1. Neurophysiological recordings were undertaken to determine how neurons in the central nucleus of the inferior colliculus (ICc) of the little brown bat, Myotis lucifugus, extract amplitude modulations that span across a series of tone pulses (i.e., signals that simulate echoes from fluttering targets). Two types of stimuli were presented to the bats. The first served as a control and consisted of an unmodulated train of tone pulses having different repetition rates (TPu, 5-400 pulses per second). The second was a train of tone pulses that were sinusoidally amplitude modulated (TPm, 5-110 Hz) across sequential pulses. The modulated trains of pulses were presented at five different repetition rates (25, 50, 100, 200, and 400 pulses per second) encompassing the range of biosonar emission rates in these bats at different stages of target-directed flight. 2. One hundred fifty-two single neurons were isolated in the ICc of M. lucifugus; their basic response properties and temporal firing patterns were characterized. The best frequencies (BFs) ranged from 10 to 80 kHz and the minimum thresholds at BF were distributed widely (10-95 dB SPL). The frequency tuning selectivity ranged widely, from very broadly tuned (Q(10db) = 1.3) to narrowly tuned (Q(10dB) 89). Units with very narrow frequency tuning (Q values > 20) were restricted to BFs of 30-50 kHz. The temporal firing pattern of ICc units could be categorized into primary-like (PL), chopper (C), onset-immediate (O-I), and onset-late (O-L). 3. In response to TPu ICc units exhibited varying degrees of response selectivities as evidenced by their count-based response functions (using the spike count as a measure) versus repetition rate. The count-based response functions of ICc units exhibited five filtering characteristics including band-pass, low-pass, high-pass, band-suppression, and all-pass characteristics. The temporal firing pattern of a unit showed certain correlations with its count-based response function. For example, the majority of O-I and O-L units, and about half of the C units, showed tuned band-pass response functions. The remaining C and onset types showed mostly low-pass response functions. In contrast, PL neurons showed mostly high-pass response functions, but one third displayed band-pass response functions. 4. The ability of ICc neurons to time-lock their discharges to the individual pulses in a train was characterized by using the synchronization coefficient (SC) as a measure. The SC was plotted against the repetition rate to construct units' synchronization-based response function. The vast majority of iCc neurons with FL, C, and O-L response patterns showed low-pass synchronization-based response functions with the remaining manifesting allpass response functions. In contrast, the majority of O-I neurons displayed all-pass synchronization-based response functions showing effective time-locking across the entire range of repetition rates tested, with the remaining units showing low-pass response functions. The average TPu cutoff frequency (the frequency at which the SC dropped to 50% of maximum value) for low-pass neurons was, in order of decreasing time-locking ability: O-I (231 pulses per second), O-L (200 pulses per second), PL(110 pulses per second), and C (94 pulses per second). 5. ICc neurons were particularly sensitive to amplitude modulations that were imposed across a sequence of sound pulses (TPm stimuli). Over half of the neurons, at all repetition rates tested, showed a preference for a specific range of across-pulse AM frequency that corresponded to the range of insect wingbeat frequencies that little
