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Responses of auditory nerve fibres in the guinea pig to noise bands of different widths
Heanng Research, 2 (1980) 327-333 © Elsevier/North-Holland Bzomedical Press
327
Auditory nerve eIecltophys~olog~cal observations ~'~ESPONSES OF AUDITORY NERVE FIBRES IN THE GUINEA PIG TO NOISE ~ANDS OF DIFFERENT 9/~DTHS
A,G. GILBERT and J.O. PICKLES
N.'uroeommunications Research Unit, Birmingham Umverstty, B:rmmgham B13 2TJ, U K
Tl~e responses of single fibres of the auditory nerve weze recorded m an,~esthet~sedguinea p]gs. The fibres were stimulated by bandpass nozse centred on the characteristic fzequenc~ As the no~se beautwidth ," as increased at constant spectral density, the firing tare me, eased, came to a perk, and m (he majority of fibres, declined at wider b~ndwzdths. The bandwidth at which the firing was at a pe~k had a mean value of 34% of the charactertshc frequency On average, the evoked ft,ang rate was suppressed by 8% at the w~est band~ of nozse used. However, 17% of fibzes showed no ~Ltppress~onat ~_i. In those that d~d, ~upptess~on was still vtsible ff the stimulus intensity was so h~h ~s to drive the ftrmg into saturatmn. Key words: auditory nerve, suppresston; nozse. INTRODUCTION One of the manifestations of cochlear nonlinearity is the phenomenon of two-tone suppression, as seen in single fibres o f the auditory nerve. In two-tone suppression, a tone in the suppressive areas which flank in frequency a fibre's excitatory response area w~ll reduce ~he response to a fust, excitatory, tone. Analogous suppression effects have been seen with more complex stimuli ~uch as bands o f noise [3,7]. A band o f noise was centred on the characteristic frequency (CF) of a neuron. As the noise band was increased in width, the firing rate at first increased, because a greater power of noise fel~ w~thhj the excitatcry response area. At wide stimulus bandwidths, howf:ver~ the firing ra~e reel/ned, presumzbly because some of the noise fell on the two-tone suppression areas. Rugg~ro [7] des:ribed the effects in fibres o f the auditory nerve, and simdar effects have beer~ seen in neuron~ o f the cochlear nucleus by Greenwood al~d G~ldberg [3]. Greenwood and Goldberg suggested that the effects seen in at l e ~ t gem0 ~,f the cells o f the cocl'dear nucleus derived from s h n ~ r phenomena ~n fibres of the ~udilory nerve. Both Greenwood and Goldberg [3] and Ruggero [7] published o ~ y illustrative urat~, presumably t h o ~ which ~ o w e d the suppre~.s/on effect par~icular|y stcongly. Moreover they lemarked that many units ~hov,ed no ~appre~ ,m ,~ ~ V¢e .note ~tereg~ed ~ d:.~o coveri'tg the l ~ l y Lmpottfff|Ce o f the effect ~ the a,20ztory r~er/~ ~ ~ wiLe.re ~ J b~_,,~ meazt.red the rra:g.~tude of the 0ffecc and ~h~ baadw,~aL~'~ ~c w.~ch ~be .~upre~sz~m occurred, over a popel~fion o f fibrez. ',~¢~"~cre ~.',~r~~ter~.z~zd to ~e~ v, het?~ec 5~e ~,~p
328
METHODS Guinea pigs were. anaesthetBed with intral~'itoneal uret~rte (1.5 g/kg b o ~ weight). The trachea was cannLdated, and the tissue over the occipital ridge and bo'.h external auditory meati reflected. The skull was opened widely through the o"..dpital ridge, and then the head was flied in a headholder with hollow earbars. The left bulla was vented dorsally. The cerebellum overlying the left cochlear nucleus was removed by aspiration, and hyperfme (100-150 Mfl) microelectrodes inserted into the internal auditory meatus, by a stereotaxic approach through the braim~.em, using the post~oventral comer of the appendicu~,ar fosse as a reference point. The ear was stimulated with a Bruel and Kjaer half.inch type 4134 microphone in a damped~ closed, sound system, c~brated by probe tube, as described by Evans [2]. Criteria for primary fibres were (i) placement of the electrode within the internal auditory meatu:,, and (it) btency within the limits described by Evans ~1]. Bandpar noise was produced in a balanced modulator. Low-pass ncir,e tqow-frequency cut-off 10 1'z, high-frequency cut-off variable, slope 12 dB/octave), was modulated by a sine wave iv a balanced modulator, resulting in a band of noise of fiat-topped spectrum with steep cut-off slopes. The cut-off slopes of the fm~ band of noi~ depended on the cel,tre frequency and the bandwidth; f~r instance a 1 ':&[z wide band centred at 10 kHz ha,t slopes of 120 dB/octave. The noise w4~ of constant total power whatever the bandwidth, to within "~0.5 dB. When a fibre was isolated, the tuning curve was t~t~,~r,e~ hy an automated procedure, and then the fibre was stimulated by a band of n.~ise arithmetically centred on the characteristic frequency. Rate-intensity func,~.ionswere obtained for noise bursts (1 s on, l s off, 20 ms rise~fall time) over a 60 dB range b~ a computer-controlled routine, m vsluch the step size was 6 dB, and in which the intensities were presented randomly, with the p~oviso that no step downward exceeded 30 dE,. The rate-intensity function was first c b~ained for the widest band of noise, which had a width in Hz approximately equal to the characteristic frequency of the fibre. Rtte-intensi y functions were then obtained fo~ noise bands of different widths, first hi decrea~in[, order of bandwidth and then increasing, and so on, so that in the end mean rate-intensity functions could be calculated from 4 -8 interleaved ~uns, the number of rues c~pending on the time for which the fibre could be held. RE~ULTS J:ig 1 shows the rate-intensity functions obtained h~ one fibre o~1 stimulation with ban.is of noise of different widths ~mt constant total pt)wer. Wide-band noise was less effcctive than narrow-band noise at driving the fibre. ~!fis probabJy occurred for two ~ea, ons. (i) Wide bands suppressed the firing, and (it) ti~e noise was of constant total pov'er, and so a smeller amount of power in the wide ~ands was able to stimulate the fibr.~. So as to separate fll~se factors, the faraily of rate-inter~sity functions wa~ reo' g~ised to relate the firing rate to flxe noise bandwidth for noBe of constant spectral den ~ky. In order to do this, it was n e c e ~ T to interpdate to 3-dB intervals the 6-dB stets of the rate-intensity functions, as two-fold changes in bandwidth, equivalent to
329
Bandwidth kHz 250=
075 15 :3
CF = i:5-8 kHz
6 12
03
17
29 41 Intensity (dB S P L )
53
65
I-~g. 1. Rate-inlem~ty functmns to bands of noise of different w;dths but constant total power, amhmetically centred on the characteristic frequency Lower curves rate m ~he 'off" period f~llowmg the 'on' perzod.
3-dB changes in spectral tensity, had been used. The derived functions relating firing rate zo noise bandwidth for noise of constant spectral density are shown for another fibre m Fig. 2A. It is apparent that a~ the noise bandwidth was increased from narrow bandwldths ".he firing rate at first increased, presumably because more power fell within the excitatory area of the fibre. The firing rate then came to a maydmur:.~, and then d,~clined, presumably because with the wider bands some of the noise fell on the two-tone supp~e~ sion a r e a s . For rite fibre in Fig, 2a, the bandwidth at v,hieh the firing rate was at a ma;~arnum, which has been called the turnover bandwidth, bec~me a little le~s a~ the noise ir tert,at7 was increased. This wa~ ~een in many fibres, and was also described by Greenwcod md Goldberg [3] for neurons of the posteroventrat coehiear nuciou~. Gen~r~y th,~,effect was as small as or smaller than the effect seen in Fig 2a, although in one case it w ~ more extreme.
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oceu,-red ~ven when the no~.e mter~i~y w~s such that the f~rmg was ~atv.rat¢,i Th~ ~ateintensity ~unetior,~ of F*_g. 1 ~ho~ t ~ to be ~o, at d~e f~nng tc fl~e wid~. band ~,.m~luz flattened out at a lower ~ate : h ~ ~o a narrow-bznd =tm='~[~s. i~ ~.~d~ca~e, ~n~c~a,~xg tl,~ intensity of the vAde-bm'~d ~timuk:~ w~uld never make tb.e r:,.re equ'M to t,%t evc_ked by an intense narrow-band stimulus. The same point ~,~made Ly ~g. 2b, ~-~c,wh,g d,e ~a~,,.
330
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CF= 13-8kHz
250
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Fig 2. Relation of f'~ing rate to no:se band~ ~dth, for noise of constant spectral densRy. Curves at 3-riB intervals of intenmy. (a) A fibre wxth a Cl," of 2,16 kHz Lowest curve - 2 5 dB SPL/IIz, h~ghJst 17 dB SPL/Hz. Co) Dam front Fig I A fibre wRh a CF of 13 8 kHz. At low intensittcs a small amount of summatton and supptes-,~on ~s vastble, At ti'e hghest mtenstty the ftfing ts saturated, and does not increase wtth increasing b~ndwidth, but ts neverthe|ess suppressed beyond the turnover bandwidth Lowest curve -15 dB SPL/~Iz, h:ghest 24 dB SPL/Hz.
data plotted in relation to noise of constant spectral density. At ~fighintensities the firing rat~ did not increare with increa~ in bandwidth, but only, beyond the turnover bandwklth, declined, Shdla~ effects w e r e seen in all fibres that showed suppression at low intensities. Thus the mean f~-ing rate, even in saturation, was able to respond to s o m e details of the stimullis, ~t th.~ case the bandwidth. The turnover bandwdt:~, calculated for all fibres showing suppression, increased in proportion to she chara,-tevsC~ frequency, and had a mean value of 34% of the characteristic frequency (Fig. 3). Overall, the turnover bandwidth was similar to the I0 dB bandwidth, but on average was about 8% larger. However, there was a statistically signific~lt (P < 0.05) tendency for the ratio of the turnover bandw/ddl to the 10 dB bandw~dth to increase as the c"haracteflstic frequency h~creased; regression analysis showed the latio el" the two bmidwidths to be equal for eh ~a:terLstic frequencies of 3.1 kHz and to cl~ange by 0.36 log units for every decade chtmge m cha~actemtic frequency, The degre~ of suopre:,'sion was defined by a suppression querier-, defined as rite proportion of the sihnulus-cvoked ftring at the tucnox'er bandwidth that was suppres~d by the va~lest bands. If R is the firing rate in~ reagent at the turnover bandwidth, mea-
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sured above the spontaneous rate, and S the reduction in firing for the bandwidth approx. nnately equal in Hz t~ the ct~arze)ensti¢ frequ~mey, the suppression quouent was SIR. Measurements were re.Me in each ease from the mean of the six f i n e s t rate-bandwidth functions below satur~.tion. If the fibre showed no suppression, and hence had no turnover bandwidth, R w;.s instead me;.smed at a bandwidth estimamd from the regression relation described in the last paiagraph. Fig. 4 shows the distribution of suppression quotients for all the low.thzeshold ~bres, namel'/, for all the fibres with best thresholds within the bottom 20 dB of th~ range at each frequency. 30/36 (83%) of the fibres showed some suppression. The mean suppression quotient over all the fibres was 0.083. In other words, as the bandwidth of noise was increased, the maximum evoked firing rate was reduced by an average of 8%. DISCUSSION
The results of this study agree wath those of Ruggero [7] in ~h¢ auditory nerve, and those of Greenwood and Goldberg [3] in monotonic units of the posteroventral nucleus of the coclflear nucb'.us, in showing that wide bands of no~se can suppress the firing of fibres. However the effect was not large; the o,terall suppression was on average 8% of the evoked firing rate increment. Moreover 17% of fibres ~owed no suppression at all even at the widest bandwidth used. The greatest effect seen in any one fibre was a reduction of 33% in rite evoked firing rate, in contrast Greenwood and Goldberg [3] illustrate a monotonic posteroventral cochlear nucleus unit in which 65% of the evoked firing was suppressed. This is more than 7 standard deviations away from the mean of the distribution shown in Fig. 4 and suggests that inhibitory mteraetiom in the cochlear nucleus may have influenced Greenwood and Coldberg's results, even for the neurons with monotonic rate-intensity functions. A furth~.r conclusion of this study is that suppression is still active when the firing of the fibres is saturated by intense stimuli. The results are relevant to the understanding of the coding in the auditory nerve of stimuli of high intensity. At intensities of 80 dB SPL and above the firing of a very high proportion of auditory nerve fibres will be saturated (see [5]), although this proportion may become smaller ff suitable search techniques, such as applying electric shocks to the cochba or the auditory nerve, are used to find fibres of high threshold and low spontaneous firing rate [4]. The question arises as to whether the proportion of fibres unsaturated ha this intensity range is sufficient to subserve the demeu~trab!e p~ychophysical abilities of the intact animal (0.g. [6,7 I). This study shows that even if only the mean firing rate is considered, the majority of fibres are nevertheless able to respond to some details of the stimulus, hero the bandwidth, oven when driven into satura¢lon. Presumably corresponding phencmena will be seen in response to any stimulus wiuch changes file balance between the stimulation of the fibre's excitatory and suppression areas Thus the majority of the fibres will be ~ble to e*hange their firing in response to an intense tone masked by imense wideband rues'king noise, albeit by only a very small :unount. It ~s possible that this is one of the ways that cochlear nonlinearity, here manifested by suppression, is able to extend the dynamic tango of the auditory system. ACKNOWLEDGEMENTS
A.G. was supported by a ~ience Rese.~rch Co,meal Studmtship.
333
2EFERENCES [ 1 ] Evans, E F. (1972) The frequency response and other properties ,3f single fibres m ~he guinea p~ cochlear nerve. L Physiol. 226,263-287. ~2] Evans, E.F. (1979): Sin~Ie urdt studies of mammahan aud:tory nerve In: A~dltory Investigations "['he Scientific and Technological Basis, pp. 324-367. E-liter" H A. Beagley. Oxford Umversity l~en, Oxford. [3] Greenwood, D.D. and Goldberg, J.M. (1970). Response of neuters m the cochlear nuclei to ~armtions m no~e bandwidth and to ~ o n e - n o ~ ~ o m b ~ d o n s . J. Acoust Soc, Am. 47, 1022.040. L4] L~enTmn, M.C. and IOang, N.Y.-S. (1978): Acoustic trauma m cats Acta Otolalyngol. Suppl. .~58. [5] Palmer, A.R. and Evans, E.F. (1980): Cochlear fibre rate-discharge functions: no evidence for basdat membrane nonlinearities. Hearing Res. 2, 319-326. [6] Pici~ies, l.O. (1977): Neural correlates of the masked threshold. In: Psychophysi~s and Physiology of Heatbtg, pp. 209-218. Editors: E.F. Evans and LP. Wilson Acactemlc Ptes~. Lon~on [7] Ruggero, M.A. (1973): Response to noise of auditory nerve fib,~rs in the squtrr~l monkey J Neuroph> siol. 36,569-587. [8] Scharf, B. and Mekeiman, C.H (1977): Critical bandwidth at high Intenslh~s In. P~ychophyslcs and Physaology of Hearing, pp. 221-232. Editors: E F. Evans and .LP. Wilson Academic Press, London.
327
Auditory nerve eIecltophys~olog~cal observations ~'~ESPONSES OF AUDITORY NERVE FIBRES IN THE GUINEA PIG TO NOISE ~ANDS OF DIFFERENT 9/~DTHS
A,G. GILBERT and J.O. PICKLES
N.'uroeommunications Research Unit, Birmingham Umverstty, B:rmmgham B13 2TJ, U K
Tl~e responses of single fibres of the auditory nerve weze recorded m an,~esthet~sedguinea p]gs. The fibres were stimulated by bandpass nozse centred on the characteristic fzequenc~ As the no~se beautwidth ," as increased at constant spectral density, the firing tare me, eased, came to a perk, and m (he majority of fibres, declined at wider b~ndwzdths. The bandwidth at which the firing was at a pe~k had a mean value of 34% of the charactertshc frequency On average, the evoked ft,ang rate was suppressed by 8% at the w~est band~ of nozse used. However, 17% of fibzes showed no ~Ltppress~onat ~_i. In those that d~d, ~upptess~on was still vtsible ff the stimulus intensity was so h~h ~s to drive the ftrmg into saturatmn. Key words: auditory nerve, suppresston; nozse. INTRODUCTION One of the manifestations of cochlear nonlinearity is the phenomenon of two-tone suppression, as seen in single fibres o f the auditory nerve. In two-tone suppression, a tone in the suppressive areas which flank in frequency a fibre's excitatory response area w~ll reduce ~he response to a fust, excitatory, tone. Analogous suppression effects have been seen with more complex stimuli ~uch as bands o f noise [3,7]. A band o f noise was centred on the characteristic frequency (CF) of a neuron. As the noise band was increased in width, the firing rate at first increased, because a greater power of noise fel~ w~thhj the excitatcry response area. At wide stimulus bandwidths, howf:ver~ the firing ra~e reel/ned, presumzbly because some of the noise fell on the two-tone suppression areas. Rugg~ro [7] des:ribed the effects in fibres o f the auditory nerve, and simdar effects have beer~ seen in neuron~ o f the cochlear nucleus by Greenwood al~d G~ldberg [3]. Greenwood and Goldberg suggested that the effects seen in at l e ~ t gem0 ~,f the cells o f the cocl'dear nucleus derived from s h n ~ r phenomena ~n fibres of the ~udilory nerve. Both Greenwood and Goldberg [3] and Ruggero [7] published o ~ y illustrative urat~, presumably t h o ~ which ~ o w e d the suppre~.s/on effect par~icular|y stcongly. Moreover they lemarked that many units ~hov,ed no ~appre~ ,m ,~ ~ V¢e .note ~tereg~ed ~ d:.~o coveri'tg the l ~ l y Lmpottfff|Ce o f the effect ~ the a,20ztory r~er/~ ~ ~ wiLe.re ~ J b~_,,~ meazt.red the rra:g.~tude of the 0ffecc and ~h~ baadw,~aL~'~ ~c w.~ch ~be .~upre~sz~m occurred, over a popel~fion o f fibrez. ',~¢~"~cre ~.',~r~~ter~.z~zd to ~e~ v, het?~ec 5~e ~,~p
328
METHODS Guinea pigs were. anaesthetBed with intral~'itoneal uret~rte (1.5 g/kg b o ~ weight). The trachea was cannLdated, and the tissue over the occipital ridge and bo'.h external auditory meati reflected. The skull was opened widely through the o"..dpital ridge, and then the head was flied in a headholder with hollow earbars. The left bulla was vented dorsally. The cerebellum overlying the left cochlear nucleus was removed by aspiration, and hyperfme (100-150 Mfl) microelectrodes inserted into the internal auditory meatus, by a stereotaxic approach through the braim~.em, using the post~oventral comer of the appendicu~,ar fosse as a reference point. The ear was stimulated with a Bruel and Kjaer half.inch type 4134 microphone in a damped~ closed, sound system, c~brated by probe tube, as described by Evans [2]. Criteria for primary fibres were (i) placement of the electrode within the internal auditory meatu:,, and (it) btency within the limits described by Evans ~1]. Bandpar noise was produced in a balanced modulator. Low-pass ncir,e tqow-frequency cut-off 10 1'z, high-frequency cut-off variable, slope 12 dB/octave), was modulated by a sine wave iv a balanced modulator, resulting in a band of noise of fiat-topped spectrum with steep cut-off slopes. The cut-off slopes of the fm~ band of noi~ depended on the cel,tre frequency and the bandwidth; f~r instance a 1 ':&[z wide band centred at 10 kHz ha,t slopes of 120 dB/octave. The noise w4~ of constant total power whatever the bandwidth, to within "~0.5 dB. When a fibre was isolated, the tuning curve was t~t~,~r,e~ hy an automated procedure, and then the fibre was stimulated by a band of n.~ise arithmetically centred on the characteristic frequency. Rate-intensity func,~.ionswere obtained for noise bursts (1 s on, l s off, 20 ms rise~fall time) over a 60 dB range b~ a computer-controlled routine, m vsluch the step size was 6 dB, and in which the intensities were presented randomly, with the p~oviso that no step downward exceeded 30 dE,. The rate-intensity function was first c b~ained for the widest band of noise, which had a width in Hz approximately equal to the characteristic frequency of the fibre. Rtte-intensi y functions were then obtained fo~ noise bands of different widths, first hi decrea~in[, order of bandwidth and then increasing, and so on, so that in the end mean rate-intensity functions could be calculated from 4 -8 interleaved ~uns, the number of rues c~pending on the time for which the fibre could be held. RE~ULTS J:ig 1 shows the rate-intensity functions obtained h~ one fibre o~1 stimulation with ban.is of noise of different widths ~mt constant total pt)wer. Wide-band noise was less effcctive than narrow-band noise at driving the fibre. ~!fis probabJy occurred for two ~ea, ons. (i) Wide bands suppressed the firing, and (it) ti~e noise was of constant total pov'er, and so a smeller amount of power in the wide ~ands was able to stimulate the fibr.~. So as to separate fll~se factors, the faraily of rate-inter~sity functions wa~ reo' g~ised to relate the firing rate to flxe noise bandwidth for noBe of constant spectral den ~ky. In order to do this, it was n e c e ~ T to interpdate to 3-dB intervals the 6-dB stets of the rate-intensity functions, as two-fold changes in bandwidth, equivalent to
329
Bandwidth kHz 250=
075 15 :3
CF = i:5-8 kHz
6 12
03
17
29 41 Intensity (dB S P L )
53
65
I-~g. 1. Rate-inlem~ty functmns to bands of noise of different w;dths but constant total power, amhmetically centred on the characteristic frequency Lower curves rate m ~he 'off" period f~llowmg the 'on' perzod.
3-dB changes in spectral tensity, had been used. The derived functions relating firing rate zo noise bandwidth for noise of constant spectral density are shown for another fibre m Fig. 2A. It is apparent that a~ the noise bandwidth was increased from narrow bandwldths ".he firing rate at first increased, presumably because more power fell within the excitatory area of the fibre. The firing rate then came to a maydmur:.~, and then d,~clined, presumably because with the wider bands some of the noise fell on the two-tone supp~e~ sion a r e a s . For rite fibre in Fig, 2a, the bandwidth at v,hieh the firing rate was at a ma;~arnum, which has been called the turnover bandwidth, bec~me a little le~s a~ the noise ir tert,at7 was increased. This wa~ ~een in many fibres, and was also described by Greenwcod md Goldberg [3] for neurons of the posteroventrat coehiear nuciou~. Gen~r~y th,~,effect was as small as or smaller than the effect seen in Fig 2a, although in one case it w ~ more extreme.
It wa~ of interest to se0 whe~er the ~.ttppre=~lo,~ o~ ~r~,g bI ,~d6~ b~.~nds ,3¢ r,~_~.~
oceu,-red ~ven when the no~.e mter~i~y w~s such that the f~rmg was ~atv.rat¢,i Th~ ~ateintensity ~unetior,~ of F*_g. 1 ~ho~ t ~ to be ~o, at d~e f~nng tc fl~e wid~. band ~,.m~luz flattened out at a lower ~ate : h ~ ~o a narrow-bznd =tm='~[~s. i~ ~.~d~ca~e, ~n~c~a,~xg tl,~ intensity of the vAde-bm'~d ~timuk:~ w~uld never make tb.e r:,.re equ'M to t,%t evc_ked by an intense narrow-band stimulus. The same point ~,~made Ly ~g. 2b, ~-~c,wh,g d,e ~a~,,.
330
(o)
(b)
CF =2"16 kHZ
CF= 13-8kHz
250
g = ,J¢
(n
6 i5 35o'
i
i
Bandwidth
o
e5
g
kHz
Fig 2. Relation of f'~ing rate to no:se band~ ~dth, for noise of constant spectral densRy. Curves at 3-riB intervals of intenmy. (a) A fibre wxth a Cl," of 2,16 kHz Lowest curve - 2 5 dB SPL/IIz, h~ghJst 17 dB SPL/Hz. Co) Dam front Fig I A fibre wRh a CF of 13 8 kHz. At low intensittcs a small amount of summatton and supptes-,~on ~s vastble, At ti'e hghest mtenstty the ftfing ts saturated, and does not increase wtth increasing b~ndwidth, but ts neverthe|ess suppressed beyond the turnover bandwidth Lowest curve -15 dB SPL/~Iz, h:ghest 24 dB SPL/Hz.
data plotted in relation to noise of constant spectral density. At ~fighintensities the firing rat~ did not increare with increa~ in bandwidth, but only, beyond the turnover bandwklth, declined, Shdla~ effects w e r e seen in all fibres that showed suppression at low intensities. Thus the mean f~-ing rate, even in saturation, was able to respond to s o m e details of the stimullis, ~t th.~ case the bandwidth. The turnover bandwdt:~, calculated for all fibres showing suppression, increased in proportion to she chara,-tevsC~ frequency, and had a mean value of 34% of the characteristic frequency (Fig. 3). Overall, the turnover bandwidth was similar to the I0 dB bandwidth, but on average was about 8% larger. However, there was a statistically signific~lt (P < 0.05) tendency for the ratio of the turnover bandw/ddl to the 10 dB bandw~dth to increase as the c"haracteflstic frequency h~creased; regression analysis showed the latio el" the two bmidwidths to be equal for eh ~a:terLstic frequencies of 3.1 kHz and to cl~ange by 0.36 log units for every decade chtmge m cha~actemtic frequency, The degre~ of suopre:,'sion was defined by a suppression querier-, defined as rite proportion of the sihnulus-cvoked ftring at the tucnox'er bandwidth that was suppres~d by the va~lest bands. If R is the firing rate in~ reagent at the turnover bandwidth, mea-
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332
sured above the spontaneous rate, and S the reduction in firing for the bandwidth approx. nnately equal in Hz t~ the ct~arze)ensti¢ frequ~mey, the suppression quouent was SIR. Measurements were re.Me in each ease from the mean of the six f i n e s t rate-bandwidth functions below satur~.tion. If the fibre showed no suppression, and hence had no turnover bandwidth, R w;.s instead me;.smed at a bandwidth estimamd from the regression relation described in the last paiagraph. Fig. 4 shows the distribution of suppression quotients for all the low.thzeshold ~bres, namel'/, for all the fibres with best thresholds within the bottom 20 dB of th~ range at each frequency. 30/36 (83%) of the fibres showed some suppression. The mean suppression quotient over all the fibres was 0.083. In other words, as the bandwidth of noise was increased, the maximum evoked firing rate was reduced by an average of 8%. DISCUSSION
The results of this study agree wath those of Ruggero [7] in ~h¢ auditory nerve, and those of Greenwood and Goldberg [3] in monotonic units of the posteroventral nucleus of the coclflear nucb'.us, in showing that wide bands of no~se can suppress the firing of fibres. However the effect was not large; the o,terall suppression was on average 8% of the evoked firing rate increment. Moreover 17% of fibres ~owed no suppression at all even at the widest bandwidth used. The greatest effect seen in any one fibre was a reduction of 33% in rite evoked firing rate, in contrast Greenwood and Goldberg [3] illustrate a monotonic posteroventral cochlear nucleus unit in which 65% of the evoked firing was suppressed. This is more than 7 standard deviations away from the mean of the distribution shown in Fig. 4 and suggests that inhibitory mteraetiom in the cochlear nucleus may have influenced Greenwood and Coldberg's results, even for the neurons with monotonic rate-intensity functions. A furth~.r conclusion of this study is that suppression is still active when the firing of the fibres is saturated by intense stimuli. The results are relevant to the understanding of the coding in the auditory nerve of stimuli of high intensity. At intensities of 80 dB SPL and above the firing of a very high proportion of auditory nerve fibres will be saturated (see [5]), although this proportion may become smaller ff suitable search techniques, such as applying electric shocks to the cochba or the auditory nerve, are used to find fibres of high threshold and low spontaneous firing rate [4]. The question arises as to whether the proportion of fibres unsaturated ha this intensity range is sufficient to subserve the demeu~trab!e p~ychophysical abilities of the intact animal (0.g. [6,7 I). This study shows that even if only the mean firing rate is considered, the majority of fibres are nevertheless able to respond to some details of the stimulus, hero the bandwidth, oven when driven into satura¢lon. Presumably corresponding phencmena will be seen in response to any stimulus wiuch changes file balance between the stimulation of the fibre's excitatory and suppression areas Thus the majority of the fibres will be ~ble to e*hange their firing in response to an intense tone masked by imense wideband rues'king noise, albeit by only a very small :unount. It ~s possible that this is one of the ways that cochlear nonlinearity, here manifested by suppression, is able to extend the dynamic tango of the auditory system. ACKNOWLEDGEMENTS
A.G. was supported by a ~ience Rese.~rch Co,meal Studmtship.
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2EFERENCES [ 1 ] Evans, E F. (1972) The frequency response and other properties ,3f single fibres m ~he guinea p~ cochlear nerve. L Physiol. 226,263-287. ~2] Evans, E.F. (1979): Sin~Ie urdt studies of mammahan aud:tory nerve In: A~dltory Investigations "['he Scientific and Technological Basis, pp. 324-367. E-liter" H A. Beagley. Oxford Umversity l~en, Oxford. [3] Greenwood, D.D. and Goldberg, J.M. (1970). Response of neuters m the cochlear nuclei to ~armtions m no~e bandwidth and to ~ o n e - n o ~ ~ o m b ~ d o n s . J. Acoust Soc, Am. 47, 1022.040. L4] L~enTmn, M.C. and IOang, N.Y.-S. (1978): Acoustic trauma m cats Acta Otolalyngol. Suppl. .~58. [5] Palmer, A.R. and Evans, E.F. (1980): Cochlear fibre rate-discharge functions: no evidence for basdat membrane nonlinearities. Hearing Res. 2, 319-326. [6] Pici~ies, l.O. (1977): Neural correlates of the masked threshold. In: Psychophysi~s and Physiology of Heatbtg, pp. 209-218. Editors: E.F. Evans and LP. Wilson Acactemlc Ptes~. Lon~on [7] Ruggero, M.A. (1973): Response to noise of auditory nerve fib,~rs in the squtrr~l monkey J Neuroph> siol. 36,569-587. [8] Scharf, B. and Mekeiman, C.H (1977): Critical bandwidth at high Intenslh~s In. P~ychophyslcs and Physaology of Hearing, pp. 221-232. Editors: E F. Evans and .LP. Wilson Academic Press, London.