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Behavioral thresholds in the cat to frequency modulated sound and electrical stimulation of the auditory nerve
EXPERIMENTAL
41,
NEUROLOGY
Behavioral Thresholds and Electrical GRAEME Child
1!%-200
in the Cat to Frequency Modulated Stimulation of the Auditory Nerve
M. CLARK,
Dcafwss UCrrersity
(1973)
Research of
HOWARD Laboratory, Melbourne,
Rercvied
G. KRANZ,
AND HARRY
Sound
MINAS
Departmewt of Otolaryrtgolog.v, Mclboume 3051, dustvalin June 11,1973
This behavioral study has helped confirm that cats can perceive low rates of electrical stimulation of the basal or high frequency end of the cochlea. The upper limit on the rate of stimulation that could be perceived was 600-800 pulse/set. It was also shown that the behavioral threshold for low rates of change of a frequency modulated electrical stimulus was similar to that of sound. In the case of an electrical stimulus it was 85 pulses/sec/sec, and in the case of sound it was 97 cycles/sec/sec.
INTRODUCTION A previous study (2) has shown the effects of rate, current, and site of electrical stimulation of the cochlea and central auditory pathways on behavioral thresholds in the cat. A significant finding was that the thresholds were lower for low rates of electrical stimulation of the basal or high frequency end of the cochlea rather than the apical or low frequency end. This indicates that the basal end of the cochlea may also be important in coding low frequency sound on the basis of a time-period code; this finding is supported by psychoacoustic studies (3). The present study has been undertaken for two reasons. Firstly, it has been carried out to help confirm that the basal end of the cochlea may code low frequency sound on the basis of a temporal code, and that electrical stimulation of this region will result in the perception of a low pitch at low rates of stimulation. Secondly, it has been undertaken to determine quantitatively the rate of change of the frequency of a sound (frequency modulation) that can just be detected. These thresholds have been compared with those obtained by stimulating the terminal auditory nerve fibers electrically. This analysis was undertaken to help determine the mechanismsunderlying the coding of frequency modulated sound and to find out whether electrical 190 Copyright All rights
0 1973 by Academic Press, Inc. of reproduction in any form reserved.
FKEQL-EXCY MOUJLATW
hlODULATED
MODUATED
WAVE
101
SOUSD CARRIER
WAVE
FIG. 1. A diagram of modulating sine and triangular xvaves and the resultant waveforms produced by modulating sine and square wave carrier frcqurncies.
stimulation could reproduce this important parameter of speechin a hearing prosthesis. METHODS This study was performed on 11 cats. In six of these animals the operation and electrode implantation were successful,and prolonged conditioning studies were completed. Surgical procedures were carried out using a closed circuit, spontaneous breathing technique. Anesthesia was induced with an intraperitoneal injection of sodium pentobarbitone (60 “g/kg) and was maintained with methoxyfluorane and halothane. In four cats a bipolar electrode was inserted into the basal turn of the cochlea through the round window, and in two, bipolar electrodes were implanted directly into the basal turn. In different animals the stimulating electrodes used were either stainless steel or silver. They were insulated except at their tips, had a diameter of 150 pm, and their impedanceswere measured over the frequency range of the stimuli used during the course of the experiment. The intensity of the current used was halfway between that required to produce a threshold and an orientating reflex and varied between 0.5-1.2 mamp. The animals were conditioned by avoidance methods using delayed conditioning techniques. The training program and conditioning sequenceused were similar to those described (2). Sounds and electrical stimuli were
192
CLARK,
KRANZ,
AND
MINAS
FIG. 2. A diagram of the experimental set-up. ATT-attenuator; COUNT-frequency counter ; CRO-cathode-ray oscilloscope ; CU-control unit ; FG-function generator ; OSC-oscillator; SFG-sweep function generator; TRANS-transformer.
modulated with sineand triangular waves as illustrated in Fig. 1. Sine waves were used as carrier frequencies for the acoustic stimuli and biphasic square waves for the electrical stimuli. The first series of experiments was performed on cats with electrodes implanted in the basal turn of the cochlea through the round window. The carrier frequencies tested were 200 pulse/set, 400 pulse/set. 800 pulse/set, 1600 yulse/sec, and 3200 pulse/set. The modulating sine wave had a frequency of 5 pulse/set, and its amplitude was adjusted to provide a percentage deviation of l’$, Sr/c, lo%, SOS, and 10070 in the carrier frequency. The experimental set-up is illustrated in the lower half of Fig. 2. The modulating frequency was produced by a Krohn Hite 4100 oscillator, the carrier frequency by a Hewlett Packard 3310A function generator, and the sound was presented through a Kola CSDOOloudspeaker. IVhen the carrier frequency was modulated, the percentage deviation in its frequency was dcterniined by the voltage of the modalatiiig wave. Fre-
~pellc~ iiiutlulation was also carried out bv a technique duce xnilditutle modulation into the signal.
that did not intro-
The second series of esl~erinients was performed on cats with electrodes placed directly into the basal turn of the cochlea. The carrier frequencies were 300 lmlse/sec and 2000 l~tilse/sec, and 25 ‘h deviations in frequency were lmducetl by triangular waves of constant aml~litude. The modulating maw \vas p-nducetl 1)~ R Svstron-Dormer 310 sweep/function generator. and . _ its slol)e 1~2s: altered to allow the presentation of graded changes in the rate of frequency modulation ( Fig. 2) . iit these rates of change the frequency of StillldtiS ~)reSelitatirJli \vas 1 Hz and it \\-a~‘ consitleretl that the animal wcmltl not respond to a small change iii tllis ~mameter in preference to the frequency niotlulatetl signal. In the alwve studies it ~vas considered important to make sure the functioning hair cells were not present. as their presence wc~ultl mean that dectrical stimulation could induce hearing through normal l)athways jelectr~~~hmiC
auditory
hearillg
)
alid
llot
lleCeSSaril\-
_
\)v-
direCt
StilnUhti~Jll
Of
tertllillal
nerve fibers. This was tlone 1)~ recording cochlear microphonic potenti;~ls. It ~2s also considered tlesirahle to make a more detailed assessmetlt of the effect of electrophonic Iwaring. This ux carried out 1)~ recording tl~reslioltls before and after the hair cells were destroyed using 1500 ratls of an ultrahigh velocity electron l)eam from a hIEL ST, 75 linear accelerator directed at the temporal bone. Xt the coml~letion of the experimental series. the cats lvere anesthetized and lm+isetl intraarterially with normal saline solution followed by 10%) formalin. The temporal hnes were removed, tlecalcitied. and embetlcled. The 1)lOCliS were serially sectioned at a thickness of 30 pi. atltl one in ten sectir 111swere stained \vith lieniatos~liii ant1 eosin.
194
CLARK,
KRANZ,
AND
TABLE CAT
Combination
Acoustic and electro-
phonic
Acoustic
and
auditory nerve
Electra phonic
and auditory nerve
Carrier frequency HZ
MIXAS
II 2
Percentage
deviation
of carrier
It25
*5
frequencf
___&SO
200 400 800 1600 3200
1.1 3.0 0.3 1.1 3.8
9.2++ 3.2 .5.5+ 14.7++
4.3+ 6.2+ 13.9+ 18.6++ 18.4++
200 400 800 1600 3200
2.0
9.0++ 38.4++
3.0 12.0++ 5.5+ 35.6++ 38.4++
13.9++ 11.9++ 45.1++ 38.4++ 35.3++
200 400 800 1600 3200
0.2 0.6 0.4 4.8+ 22.5++
0.1 0.3 0.6 17.4++ 8.5++
3.5 1.1 1x9++ 6.0+ 4.4+
1.1 0.0
1.1
f2.5 23.3-’ 13.1++ 22.F 15.0++
10.3++ 32.9+*
fO.5 0.3 0.0 4.6+ 17.1++ 17.1++
1.5.0++
0.8 0.4
35.3++ 27.9+* 32.-I+’
14.3++ 26.8++
1.6 0.1 2.5 2.8 8.5++
11.4++
1.0 0.4
0.9 1.0 1.7
Chi-square test of significance. +x2 1 df (3.8), P < 0.05. ‘+xX” 1 df (6.6), I’ < 0.01.
RESULTS The first series of experiments was mldertaken to compare the difference limina for frequency, for sound, and electrical stimulation of the basal auditory nerve fibers. This was carried out using sinusoidally modulated stimuli, and was done to help determine the importance of the basal turn of the cochlea in coding low frequency sound. The cats were first conditioned to sinusoidally modulated sound, and when their performance levels reached an asymptote, the acoustic stimulation was replaced by sinusoidally modulated electrical stimulation. At each daily session the cat was tested to a descending series of five percentage deviations in the carrier frequency from 100% to 1%. The spontaneous response rate was also sampled in random order during these sessions. Each day the carrier frequency was changed in an ascending, descending and ascending series from 200 pulse/set to 3200 pulse/set until 30 stimulus presentations had been given for each possible carrier frequency-percentage deviation combination.
FKEQUENCT
JlOI~UL.ZTEI)
SOUNI)
195
The behavioral responses for each carrier frequency-percentage deviation combination were tested for significance against the spontaneous responding rates using a Ch-square analysis. and a 5% probability level was accepted. The results are summarized in Table 1, which shows the thresholds for different percentage deviations of the carrier frequencies obtained for cats 1 and 2 \vhrn using acoustic, electrophonic, and auditory nerve stimulation. I;rom this it can be seen that the thresholds for electrophonic and auditory nerve stimulation were higher than those obtained for sounds hal-ing the same carrier frequency. These differences in the behavioral responses for acoustic. electrophonic, and auditory nerve stimulation were tested for statistical significance using a Chi-square analysis, and the results for cat 2 are she\\-n in Table 2. These show that the differences between acoustic and auditory nerve stimulation, and between acoustic and electrophonic stimulation were significant for most carrier frequencies and percentage deviations in frequency. \\%en electroplionic and auditory nerve stimulation was compared the responses were only significantly different for stimuli with high carrier frequencies. From the thresholds for percentage deviation in different carrier frequencies it was possible to estimate the niasimunt rate of electrical stimulation of the basal end of the cochlea that could produce a low pitched sound on the basis of a time-period code. It was assumed that for each carrier frequency the cat would perceive a change in frequency at the ion-est point in the modulation cycle. On this basis it can be seen in Table 1 that for electrophonic hearing the highest frequency that could be perceived occurred at a * 257, deviation in a carrier frequency 3200 pulse/set for cat 1, and at a * SO7c deviation at 3200 pulse/set for cat 2. The rates of stimulation perceived at these carrier frequency-percentage deviation thresholds were at least 2400 pulse/ set for cat 1 and 1600 pulse/set for cat 2. Similarly for electrical stimulation of the auditory nerve the highest rates of electrical stimulation that could be perceived occurred at a carrier frequency of 800 pulse/set and 25 s deviation in frequency for cat 1, and a carrier frequency of 1600 pulse/set and SOY deviation in frequency for cat 2. The rates of stimulation perceived at these carrier frequency-percentage deviation combinations were GO0 pulse/set for cat 1 and 800 pulse/set for cat 2. The second series of experiments was designed to make a more accurate assessment of thresholds for rate of change of frequency. In this series amplitude was kept constant, and triangular waves were used to modulate the sound. The thresholds for frequency modulated sound were then compared with those obtained for electrical stimulation. The cats were conditioned to a modulated sound and electrical stimulus and, \\hen their performance levels reached an asymptote, they were tested
196
CLARK,
KRANZ,
AiYD
MINAS
-
200Hz
x----x
0
200
100
300
GRADIENT
Sound
2OOPulse/sec Electrical
400
500
(Hz/set)
graph of the behavioral responses for rate of change of frequency ior acoustic and electrical stimulation with carrier frequencies of 200 Hz. FIG.
3. A
at various rates of frequency change. These rates were varied in seven graded steps from 1 pulse/sec/sec to 500 pulse/sec/sec, and 10 pulse/set/ set to 5000 pulse/sec/sec for carrier frequencies of 200 pulse/set and 2000 pulse/set. respectively. They were presented in an ascendingand descending series of 13 stimuli, and at each sessionthe spontaneous responding rate was also determined. These sessions were repeated until five stimulus presentations had been given for each rate of frequency change, The spontaneous responseswere then subtracted and the result expressed as a percentage. The percentage responsesfor different rates of frequency change were plotted graphically for sound and electrical stimulation having a carrier frequency at 200 Hz, and the results are shown in Fig. 3. The responses for different percentage rates of frequency change for sound having carrier frequencies of 200 Hz and 2000 Hz are shown in Fig. 4. The thresholds were determined by plotting the rate of frequency change equivalent to a SO% behavioral
response.
In Fig.
3 it can be seen that the thresholds
for
acoustic and electrical stimuli having a carrier frequency of 200 Hz were 97 Hz/set and 85 pulse/sec/sec, respectively. In Fig. 4 the thresholds for acoustic
stimuli
having
carrier
frequencies
of 200 Hz and 2000
Hz were
47%’ Hz/set and 3570 Hz/set, respectively. J)ISCUSSION A previous study (2) showed that the cat could respond to low rates of electrical stimulation of the terminal auditory nerve fibers passing to the basal turn of the cochlea, but the difference limina were greater then those
197
-
200
Z+ -- --X 2000
GRADIENT
FIG. 3. stimuli
Hz Sound Hz Sound
( ~Hz /SW
A graph of the behavioral responses for rate of percentage with carrier frequencies of 200 Hz and 2000 Hz.
change
of acoustic
obtained for sound by Frazier and Elliott (5 j in cats, and Shower and Hiddulpl~ ( 10 j in human subjects. If the hasal turn of the cochlea plays a role in the coding of low frequency sound, difference limina for electrical stimulation should be similar to those for sound. In this previous study (2) a tlirect comparison of the difference limina for sound and electrical stimulation in the same experimental animal ~2s not made. 17urtlierniore, the difference limina were measured h\- incrementing a steady state frequency to another steady state frequency without an intervening glide between the two. Although this method avoids introducing additional frequency cues during a glide. most of the previous difference limen measurements have been made using a frequency which was sinusoidally modulated from one to the other. Consequently, in the present study, thresl~oltls for sinusoidally modulatetl soiuitl and electrical stimuli have been measured. These threshold measurements also made it possible to estimate the maximum rates of electrical stimulation that could he perceived by exciting the auditory nerve fibers to the basal turn of the cochlea. The results of the comparison between the thresholds for percentage tleviations in carrier frequencies for sound and electrical stimulation of the auditory nerve are shown in Table 2. The statistical tests indicate that tllere were significant differences for most carrier frequencies and percentage deviations. This suggests that perceptual acuity is greater for acoustic stimulation of the apical turn of the cochlea rather than electrical stimulation of the basal turn. I;urthermore. there were significant differences in the responses for electroplwtiic arid auditory nerve stimulation at l~iglier carrier frequencies.
198
CLARK,
XRANZ,
AND
MINAS
This probably occurred because all the cochlear hair cells were not destroyed by the introduction of the round window electrode, and because electrical stimulation vibrated the skull by a capacitance effect. As a result sound waves were transmitted to the residual hair cells by bone conduction. This further emphasizes the importance of making sure that viable hair cells are not present in all studies on electrical stimulation of the auditory nerve so that hearing may not be produced by the electrophonic effect. The upper limit on the rate of electrical stimulation that can still be perceived as a distinct frequency was estimated from the threshold for the percentage deviations in the highest carrier frequency tested. It was assumed that if the cat could perceive a changing frequency it must be able to detect a rate of stimulation at the lowest level of this frequency modulation range. The results showed that the upper limit for electrical stimulation of the auditory nerve fibers is 600-800 pulses/set. This is higher than the rate ‘determined in previous acute behavioral experimental studies (1, 2) where the findings indicated an upper limit of 200-400 pulses/set. The difference in the results could be due to the use of a biphasic square wave stimulus in the present series instead of the monophasic one used in previous studies (1, 2). A biphasic stimulus has been shown to produce increased auditory nerve firing during one-half of the cycle, and decreased firing during the other half, ( 12). Consequently, a biphasic electrical stimulus is more likely to result in nerve firing which is phase-locked to the stimulus, and distinguishable from spontaneous activity. As a result higher rates of excitation would be more readily perceived. The underlying neural mechanism involved in the perception of a frequency modulated tone are not clearly understood. Psychoacoustic studies (7,s) suggest that the perception of a changing frequency may be related to the frequency content of the Fourier analysis of the signal. In which case it would be perceived on the basis of the spatial pattern of excitation along the cochlea produced by the different frequencies. It seems more probable, however, that a time-period code operates, particularly for small variations in rate of frequency change, as experimental studies have shown the coding of steady state frequencies and complex sounds can be explained on this basis (6, 9). If a time-period code is important, frequency modulated electrical stimulation of terminal auditory nerve fibers should produce thresholds similar to those for sound, This comparison was undertaken in the present study and, as the hair cells were nonfunctioning, it was assumed there would be little .chance of the frequency side bands in the stimulus producing a spatial pattern of excitation in the cochlea. The results shown in Fig. 3 indicate that there was little difference in the thresholds obtained for frequency modulated sound or electrical stimuli with
FREQUEKCY
XODULATEI)
SOUr\‘l)
199
carrier frequencies of 200 Hz. The threshold was 97 Hz/set for sound, and 55 pulse/sec/sec for electrical stimulation. These results suggest that a varying time-period code is important in detecting a frequency modulated signal, Nevertheless it seems unlikely that this mechanism will operate for high carrier frequencies and modulation rates as an upper limit will be set by the refractory period of nerve fibers. In this study the threshold for rate of change of frequency was lower for electrical stimulation than for sound, but for steady state stimuli the reverse applied. ;Iii inverse relation between absolute, and differential sensitivity has also been shown when comparing thresholds for acoustic stimuli in cats and men (4) . The innervation of the cochlea has been studied by Spoendlin (11)) and he has shown that a single intier hair cell is connected to about 20 ganglion cells, on the other hand about ten outer hair cells innervate one ganglion cell. Consequently, if electrical stimulation is to produce a perceptual sensation similar to that of a steady state tone then the pattern of excitation of auditory nerve fibers is likely to be important. As this could not be simullatecl with the bipolar electrodes used in a previous study (3 j this would explain the discrepancies between the thresholds for sound and electrical stimulation. On the other hand, if the ganglion cell density is important in the perception of a change in frequency, and one intier hair cell is connected to 20 ganglion cells then rate of stimulation is more likely to be important. This would help explain why in the present study the difference limen for an electrical stimulus with a changin g frequency was the same or lower than that for sound. In the present study, the thresholds for rate of change of frequency were also compared for acoustic stimuli with carrier frequencies of 200 Hz and 2000 Hz. Although the thresholds were quite different when measured as Hertz per second, they were practically the same when considered as a percentage change of the carrier frequency. This suggests that for a certain range of carrier frequencies the thresholds may be the same when measured this way. A more detailed study needs to be carried out, however, to substantiate this finding. ACKXOWLEDGMENTS This study was supported by grants from the National Health and Medical Research Council of Australia, the Bushells Trust, the Felton Bequest, and the Sunshine Foundation. FVe thank Dr. R. Kerr from the Peter biacCallum Clinic for irradiating the cats temporal bones, Miss B. Laby of Melbourne for from the Department of Statistics, at the University statistical advice, and Mr. R. J. ~~~alkerclen for help with electronics.
200
CLARK,
KRANZ,
AND
MINAS
REFERENCES 1. CLARK, G. M. 1969. Responses of cells in the superior olivary complex of the cat to electrical stimulation of the auditory nerve. Exp. Ncrwol. 24: 124-136. 2. CLARK, G. M., J. M. NATHAR, H. G. KKANZ, and J. S. MARITZ. 1972. A behavioral study on electrical stimulation of the cochlea and central auditory path\vays of the cat. E.rp. Ncwol. 36: 3.50-361. 3. DALLOS, P.. and R. H. SWEETRIAN. 1969. Distribution pattern of cochlear harmonics. J. .4coust. Sot. Amer. 45: 37-46. 4. ELLIOTT, D. N., L. STEW, and M. L. HARRISON. 1960. Determination of absoluteintensity thresholds and frequency difference thresholds in cats. J. .4coust. Sot. Anzrr. 32 : 380-384. 5. FRAZIER, L. F., and D. N. ELLIOTT. 1963. Size and reliability of frequency-difference thresholds determined with operant tracking procedure. J. Ex). ilrral. Bchrra. 6 : 189-192. 6. HIKD, J. E., D. J. AKDERSON, J. ‘F. BRUXGE, and J. E. ROSE. 1967. Coding of information pertaining to paired low frequency tones in single auditory nerve fibers of the squirrel monkey. J. Nrwophysiol. 30: 794-816. 7. MCCLELLAIGD, K. I)., and J. F. BRAKDT. 1969. Pitch of frequency-modulated sinusoids. J. =Icmst. Sor. Amer. 45 : 1489-1498. 8. NABELEK, I. V., A. K. NABELEK, and I. J. HIRSH. 1970. Pitch of tone bursts of changing frequency. J. .qro~st. Sot. Anzcr. 48: 536-553 9. ROSE, J. E., J. F. BRUGGE, D. J. AKDERSON, and J. E. HIKD. 1967. Phase-locked response to low-frequency tones in single auditory nerve fibers of the squirrel monkey. J. Ncurophysioi. 30: 767-793. 10. SHOWER, E. G., and R. BIIKJULPH. 1931. Differential pitch sensitivity of the ear.
J. Am&. 11. SPOEKDLIN,
SOL.. Anzcr. 2: 275287. H.
1971.
Degeneration
behaviour
of the
cochlear
nerve.
Arch. k’lin.
Exp. Oh., Nas. U. k’rhlk. Hclzlk. 200: 275-291. 12. TEAS, D. C., T. KONISHI, and J. S. WARNICEE. 1970. Effects of electrical current applied to cochlear partition on discharges in individual auditory-nerve fibers. II. Interaction of electrical polarization and acoustic stimulation. J. Acwst SOL-. Amr. 47: 6, 1527-1537.
41,
NEUROLOGY
Behavioral Thresholds and Electrical GRAEME Child
1!%-200
in the Cat to Frequency Modulated Stimulation of the Auditory Nerve
M. CLARK,
Dcafwss UCrrersity
(1973)
Research of
HOWARD Laboratory, Melbourne,
Rercvied
G. KRANZ,
AND HARRY
Sound
MINAS
Departmewt of Otolaryrtgolog.v, Mclboume 3051, dustvalin June 11,1973
This behavioral study has helped confirm that cats can perceive low rates of electrical stimulation of the basal or high frequency end of the cochlea. The upper limit on the rate of stimulation that could be perceived was 600-800 pulse/set. It was also shown that the behavioral threshold for low rates of change of a frequency modulated electrical stimulus was similar to that of sound. In the case of an electrical stimulus it was 85 pulses/sec/sec, and in the case of sound it was 97 cycles/sec/sec.
INTRODUCTION A previous study (2) has shown the effects of rate, current, and site of electrical stimulation of the cochlea and central auditory pathways on behavioral thresholds in the cat. A significant finding was that the thresholds were lower for low rates of electrical stimulation of the basal or high frequency end of the cochlea rather than the apical or low frequency end. This indicates that the basal end of the cochlea may also be important in coding low frequency sound on the basis of a time-period code; this finding is supported by psychoacoustic studies (3). The present study has been undertaken for two reasons. Firstly, it has been carried out to help confirm that the basal end of the cochlea may code low frequency sound on the basis of a temporal code, and that electrical stimulation of this region will result in the perception of a low pitch at low rates of stimulation. Secondly, it has been undertaken to determine quantitatively the rate of change of the frequency of a sound (frequency modulation) that can just be detected. These thresholds have been compared with those obtained by stimulating the terminal auditory nerve fibers electrically. This analysis was undertaken to help determine the mechanismsunderlying the coding of frequency modulated sound and to find out whether electrical 190 Copyright All rights
0 1973 by Academic Press, Inc. of reproduction in any form reserved.
FKEQL-EXCY MOUJLATW
hlODULATED
MODUATED
WAVE
101
SOUSD CARRIER
WAVE
FIG. 1. A diagram of modulating sine and triangular xvaves and the resultant waveforms produced by modulating sine and square wave carrier frcqurncies.
stimulation could reproduce this important parameter of speechin a hearing prosthesis. METHODS This study was performed on 11 cats. In six of these animals the operation and electrode implantation were successful,and prolonged conditioning studies were completed. Surgical procedures were carried out using a closed circuit, spontaneous breathing technique. Anesthesia was induced with an intraperitoneal injection of sodium pentobarbitone (60 “g/kg) and was maintained with methoxyfluorane and halothane. In four cats a bipolar electrode was inserted into the basal turn of the cochlea through the round window, and in two, bipolar electrodes were implanted directly into the basal turn. In different animals the stimulating electrodes used were either stainless steel or silver. They were insulated except at their tips, had a diameter of 150 pm, and their impedanceswere measured over the frequency range of the stimuli used during the course of the experiment. The intensity of the current used was halfway between that required to produce a threshold and an orientating reflex and varied between 0.5-1.2 mamp. The animals were conditioned by avoidance methods using delayed conditioning techniques. The training program and conditioning sequenceused were similar to those described (2). Sounds and electrical stimuli were
192
CLARK,
KRANZ,
AND
MINAS
FIG. 2. A diagram of the experimental set-up. ATT-attenuator; COUNT-frequency counter ; CRO-cathode-ray oscilloscope ; CU-control unit ; FG-function generator ; OSC-oscillator; SFG-sweep function generator; TRANS-transformer.
modulated with sineand triangular waves as illustrated in Fig. 1. Sine waves were used as carrier frequencies for the acoustic stimuli and biphasic square waves for the electrical stimuli. The first series of experiments was performed on cats with electrodes implanted in the basal turn of the cochlea through the round window. The carrier frequencies tested were 200 pulse/set, 400 pulse/set. 800 pulse/set, 1600 yulse/sec, and 3200 pulse/set. The modulating sine wave had a frequency of 5 pulse/set, and its amplitude was adjusted to provide a percentage deviation of l’$, Sr/c, lo%, SOS, and 10070 in the carrier frequency. The experimental set-up is illustrated in the lower half of Fig. 2. The modulating frequency was produced by a Krohn Hite 4100 oscillator, the carrier frequency by a Hewlett Packard 3310A function generator, and the sound was presented through a Kola CSDOOloudspeaker. IVhen the carrier frequency was modulated, the percentage deviation in its frequency was dcterniined by the voltage of the modalatiiig wave. Fre-
~pellc~ iiiutlulation was also carried out bv a technique duce xnilditutle modulation into the signal.
that did not intro-
The second series of esl~erinients was performed on cats with electrodes placed directly into the basal turn of the cochlea. The carrier frequencies were 300 lmlse/sec and 2000 l~tilse/sec, and 25 ‘h deviations in frequency were lmducetl by triangular waves of constant aml~litude. The modulating maw \vas p-nducetl 1)~ R Svstron-Dormer 310 sweep/function generator. and . _ its slol)e 1~2s: altered to allow the presentation of graded changes in the rate of frequency modulation ( Fig. 2) . iit these rates of change the frequency of StillldtiS ~)reSelitatirJli \vas 1 Hz and it \\-a~‘ consitleretl that the animal wcmltl not respond to a small change iii tllis ~mameter in preference to the frequency niotlulatetl signal. In the alwve studies it ~vas considered important to make sure the functioning hair cells were not present. as their presence wc~ultl mean that dectrical stimulation could induce hearing through normal l)athways jelectr~~~hmiC
auditory
hearillg
)
alid
llot
lleCeSSaril\-
_
\)v-
direCt
StilnUhti~Jll
Of
tertllillal
nerve fibers. This was tlone 1)~ recording cochlear microphonic potenti;~ls. It ~2s also considered tlesirahle to make a more detailed assessmetlt of the effect of electrophonic Iwaring. This ux carried out 1)~ recording tl~reslioltls before and after the hair cells were destroyed using 1500 ratls of an ultrahigh velocity electron l)eam from a hIEL ST, 75 linear accelerator directed at the temporal bone. Xt the coml~letion of the experimental series. the cats lvere anesthetized and lm+isetl intraarterially with normal saline solution followed by 10%) formalin. The temporal hnes were removed, tlecalcitied. and embetlcled. The 1)lOCliS were serially sectioned at a thickness of 30 pi. atltl one in ten sectir 111swere stained \vith lieniatos~liii ant1 eosin.
194
CLARK,
KRANZ,
AND
TABLE CAT
Combination
Acoustic and electro-
phonic
Acoustic
and
auditory nerve
Electra phonic
and auditory nerve
Carrier frequency HZ
MIXAS
II 2
Percentage
deviation
of carrier
It25
*5
frequencf
___&SO
200 400 800 1600 3200
1.1 3.0 0.3 1.1 3.8
9.2++ 3.2 .5.5+ 14.7++
4.3+ 6.2+ 13.9+ 18.6++ 18.4++
200 400 800 1600 3200
2.0
9.0++ 38.4++
3.0 12.0++ 5.5+ 35.6++ 38.4++
13.9++ 11.9++ 45.1++ 38.4++ 35.3++
200 400 800 1600 3200
0.2 0.6 0.4 4.8+ 22.5++
0.1 0.3 0.6 17.4++ 8.5++
3.5 1.1 1x9++ 6.0+ 4.4+
1.1 0.0
1.1
f2.5 23.3-’ 13.1++ 22.F 15.0++
10.3++ 32.9+*
fO.5 0.3 0.0 4.6+ 17.1++ 17.1++
1.5.0++
0.8 0.4
35.3++ 27.9+* 32.-I+’
14.3++ 26.8++
1.6 0.1 2.5 2.8 8.5++
11.4++
1.0 0.4
0.9 1.0 1.7
Chi-square test of significance. +x2 1 df (3.8), P < 0.05. ‘+xX” 1 df (6.6), I’ < 0.01.
RESULTS The first series of experiments was mldertaken to compare the difference limina for frequency, for sound, and electrical stimulation of the basal auditory nerve fibers. This was carried out using sinusoidally modulated stimuli, and was done to help determine the importance of the basal turn of the cochlea in coding low frequency sound. The cats were first conditioned to sinusoidally modulated sound, and when their performance levels reached an asymptote, the acoustic stimulation was replaced by sinusoidally modulated electrical stimulation. At each daily session the cat was tested to a descending series of five percentage deviations in the carrier frequency from 100% to 1%. The spontaneous response rate was also sampled in random order during these sessions. Each day the carrier frequency was changed in an ascending, descending and ascending series from 200 pulse/set to 3200 pulse/set until 30 stimulus presentations had been given for each possible carrier frequency-percentage deviation combination.
FKEQUENCT
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The behavioral responses for each carrier frequency-percentage deviation combination were tested for significance against the spontaneous responding rates using a Ch-square analysis. and a 5% probability level was accepted. The results are summarized in Table 1, which shows the thresholds for different percentage deviations of the carrier frequencies obtained for cats 1 and 2 \vhrn using acoustic, electrophonic, and auditory nerve stimulation. I;rom this it can be seen that the thresholds for electrophonic and auditory nerve stimulation were higher than those obtained for sounds hal-ing the same carrier frequency. These differences in the behavioral responses for acoustic. electrophonic, and auditory nerve stimulation were tested for statistical significance using a Chi-square analysis, and the results for cat 2 are she\\-n in Table 2. These show that the differences between acoustic and auditory nerve stimulation, and between acoustic and electrophonic stimulation were significant for most carrier frequencies and percentage deviations in frequency. \\%en electroplionic and auditory nerve stimulation was compared the responses were only significantly different for stimuli with high carrier frequencies. From the thresholds for percentage deviation in different carrier frequencies it was possible to estimate the niasimunt rate of electrical stimulation of the basal end of the cochlea that could produce a low pitched sound on the basis of a time-period code. It was assumed that for each carrier frequency the cat would perceive a change in frequency at the ion-est point in the modulation cycle. On this basis it can be seen in Table 1 that for electrophonic hearing the highest frequency that could be perceived occurred at a * 257, deviation in a carrier frequency 3200 pulse/set for cat 1, and at a * SO7c deviation at 3200 pulse/set for cat 2. The rates of stimulation perceived at these carrier frequency-percentage deviation thresholds were at least 2400 pulse/ set for cat 1 and 1600 pulse/set for cat 2. Similarly for electrical stimulation of the auditory nerve the highest rates of electrical stimulation that could be perceived occurred at a carrier frequency of 800 pulse/set and 25 s deviation in frequency for cat 1, and a carrier frequency of 1600 pulse/set and SOY deviation in frequency for cat 2. The rates of stimulation perceived at these carrier frequency-percentage deviation combinations were GO0 pulse/set for cat 1 and 800 pulse/set for cat 2. The second series of experiments was designed to make a more accurate assessment of thresholds for rate of change of frequency. In this series amplitude was kept constant, and triangular waves were used to modulate the sound. The thresholds for frequency modulated sound were then compared with those obtained for electrical stimulation. The cats were conditioned to a modulated sound and electrical stimulus and, \\hen their performance levels reached an asymptote, they were tested
196
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MINAS
-
200Hz
x----x
0
200
100
300
GRADIENT
Sound
2OOPulse/sec Electrical
400
500
(Hz/set)
graph of the behavioral responses for rate of change of frequency ior acoustic and electrical stimulation with carrier frequencies of 200 Hz. FIG.
3. A
at various rates of frequency change. These rates were varied in seven graded steps from 1 pulse/sec/sec to 500 pulse/sec/sec, and 10 pulse/set/ set to 5000 pulse/sec/sec for carrier frequencies of 200 pulse/set and 2000 pulse/set. respectively. They were presented in an ascendingand descending series of 13 stimuli, and at each sessionthe spontaneous responding rate was also determined. These sessions were repeated until five stimulus presentations had been given for each rate of frequency change, The spontaneous responseswere then subtracted and the result expressed as a percentage. The percentage responsesfor different rates of frequency change were plotted graphically for sound and electrical stimulation having a carrier frequency at 200 Hz, and the results are shown in Fig. 3. The responses for different percentage rates of frequency change for sound having carrier frequencies of 200 Hz and 2000 Hz are shown in Fig. 4. The thresholds were determined by plotting the rate of frequency change equivalent to a SO% behavioral
response.
In Fig.
3 it can be seen that the thresholds
for
acoustic and electrical stimuli having a carrier frequency of 200 Hz were 97 Hz/set and 85 pulse/sec/sec, respectively. In Fig. 4 the thresholds for acoustic
stimuli
having
carrier
frequencies
of 200 Hz and 2000
Hz were
47%’ Hz/set and 3570 Hz/set, respectively. J)ISCUSSION A previous study (2) showed that the cat could respond to low rates of electrical stimulation of the terminal auditory nerve fibers passing to the basal turn of the cochlea, but the difference limina were greater then those
197
-
200
Z+ -- --X 2000
GRADIENT
FIG. 3. stimuli
Hz Sound Hz Sound
( ~Hz /SW
A graph of the behavioral responses for rate of percentage with carrier frequencies of 200 Hz and 2000 Hz.
change
of acoustic
obtained for sound by Frazier and Elliott (5 j in cats, and Shower and Hiddulpl~ ( 10 j in human subjects. If the hasal turn of the cochlea plays a role in the coding of low frequency sound, difference limina for electrical stimulation should be similar to those for sound. In this previous study (2) a tlirect comparison of the difference limina for sound and electrical stimulation in the same experimental animal ~2s not made. 17urtlierniore, the difference limina were measured h\- incrementing a steady state frequency to another steady state frequency without an intervening glide between the two. Although this method avoids introducing additional frequency cues during a glide. most of the previous difference limen measurements have been made using a frequency which was sinusoidally modulated from one to the other. Consequently, in the present study, thresl~oltls for sinusoidally modulatetl soiuitl and electrical stimuli have been measured. These threshold measurements also made it possible to estimate the maximum rates of electrical stimulation that could he perceived by exciting the auditory nerve fibers to the basal turn of the cochlea. The results of the comparison between the thresholds for percentage tleviations in carrier frequencies for sound and electrical stimulation of the auditory nerve are shown in Table 2. The statistical tests indicate that tllere were significant differences for most carrier frequencies and percentage deviations. This suggests that perceptual acuity is greater for acoustic stimulation of the apical turn of the cochlea rather than electrical stimulation of the basal turn. I;urthermore. there were significant differences in the responses for electroplwtiic arid auditory nerve stimulation at l~iglier carrier frequencies.
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AND
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This probably occurred because all the cochlear hair cells were not destroyed by the introduction of the round window electrode, and because electrical stimulation vibrated the skull by a capacitance effect. As a result sound waves were transmitted to the residual hair cells by bone conduction. This further emphasizes the importance of making sure that viable hair cells are not present in all studies on electrical stimulation of the auditory nerve so that hearing may not be produced by the electrophonic effect. The upper limit on the rate of electrical stimulation that can still be perceived as a distinct frequency was estimated from the threshold for the percentage deviations in the highest carrier frequency tested. It was assumed that if the cat could perceive a changing frequency it must be able to detect a rate of stimulation at the lowest level of this frequency modulation range. The results showed that the upper limit for electrical stimulation of the auditory nerve fibers is 600-800 pulses/set. This is higher than the rate ‘determined in previous acute behavioral experimental studies (1, 2) where the findings indicated an upper limit of 200-400 pulses/set. The difference in the results could be due to the use of a biphasic square wave stimulus in the present series instead of the monophasic one used in previous studies (1, 2). A biphasic stimulus has been shown to produce increased auditory nerve firing during one-half of the cycle, and decreased firing during the other half, ( 12). Consequently, a biphasic electrical stimulus is more likely to result in nerve firing which is phase-locked to the stimulus, and distinguishable from spontaneous activity. As a result higher rates of excitation would be more readily perceived. The underlying neural mechanism involved in the perception of a frequency modulated tone are not clearly understood. Psychoacoustic studies (7,s) suggest that the perception of a changing frequency may be related to the frequency content of the Fourier analysis of the signal. In which case it would be perceived on the basis of the spatial pattern of excitation along the cochlea produced by the different frequencies. It seems more probable, however, that a time-period code operates, particularly for small variations in rate of frequency change, as experimental studies have shown the coding of steady state frequencies and complex sounds can be explained on this basis (6, 9). If a time-period code is important, frequency modulated electrical stimulation of terminal auditory nerve fibers should produce thresholds similar to those for sound, This comparison was undertaken in the present study and, as the hair cells were nonfunctioning, it was assumed there would be little .chance of the frequency side bands in the stimulus producing a spatial pattern of excitation in the cochlea. The results shown in Fig. 3 indicate that there was little difference in the thresholds obtained for frequency modulated sound or electrical stimuli with
FREQUEKCY
XODULATEI)
SOUr\‘l)
199
carrier frequencies of 200 Hz. The threshold was 97 Hz/set for sound, and 55 pulse/sec/sec for electrical stimulation. These results suggest that a varying time-period code is important in detecting a frequency modulated signal, Nevertheless it seems unlikely that this mechanism will operate for high carrier frequencies and modulation rates as an upper limit will be set by the refractory period of nerve fibers. In this study the threshold for rate of change of frequency was lower for electrical stimulation than for sound, but for steady state stimuli the reverse applied. ;Iii inverse relation between absolute, and differential sensitivity has also been shown when comparing thresholds for acoustic stimuli in cats and men (4) . The innervation of the cochlea has been studied by Spoendlin (11)) and he has shown that a single intier hair cell is connected to about 20 ganglion cells, on the other hand about ten outer hair cells innervate one ganglion cell. Consequently, if electrical stimulation is to produce a perceptual sensation similar to that of a steady state tone then the pattern of excitation of auditory nerve fibers is likely to be important. As this could not be simullatecl with the bipolar electrodes used in a previous study (3 j this would explain the discrepancies between the thresholds for sound and electrical stimulation. On the other hand, if the ganglion cell density is important in the perception of a change in frequency, and one intier hair cell is connected to 20 ganglion cells then rate of stimulation is more likely to be important. This would help explain why in the present study the difference limen for an electrical stimulus with a changin g frequency was the same or lower than that for sound. In the present study, the thresholds for rate of change of frequency were also compared for acoustic stimuli with carrier frequencies of 200 Hz and 2000 Hz. Although the thresholds were quite different when measured as Hertz per second, they were practically the same when considered as a percentage change of the carrier frequency. This suggests that for a certain range of carrier frequencies the thresholds may be the same when measured this way. A more detailed study needs to be carried out, however, to substantiate this finding. ACKXOWLEDGMENTS This study was supported by grants from the National Health and Medical Research Council of Australia, the Bushells Trust, the Felton Bequest, and the Sunshine Foundation. FVe thank Dr. R. Kerr from the Peter biacCallum Clinic for irradiating the cats temporal bones, Miss B. Laby of Melbourne for from the Department of Statistics, at the University statistical advice, and Mr. R. J. ~~~alkerclen for help with electronics.
200
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AND
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REFERENCES 1. CLARK, G. M. 1969. Responses of cells in the superior olivary complex of the cat to electrical stimulation of the auditory nerve. Exp. Ncrwol. 24: 124-136. 2. CLARK, G. M., J. M. NATHAR, H. G. KKANZ, and J. S. MARITZ. 1972. A behavioral study on electrical stimulation of the cochlea and central auditory path\vays of the cat. E.rp. Ncwol. 36: 3.50-361. 3. DALLOS, P.. and R. H. SWEETRIAN. 1969. Distribution pattern of cochlear harmonics. J. .4coust. Sot. Amer. 45: 37-46. 4. ELLIOTT, D. N., L. STEW, and M. L. HARRISON. 1960. Determination of absoluteintensity thresholds and frequency difference thresholds in cats. J. .4coust. Sot. Anzrr. 32 : 380-384. 5. FRAZIER, L. F., and D. N. ELLIOTT. 1963. Size and reliability of frequency-difference thresholds determined with operant tracking procedure. J. Ex). ilrral. Bchrra. 6 : 189-192. 6. HIKD, J. E., D. J. AKDERSON, J. ‘F. BRUXGE, and J. E. ROSE. 1967. Coding of information pertaining to paired low frequency tones in single auditory nerve fibers of the squirrel monkey. J. Nrwophysiol. 30: 794-816. 7. MCCLELLAIGD, K. I)., and J. F. BRAKDT. 1969. Pitch of frequency-modulated sinusoids. J. =Icmst. Sor. Amer. 45 : 1489-1498. 8. NABELEK, I. V., A. K. NABELEK, and I. J. HIRSH. 1970. Pitch of tone bursts of changing frequency. J. .qro~st. Sot. Anzcr. 48: 536-553 9. ROSE, J. E., J. F. BRUGGE, D. J. AKDERSON, and J. E. HIKD. 1967. Phase-locked response to low-frequency tones in single auditory nerve fibers of the squirrel monkey. J. Ncurophysioi. 30: 767-793. 10. SHOWER, E. G., and R. BIIKJULPH. 1931. Differential pitch sensitivity of the ear.
J. Am&. 11. SPOEKDLIN,
SOL.. Anzcr. 2: 275287. H.
1971.
Degeneration
behaviour
of the
cochlear
nerve.
Arch. k’lin.
Exp. Oh., Nas. U. k’rhlk. Hclzlk. 200: 275-291. 12. TEAS, D. C., T. KONISHI, and J. S. WARNICEE. 1970. Effects of electrical current applied to cochlear partition on discharges in individual auditory-nerve fibers. II. Interaction of electrical polarization and acoustic stimulation. J. Acwst SOL-. Amr. 47: 6, 1527-1537.