A single-unit analysis of inferior colliculus in unanesthetized bats: response patterns and spike-count functions...

1. Single-unit activity evoked by constant-frequency (CF) and frequency-modulated (FM) sounds was recorded from the inferior colliculus of unanesthetized Mexican free-tailed bats. The FM bursts were designed to mimic the natural orientation cries emitted by this species. 2. The feature of greatest concern in this study is the response patterns evoked by acoustic signals. Four major types of response patterns are recognized: a) the phasic on patterns where up to 4 spikes were evoked with a total firing duration occurring within a period of about 5 ms, b) the on-off patterns characterized by a phasic on-response followed by a silent interval with a brief burst of impulses occurring to the off-set of the signal, c) the phasic burst patterns where the unit typically fired 3-7 spikes over a 5-10 ms (or longer) duration with the same response pattern being evoked by a wide range of signal durations, d) the tonic or sustained patterns characterized by a sustained firing whose duration faithfully followed the signal duration. 3. Constant frequency and FM signals were not equally effective for eliciting the various response patterns. While all of the major response categories and most of the subtypes were evoked with CF stimulation, FM signals, which mimicked the natural echolocation cries, evoked predominantly phasic on-responses and a much smaller number of phasic bursters. Tonic and on-off patterns were never observed with FM signals. 4. Many units exhibited a particular response pattern with CF signals which differed from the pattern evoked by FM signals. This finding demonstrated that utilizing CF signals to investigate encoding features relevant for echolocation is an inappropriate approach and can result in misleading conclusions. 5. In many electrode penetrations most units, and in some cases all units, had the same response pattern. This was observed for phasic constant-latency responders (pELRs), phasic erratic-latency responders (pELRs), phasic burst, and tonic units, and provides evidence that units having a particular response pattern are organized in vertical arrays within the volliculus. 6. Several of the response patterns were strongly correlated with a particular spike-count function. The pCLRs almost always had a steeply rising monotonic function, phasic bursters were always nonmonotonic but never upper threshold, while the tonic units typically had monotonic functions. The pELRs were heterogeneous with regard to spike counts having monotonic, nonmonotonic, and upper-threshold functions. Upper-threshold functions were observed only in pELRs.