Biomedical subjects

J Syka

Publications and source records attributed to J Syka.

At least 19 recordsLinked to original sources

PubMed2005

Effect of noise exposure on gap detection in rats.

The effects of intense (110-120 dB) noise exposure (broadband noise for one hour) on temporal resolution was estimated in rats by measuring the behavioural gap detection threshold (GDT). Changes in GDT after 120 dB noise exposure were compared with changes in the threshold and amplitude of middle latency responses (MLR) recorded in response to tone stimuli. GDT values increased from 1.6 to 4.3 or 7.8 ms after exposure to 110 or 115 dB SPL, respectively; GDT recovered to pre-exposure values in 3-7 days. Three main types of noise-induced changes were observed after 120 dB SPL exposure: (I) GDT changes similar to those following noise exposure to 115 dB SPL and maximal hearing threshold shifts (TSs) at high frequencies of about 45 dB; (II) more pronounced changes in GDT (up to 60 ms) with maximal hearing threshold shifts of about 65 dB and (III) a lack of reliable responses to gap during the first weeks post-exposure with maximal hearing threshold shifts of about 80 dB. An increased GDT was present two months after noise exposure in animals with types II and III post-exposure changes; enhanced MLR amplitudes were also found in most of these in the first post-exposure week. The pronounced deficit in gap detection in some rats after 120 dB SPL noise exposure may signal the presence of a noise-induced tinnitus.

Animals↗

PubMed2003

NADPH-diaphorase-positive neurons in the auditory cortex of young and old rats.

Age-related changes in NADPH-diaphorase (NADPH-d)-positive neurons were examined in the auditory cortex of young (3 months old) and very old (36 months old) rats (strain Long Evans). In very old rats a significant reduction was found in the thickness of the auditory cortex, to 54% of that in young animals, as well as changes in the shape and configuration of nerve cell bodies and dendrites. Quantitative analysis demonstrated an age-related increase in the number of dendritic segments and dendritic branching points. The length of dendrites in NADPH-d-positive neurons and their density increased in very old rats. The total number of NADPH-d-positive neurons within the Te 1 and Te 3 fields was 13% lower in the old rats than in the young.

Aging↗

PubMed2003

Changes in neuronal activity of the inferior colliculus in rat after temporal inactivation of the auditory cortex.

The role of cortico-tectal pathways in auditory signal processing was studied in anesthetized rats by comparing the extracellular single unit activity in the inferior colliculus (IC) before and after functional ablation of the auditory cortex (AC) by tetrodotoxin (TTX). The responses of several IC neurons to sound stimuli were simultaneously recorded with a 16-channel electrode probe introduced into the IC. Click-evoked middle latency responses (MLR) recorded from the AC were suppressed for several hours after TTX injection. During AC inactivation the firing rate of IC neurons increased (40 % of neurons), decreased (44 %) or did not change (16 %) in comparison with control conditions. In several IC neurons, TTX injection resulted in alterations in the shape of the rate-level functions. Response thresholds, tuning properties and the type of discharge pattern of IC neurons were not altered during AC inactivation. However, in one-third of the neurons, the initial part of the response was less altered than the later, sustained part. In two-thirds of neuronal pairs, functional decortication resulted in a change in the cross-correlation coefficient. The results reveal the complex changes that appear in IC neuronal activity after functional ablation of the ipsilateral auditory cortex.

Acoustic Stimulation↗

PubMed2002

Effects of electrical stimulation of the inferior colliculus on 2f1-f2 distortion product otoacoustic emissions in anesthetized guinea pigs.

The effects of electrical stimulation of the inferior colliculus (IC) on the activation of olivocochlear nerve fibers were investigated in guinea pigs in which the 2f1-f2 distortion product otoacoustic emissions (DPOAE) were recorded. Animals were anesthetized with ketamine (33 mg/kg) and xylazine (6.6 mg/kg). Bipolar electrical stimulation of the IC by a train of pulses with currents less than the threshold for evoking muscle twitches resulted in a small depression of the DPOAE amplitude by 0.1-2 dB. The maximal effect was observed when the stimulating electrodes were located in the rostro-medial or ventral parts of the IC. The suppression of electrically evoked DPOAE was similar to the DPOAE suppression produced by acoustical stimulation of the contralateral ear by a broad-band noise. Suppression of DPOAE amplitude in response to both acoustical and electrical stimulation was abolished 1-2 h after a single intramuscular injection of gentamicin (210-250 mg/kg). The results indicate that electrical stimulation of the IC can activate the efferent system and produce DPOAE changes by similar mechanisms as does acoustical stimulation of the contralateral ear.

Animals↗

PubMed2002

Gap detection threshold in the rat before and after auditory cortex ablation.

Gap detection threshold (GDT) was measured in adult female pigmented rats (strain Long-Evans) by an operant conditioning technique with food reinforcement, before and after bilateral ablation of the auditory cortex. GDT was dependent on the frequency spectrum and intensity of the continuously present noise in which the gaps were embedded. The mean values of GDT for gaps embedded in white noise or low-frequency noise (upper cutoff frequency 3 kHz) at 70 dB sound pressure level (SPL) were 1.57+/-0.07 ms and 2.9+/-0.34 ms, respectively. Decreasing noise intensity from 80 dB SPL to 20 dB SPL produced a significant increase in GDT. The increase in GDT was relatively small in the range of 80-50 dB SPL for white noise and in the range of 80-60 dB for low-frequency noise. The minimal intensity level of the noise that enabled GDT measurement was 20 dB SPL for white noise and 30 dB SPL for low-frequency noise. Mean GDT values at these intensities were 10.6+/-3.9 ms and 31.3+/-4.2 ms, respectively. Bilateral ablation of the primary auditory cortex (complete destruction of the Te1 and partial destruction of the Te2 and Te3 areas) resulted in an increase in GDT values. The fifth day after surgery, the rats were able to detect gaps in the noise. The values of GDT observed at this time were 4.2+/-1.1 ms for white noise and 7.4+/-3.1 ms for low-frequency noise at 70 dB SPL. During the first month after cortical ablation, recovery of GDT was observed. However, 1 month after cortical ablation GDT still remained slightly higher than in controls (1.8+/-0.18 for white noise, 3.22+/-0.15 for low-frequency noise, P<0.05). A decrease in GDT values during the subsequent months was not observed.

Acoustic Stimulation↗

PubMed2002

Changes in the acoustically evoked activity in the inferior colliculus of the rat after functional ablation of the auditory cortex.

The role of the cortico-tectal pathways in the processing of auditory signals was investigated by recording the click-evoked responses and extracellular multiple unit activity in the inferior colliculus (IC) after functional ablation of the auditory cortex (AC) by local intracortical application of a sodium channel blocker, tetrodotoxin (TTX). Click-evoked IC responses (IC-ER) and multiple unit activity in response to tone bursts were recorded with implanted electrodes in the IC of rats lightly anaesthetized with xylazine. Neural activity was recorded before and after the application of TTX into the ipsilateral auditory cortex (AC) through three implanted cannulas in a total dose of 30 ng. The functional status of the AC was monitored by recording click-evoked middle latency responses from a ball electrode implanted on the AC. During inactivation of the AC, IC-ER amplitudes were either increased (48 % of the cases), decreased (32 % of the cases) or not evidently changed (20 % of the cases). Corresponding effects were observed in the firing rate of IC neurons. Functional ablation of the AC also resulted in a significant prolongation of the latencies of individual waves of the IC-ER. However, the discharge pattern of the multiple unit responses, response thresholds and tuning were not altered during AC inactivation. IC neural activity recovered within several hours, and maximally during 2 days. The results reveal principles of the interaction of cortico-tectal pathways with IC neuronal activity.

Acoustic Stimulation↗

PubMed2001

Effect of auditory cortex lesions on NADPH-diaphorase staining in the inferior colliculus of rat.

Projections from the auditory cortex (AC) in the rat terminate in the dorsal cortex (DC) and in the external cortex (EC) of the inferior colliculus (IC), areas which exhibit a moderate number of nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive neurons. NADPH-d co-localizes with nitric oxide synthase, which is responsible for the production of the transcellular messenger, nitric oxide. Changes in NADPH-d staining in the IC were found after unilateral lesions of the AC. Lesions resulted in a reduction in NADPH-d staining in neurons and neuropil within the ipsilateral DC and EC with the maximum reduction occurring 3-4 days after lesion. The reduction in NADPH-d staining in the contralateral IC was less pronounced. Lesions affecting auditory areas Te 1 and Te 3 produced the largest decrease in NADPH-d staining in neurons and neuropil. This finding may be related to the abolition of the influence of glutamatergic corticocollicular and commissural pathways.

Animals↗

PubMed2001

Effects of contralateral acoustical stimulation on three measures of cochlear function in the guinea pig.

The magnitudes of suppression of the click-evoked compound action potential of the auditory nerve (CAP), transient click-evoked otoacoustic emissions (TEOAEs) and ensemble background activity of the auditory nerve (EBA), elicited by contralateral acoustical stimulation, were compared in awake or lightly sedated guinea pigs. The contralateral ear was stimulated either by continuous broad-band noise or by low-pass or high-pass noise (intensity 41-62 dB SPL) with cut-off frequencies of 2, 8 and 12 kHz. The maximal suppression of TEOAEs was achieved by contralateral noise containing mainly low frequencies, whereas for suppression of the CAP it was necessary for middle frequencies to be present in the contralateral noise (less than 8 kHz). In contrast to this, EBA was suppressed mainly by high-frequency noise (higher than 8 kHz) whereas low- and middle-frequency noise was ineffective in suppressing EBA. Evaluation of the root mean square voltage of the EBA (filtered in frequency range 0.75-1.25 kHz) enabled the evaluation of fast and slow components of olivocochlear activation. Both fast and slow effects were proportionally suppressed by individual types of contralateral stimulation. The mechanisms of TEOAEs and CAP generation has been confirmed in many earlier studies, but the origin of EBA has yet to be fully elucidated. The obtained data support the hypothesis that a large part of EBA is formed by spontaneous activity of high-frequency-tuned auditory nerve fibres. Suppression of the EBA magnitude during contralateral stimulation may be caused either by a reduced spontaneous firing rate or by a decrease in possible synchronised neuronal firing.

Acoustic Stimulation↗

PubMed2001

Susceptibility to noise exposure during postnatal development in rats.

Susceptibility to noise-induced hearing loss was studied during maturation in 20 female pigmented rats (strain Long-Evans). Young rats, 3, 4, 5 and 6-7 weeks old, were exposed for 1 h to a broad-band noise with an intensity of 120 dB SPL. The thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. The results were compared with data from our previous study, in which the effects of broad-band noise exposure on MLR were investigated in adult rats [Syka, J. and Rybalko, N. (2000) Hear. Res. 139, 59-68]. The hearing thresholds of 3-7 week old rats before noise exposure were within the normal adult range. Noise exposure in young rats produced an adult-like pattern with an elevation of hearing thresholds. One-two weeks post-exposure a recovery of MLR thresholds was observed, though full recovery only occurred in the low frequency range. Recovery of hearing thresholds in the high frequency range depended on the age of the animal at the time of exposure. In all animals aged less that 6-7 weeks, exposure resulted in a permanent threshold shift in the range of 4-32 kHz. The mean values of permanent threshold shifts at 16 kHz (the frequency of maximal hearing loss) were 53.0+/-4.5, 47.6+/-9.6, 37.5+/-7.5 and 27+/-10 dB for rats exposed at 3, 4, 5 and 6-7 weeks of age, respectively. Similar to adult rats, young rats exposed to noise exhibited an enhancement of MLR amplitudes. This amplitude enhancement was more pronounced in the high frequency range. In several rats exposed at 3-5 weeks of age, the recovery period to normal amplitudes was substantially prolonged and lasted 4-8 weeks in comparison with 1-2 weeks in adult rats. These results demonstrate a greater susceptibility to noise exposure in rats during the first 5 postnatal weeks.

Age Factors↗

PubMed2000

Response properties of neurons in the central nucleus and external and dorsal cortices of the inferior colliculus in guinea pig.

The inferior colliculus (IC) represents a mid-brain structure which integrates information from many ascending auditory pathways, descending corticotectal projections and intercollicular pathways. The processing of information is different in each of the three main subdivisions of the IC--the central nucleus (CNIC), the dorsal cortex (DCIC) and the external cortex (ECIC)--which may be distinguished morphologically as well as by different inputs and outputs. To assess the differences in information processing we compared the response properties of single neurons in individual subnuclei of the IC in anesthetized guinea pigs. In comparison with DCIC and ECIC neurons, the CNIC neurons as a group were characterized by a sharper frequency tuning (as expressed by Q10 values), a lower average threshold, a shorter average first-spike latency of response to tones at the characteristic frequency (CF), a higher occurrence of non-monotonic rate/level functions and a higher rate of spontaneous activity. CNIC neurons and DCIC neurons reacted to tones at the CF more frequently by a sustained type of response than did ECIC neurons. The difference between the parameters of DCIC neuronal activity and ECIC neuronal activity was found to be smaller. The frequency tuning (expressed in Q10 values), spontaneous activity and dominance of monotonic rate/level functions were very similar in both structures; ECIC neurons expressed a higher average threshold and a shorter average first-spike latency than did DCIC neurons. Responsiveness expressed as the average maximal firing rate to tones at the CF was significantly higher in the CNIC than in the ECIC. The results give additional support to the idea that the CNIC is a part of a fast, frequency-tuned, low threshold and intensity-sensitive ascending pathway, whereas the other two subdivisions are involved in additional processing of information that involves feedback loops and polysensory pathways.

Acoustic Stimulation↗

PubMed2000

Neuronal connections in the medial geniculate body of the guinea-pig.

The spontaneous and evoked activities of individual pairs of single units were recorded simultaneously with the same microelectrode in the medial geniculate body (MGB) of ketamine-xylazine-anaesthetised guinea-pigs. Cross-correlograms (CCGs) of spike train pairs were computed and divided on the basis of correlation peak shape into four classes [a unilateral narrow (UN) peak, a centrally positioned wide (CW) peak, a complex peak and no significant peak] interpreted in terms of the functional connection between neighbouring neurones. The shift predictor procedure was applied with the aim of removing the effect of the stimulus on the final CCG shape. The occurrence of correlation peak types and the distribution of correlation coefficients were found to be similar for the spontaneous activity during silent periods following acoustical stimulation and for the long-lasting recording of spontaneous activity. CCGs in 38% of pairs computed during silent interstimulus intervals contained a UN peak, suggesting a monosynaptic excitatory connection. Almost 20% of all pairs expressed a CCG shape typical for a common input, i.e. a CW peak. In 5% of cases multiple, so-called complex peaks, were found. About 20% of the CCGs contained no significant correlation peak in the interstimulus period, which is typical for a very weak or absent functional connection between recorded neurones. No inhibitory interaction (groove in the CCGs) between recorded pairs was observed. The distribution of correlation peak shapes was similar when calculated during acoustical stimulation and during silent interstimulus intervals. CCGs computed during presentation of four acoustical stimuli (pure tone bursts, noise bursts, natural call whistle and artificially inverted whistle) showed most frequently a UN peak (28-37%) followed by CCGs with no significant peak (18-28%) and with a UN/CW peak (14-23%). On average, the occurrence of UN peaks tended to be less frequent during stimulus presentation than in silent conditions, but the difference was not statistically significant. The most frequent occurrence of clear UN peaks was found in the medial part of the MGB (from 52-64% of pairs depending on the type of acoustical stimulus), while the least was observed in the ventral part of the MGB (12-22%). In contrast, CW peaks were most frequently expressed in pairs located in the ventral part of the MGB (18-33%), while neuronal pairs in the medial part revealed a very low occurrence of CW peaks (0-7%). The occurrence of independently firing neurones was lowest in the medial part of the MGB (8-20% of pairs) in comparison with the ventral (31-39%) and dorsal (12-41%) parts. In 20% of pairs acoustical stimulation produced a change in the type of correlation peak present during spontaneous activity. Most frequently, a CW peak (shared input) changed to a flat CCG, which represents independently firing neurones. In some pairs higher connection strengths (as expressed by the value of the correlation coefficient) were found for silent interstimulus intervals than for acoustical stimulation. This suggests that in the MGB the stimulus may desynchronise the spontaneous activity of simultaneously firing units in neuronal pairs.

Acoustic Stimulation↗

PubMed2000

Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats.

The effect of exposure to various types of noise (broadband, high-frequency or low-frequency) was studied in adult pigmented rats. Thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. Exposure to noise with intensities ranging from 105 to 120 dB for 1 h produced only temporary threshold shifts (TTS). Exposure to broadband noise produced TTS throughout the whole frequency range of the rat's hearing, mostly expressed at frequencies of maximal hearing sensitivity (16-32 kHz). Hearing loss produced by high- or low-frequency noise exposure was related to the spectral characteristics of the noise. The exposure to high-intensity noise may also result in amplitude enhancement of the MLR. This phenomenon was seen mainly after broadband noise exposure and occurred in response to both low-frequency and high-frequency test stimuli. High-frequency and low-frequency noise produced amplitude enhancement mainly at frequencies which corresponded to the maximum exposure energy. In contrast to the relatively similar values of TTS obtained in different rats under the same conditions of noise exposure, great inter-individual variability was found in the MLR amplitude enhancement. In all rats the dynamics of recovery functions for amplitude enhancement were different from those for MLR thresholds. The data indicate that whereas post-exposure TTS are related to peripheral changes, the post-exposure MLR amplitude enhancement is most probably connected with a change in the processing of auditory information in the central nervous system.

Acoustic Stimulation↗

PubMed2000

Discharge properties of neurons in subdivisions of the medial geniculate body of the guinea pig.

The activity of 194 neurons was recorded in three subdivisions of the medial geniculate body (74 neurons in the ventral, 62 in the medial and 44 neurons in the dorsal subdivision, i.e. vMGB, mMGB and dMGB) of guinea pigs anesthetized with ketamine-xylazine. The discharge properties of neurons were evaluated by means of peristimulus time histograms (PSTHs), interval histograms (INTHs) and auto-correlograms (ACGs). In the whole MGB, the most frequent PSTH responses to pure tone stimuli were onset (43%) or chopper (32%). The onset responses were mostly present in the vMGB, whereas chopper responses dominated in the dMGB. In the whole MGB Poisson-like and bimodal INTHs were found in 46% and 40% of neurons, respectively. The mMGB revealed fewer bimodal and more symmetrical types of INTH. In the whole MGB, 60% of units were found to have ACGs typical for short bursts (<100 ms), 23% for long bursts (>100 ms) and 15% of units fired without bursts. Neurons in the vMGB were characterized by short bursting, whereas those in the mMGB and dMGB expressed more activity in the long bursts. The results demonstrate that the type of information processing in the vMGB, which belongs to the "primary" auditory system, is different from that in two other subdivisions of the MGB.

Acoustic Stimulation↗

PubMed1999

Acoustically and electrically evoked contralateral suppression of otoacoustic emissions in guinea pigs.

It is generally accepted that stimulation of the efferent auditory system results in changes of cochlear activity. A simple method of activating the olivocochlear pathway by contralateral electrical stimulation of the round window (ES-RW) was used in this study with the aim of comparing the efficacy of acoustically and/or electrically evoked contralateral suppression. The suppression of transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) was elicited by contralateral acoustic stimulation (AS) (61 dB SPL continuous white noise), and/or by electrical stimulation of an electrode implanted at the contralateral round window (monopolar rectangular pulses 0.1 ms, repetition rate 300 Hz, intensity 50-100 PA) in 12 guinea pigs. The average value of contralateral suppression of TEOAEs amounted to 1.04 +/- 0.48 dB for acoustic stimulation and 0.97 +/- 0.53 dB for round window electrical stimulation. The simultaneous presentation of both acoustic and electrical stimulation had only a slight additive effect and resulted in 1.27 +/- 0.79 dB diminution of TEOAEs. The suppression of DPOAEs during contralateral acoustic and electrical stimulation was evident mainly at low and middle frequencies (14 kHz). In two guinea pigs the maximum DPOAE suppression was present at high frequencies. The average values of contralateral suppression measured at individual f2 frequencies of DPOAEs were similar to those calculated from 1/4 octave power spectrum analysis of the TEOAEs in half of the animals. The results demonstrated that contralateral ES-RW had a similar suppressive effect on TEOAEs and DPOAEs as did contralateral AS and simultaneous AS+(ES-RW). The results of spectral analysis suggested that both modes of contralateral stimulation excited similar sensory cochlear elements and induce comparable suppression of both TEOAEs and DPOAEs.

Acoustic Stimulation↗

PubMed1997

Projections of auditory cortex onto the inferior colliculus in the rat.

The organization of the neocortical projection to the inferior colliculus (IC) was studied in 36 rats using retrograde transport of horseradish peroxidase (HRP) or horseradish peroxidase conjugated with lectin (WGA-HRP). Projection to the external and dorsal cortices originates in the temporal neocortical areas Te 1, Te 2 and Te 3 and in the parietal area Par 2. The corticocollicular projection is predominantly ipsilateral with a weak contralateral contribution. Projection to the rostromedial and rostrolateral part of the external cortex (EC) of the IC arises mainly from the areas Par 2 and Te 1. The participation of the cortical areas Te 2 and Te 3 in this projection is only small. The fibres to the caudobasal part of the external cortex descend from the caudal parts of areas Te 1, Te 2, and Te 3. The corticocollicular projections to the dorsal part of the IC are more numerous than the projections to the EC and originate in all temporal areas, i.e. in area Te 1, Te 2 and Te 3. However, the topographical organization of the corticocollicular projection is more pronounced in the part which projects to the EC. We suggest that the topographical organization of the projections to the EC corresponds with the map of auditory space in the EC. The source of corticocollicular fibres are exclusively neurones of lamina V of all cortical areas sending their fibres to the IC.

Animals↗

PubMed1996

Auditory frequency and intensity discrimination in pigmented rats.

Auditory function was investigated in seven pigmented hooded rats (strain Long-Evans) with the aid of an operant conditioning procedure. Frequency difference limen was measured at frequencies from 0.5 to 64 kHz at 50 dB sensation level (SL). Weber ratios (frequency difference limen/frequency) in this range varied between 3.7 and 7.3%. The decline in the intensity of the stimulus from 50 to 10 dB SL was accompanied by a slight increase in the frequency difference limen. The frequency difference limen values were similar for frequency shifts upwards or downwards. Intensity discrimination was measured at 50 dB SL at frequencies of 2, 8 and 32 kHz. Intensity difference limen was frequency independent and amounted to 2.9 +/- 0.5 dB in conditions of upward intensity shift. The values of intensity difference limen measured in conditions of downward intensity shift were significantly larger and amounted to 6.5 +/- 1.6 dB. The characteristics of hearing function found in these experiments correspond with those described by other authors in albino rats and indicate that albinism in the rat has no significant influence on auditory frequency and intensity discrimination.

Acoustic Stimulation↗

PubMed1995

Middle latency responses to acoustical and electrical stimulation of the cochlea in cats.

The middle latency responses (MLR) to acoustical stimulation (A-MLR) as well as to electrical stimulation (E-MLR) of the inner ear were recorded in pentobarbital-anaesthetised cats. Monopolar and bipolar MLR recordings were performed with electrodes located at different places on the primary auditory cortex (AI). The cochlea was electrically stimulated (ES) through a single round-window electrode or through a multichannel intracochlear implant. The slope of amplitude-intensity functions of the A-MLR was steeper when the stimulus frequency of the acoustical stimuli corresponded to the tonotopical recording place on the auditory cortex. Other response properties (waveshape, thresholds and latencies) were related to the recording site and stimulus frequency in only two-thirds of animals. Parameters of E-MLRs evoked by high-frequency ( > 4 kHz) and low-intensity ES in hearing cats, which produced an electrophonic effect, were similar to parameters of acoustically evoked MLRs. In deafened cats, the properties of responses to extracochlear ES were different from those recorded to acoustical stimulation and they were almost uniform in all cortical places. Variations in thresholds, in latencies and in the slope of the amplitude-intensity functions of the E-MLRs recorded in individual tonotopical cortical places were observed when the auditory nerve was stimulated with different configurations of electrodes through a multichannel intracochlear implant.

Acoustic Stimulation↗

PubMed1994

The responses of neurons in subdivisions of the inferior colliculus of cats to tonal, noise and vocal stimuli.

The aim of this study was to gain information from anesthetized cats about the differential coding properties of neurons in the three major subdivisions of the inferior colliculus: the central (CNIC) and external (EN) nuclei and dorsal cortex (DC). Stimuli were presented in the free field from a speaker facing the contralateral pinna. For each unit, the characteristic frequency (CF, where threshold was lowest) was determined, and impulse rates to CF tone bursts, noise bursts and four feline vocal stimuli were measured as a function of increasing sound pressure level (rate/level functions). Peristimulus-time histograms were computed for responses to all stimuli. Sustained firing patterns to CF stimuli were observed for 81% of units in CNIC, for 50% of units in EN and 27% of units in DC. Sustained discharges were evoked by noise in 78-100% of units in all regions, and by at least one vocal stimulus in 86% of units in CNIC, 82% in EN and 55% in DC. In the CNIC, non-monotonic rate/level functions to CF stimuli were more common (41%) than either monotonic or plateau functions, whereas the reverse was the case with noise and vocal stimuli. Non-monotonic functions were uncommon to any stimulus in EN and DC (21-24%). Vocal stimuli were more effective in terms of higher firing rates than noise or CF stimuli in 27% of units in CNIC, 82% in EN and 72% in DC. There were no units that responded exclusively to one vocal stimulus, but a high proportion of units in EN responded strongly to broad band stimuli, and some of these showed clear preferences for one vocal stimulus over others.

Acoustic Stimulation↗