We recorded extracellular activity from 402 single units located in the inferior colliculus (IC) of barbiturate-anesthetized albino mice. The stimuli were pure tones at characteristic frequency (CF) with durations of 10, 40 and 100 ms and intensities ranged from 5 to 25 dB above unit’s minimum threshold (MT). The tones were presented with different repetition rates (RRs) ranging from 0.2 to 20.0 Hz. At low intensities (5 dB above MT, determined at RR of 0.5 Hz) the great majority of units exhibited a strong decline of their responses when the stimulus RR was increased. About one-half of the units did not respond to 40 ms tones when they were stimulated with the RR of 3.0 Hz. This effect was even more pronounced for 100 ms tones. Generally, the increase in stimulus intensity led to an increase in the high-frequency border of RR. Nevertheless, even at intensities of 20-30 dB above MT, some units showed no response when the RR exceeded 5.0 Hz. In many cases the band-pass or high-pass duration tuning of the single unit was transformed to low-pass or all-pass when the rate was low enough to guarantee the independence of successive presentations of the stimuli. Responses of a very small group of IC units, however, were enhanced when the RR was increased. Our data have shown that the changes in the RR radically modify many features of the neural response (number of spikes, latency, discharge pattern, duration selectivity). We suggest that long-lasting inhibitory processes may be induced by low intensity stimuli in many units of the IC.

In acoustic communication, animals must extract biologically relevant signals that are embedded in noisy environment. The present study examines how weak noise may affect the auditory sensitivity of neurons in the central nucleus of themouse inferior colliculus (IC)which receives convergent excitatory and inhibitory inputs from both lower and higher auditory centers. Specifically, we studied the frequency sensitivity and minimum threshold of IC neurons using a pure tone probe and a weak white noise masker under forward masking paradigm. For most IC neurons, probe-elicited response was decreased by a weak white noise that was presented at a specific gap (i.e. time window). When presented within this time window, weak noise masking sharpened the frequency tuning curve and increased the minimum threshold of IC neurons. The degree of weak noise masking of these two measurements increased with noise duration. Sharpening of the frequency tuning curve and increasing of the minimum threshold of IC neurons during weak noise masking were mostly mediated through GABAergic inhibition. In addition, sharpening of frequency tuning curve by the weak noise masker was more effective at the high than at low frequency limb. These data indicate that in the real world the ambient noise may improve frequency sensitivity of IC neurons throughGABAergic inhibitionwhile inevitably decrease the frequency response range and sensitivity of IC neurons.

Natural auditory environment consists of multiple sound sources that are embedded in ambient strong and weak noise. For effective sound communication and signal analysis, animals must somehow extract biologically relevant signals from the inevitable interference of ambient noise. The present study examined how a weak noise may affect the amplitude sensitivity of neurons in the mouse central nucleus of the inferior colliculus (IC) which receives convergent excitatory and inhibitory inputs from both lower and higher auditory centers. Specifically, we studied the amplitude sensitivity of IC neurons using a probe (best frequency pulse) and a masker (weak noise) under simultaneous masking paradigm. For most IC neurons, weak noise masking increases the minimum threshold and decreases the number of impulses. Noise masking also increased the slope and decreased the dynamic range of the rate amplitude function of these IC neurons. The strength of this noise masking was greater at low than at high sound amplitudes. This variation in the amplitude sensitivity of IC neurons in the presence of the weak noise was mostly mediated through GABAergic inhibition. These data indicate that in the real world the ambient weak noise improves amplitude sensitivity of IC neurons through GABAergic inhibition while inevitably decreases the range of overall auditory sensitivity of IC neurons.

Echolocation calls and neurophysiological correlations with auditory response properties in the inferior colliculus of Pipistrellus abramus (Microchiroptera: Vespertilionidae). Zoological Studies 46(5): 622-630. The present study examines the echolocation calls and auditory responses of single neurons in the inferior colliculus (IC) of Pipistrellus abramus (Microchiroptera: Vespertilionidae). The data showed that there was a neurophysiological correlation of the auditory response properties with echolocation calls in IC neurons. The echolocation calls of P. abramus were broad-band swept from 86.6 to 43.2 kHz. The ending frequencies of the first harmonics which centered around 40 (average, 43.2; range, 37.0-47.0) kHz, were relatively more stable than the initial high frequencies. The average peak frequency was 52.1 (range, 43.3-57.6) kHz of which the majority (81%, 154 of 190 calls) ranged from 50.1 to 60 kHz. We recorded the responses of 75 single IC neurons to pure tones. Most IC neurons had the best frequency (BF) at between 30 and 50 kHz (centered around 40 kHz) (73%, 54 of 75) and between 50.1 and 60 kHz (19%, 14 of 75), respectively corresponding to the ending frequencies of the first harmonics and peak frequencies. The minimum threshold (MT) distribution was wider, and the average MT was significantly higher for neurons with a BF of 30-50 kHz than for neurons with a BF of 50.1-60 kHz (62±11vs. 49±8 dB SPL, p<0.001, t-test). The latency distribution was also slightly wider for neurons with a BF of 30-50 kHz (71% between 6.1 and 8.0 ms) than for neurons with a BF of 50.1-60 kHz (79% between 4.0 and 6.0 ms). Our study of echolocation calls and auditory response properties of IC neurons suggests that the IC of P. abramus can effectively process emitted pulses and echoes during hunting.

This study examined the effect of bicuculline application on the auditory response properties in the auditory cortex of the big brown bat, Eptesicus fuscus. All auditory cortical neurons studied discharged either 1-2 or 3-7 impulses to 4 ms sound stimuli. Cortical neurons with high best frequencies tended to have high minimum thresholds. Bicuculline application increased the number of impulses and shortened the response latencies of all cortical neurons as well as changing the discharge patterns of half of the cortical neurons studied. Bicuculline application raised the rate-intensity functions but lowered the latency-intensity functions to varying degrees. Threshold-frequency tuning curves (FTCs) were either V-shaped, upper threshold or double-peaked. Threshold-FTCs and impulse-FTCs were mirror-images of each other. Bicuculline application expanded and raised the impulse-FTCs but lowered the threshold-FTCs, resulting in significantly decreased Qn values. Threshold^FTCs of cortical neurons determined within an orthogonally inserted electrode were very similar and expanded FTCs during bicuculline application were also very similar. Possible mechanisms for the contribution of GABAergic inhibition to shaping these response properties of cortical neurons are discussed.