Characteristics of phasic on neurons in inferior colliculus of unanesthetized bats with observations relating to mechanisms for echo ranging

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Abstract
Single units with phasic-on firing patterns were recorded from the inferior colliculus of unanesthetized Mexican free-tailed bats. A population of neurons was identified whose response characteristics were suitable for coding the time interval between an emitted pulse and echo, which would be of value for echo-ranging. Three categories of units were distinguished on the basis of the consistency of the initial spike latent periods elicited by repetitive signals. Most important are the phasic constant latency responders (pCLR) in which the dispersion of initial spike latencies was at most .+-. 250 .mu.s at any intensity, and where the mean latency changed by at most 1.75 ms when the signal level was increased from 3-6 dB to 30-40 dB above threshold. The initial spike latencies in most pCLR were erratic with tone bursts but became highly synchronous with frequency-modulated (FM) signals resembling the natural chirps of this species. Each of these units exhibited an independent-like recovery pattern where the presentation of an initially loud pulse did not affect the response to a simulated echo following shortly thereafter. The response and recovery characteristics of the pCLR were correlated with behavioral data reported in previous studies, and it was shown that the pCLR have the capacity to code for the time interval between a pulse and the returning echo.