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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.

Neurons in the auditory cortex (AC) receive convergent excitatory and inhibitory inputs from the lower auditory nuclei. Interaction between these two opposing inputs shapes different response properties of AC neurons. In this study, we examined how this interaction might affect the frequency tuning curves (FTCs), number of impulses and latency of AC neurons in the big brown bat, Eptesicus fuscus, using a probe (excitatory tone) and a masker (inhibitory tone) under different stimulation conditions. Excitatory FTCs of AC neurons were either V-shaped, closed (i.e. upper threshold) or double-peaked. Inhibitory FTCs were obtained either at both flanks or only at the low or high flank of excitatory FTCs. Application of bicuculline, an antagonist for gamma-aminobutyric acid A receptors, produced expansion of excitatory FTCs into predrug inhibitory FTCs. Inhibition of probe-elicited responses occurred when a masker was presented at certain intertone intervals. Maximal inhibition typically took place when a masker was presented within 4 ms prior to the probe. During maximal inhibition, a neuron had the minimal number of impulses and the longest response latency. Inhibition became stronger with increasing masker intensity but became weaker with increasing intertone interval. Biological significance of these data is discussed.

This study examined auditory responses of two simultaneously recorded neurons in the central nucleus of bat inferior colliculus (IC) under two-tone stimulation conditions. We specifically examined how a sound within the excitatory frequency tuning curve (FTC) of one IC neuron might affect responses of the other IC neuron in amplitude and frequency domains. Under this specific two-tone stimulation condition, responses of 82% neurons were suppressed and their excitatory FTCs sharpened. Responses of the other 18% neurons were facilitated and their excitatory FTCs broadened. Two-tone suppression was greater at low than at high stimulus amplitudes. Two-tone suppression also decreased with increasing recording depth and best frequency (BF) difference between each pair of neurons. The suppressive or facilitatory FTC of a neuron plotted under two-tone stimulation conditions was always within the excitatory FTC of the other neuron. Two-tone suppression or two-tone facilitation was weak near the BF but became increasingly strong with frequencies away from the BF. Biological significance of these findings is discussed.