1. Encoding of temporal stimulus parameters by inferior collicular (IC) neurons of Eptesicus fuscus was studied by recording their responses to a wide range of repetition rates (RRs) and durations at several stimulus intensities under free field stimulus conditions. 2. The response properties of 424 IC neurons recorded were similar to those reported in previous studies of this species. 3. IC neurons were classified as low-pass, band-pass, and high-pass according to their preference for RRs and/or durations characteristic of, respectively, search, approach, or terminal phases of echolocation. These neurons selectively process stimuli characteristic of the various phases of hunting. 4. Best RRs and best durations were not correlated with either the BFs or recording depths This suggests that each isofrequency lamina is capable of processing RRs and durations of all hunting phases. 5. Responses of one half of IC neurons studied were correlated with the stimulus duty cycle. These neurons may preferentially process terminal phase information when the bat's pulse emission duty cycle increases. 6. While the stimulus RR affected the dynamic range and overall profile of the intensity rate function, only little effect was observed with different stimulus durations.

The directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, was studied under free field stimulation conditions with 3 temporally patterned trains of sound pulses which differed in pulse repetition rate and duration. The directional sensitivity curves of 92 neurons studied can be described as hemifield, directionally-selective, or non-directional according to the variation in the number of impulses with pulse train direction. When these neurons were stimulated with all 3 pulse trains, the directional sensitivity curves of 50 neurons was unchanged but that of the other 42 neurons changed from one type into another. When these pulse trains were delivered at high pulse repetition rate and short pulse duration, they significantly sharpened the directional sensitivity of two thirds of the neurons examined by reducing the angular range and increasing the slope of their impulse directional sensitivity curves. These pulse trains also sharpened the slope of the threshold directional sensitivity curves of 25 neurons studied. However, when directional sensitivity of collicular neurons was determined with pulse trains that differed only in pulse repetition rate or in pulse duration, significant sharpening of directional sensitivity was rarely observed in all experimental conditions tested. Possible mechanisms underlying these findings are discussed.

Using the big brown bat, Eptesicus fuscus, as a model mammalian auditory system, we studied the effect of GABAergic disinhibition by bicuculline on the responses of inferior collicular (IC) neurons to temporally patterned trains of sound pulses delivered at different pulse repetition rates (PRRs) under free-field stimulation conditions. All 66 neurons isolated from eight bats either discharged one to two impulses (phasic on responders, n = 41, 62%), three to eight impulses (phasic bursters, n = 19, 29%), or many impulses throughout the entire duration of the stimulus (tonic responders, n = 6, 9%). Whereas 50 neurons responded vigorously to frequency-modulated (FM) pulses, 16 responded poorly or not at all to FM pulses. Bicuculline application increased the number of impulses of all 66 neurons in response to 4 ms pulses by 15-1,425%. The application also changed most phasic on responders into phasic bursters or tonic responders, resulting in 12 (18%) phasic on responders, 34 (52%) phasic bursters, and 20 (30%) tonic responders. Response latencies of these neurons were either shortened (n = 25, 38%) by 0.5-6.0 ms, lengthened (n = 9, 14%) by 0. 5-2.5 ms or not changed (n = 32, 48%) on bicuculline application. Each neuron had a highest response repetition rate beyond which the neuron failed to respond. Bicuculline application increased the highest response repetition rates of 62 (94%) neurons studied. The application also increased the highest 100% pulse-locking repetition rates of 21 (32%) neurons and facilitated 27 (41%) neurons in response to more pulses at the same PRR than predrug conditions. According to average rate-based modulation transfer functions (average rate MTFs), all 66 neurons had low-pass filtering characteristics both before and after bicuculline application. According to total discharge rate-based modulation transfer functions (total rate MTFs), filtering characteristics of these neurons can be described as band-pass (n = 52, 79%), low-pass (n = 12, 18%), or high-pass (n = 2, 3%) before bicuculline application. Bicuculline application changed the filtering characteristics of 14 (21%) neurons. According to synchronization coefficient-based modulation transfer functions, filtering characteristics of these neurons can be described as low-pass (n = 41, 62%), all-pass (n = 11, 17%), band-suppression (n = 7, 10.5%), and band-suppression-band-pass filters (n = 7, 10.5%). Bicuculline application changed filtering characteristics of 19 (29%) neurons.

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