Interaural delay sensitivity to tones and broad band signals in the guinea-pig inferior colliculus

Hearing Research, 50 (1990) 71-86 Elsevier

HEARES

71

01461

Interaural

delay sensitivity to tones and broad band signals in the guinea-pig inferior colliculus A.R. Palmer

‘, A. Rees 1-2and D. Caird 3

’ MRC Institute of Hearing Research, University of Nottingham, Nottingham, U.K. 2 Department of Physiological Sciences, The Medical School, Universify of Newcastle-upon-Tyne, Newcastle-upon-Tyne, U.K. ’ Zentrum der Physiologic. Frankfurt am Main, F. R.G. (Received

7 February

1990; accepted

18 May 1990)

We have measured the sensitivity of 243 low-frequency cells in the central nucleus of the guinea pig to the interaural time delay of best frequency (BF) tones, wideband noise and synthetic vowels. The highest rate of firing for the majority of cells occurred when the stimulus to the contralateral ear arrived 100-400 ps before that to the ipsilateral ear. The best delays for tones and noise measured in the same cell were highly correlated. In contrast to the tone delay functions, the majority of the delay functions obtained in response to tideband signals did not cycle, but were characterized by a single dominant peak or trough. The response frequency calculated from the delay functions to the vowel often did not correspond to the unit’s BF, suggesting that the unit was responding to a component close to the first formant frequency (730 Hz) of the vowel. Phase-locked responses, on the other hand, only occurred to the fundamental frequency of the vowel (100 Hz) and not to higher frequency components. The responses to delayed tone and noise signals in the guinea pig are very like those obtained in the cat and other mammals. The similarity of the range of best delays for the guinea-pig with those reported for the cat, despite the difference in head size in these two species, suggests that the sensitivity to interaural delays reflects the properties of the binaural pathways rather than an adaptation to the delays normally experienced by the animal.

Guinea

pig; Inferior

colliculus;

Interaural

time delays;

Single unit recording

Introduction The majority of low-frequency cells in the cat inferior colliculus are sensitive to interaural time differences. This sensitivity reflects processing in the superior olivary complex, where the phaselocked inputs from the two ears are first combined (Goldberg and Brown, 1969; Moushegian et al., 1971; Crow et al., 1978; Caird and Klinke, 1983). In effect, the inferior colliculus cells respond to interaural phase differences (IPDs) in lowfrequency components of the signal (Kuwada and Yin, 1983). With dichotic (i.e. closed sound field) stimulation, the spike rate of these low-frequency cells (here called IPD cells for convenience)

Correspondence to: A.R. Palmer, MRC Institute Research, University of Nottingham, University tingham, NG7 2RD, U.K. 0378-5955/90/$03.50

of Hearing Park, Not-

0 1990 Elsevier Science Publishers

changes as a function of IPD. A plot of spike-rate versus IPD is usually a cyclical function, with a period of either the reciprocal of the stimulation frequency or, for noise stimulation, of the best frequency (Yin and Kuwada, 1983; Yin et al., 1986). The main peak of the IPD curve is often at a physiologically realistic interaural time delay and usually corresponds to a sound source position in the contralateral hemifield (Kuwada and Yin, 1983, Yin et al., 1986, Caird and Klinke, 1987). Furthermore, the position of this peak often remains constant for different stimulus frequencies. Rose et al. (1966), who first described this phenomenon, introduced the concept of the ‘characteristic delay’, i.e. that interaural time delay value at which the cell’s relative output was the same irrespective of the stimulus frequency. Yin, Kuwada and co-workers (see Yin and Chan, 1988 for review) confirmed and extended the observations by Rose et al. (1966) in extensive parametric

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Division)

72

studies in the cat. The majority of IPD cells were found to have a characteristic delay but, rather surprisingly, the characteristic delay of most cells did not correspond to the peak of the IPD curve; rather it was situated on one of the flanks (Yin and Kuwada. 1983) i.e. in such &Is the position of the peak of the delay curve changed with stimulus frequency while a point on the flank remained constant. The characteristic delay may not, however, be the most relevant descriptor of the cell’s output. Yin and Kuwada (1983b) have suggested that the peak of the delay function to wideband signals may be the more important indicator. This peak (which we refer to here as the best delay) is less affected by signal level than the characteristic delay. Interestingly, in rabbits (Kuwada et al., 1987) and owls (Takahashi and Konishi, 1986), the characteristic delay does appear to be the same as the best delay. Stimulation with signals having different degrees of interaural coherence has shown these IPD cells to be acting as binaural correlators, specific for signals with a given interaural time delay value (Chan et al., 1987; Yin et al., 1987), as originaIly proposed by Jeffress (1948). In experiments using free-field stimulation, IPD cells in the cat inferior colIicuIus have been shown to be directionally sensitive, with spike rate/signal azimuth functions that look very similar to dichotic spike rate/IPD functions (Aitkin et al., 1985). This suggests that the IPD cells may be spatial channels preferentially sensitive to (low-frequency) signals coming from a particular direction. IPD interactions are very similar in the inferior colIicuIus of species as dissimilar as the owl (Takahashi and Konishi, 1986), the cat and the rabbit (Kuwada et al., 1987). Thus, although no data are available on binaural effects in the guinea-pig, one would expect that low-frequency cells in the guinea-pig inferior colIicuIus would show binaural and IPD effects simihu to those of the cat. However, extensive measurements of best delay are only available for the cat. This being so, one of our main aims was to measure the best delays of a large sample of neurones in the guineapig to aIIow proper comparison of the cat data with those from another mammal with a smaller head width. If the best delays of IPD cells provide the basis for coding sound position, one might

expect some correspondence between the head size and the range of best delays. In addition, as a preliminary to a series of experiments investigating binaural masking effects, we wanted to compare the responses of IPD cells to tones, noise and other complex stimuli. In those cells where best delay is not the same as the characteristic delay, the position of the peak of the delay curve will be affected by the frequency of the signal. This is particularly significant for broad-band signals which do not have a flat spectrum, but show strong spectral peaks. To investigate this, we have used synthetic vowels as test stimuli which, while relatively broadband, have strong spectral patterning. Most IPD cells do not phase-lock to bestfrequency tones, and those that do nearly always have best frequencies below 600 Hz (Kuwada et al., 1984). However, it has been shown that inferior colhculus cells respond well to modulations of the signal envelope irrespective of best frequency (Rees and Meller, 1983, 1987; Rees and Palmer, 1989; Yin et al., 1984). The vowel signal we have used is strongly modulated at its fundamental frequency, thus enabling a measure of the temporal patterning of IPD cell output to be obtained. In addition, the particular vowel chosen has the advantage that it has been used in studies at lower levels of the guinea-pig auditory pathway (Palmer et al., 1986). Methods Anaesthesia and surgical preparation The experiments were performed

on mature, pigmented guinea-pigs weighing between 250 and 450 g. For most of the experiments described here, fulI surgical anaesthesia was achieved using the following regime: sodium pentabarbitone 30 mg/kg IP, droperido14 mg/kg IP and phenoperidine 1 mg/kg IM. For maintenance sodium pentobarbitone 6 mg/kg IP and phenoperidine 1 mg/kg IM were administered on indication (see Evans, 1979). Atropine sulphate (0.06 mg SC) was administered to reduce bronchial secretions. However, in an attempt to achieve more stable recordings we also used two other regimes: (a) as above for induction and surgical preparation, followed by alpha-chloralose, 40 mg/kg at Chourly inter-

13

vals or (b) induction: 1 part Hypnorm (fentanyl and fluanisone), 2 parts water and 1 part midazolam (5 mg/ml) administered 8 ml/kg IP. Maintenance, Hypnorm (1 mg/kg IM) hourly and midazolam 4-hourly (see Flecknell, 1987). We found no striking differences between the data obtained under these different anaesthetic regimes. A tracheotomy was performed and the animal’s core temperature was monitored with a rectal probe and maintained at 38°C with a thermostatically controlled blanket. A midline sagittal incision was made in the scalp, the skin reflected and temporalis muscles were removed on both sides. The left and right auditory meati were transected about 1 mm from the bony rim. The animal was placed in a modified stereotaxic frame, in which perspex speculi that fitted tightly into each auditory meatus were substituted for the ear bars. A craniotomy extending 2 mm rostra1 and caudal to the interaural plane, and 3 mm lateral to the midline was made on the right side. The dura was reflected, and the brain surface covered with 2% agar to prevent desiccation. A small hole was made in the auditory bulla on each side into which a 10 cm length of 0.5 mm diameter polythene tubing was sealed to allow pressure equa~zation of the middle ears. Stimulus presentation The animal was placed inside a sound-attenuating room and the stimuli were delivered dichotitally through sealed acoustic systems consisting of 12.7 mm condenser microphones (Bruel and Kjaer 4134) driven as earphones, coupled to damped 4 mm diameter probe tubes which fitted into the perspex speculi. Distortion compensation networks in the driver amplifiers ensured that all the higher harmonics of a pure tone were at least 40 dB below the fundamental component in the acoustic signal at the highest output level. The output of the sound system at each ear was calibrated 3 to 4 mm from the tympanic membrane using a Bruel and Kjaer 4134 microphone fitted with a calibrated 1 mm probe tube. The output of the sound system on both sides was flat within +2 dB from 0.1 to 10 kHz. The difference in output between the two sides was less than 2 dB

at all frequencies measured. The spectrum level of the noise signal was calculated by measuring its RMS sound pressure in a 1 Hz band with a spectrum analyzer (Hewlett Packard 3561A) and expressing this value in dB re 20 pPa/Hz. StimuI~ generation Three stimuli were used in these experiments, pure tones, noise and synthetic vowel sounds. The pure tones were generated by a Hewlett Packard 3325A Waveform Synthesizer. The broadband noise had a high-frequency cut-off at 20 kHz with a spectrum flat to within &OS dB. The onsets and offsets of the tone and noise stimuli were shaped with 5-ms rise-fall times. The speech signal used in these experiments was a 50-ms segment of a steady-state approximation to the vowel /a/, with a fundamental frequency of 100 Hz, presented at 5/s. The four formant vowel was generated by a software cascade synthesizer (Klatt, 1980). The frequencies of the first three formants (defined by the harmonics of 100 Hz) were 730, 1090 and 2440 Hz and the bandwidths were 90, 110 and 170 Hz. The spectrum of this vowel is shown in Fig. 3~. The signals were output through a digital to analogue converter at a sampling frequency of 10 kHz and an anti-aliasing filter (Kemo VBF16, cut-off slope 135 dB/octave) set to give 20 dB of attenuation at the Nyquist frequency of 5 kHz. The signal path was split into two channels, one to each ear. A two-channel digital delay line enabled the interaural delays of the signals to be set between the two ears with a resolution of 1 ps. A digitally controlled attenuator in each of the left and right ear channels permitted control of the final output levels of the stimuli. Response recording ~orn~ou~d action potential In experiments where the compound action potential (CAP) was recorded, a wire electrode was positioned on the round window and the bulla re-sealed. The electrical potentials were amplified ( x 10 000) and filtered (0.3-l kHz) prior to display on an oscilloscope. CAP theshold was estimated by observing the waveform on the oscilloscope display while adjusting the intensity of a tone burst.

Single units

The activity of single units in the inferior colhculus was recorded with glass-coated tungsten microelectrodes (Merrill and Ainsworth, 1972; Bullock et al., 1988). Vertical electrode penetrations were made stereotaxically into the colliculus through the cerebral cortex. After initial placement to 2 mm above the inferior colliculus, the electrode was advanced by a hydraulic microdrive (Kopf 607WCP). The extraeellularly recorded neural action potentials were amplified (~1000) and filtered (0.3-3 kHz) by a preamplifier (WPI DAM-SA) and converted to logic pulses by an amplitude discriminator. A micro-computer was used to record the time of occurrence of the spikes with respect to synchronising pulses or to perform gated spike counts. Visual monitoring of the spike waveform on a digital storage oscilloscope ensured that only spikes from a single unit were recorded at any one time. ‘To enable verification of the recording site, electrolytic lesions were made by passing 10 PA of current for 10 s through the electrode.

Two methods have been used to estimate the best delays from the averaged responses: (i) a simple visual estimation of the position of the largest peak; the accuracy of this method is limited to the size of the delay step, and (ii) when the abscissa extended over at least one cycle of response, the visual estimate of the best delay was used as the centre of a single cycle vector strength analysis (see Goldberg and Brown, 1969) to determine the phase of the largest response (cf Kuwada and Yin, 1983; Yin et al., 1986). The best delay was then estimated by combining the response phase and the response period. The response period, and hence the response frequency to noise and vowels, was estimated using both peaks and troughs in the average curves. In cases where the response only consisted of one peak and one trough, the response period was taken as twice the peak-to-trough delay. The times at which spikes occurred during the presentation of the speech stimulus were used to construct a period histogram locked to the pitch period (10 ms) of the vowel. Histology

Data collection and analysis

Single neurones were isolated using 50-ms noise or tone bursts as search stimuli. The lowest threshold and the frequency at which it was obtamed (the best frequency, BF) were routinely determined. The delay sensitivity of neurones with BFs below 2.5 kHz (and occasionally below 4 kHz) was measured in response to three stimuli: SO-ms bursts of BF tone, noise, and speech, presented with a repetition rate of 5/s. The unit’s delay sensitivity was measured by varying randomly the intensity and interaural delay of the stimuli over a 90 dB range of intensity (in 5 dB or 10 dB steps) and 11 steps of interaural delay (step size 50-700 ps). Firing rates as a function of delay were calculated for each intensity to establish the intensity showing the greatest interaural delay sensitivity. The anaIysis was then performed with the intensity fixed at the most effective level, but with 19 or 21 repetitions of each interaural delay in random order to obtain an average response (see Fig. 2).

At the termination of each experiment, the animal was perfused transcardially with 0.9% saline followed by 10% form01 saline. The brain was removed from the skull and placed in 10% formol saline and sucrose solution until it sank. Frozen sections of 25 pm thickness were cut through the inferior colliculus and the sections were stained with cresyl violet. The electrode penetrations, and the positions of the recorded cells were reconstructed from the electrolytic lesions made during each experiment. The boundaries of the central nucleus were identified following the criteria described by Morest and Oliver (1984) in the cat. All of the data described in the results are from neurones isolated in the central nucleus of the inferior colbculus. Results ~~tera~al cross-talk in the guinea-pig Since the guinea-pig has a small head

and relatively large bullae, it was important to ensure that

stimulation. The difference between left and right ear values probably reflects small differences in the electrode placement. The difference between the ipsi- and contra-lateral thresholds gives a measure of the interaural attenuation and is shown in Fig. lb. At frequencies from 500 Hz to 10 kHz the attenuation exceeds 50 dB and for higher frequencies it is greater than 45 dB. These values are similar to those reported in the guinea-pig by Teas and Nielsen (1975) and Popelar et al (1988). In the current experiments, the interaural intensity disparities were never greater than a few decibels and thus our results are unlikely to be contaminated by cross-talk.

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Frequency (kHz) Fig, 1. Cross-talk in the guinea-pig measured using our sealed sound system. Fig. la shows the thresholds of the gross co&ear action potential at each ear to shaped tone pips presented ipsilaterally and contralaterally. The symbols are as follows: left ear stimulation (open circles), right ear stimulation (crosses). Fig. lb shows the difference between the thresholds measured to ipsilateral and contralateral stimuli at the left (open circles) and right (crosses) ears. The arrows show points for which no contralaterally evoked CAP could be obtained. In these cases, the values shown are minimum interaural attenuations obtained by subtracting the CAP threshold for ips~ater~ stimulation from the maximum sound level presented to the contralateral ear. The maximum output of the sound system was approximately 100 dB SPL at each ear {see methods).

the data collected in this study were not contaminated by cross-talk. The degree of cross-talk depends on the nature of the sound delivery system and the means of sealing to the meatus. We therefore estimated the ma~tude of cross-talk in the guinea-pig with our sound system using CAP thresholds. We measured the CAP thresholds at the round window in response to shaped tone pips (see methods) presented either ipsilaterally or contralaterally. The results for both cochleae from one guinea-pig are shown in Fig. la; the ipsilateml CAP thresholds for the two ears are within a few dB of each other, as are those for contralateral

Response as a function of signaE delay Delay sensiti~ty was measured in 243 neurones in the central nucleus of the inferior colliculus. The best frequencies of this sample of cells ranged from 120 to 4000 Hz, but no delay sensitivity was found for a BF tone in any cell whose BF exceeded 1800 Hz. Responses to tones us a function of interaura~ delay We first measured the response as a function of both delay and mean binaural intensity level using a single stimulus presentation at each delay/ intensity value. Examples of this procedure using best-frequency tone stimuli are shown in Figs. 2a and 2d for two cells which responded non-monotonically as a function of the sound level. This non-monotonicity is readily seen where the bars representing the number of spikes evoked by a single tone presentation are longer for sound levels near threshold (50-70 dB attenuation) than at higher sound levels. The contour lines in Figs. 2a and 2d further accentuate the regions of higher evoked activity near threshold for both cells. Using this analysis, non-monotonic responses were frequently observed, but we have made no attempt to quantify the effect further. The plots shown in Figs. 2b and 2e of spike rate as a function of delay allowed us to establish the level which gave the greatest variation in response as a function of ITD. For the cell in Fig. 2a this was determined as 70 dB attenuation (just above threshold) and for that in Fig. 2d at 40 dB attenuation. We then measured the delay function at that intensity with repeated presentations of each delay in random

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Fig. 2. (a-c) The response of a oeli of best-frequency 1500 Hz to best fquency tonea as a function of both interaural time delay and stimuhas level. (a) Each combination of interaural time delay and level was preaenkxi once. The number of discharges evoked is indicated as the length of the vertical line at the appropriate position. The contour line shows 3 spikes/stimulus. Zero dB for the binaural level shown corresponds 10 107 dB SPL. (b) Shows the delay functions derived from the data of Fig. 2a. Only three binaural levels are shown for ctarily: 0, - 40, - 60. (c) The delay function derived by averaging 21 repetitious at each delay a1 a fixed binaural stimulation level (-40 dB) (d, e and f) As above for another cell with best-frequency of 645 Hz. The contour line shows 2.5 spikes/stimulus. Zero dB for the binaural levels shown corresponds to 104 dB SPL. Levels in (e) were - 30, - 50 and -. 70 dB. Stimulation level in (f’j was -40 dB. The mows show the position of the peaks used localculate the best deiay to best-frequency tones.

order (see methods) as illustrated in Figs. 2c and 2f. As has been described in other species, the discharge of the units shown in Fig. 2 increases and decreases as the delay is varied. The best frequency of the cell shown in Figs. 2a-c was 1500 Hz and the period of the cycling of the discharge in Fig. 2c is 650 ps, which is equal (within the resolution of the analysis) to l/BF for this cell (666 ps) Similarly, the mean period of the cycling

in Fig. 2f is 1600 ps which is close to l/BF for this cell (l/645 Hz = 1550 ps). We have observed cyclic behaviour of the sort shown in Fig. 2 to best frequency tones up to 1.8 kHz. Of the low-frequency cells ( < = 1.8 kHz) anslyssd in this study with BF tones, 28% (48/171) did not show sensitivity to the interaural delay. Fig. 3a shows the response of a single cell with best frequency of 1.2 kHz to tones from 0.4 kHz to 1.8 k.Hz. The filled circles on the curves are

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Fig. 3. Fig. 3a shows the response of a single inferior colliculus neurone (best frequency 1.2 kHz) as a function of the interaural delay of 85 dB SPL tones from 0.4 kHz (top curve) to 1.8 kHz (bottom curve) in 0.2 kHz intervals. The ordinate markers represent 1 spike per stimulus presentation for all but the 0.4 and 0.6 kHz curves for which it represents 2 spikes per presentation. The zero for each subsequent curve has been displaced by one tick. The dots on the curves indicate the period of the stimulating sinusoid: the first dot was located on the largest peak and the rest were then positioned at intervals corresponding to the stimulus period. Fig. 3b shows delay functions for the same cell as in Fig. 3a in response to 38 dB SPL spectrum level wideband noise (crosses) and the vowel /a/ (asterisks) at 84 dB SPL, along with a curve obtained by simple summation of the curves in Fig. 3a (open circles). The noise and /a/ curves have been scaled to match the maximum value of the summed curve; the noise and vowel were presented at roughly the same rms sound level as the individual tones and thus each component will be less intense than the single tone. The scaling factors were 3.18 for the noise and 4.46 for the vowel. Fig. 3c shows the spectrum of the vowel /a/ which consists of the harmonics of 100 Hz with peaks at the formant frequencies as shown by the arrows.

separated by the period of the stimulating tone, positioned with respect to the highest point on each curve. It is clear from this figure that the period of the cyclic behaviour is determined by the period of the stimulating tone. The majority of the results described in the following sections are derived from measurements

at a single intensity. To calculate the best delay (see methods for details) for cells showing these cyclic responses to tones, we have taken the peak nearest zero interaural delay as shown by the arrows in Fig. 2c and 2f (this peak also usually corresponds to the largest peak in the response to noise as shown in Fig. 4).

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Responses to noise as a function of interaural delay

Figure 4 shows the delay functions for two cells in response to best frequency tones and wideband noise. For these cells, the best delays for noise and tones are the same (500 ps and 300 ~.rs respectively). However, unlike the responses to pure tones, the fluctuations in the noise delay functions become less pronounced at larger delay values. Indeed, in the two examples shown there is only one peak in the noise delay function. Yin et al (1986) have shown that in cat IPD cells, the delay function for a noise stimulus is very similar to the function obtained by summing the delay functions to a series of excitatory tones. A similar result is obtained by summation of the

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tone delay functions shown in Fig. 3a. The composite curve so obtained is shown by the open circles in Fig. 3b and has the same shape as the delay functions obtained with either wideband noise (crosses) or with the vowel /a/ (asterisks). The shapes of the noise delay curves from different cells varied significantly, as illustrated by the examples shown in Fig. 5. Generally, the peak of the delay curve was flanked by one or two troughs. This suggests that facilitation occurs at favourable delays, with some degree of suppression or inhibition of the firing rate below the mean value at nonfavourable delays. However, in the delay curves from some cells, only a peak appeared to be present (Fig. 5d) and in other cells only a trough (Fig. 5e). For these latter neurones the major effect of delaying the noise was to produce a reduction of the output near zero delay, without facilitation at other delays. For these cells (4%, 10/237) we have used the position of the minimum in the delay function as our estimate of the best delay. The range of best delays in our sample of neurones is shown as a histogram in Fig. 6, for both BF tone and noise signals. The majority of the neurones responded best when the stimulus at the contralateral ear occurred 100-400 ps before that to the ipsilateral ear. This range of delays would correspond to sound sources in the contralateral hemifield. In many cells we were able to measure the best delay values for both noise and tones. The relationship between these measures is shown in Fig. 7. The solid line shows the fit to the points by linear regression, and the dashes show the line of identity. The best delay obtained using the two signals is highly correlated (correlation coefficient, r= 0.78; N = 73; significant, P < 0.001). The crosses represent the points measured from the delay functions in which the best delay was estimated from the position of the minimum. These points also he close to the regression line.

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Fig. 4. Rcspomc as a fun&on of the interaural delay of the stimulus for two diffctvnt nauc4tcs. Rcspoms to best frequency tones ((a) 320 Hz 45 dB SPL and (b) 760 Hz 55 dB SPL) are shown by the open circles and the uosrm show the response to wideband noise (38 and 28 dB SPL apa%rum lml respectively).

Responses &lay

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of interaural

In 64 cells we also measured the best delay to a 50-ms sample of a vowel stimulus (see methods). Examples are shown in Fig. 8 of delay functions to best frequency tones, noise and vowels for three

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Fig. 5. Response as a function of the interaural delay of a wideband noise stimulus for 10 different neurones to illustrate the different shapes of the delay functions obtained in response to wide band noise. Notice that in Fig. 5e the major response was a reduction of the discharge near zero delay with no corresponding facilitation. (a) open circles BF 826 Hz, noise 38 dB spectrum level crosses BF 400 Hz. noise 38 dB spectrum level; (b) open circles BF 928 Hz, noise 18 dB spectrum level crosses BF 645 Hz, noise 28 dB spectrum level; (c) open circles BF 1500 Hz, noise 38 dB spectrum level crosses BF 1270 Hz, noise 18 dB spectrum level; (d) open circles BF 600 Hz, noise 28 dB spectrum level crosses BF 910 Hz, noise 38 dB spectrum level; (e) open circles BF 600 Hz, noise 8 dB spectrum level crosses BF 990 Hz, noise 18 dB spectrum level.

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Fig. 6. Histogram of the number of neurones with different best delays. The solid bars are data obtained with best frequency tones and the unshaded bars are with wideband noise. Only cells with best frequencies below 1.8 kHz are included in the data obtained with pure tones. NDS indicates neurones not delay sensitive.

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Fig. 7. Best delay to best function of the best delay to line of identity and the solid linear regression. The crosses minima of delay functions

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Fig. 8. (a) Response as a function of the utteraural delay of a 55 dB SPL tone at best frequency (1.2 kHz, open circles). wideband noise at 18 dB SPL spectrum level (crosses) and the vowel /a/ at 44 dB SPL (asterisks). (b) as (a) for 0.303 kHz, 60 dB SPL tone, 8 dB SPL spectrum level noise and 74 dB SPL vowel /a/. (c) as (a) for 0.645 kHz, 34 dB SPL tone, 28 dB SPL spectrum level noise and 54 dB SPL vowel /a/. The response to the tone was stronger than to the wideband stimuli and to facilitate comparison of the shapes of the delay functions these data have been reduced to l/3 of their actual values.

different neurones. For the cell in Fig. Sa, the delay functions for all three stimuli are similar, except that the vowel function has a longer period than those to the 1.2 kHz tone and the noise. The periods for tone and noise are about 800 ,us, corresponding to a frequency of 1.2 kHz. The period to the vowel is about 1100 or 1200 ps corresponding to a frequency of 800-900 Hz, which is close to tire first formant pe.ak of the vowel /a/ at 740 Hz. Following earlier studies.

the period of the delay function will be referred to as the response frequency (see Yin and Chan 1988). Figs. 8b and 8c show data from two more cells with best frequencies of 303 and 645 Hz respectively. The response frequency to the vowel is higher than to the tone or noise in Fig. 8b and 8c (666 Hz and 833 Hz respectively). There was as much variation in the shape of the delay function to the vowel as there was to the noise. We illustrate this variation in Fig. 9 with delay functions in response to the vowel for 10 different cells. The functions in Fig. 9 have been selected to allow direct comparison with those in Fig. 5. It is clear from a comparison of Figs. 5 and 9 that the shapes of the delay functions to both of the wideband stimuli are similar. The response frequency (see methods) to the vowel is plotted against the cell’s best frequency in Fig. 10. Many of the cells did not respond to the component of the vowel closest to their best frequency (as indicated by the dotted line), instead they responded to a component of the vowel close to the fist or second formant frequencies (the first formant of /a/ is at 730 Hz and is defined by two equally strong harmonics at 700 and 800 Hz. The positions of these two components and that of the second formant at 1090 Hz are shown by the dashed lines). The resolution of our analysis was 100 ps and the points plotted at 833 Hz are the closest our analysis would allow us to estimate 800 Hz. Fig. 10 shows that even for cells with similar best frequency, some responded to the first formant of the vowel, while others responded to the component nearest the best frequency. Presumably, those responding to the best frequency component are more narrowly tuned and are thus less affected by the lower frequency energy at the first formant of the vowel. However, we have no data to address this question directly. There is clearly at least one anomalous cell which cannot be explained in these terms (that responding at 400 Hz with best frequency of 1.34 kHz; see Discussion). The frequency effective in driving these neurones appears, therefore, to be a result of both the patterning of the harmonic spectrum and the filtering due to the neurone’s response area. For those cells where the best delay to both noise and the vowel was measured, the absolute difference between the two values was calculated.

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Fig. 9. Response as a function of the interaural delay of the vowel /a/ for 10 different neurones to illustrate the different shapes of the delay functions obtained in response to the vowel (c.f. Fig. 5). Notice that in Fig. 9e the major response was a reduction of the discharge near zero delay with no corresponding facilitation. (a) open circles BF 645 Hz, /a/ 54 dB SPL; crosses BF 928 HZ, /a/ 64 dB SPL. (b) open circles BF 400 Hz, /a/ 74 dB SPL (these data have been multiplied by four to facilitate shape comparisons); crosses RF 400 Hz, /a/ 64 dB SPL. (c) open circles BF 626 I-Lz, /a/ 64 dB SFL, crosses BF 1500 Hz, /a/ 54 dB SPL. (d) open circles BF 600 Hz. /a/ 74 dB SPL (these data have been multiplied by two to facilitate shape comparisons): crosses BF 340 Hz, /a/ 54 dB SPL. (e) open circles BF 950 Hz, /a/ 44 dB SPL crosses BF 400 Hz. /a/ 44 dB SPL.

Fig. 10. Response frequency to the vowel /a/ as a function of best frequency. Dots show the line of identity. Dashed tines show the positions of the 700 and 800 Hz harmonics which are located around the first formant frequency of the vowel, and the 1100 Hz harmonic which is close to the frequency of the second formant of the vowel.

Fig. 11. Difference between the best delay values to the noise and vowel stimuli as a function of best frequency. Dashed line shows the position of the first formant of the vowel /a/. The crosses represent values derived from the minima of delay functions like those illustrated in Fig. Se and 9e.

82

0

e

18

24

40

32 Time

finsf

0

1000

2000

3000

4000

Frequency

6000

WI

Fig. 12. Fig. 12~ shows a histogram (binwidth 200 ps), locked to the pitch per&i, of the responses of a cell (best frequency 600 Hz) 10 the vowel /a/ at 84 dB SPL. Fig. 12b shows the Fourier transformation of the period histogram.

This difference is plotted against the cell’s BF for the vowel in Fig. 11. At all best frequencies, some cells showed zero difference; i.e. the best delay to noise and to the vowel were the same, as in Fig. 8a However, some of the cells which did not have BFs near the first formant of the vowel at 730 Hz, showed considerable differences (Fig. 10). In addition to these spike rate effects, we also observed that the responses of most of these cells were synchronized to the fundamental frequency of the vowel. An example is shown in Fig. 12 in which the timing of the impulses has been used to construct a period histogram locked to the pitch period of the vowel. There are clear peaks in the period histogram every 10 ms, corresponding to the period of the 100 Hz fundamental. This is confirmed in Fig. 12b in which we show the Fourier transform of the period histogram with the largest peek at 100 Hz. The best frequency for this unit was 600 Hz, and the fist formant of the vowel /a/ is at 730 Hz. There is, however, no evidence of a peak at or near the formant frequency in the Fourier transform in this or any other cell analysed. Discussion In the present study we have described the interaural delay functions in the guinea pig in response to tones, noise and vowel stimuli. The shape of these curves, and the positions of their

peaks are very similar to those in the cat (Kuwada and Yin 1983, Yin et al., 1986). Although most data on inferior colliculus IPD cells have been obtained in cats, a comparison with our data and those available for other mammals shows no significant species differences (rabbit: Aitkin et al., 1972; Kuwada et al., 1987; kangaroo rat: Moushegian et al., 1971; Stilhnan 1971). Even the barn owl, which shows IPD sensitivity up to 8 kHz (Moiseff and Konishi, 1981), has the same basic IPD coding mechanisms as the cat (Yin et al., 1986). Tbis remarkable similarity of IPD interactions, even across vertebrate classes, strongly suggests that these effects are general rather than species-specific. While the original concept of a ‘characteristic delay’ for individual IPD cells (Rose et al., 1966) has been refined and modified by Yin and coworkers (see Yin and Ghan 1988 for review), they have suggested that the best delay appears to be the functionally relevant parameter of the cell’s output (Yin and Kuwada 1983). The extent to which the best delay is relevant under physiological conditions is not entirely clear. In the cat, where the maximum ITD is about f 400 ~.ls(Roth et al., 1980), at least 30-3546 of the cells have best delays outside the physiological range (Kuwada and Yin, 1983; Yin et al., 1986). For many of the remainder, the sound source would have to be close to 90 degrees from the midline in order to generate a delay close to their best delay. If these

83

cells represent spatial channels tuned for particular best delays one might expect a more even distribution of best delay values across the azimuthal range. The head size of the guinea-pig is considerably smaller than the cat, and one would predict that, if best delay is the functionally relevant parameter, the distribution of best delays in the guinea-pig would be over a narrower range than in the cat. The maximum interaural delay value for the guinea-pig, calculated using the model first described by Woodworth (1938) is 90 ps (based on a maximum interaural distance of 24 mm: Withington-Wray et al., 1989). As in the cat, where the measured and calculated m~mum delays are 350 and 210 p.s repectively (Roth et al., 1980), the model is likely to underestimate the actual value. Nevertheless, the maximum delay is smaller in the guinea-pig than in the cat. This difference is, however, not reflected in the histograms of best delay. Statistical comparison of the distributions of best delay obtained with both tone and noise obtained here in the guinea-pig (Fig. 6) with data in the cat (Kuwada and Yin, 1983, Fig. 4c; Yin et al., 1986, Fig. 2.) showed no statistical difference between the mean (by Student’s r-test) and variances (by variance ratio) in these dist~butions [t = 1.74 (P > 0.05). F= 1.12 (P > 0.05) for the tone data, t = 1.07 (P > 0.05), F = 1.19 (P > 0.05) for the noise]. This evidence suggests that the best delays are a reflection of the physiology of the binaural processing system (e.g. frequencies at which phase locking ocurrs in the periphery, path lengths and number of synapses) rather than being specially adapted to the acoustic environment experienced by the animal. The way in which the position of a sound source is coded by cells whose best delay is outside the physiolo~cal range is not clear. However, McFadden (1973) has offered a number of possible roles for these cells such as the suppression of reverberation or even periodicity analysis. The shapes of the guinea pig noise delay curves are also like those of cat IPD cells. Typically, they have a main peak, flanked by troughs. For responses to noise, the cycling at the stimulation frequency seen in the tone response delay curves disappears at higher delay values. This is because the different frequency components of the noise

signal which contribute to the delay curve move out of phase at high delay values and cancel one another (Chan et al., 1987, Yin et al., 1986). As in cat IPD cells, the degree of cycling and the apparent levels of facilitation and suppression at the peaks and troughs vary from cell to ceil. The extreme case, seen in those cells where inhibition appeared to be predominant, giving a minimum in the noise delay curve, is also seen in cats (Yin et al., 1986, Chan et al., 1987). The constant position of this minimum with tone, noise and vowel stimulation suggests that this ‘worst delay’ is nevertheless an important feature of these cells responses. In the cat, the response frequency of IPD cells to noise is often not the same as their best frequency. The response frequency to noise is determined by the strength of the cell’s IPD synchronization at different frequencies rather than the frequency at which threshold is lowest (Yin and Chan, 1988, Chan et al., 1987). Because phase-locking decreases with increasing frequency, this becomes more important for cells with higher best frequencies. In addition, Chan et al. (1987) have shown that the response frequency obtained when the cell is stimulated with different filtered noises can be predicted from a combination of the noise spectrum and the binaural synchronization rate curve for the cell. These effects will also contribute to the responses we have described to vowel signals, which are broadband but do not have flat spectra. For the majority of cells, the response frequency of the vowel delay curve is strongly influenced by the harmonics around the first formant of the vowel (700-800 Hz). The response frequency of a few cells with best frequencies above this range is, however, the same as their best frequency. Most of the cells with low best frequencies responded to lower harmonics of the vowel signal. One cell showed a somewhat anomalous vowel delay curve (Fig. 13a). For this cell, the interaural delay function for the vowel /a/ is quite different from that for both the wideband noise and the 1.34 kHz BF tone. The tone and noise functions are cyclic at a period corresponding to that of the BF tone. The response to the vowel, if cyclic at all (which is not clear on this restricted range of interaural delays), has a much longer period of the order of 2 ms or more. Presumably in this case the neurone is re-

1 -500 -YOOO

8 0.745kifr

Delay

/

0

(us

500

too0

1

contra

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Fig. 13. (a) Response as a function of the interaural delay of a 6.5 dB SPL tone at best frequency (1.34 kHz, open circles), wideband noise at 18 dB SPL spectrum level (crosses) and the vowel /a/ at 54 dB SPL (asterisks). The spike counts for the vowel were much higher than to the other stimuli and to enable the shapes of the delay functions to be compared the vowel data have been multiplied by 0.33. (b) as (a) for 0.745 kHz, 50 dB SPL tone, 38 dB SPL spectrum level wideband noise and 64 dB SPL vowel /a/. For this cell the tone response was stronger than the two wideband signals and to allow comparison of the shapes of the delay functions the tone data were multiplied by 0.6.

sponding to one of the low-frequency harmonic components of the vowel or the fundamental at 100 Hz. Although this cell was sensitive to IPDs at higher frequencies, the relatively low level of such frequencies in the vowel may have allowed the response to be dominated by lower frequency components. In the other anomalous cell (Fig. 13b), both noise and vowel delay curves showed a higher

apparent response frequency than the best frequency tone delay curve. Presumably the cancellation by different components responding in opposite phase (very strong to both noise and vowel signals) is such that broad band signals increase the apparent ‘sharpness’ of the main peak of the delay curve in this case. For cells whose best frequency was not near the first formant of the vowel, the best delay to the vowel was sometimes very different to the noise best delay. The simplest explanation of this phenomenon is to assume that some IPD cells in the guinea pig, like those of the cat, have best delay values that are not the same as their characteristic delay values. In such cases, stimulation with signals whose largest frequency component is different from best frequency will lead to shifts in the peak of the delay curve (Yin and Kuwada, 1983). For such cells, the best delay would be an unreliable parameter to describe the cell’s function when using signals with strongly shaped spectra. In the cat inferior colliculus, Kuwada et al (1984) showed that the output of the IPD cells is generally not locked to the phase of pure tone stimuli. Only 18% of their sample showed phaselocking, and it was seldom seen for frequencies above 600 Hz. This contrasts with the IPD effects themselves, which depend on ph~e-loc~ng in the periphery, and occur up to about 3 kHz (Kuwada and Yin, 1983). Although we did not measure phase-locking to tones in this study, it is highly unlikely that it is better in the guinea pig than in the cat, especially since phase-locking in the guinea pig auditory nerve has a lower cut-off frequency than in the cat (Palmer and Russell, 1986). Our sample of units did, however, show strong phase locking to the fundamental frequency of the vowel sounds. The suggestion of Chan et al. (1987) that spike rate is the only significant output parameter for these IPD cells may only apply to pure tones. The timing of the discharges could be an important feature of the IPD cells’ output when more complex sounds with low fundamental frequencies are presented. However, in contrast to the auditory nerve and cochlear nucleus (Palmer et al., 1986), we only saw phase-locking to the fundamental frequency of 100 Hz, which confirms that there is significant loss of timing information at the output from the colliculus.

85

Acknowledgements

We thank Padma Moorjani for assistance throughout this study. Dr. I.M. Winter and Professor M.P. Haggard provided very useful comments on the manuscript. The study was supported by the Medical Research Council, the Deutsche Forschungsgemeinschaft (SFB45/Bl2~, by twinning grant from the European Training Program in Brain and Behavioural Science to ARP and DC, and a grant from the Small Grants Research Sub-Committee of the University of Newcastle-upon-Tyne to AR. References Aitkin, L.M., Blake, D.W., Fryman, S. and Bock, G.R. (1972) Responses of neurones in the rabbit inferior colliculus. II Influence of binaural tonal stimuluation. Brain. Res. 47. 91-101. Aitkin. L.M.. Pettigrew, J.D.. Calford, M.B., Philips, SC. and Wise, L.Z. (1985) Representation of stimulus azimuth by low-frequency neurons in inferior colliculus of the cat. J. Neurophysiol. 53, 43-59. Buliock, D., Palmer, A.R. and Rees, A. (1988) A compact and easy to use tungsten-in-glass microelectrode manufacturing workstation, Med. Biol. Eng. Computing 26, 669-672. Caird, D.M. and Klinke, R. (1983) Processing of binaural stimuli by cat superior olivary complex neurons. Exp Brain Res. 52, 385-399. Caird, D.M. and Khnke, R. (1987) Processing of interaural time and intensity differences in the cat inferior colliculus. Exp. Brain Res. 68, 379-392. Chan, J.C.K., Yin, T.C.T. and Musicant, A.D. (1987) Effects of interaural time delays of noise stimuli on low-frequency cells in the cat’s inferior colliculus. If. Responses to bandpass filtered noises. J. Neurophysiol. 58, 543-561. Crow, G.. Rupert, A.L. and Moushegian, G. (1978) Phase-locking in monaural and binaural medullary neurons, implications for binaural phenomena. J. Acoust. Sot. Am. 64, 493-501. Evans, E.F. (1979) Neuroleptanaesthesia for the guinea-pig. Arch. Otolaryngol. 105, 185-186. Flecknell. P.A. (1987) Laboratory animal anaesthesia. Academic Press, London. Goldberg, J.M. and Brown, P.B. (1969) Response of binaural neurons of dog superior olivary complex to dichotic tonal stimuli: some physiolo~cal mechanisms of sound localisation. J. Neurophysiol. 32, 613-636. Jeffress, L.A. (1948) A place theory of sound localization. J. Comp. Psychol. 44, 35-39. Klatt, D.H. (1980) Software for a cascade/parallel formant synthesiser. J. Acoust. Sot. Am. 67, 971-995. Kuwada, S. and Yin, T.C.T. (1983) Binaural interaction in low-frequency neurons in inferior colliculus of the cat. I.

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