESULTS
G
r)rq -LF
;
G
2
3
4
G
4
=4J
=I
x’ =x
I,
Y"Y -Y I
The system has been employed to test the response characteristic of auditory neurons in the inferior colliculus, the largest brainstem nucleus, of adult cats under a-chloralose anaesthesia (80 mg/kg). Typical examples of the neural responses are shown in Figures 9-10. Each diagram represents the result of about 4 minutes of data acquisition, including display on the CRT. The following observations are noted:
I
Ql Em7 Yes
x-y
cali freauency synthesizer
_I
driving orders in a table list, value Indexed by X’ . Y’
driving subroutine 1 activate interrupt handling
1. The results are generally reproducible. The preparation usually produced stable results over 4 hours of continuous recordings. 2. Neurons showed differential responses to binaural inputs. For instance, cell response dependend on binaural inputs but was active only monaurally (Figure 9a) or could be driven by both ears differentially (Figure 9b). 3. Neurons were found to respond to the binaural stimulation in a complex manner not shown previously (Figwe IOU).
4
RETURN TO LEVEL I MONITOR
6
b
Overall flowchart of dual channel generator: Parameters: 1. DPROG, DPHASE, DMODE; 2. subroutines
Figure 8
MULT, DMDE,
BASIC
UTILITIES
ARAD; 3. INTERRUPT HANDLER.
4. The response was found to depend on the stimulus walking pattern. In general, cells in the inferior colliculus were most sensitive to binaural tones of rapidly changing frequencies (Figure 1 Ou, walk pattern_ C) than to tones of more steady frequencies (Figure lob, walk pattern A). Furthermore, it was found that many cells consistently showed differential sensitivity to the direction of frequency change. For example, response is greater when the tone changed from a high towards low frequency but less so in the reverse direction (note the staggered peaks in Figure 9a).
5. There is no observable effect caused by the Figure 7
JI)E
6:3-D
Software organization.
change of phase angle introduced between the two channels. For the population of neurons examined,
J. BiomedEng. 1984, Vol. 6, July 217
Binaural measurement:
Figure9 I..\ -^-
^.._^
F.H. Y. Ghan et al
1
the characteristic frequencies are much higher than 500 Hz, and according to previous studies on the neurons sensitivity to binaural phase difference, our results are consistent with the existing literature that interaural phase difference has little effect on the responsiveness of cells in the frequency range above 500 Hz. However, the effects of phase shift between the two sides may become significant when cells having characteristic frequencies lower than 500 Hz are being studied. REFERENCES Harrison, J.M. and Howe, M.E. anatomy of the afferent auditory nervous system of mammals in Handbook of Sensory Physiology Vol. V/l: (Eds W.D. Keidel and W.D. Nell) Springer-Verlag, N.Y. 1974, 283-336. Morest, D.K. The collateral system of the medial nucleus of the trapezoid body of the cat, its neuronal architecture and relation to the olivo-cochlear bundle. Brain Bes. 1968, 9: 288-311. Semple, M.N. and Aitkin, L.M. Representation of sound frequency and laterality by units in the central interaction in low-frequency neurons in inferior colliculus of the cat II. Effects of chanaina rate and direction of interaural phase.J. NeurOphys%. y979, 50: 1006-1019. Celfand, S.A. Heating an introduction to psychological and physiological acoustics. (Ed S.A. Celfand) Marcel Dekker, N.Y. 1981. Kuwada, S. and T.C.T. Binaural interaction in lowfrequency neurons in inferior colliculus of the cat I. Effects of long interaural delays, intensity and repetition rate on interaural delay function.J. Neurophysiol. 1983, 50: 981-999. Yin, T.C.T. and Kuwada, S. Binaural interaction in low frequency neurons in inferior colliculus of the cat III. Effects of changing frequency J. Neurophysiol. 1983, 50: 1020-1042.
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Figure 16 Effect of walking pattern on response (a) walk pattern C; (b) walk pattern A.
of one cell:
CCNCLUSIONS A practical microcomputer-based binaural measurement system has been successfully developed and tested. It enables accurate, repeatable and controllable input stimulus to be generated, and for the results to be quickly measured. Some interesting results are obtained and the 3-D plot proves to be very useful in showing the inter-relationship between various parameters. The same system can also be used for general auditory experiments in measuring frequency response and the centre-frequency of auditory neurons. 7
8
9 10
11
12
13
Butler, R.A. The influence of the external and middle ear on auditory discriminations in Handbook of sensory physiology Vol. V/2: (Eds W.d. Keidel and W.D. Neff), SpringerVerlag, N.Y. 1975, 247-260 Nuetzel, J.M. and Hafter, E.R. Discrimination of interaural delays in complex waveforms: Spectral effects. J. Acoust. Sot. Am. 1981 69: 1112-1118. Moore, B.C.J. Introduction to thepsychology of hearing (Ed. B.C.J. Moore) University Park Press, Baltimore, 1977 Aitkin, L.M., Anderson, D.J. and Brugge, J.F. Tonotopic organization and discharge characteristics of single neurons in the nuclei of the lateral lemniscus of the cat. J. Neur@tysiol. 1970, 33, 421-440. Arthur, R.M., Puffer, R.R. and Suga, N. Properties of “Two-tone inhibition” in primary auditory neurons. J. Physiol. 197 1, 212: 593-609. Li, H.F., Chan, F.H.Y., Hwang, J.C. Microcomputer-based spike train analysis. Computer Programs in Biomedtine: 198 1, 13, 61-72. Chan, F.H.Y., Li, H.F., Poon, P.W.F., and Hwang, J.C. On-line micro-computer analysis of response characteristics of auditory neurons to synthetic tones. Rot. IEEE Conf Frontiers of Computers in Medtiinz, 1981, 87-89.