Processing of noise by single units of the inferior colliculus of the bat Rhinolophus ferrumequinum

Hearing Research, 3 (1980) 285-30,9 © Elsevier/North-HollandBiomedical Press

285

PROCESSING OF NOISE BY SINGLE UNITS OF THE INFERIOR COLLICULUS OF T ~ E BAT R?tlNOLOPHU$ FERR UMEQ UINUM

R, ENGELSTJ~TTER,M. VATER and G. NEUWEILER

Arbetrskreis Neuro.~und~Rezeptorphysiologie, Fachbereieh Biologic der £- W.-G~ethe-Universitdt, D 6000 Franlcfurt/Main, F.R;(7. (Received 4 August 1980; accepted 10 September 1980)

For inferior colliculus units the response patterns and the thresholds fo~ pure tc,nes and noise of variable bandwidth were determined. In a threshold-bandwidth plot the nc.ise thresholds usually fell along two regression lines whose point of intersection established the size: of the neuronal critical bandwidth (nOB). The relevance of the small nCBs (0.2-0.4 kHz) obtained for the fr~:quency range of the constant frequency part of the orientation Callis discussed. No fixed relation was found effher between the nCBs and the neuronal critical rattles or between the size of nCB and the width of the tuning curve 3 dB above threshold of the best frequency. • ~

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Key words: inferior colliculus; neuronal critical bandwidth; neuronal critical ratio; frequency selectivity.

IN'~RODUCrlON The bat Rhinolophus ferrumequinum uses an echolocation call which consists of a long constant frequency (CF) component and a subsequent short frequency modulated (FM) component [24]. The frequency of the CF part is lowered in ihe presence of Doppler-shifted echoes so that the echo frequency is kept constant at the so called 'reference frequency' [251. The hearing curves obtained with neurophysiological [19] and behavioural methods [16] show an unusual sharp f'dter in the region of the 'reference frequency' and it seems clear from the work of Bruns [3,4] that the filter is the result of mechanical specialisations of the basilar membrane and its anchoring system. Under natural conditions, Rhinolophus ferrumequinum hears not only its outgoing orientation calls and the returning echees from its prey (wing-beating insects) but also 'background noise' which could mask the echoes from prey. In order 1-o achieve a gooe separation of the relevant information carrying signals from the 'cluttering noise', it is necessary that th,~ auditory system in the frequency range around the f'dtet region be as little disturbed by noise as possible. The effect of noi~ on hearing kis been studied mainly in psychophysical experiments in man and different groups of animals (reviewed in [6,23]). If a pure tone is overlaid by white noise, not all spectral comportents will contribute to the masking effect; only a narrow band of flequencies (critical band, CB) including the signal frequency is effective (reviewed in [23]). This CB is the result of a filter process assumed to take l~lace within

286

the cochlea [8,9] or in more central stations of the auditory system [21]. The size of the CBs can be measured in different ways, e.g. by the masking of a response to a pure tone by noise of variable bandwidtl~s (band.narrowing technique) or by calculating the signal-to-noise ratio (SNR) between the masked absolute threshold for a pure tone an~l the spectrum level (P) of a broadband masking noise. This ratio, called 'critical ratio' (CR), gives aa indirect estimate of the size of the CB. Psychophysical CRs have been measured in the bat Rhinolophusferrumequinum [ 14]. i~e*ween 40 and 80 kHz ~Lndabove tile 'reference frequency' (about 83.3 kHz) the CRs increase linearly with the logarithm ~f frequency. But in contrast to all mammals studied so far, where the maxima of sensitivity of the audiogram are not reflected in the CR curve [5 ], a minimum in the CR curve ofRhinolophus fermmequinum corresponds to the f'dter range around the 'reference frequency'. The advantage of these small CRs is the greater ability to detect these frequencies in the presence of noise [ 14]. The low sen~;itivity for noise within t?te filter region has also been demonstrated in neurophysiologi~:al expedients in which the respon~ to a pure tone, delivered at the best frequencies (BFs) of single units in the cochlear nucleus (CN) was masked by broadband noise (20 kHz bandwidth, centered around the BF) [29]. Units with a BF within the filter region (i.e. a BF +-3 kHz around the reference frequency)showed an especially small SNR. Therefore a small CB for single units with BFs in this frequency range can be expected. Tile aim of this work was to determine the frequency resolution capacities of single units in the inferior colliculus (IC)of Rhinolophus ferrumequinum and to compare the results with "~hose from single units of the CN (Neuweiler and Vater, unpublished). In addition to the neuronal critical bands (nCBs), obtained by using the band-narrowing technique, neuronal critical ratios (nCks) were studied. Both, nCBs and nCRs will be discussed in rela~:ionto the psychophysical result~ of Long [ 14,15]. MA'I'ERIAL AND METHODS

Single unit responses were recorded from the IC of five Rhinolophusferrumequinum. For surgery arid during recordings the bats were anaesthetized with Nembutal (25 mg/kg body weight). Recordings on each animal were made over several consecutive days. The surgical procedure and the recording and stimulation set up was the same as described by Vater et al. [30] (free field stimulation, loudspeaker flat within +2 dB SPL between l 5 and 90 kHz). The stimuli were presented at repetition rates of 3 Hz from a loudspeaker located at the best horizontal angle to the contralatt~ral ear. The pure tone pulses lasted for 20 ms and the noise pulses for 80 ms; the rise-fall time for all pulses was 1.0 ms. TIRepu~e tone pulses were delayed 30 ms relative to the noise onset. Pseudorandom noise was generated by a Wavetek 132 VCG/Noise Generator, low.pass filtered a~ multiplied with the pulsed carrier frequency. The noise bandwidth (B)was varied between 0.1 and 20 kHz. The overall intensity In was dependent on bandwidth (for B = 20 kHz, IN = 90 dB SPL; for B = 0.1 kHz, IN = 80 dB SPL) and thus the spectral density (P) in dB/Hz was used for the evaluation of nCBs and nCRs. P was ea|culated according to P= IN - 10 IogB/Bl (BI = 1 Hz) [1]. Frequ~.~ncv-threshold curves of single units and the thresholds for noise of variable

287 bandwidths, centered around the BF, were measured using audiovisual criteria. The response patterns to these stimuli were judged from PST histogram,; (binwidth 0.5 ms; 50 stimuli) constructed on-line by a PDP-11/40 con~pt,'.er (programs: H. Z611er). For measurement of threshold SNRs, the noise intensity was increased in 5.dB steps until the response to a pure tone BF stimulus, 20 dB above threshold, was masked. The wdues of masked thresholds were determined by off-line examination of PST histograms. The threshold values for no[.se and masked tones were fitted by regression lines by using the method of least square~.. In most cases the thresholds for different bandwidths could be described by two strail;ht lines whose intersectior~ gave an estimate of the noise bandwidth responsible for rnasking [22]. In psychophysical studies this noise bandwidth has been called 'critical band', but in the present study, dealing with single units, it is denoted as 'neuronal critical bandwidth" (nCB). It should be noted, however, that this term should not be taken to hr~ply that all the single unit data are in agreement with the psychoacoustical model. The nCRs were calculated for noise of 20 kHz b~Jndwidth according to., CR (in dB) = T - P

(T = threshold of the pure tone in dB SPL; P = spectral density in dB/Hz)

CR (in Hz) = antilog (10 -1 CR (in dB))

(from [231)

RESULTS

1. Response patterns 106 units were sampled from the IC. According to the terminology of Neuweiler and Vater [20] the response patterns to l,ure tones were classified into one of four categories: sustained, transient, complex and :~egative responders. However, since changes could occur in the response patterns of transient, sustained and negative responders to ~ure tones, depending on intensity and/or the bandwidth of noise, these units were defined as complex responders to noise. Whereas most of the units (72.7%) responded to pure tones in a transient way~ they often changed their pattern when ex~:ited by noise. With 74..5% of the units the sustained response (mostly 'Pauser') was the most common pattern to noise, followed by the complex response pattern (18.7%). A trarsient or negative response pattern rarely occurred (6 and 1%, respectively). One unit did not respond to pure tones but was excited by n~ise. In cochlear nucleus units displaying a transient, sustained or negative pattern the response to both pure tones and noise of different bandwidths are reported to be the same [28,29]; in collicular units, however, the response to noise cannot be predicted from the pure tone response pattern. This shall be demonstrated for three neurons which responded to a pure tone with a reduction of spontaneous activity for 20.-60 ms (negative responders) (Fig. la); however, noise of any bandwidth led to different effects: - Unit 190 (Fig. lb) only showed o n - o f f inhibition; the degree of inhibition was dependent on the intensity of noise, Unit 116 (Fig. 1c) responded with excitation (Pauser pattern), -

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2. Estimation of neuronal critical bands (nCBs) 2.1 Determination o f noise thresholds For 77 neurons the thresholds for noise of different bandwidths (B = O. 1,0.2, 0.5, 1.0, 2.0, 5.0, 10 and 20 kHz, centered around the BF) were measured, expressed in spectral density P and plotted as a function of bandwidth B. In 50 neurons the thresholds fm different noise bandwidths followed the course to be expected from psychophysical experiments. First of all, the thresholds declined ~vith increasing B. As the noise bandwidth increased within the boundaries of the tuning curve, summation occurred and less spectral density was necessary to elicit an excita~:ory response. With a further increase of bandwidth, summation ceased and the thresholds remained constant. Two straight lines in a noise threshold-bandwidth plot describe this behaviour (Fig. 2): a descending line (slope between - 2 and -11.5 dB/octave) and a horizontal line (slope 0 dB/octave). Their point of intersection defines the size of nCB. For a few units the slope of the straight line that could be fitted for bandwidths larger than nCB was different from zero, but ranged between 3 and - 2 dB/octave. If thresholds declined continuously down to B = 20 kHz, the size of nCB was categorized as larger than or equal to 20 kHz (Fig. 2, Unit 77). Fig. 2 also shows a f'flter unit (Unit 215, BF = 83.0 kHz) in which noise bandwidths g','eater than 1 kHz did not elicit any responses. Out of 17 filter u~aits 7 showed a similar behaviour. Except for one unit (Unit 27, Fig. 8), they were not spontaneously active and thus it could not be decided whether the unresponsiveness ~as due to the stimulation of sbrrounding inhibitory areas. Units with a BF below 80 kHz (n = 60) commonly had a more ~imple response behaviour. All were excited by noise irrespective of bandwidth with four exceptions. Two of

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these did not respond to nc,ise bands larger than :5 kHz. However, in a second trial both units responded well to signals with bandwidths up to 20 kttz. This behaviour might be explained by Nembutal-induced reversible threshold shifts. For tho:;e units that resFonded to broadband noise (B = 20 kHz), the comparison of thresholds !~or pure tones a! the BF and for (~oise disclosed that thresholds for noise can vary from -36 to +57 dB relative to tone threshold, yet noi~ thresholds in the range from +1 to +20 dB were m:)st common, ~.e. thresholds for noise usually arehigher than those for pure tones. The nCBs plotted as a function of t',~v BF of the neurons (Fig. 3) revealed that irt the frequency ~:ar~gefrom 15 t~ 80 kHz the size of nCB can vary unsystematically between extreme values from. 0.27 Io ~20 kHz. Furthermore, there was no correlation between the size of nCB and BF. In contrast, neurons with BFs in the frequency range aiound the CF part of the echolocatica ear (about 83 kHz) characteristically had extremely small nCBs (0.2-0.4 kHz) and irl this frequency range the .-,ample of the nCB values showed littie scatter. Intennediate ~alues of nCBs were obtained in units with BFs around 81.5 and 85 kHz; nCBs larger than 3 kllz never occurred. No data are available for units with BFs large~ than 86.5 kltz. 2.2 Determination of maslce4 thresholds For 15 of these 50 units which showed th¢~ expected r~oi.se thre~old course tho masked thresholds were als,3 determined. Fig. 4b Rows the threshold SNR of a tone at the BF as a function of the, bandwidth of the masking si~jaat. The curves for noise ~resholds (Fig. 4a) and the threshold SNR (Ffg. 4b) are, with two exceptio~s, mirror images;

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within the nCB the masked thresholds increased with increasi:tg B (slope 2.5 to 10 dB/ t~ctave); for bandwidths larger than the nCB a horiz,~ntai line could be fi~ted. Thus, noise lying outside a defined critical bandwidth does not contribute to masking. The aCBs estimated with these two me~hods (by determining noise and masked tt,~esholds) are, with one exception, it~ good agreeme,,~t (Fig. 5). 2.3 Variations oi" the noise threshold course As already mentioned, in 50 out of 77 neurons the, expected correlation bet:~'een noise

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threshold,,; and noise bandwidths (B = 0 . ! - 2 0 kHz) was not found. The other 27 urdts showed a different behaviour which will be illustrated with two examples. In six units thresholds declined slightly for narrow bandwidths (slope -1 to - 3 dB/ octave), followed by a sudden steep drop at a particular bandwidth (Fig. 6a). Larger bandwidths caused an increase of ~Lresholds (slope 3 to 10 riB/octave). In contrast to the norma~ c~se the two fitted curves for decreasing and increasing noise thresholds never intersected within the range of bandwidths tested. Fo~ two of these six neurons the noise thresholds were determined again about one hour :ffter the first measurement; these second curves showed the norm'~l relation (Fig. 6b). It is striking that at this time the intersection falls into the area of t}le sudden drop. For these two neurons the size of nCB also could be confirmed by the masked thresholds (Fig. 6c). In two further neurons of thi.'s type masked thresholds were also detenr dned, and in both cases the ir~tersection of the straight lines again corresponded to the region of discontbmity. Fig. 7~Lshows another variant found in five neurons. For small bandwid~Lhs the noise thresholds; remai]~ed constant; however, they decreased for bandwidths ~(}.5 kHz. The intersection of the two lines corresponds, according to the masked thresholds (Fig. 7b), to th~ n£B. Nevertheless, the dec]iiJae (slope - 7 to - 1 7 dB/octave) does not occur continously; rot B - 5 kHz there is an increase, which amounts to 8-15 dB/Hz depending on the unit. Fhe thresholds for futth,,,r increasing bandwidths can be described by a second declining line, which is shifted para]llel to the first one or declines with a stronger slope. This variant r',ormally cannot be i~)und in the masked thresholds; only in one unit did a

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mirror relation exist between noise and masked thresholds. The remaining units displayed a still different cou, se, but th(.'se variants from the normal behaviour never were reflect~.~dwithin the masked thresholds. The nCBs estimated for these variants are also plotted in Fig. :!;, but only in those cases where the points of intersection, obtained by noise thresholds, could be confirmed by nCBs from masked thresholds. The two exceptions from the common mirror.image cc~urs,: of masked thresholds, mentioned in section 2.2, will be described now. In one of the~e neurons th(; thresholds increased again between 2 and 5 kHz bandwidth; that means fi~at noise of these bandwidths contributed again to the masking process, but a furth~*.r increase of B had no influence (Fig. 8b). Stimulation with noise alone caused inhibition for bandwidths equal to or larger than 5 kHz (Fig. 8a), which might be responsible fo' the variation in masked thre~olds. The second exception shc~wed a similar behaviour wh,.'n the response to a pure

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tone was masked with noise, althou~ noise did noll elicit any response for bandwidti~s larger then 1 kHz. 3. Nonlinear behaviour The width of the tuning curve and the size of the nCB are both measures for the spectral resolation. M~ller [ 1 7 ] dcmonstrated for coch!ear nucleus units of the rat displaying a sustain,.~d firing pattern that the average width of che tuning curves a.t 3 d ~ points is in good agrc;ement with the frequency range over which a 20 kHz noise band is hltegrated. , ' ~

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W (kHp.) Fig. 9. Size of neuronal critical bandwidth (nCB), determined with noise thre,,;hotds(I,O, plotted for 45 units as a function of the width (149of the tuning curve 3 dB above, threshold o'/BF. The straight line shows the relation expected if the behaviour of the single units were linear. Fc.r urits with a nCB 20 kl-,,z, a nCB value of 20 kHz is plotted.

I n a(:cordance with F,vans and Wilson [7], who calculated "etfective bandwidths' for auditory nerve fibers of the cat, M611er [17] found a linear filler process for a limited intensity range. To test whether this is also valid for collicular units, the wJdth (lit) of the tuning curve 3 dB above threshold of BF, calculated according to M~llLer [ 17] ~h;df-width 10 dB above threslaold of BF) was compared with the nCB values (Fig. 9). C)xly some of the neurons hwestigated exhibited linear properties. In most cases more (:omplex relations were recorded: e.g. for an extremely broad tuning of W = 10 kHz, aCBs of between 0.34 and ~ 2 0 kHz were found.

4. Neuronal critical ratios (nCRs ) For all units for which masked thre:;holds for B = 20 kHz were determined, nCRs were calculated. Only those units were sele.::ted, whose masked thresholds showed the normal course (see Section 2.2) and displayed ;t nCB smaller than 20 kHz. Fig. 10 compares nCRs with nCBs for 21 coll;cular units. In contrast to the psychophysical 6ata in Rhinolophus ]en'umequinurn, according to which CBs between 20 andSO kHz ~.re twice as large, as CRs [15], such a fixed relation does not exist for single co!licular units. The nCRs range from twenty times smaller up to six times greater than the nCBs. In units deviating from the expected dependency of ma~ked l:hresholds on bandwidth (Fig. 8b), it is already obvious from the course of the masked thresholds that the nCRs

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:Fig. 10. Comparison of nCB(S/N) and nCR. Only those units are included whose masked thr~holds showed the normal course and in which nCBs(S/N) were smaller than 20 kHz.

will not give an estimate of the nCBs. The wdues derived are, although lying outside of the nCB, dependent on B; whereas from the 5 to 20 kHz bandwidths a nCR of 12.59 ktlz can be calculated, for B :: 2 kHz (a bandwidt]h which is normally not used to define CRs) nCR is 1.0 kHz. For co~parison nCB, determined by masked thresholds, was 0.44 kHz. In this case small bandwidths gave a more reasonable estimate for the nCB. This was due to the diminished influence of factor,~,which increased the SNR and thus also the nCR. DISCUSS[ON

Both van Gisbergen [28] (cat) and Vater [29] (Rhinolophusferrumequinum) ha'qe reported tile same response pattern for noise ;rod pure tones in cochlear nucleus units dits. playing a transient, ~;ustained or negative response pattern, but M¢ller [17] (rat) has described a few units which were excited by imre tones but inhibited by broadband nobe (/7 = 20 kHz). Thus a few neurons of the CN display a behaviour which seems to characterize IC units, i.e. an ind:ependent response pattern to noise (B = 0.1-20 kHz) from that to pure 1:ones. This :.s probably due to more complex neuronal interactions in the IC th;m in the CN with an increase of inhibitory influences [18]. For example, the respoase behavio~r cf neurons, which are excited by noise but show a reduction of spontaneous activity J~n response to pure tones, can be explained by assuming that they receive at le~,Lt one excitatory and one inhibitory input. If the inhibitory input dominates, the unit will respond with a negatiive pattern to ~,. single tone, but if these two inputs have differ.f.'nt threshohls :for noise alad pure tones, then the influence of the excitatory input can dominate in the presertce of noise and elich a positivie response pattern. ~oung ~nd Bl~owm.,]l 1I:31] described CN units of the cat which had small excitatory but larg,; inhibitory ~reas (cell type IV) and which also responded with excitation to noise. "Iheir inhibitory input might come from cells labeled type II/III which only respondc:d weakly or not at all to broi~dband stimuli.

297 IC neurons ~that did:not respond to p~lre tones butwere excited in the presence of noisewerealso described by Suga [16] [~I~'pure tone..deaf' or 'neise specialized'. This behaviour, too, must result from unbalanced excitatory ~md inhibitory inputs. Inhibition probably al~o was the reason why about half of the filter units did not respond to broadband noise. Despite the prcminent inhibitory areas in units with BFs below 80kHz [18], the present study shows that the behaviour of most of these units (58 out of 60)issimple, i.e. noise of any b~kndwidth evokes excitation. In agreement with these findings,Greenwood and Maruyama [lO] observed ~;ome units in the CN of the cat which responded with excitation to noise ,.~timuli of smal~ ban~Jwidth, yet inc,'easing the bandwidth so that it contained frequencies of the inhibitory areas led to a disappearance of the response;however, not 'all units surrouaded by inhibitory areas showed the same behaviour. Evans and Wilson [7] studying auditory nerve fibers in cats, and M~ller [ 17] working on the CN of rats, found that the threshold differences for pure tones at the BF and noise of 20 kHz bandwidth are in good agreement with the 'effective bandwidth' of the tuning curves. These neurons behaved, at least at '.low intensities;, as linear filters, and their frequency resolution capacity for noise can be: described by the 3 dB b~tndwi:tth. This, however, seems not to be a valid description of the response b~,~haviourof most collicular units in Rhinolophus ferrumequinum. First of al~, title scatter of threshold differences between noise and pure tones is quite large ( - 3 6 to +5'7 dB). Furfhennore, the cor~lparison of the nCB value with the bandwidth 3 dB above threshold of BF reveals that only about one third of the coUicular units had the properties of linear filters, at k'.ast at this intensity range. Most of the units integrated over smaller' or larger ~'requency ranges, the amount of which was probably fixed by the amount of inhibition. In the following, the neurophysiological data on th¢~ nCB a'~d the nCR will be discussed in relation to the psychophysical measurements of Long [ 14,15] in Rhb~olophus

ferrumequinum. The psychophysical data demonstrate tha~: between 40 and 80 kHz and above the 'reference frequency' CRs increase linearly with the logarithm of frequency [ 14], whereas the neurophysiological results from IC ~s well from CN units (Neuweiler and Vater, unpublished) indicate that the size of the nCB is independent of frequency. Furthermore, since psychophysical CBs at frequencies up to 80 kHz are twice as large as CRs [ 15], there is no col:relation between nCB and CB. The suggestion of Pickles ~.nd Comis [22] that at the level of CN and beyond the nCBs could covceivably be related to the CB is not valid for the IC ofRhinolophus ferrumequirmm. A good agreement with the psychophysical :results was obtained for the filter region; according to both neurophysiological and behavioural data Rhinolophusferrumequinum has the ability to resolve fine frequency differences in the filter regior,. Consequently the possibility of detecting these frequencies in th~ presen~:e of noise is. high. However, the maximum of the CR curve around 82 kt~z [ 14] is nei~ler reflected in single units of the IC nor in those of the CN; between 80 and 82.5 kHz nCBs larger than 3 kHz newer were found, i.e. in this frequency range a traz~sition in neuronal properties seems to takv place which is similar to that of the tuning curves of singl,~ units (e.g. [27].). Nevvrtheless, according to new psychophysical experiments with mare detailed measurements in the filter area, the course of CRs was confirmed and the m ~ i m u m was even more prominent [151.

298

A possible explanation for this discrepancy can be offered. By masking cochlear nucleus units with broadband noise, Vater [29] found that the SNR varies with the BF; neurons with a BF just below the f'dter range have significantly higher SNRs than units with a BF around 83 kHz. According to the psychophysical experiments in Rhinolophus femtmequinum [ 15], it can be assumed that this is also valid for the whole animal. Therefore high CRs should be obtained around 82 kHz because of high SNRs and not because of high CBs, i.e. a heavily reduced frequency resolution. It: contrast to psychoacoustics where CBs and CRs seem to have a fLxed relationship [6], in single units of IC and CN the nCRs cannot give an estimate for the size of the nCBs. The nCR obtained fc,r units with a nCB of identical size can vary over a large range. Since the SNR, from which the nCR is calculated, may be different, the discrepancy is easy to explain. Imagine two masked threshold curves shifted parallel for 10 dB; the size of the nCB would be the same in this case, but the nCRs would differ by a factor of ten. It follows that for units with a masked threshold curve deviating from the normal course (e.g. Unit 2'7, Fig. 8) the estimation of ~the nCB from the nCR will show even more discrepancy. In agreement with Pickles and Comis [22], it was found that some units have a nCR smaller than the psychophysical CR. Since the CR is a direct measurement ef sensitivity, thi.s means that even if some of the neurons are able to det~;ct the stimulus the animal as a whole is not [22]. The course of the noise threshold curves of single units was extremely variable. About one third of the units deviated from the expected beh~.viour. Furthermore, the masked threshold curves did not necessarily reflect these variations. Since there was no fixed relationship between the noise threshold curves and other characteristics of the units (e.g. BF, kind of tuning curve, inhibitory areas), the variability is difficult to explain. A pos. sible interpretation is that at least a part of the variants might be caused by influences like anaesthesia, body temperature, alterations in blood pressure, blood supply of the brain, genera~ physiological conditior~, etc.. factors known to influence hearing (see e.g. [2, lC)]). Such influences might be responsible for the response I:ehaviour of the unit docurnerated in Fig. 6. The peculiarity here is the discontinuity of the two fitted lines which co~ responds exactly to the nCB. The measur,~ment (Fig. 6b) where the unit behave, l 'nor. really', was done one hour later when the depth of anaes-~hesia was, judging from behav. lout, markedly reduced. Since the physiological state of ~he animals was not controlled and not all effects of anaesthesia are known, it is difficult to judge the relative importance of t~le::e factors. If, however, anaesthesia is responsible fl~r this kind of response behavio~r, lit is :surprising that it did not occar more frequently Different kinds of threshold curve:~; are not characterisl ic for IC units as demonstrated in ~he r,~.sults of Neuweiler and ~¢ater (unpublished) frora CN units. However, the relation b,;tween noise thresholds and nCBs seems to be more complex in the IC. AlOlough both psychophysical [14] and neurophysic, logical data indicate that the filter rt;gion is characterized by h~gh frequency :resolutio-I abilities, an influence of low frequea,:y noise cannot be excluded. Kiang and Moxon [13]described for auditory nerve fibers of the cat with a high BF (around 10 kHz) that narrowband (B = 0.5 kHz) low frequency noise presented at a stimulus level just high e n o u ~ to elicit a response led to a marked ei~evation of the low threshold tip of the tuning curve. This noise, if one takes

299

into account the results of Evans and Wilson [7] and P~ckles and Camis [ 22], was outside the effective bandwidth. Probably forward masking occurred due to ~:he frequency representation on the basilar membrane. Similar influences have to be ex:pected for ]C filter unEs of Rhinolophus ferrumequinum since some of these cel.ls a]so have low frequency t~ils [ 18]. On the; other hane,, to overcome masking effects the bat has some possibilities which facilitate the detection of a signal in noise. Beuter [ 1] described that in the presence of narrowband noise (B = 0.4 kHz), centered around the; 'reference frequency', an incom.. plete Doppler-shift compensation occurs in Rhinolophus ferrumequinum, i.e. the echo is kept above the 'reference frequency'. By this behavioural response the masking effect can probably be reduced. Furthermore, the pinnae operate as filters for high frequencies and, additionally, varying pinnae positions as well as binaural mechanisms may rule out some masking effects [ 11,12 ]. ACKNOWLEDGEMENTS

The authors wish to thank the members of the AK Neuro- und Rezeptorphysiologie for their interest and discussions and H. Hahn for providing technical assistance. Sp~.'cial [ thanks are due to Dr. G.R. Long and Dr. O.W. E[enson for carefully reading and correcting the English version of the manuscript and tbr their many helpful suggestions and criticisms. This work was supported by Deutsche Forschungsgemeinschaft grants Br 593/2, Ne 146/8 and Ne 146[11. REFERENCES

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[31 Bran.,;, V. (1976): Peripheral auditory tuning for fine frequency analysis by the CF-FM bat, Rhinolophus ferrumequinum. 1. Mechanical specialisafions of the cochlea. J. Comp. Physiol. 106, 77-86.

[41 Br~ns, V. (1976): Peripheral auditory tuning for fine frequency analysis by the CF-FM bat, Rhinolophus ferrumequinum. 2 Frequency mapping in the cochlea. J. Comp. Physiol. 106, 8797.

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