FREQUENCY TUNING, LATENCIES, AND RESPONSES TO FREQUENCY-MODULATED SWEEPS IN THE INFERIOR COLLICULUS OF THE ECHOLOCATING BAT, EPTESICUS-FUSCUS

Neurons in the inferior colliculus (IC) of the awake big brown bat, Ep tesicus fuscus, were examined for joint frequency and latency response properties which could register the timing of the bat's frequency-mod ulated (FM) biosonar echoes. Best frequencies (BFs) range from 10 kHz to 100 kHz with 50% tuning widths mostly from 1 kHz to 8 kHz. Neurons respond with one discharge per 2-ms tone burst or FM stimulus at a cha racteristic latency in the range of 3-45 ms, with latency variability (SD) of 50 mu s to 4-6 ms or more. BF distribution is related to bioso nar signal structure. As observed previously, on a linear frequency sc ale BFs appear biased to lower frequencies, with 20-40 kHz overreprese nted. However, on a hyperbolic frequency (linear period) scale BFs app ear more uniformly distributed, with little overrepresentation. The cu mulative proportion of BFs in FM1 and FM2 bands reconstructs a scaled version of the spectrogram of FM broadcasts. Correcting FM latencies f or absolute BF latencies and BF time-in-sweep reveals a subset of IC c ells which respond dynamically to the timing of their BFs in FM sweeps . Behaviorally, Eptesicus perceives echo delay and phase with microsec ond or even submicrosecond accuracy and resolution, but even with use of phase-locked FM and tone-burst stimuli the cell-by-cell precision o f IC time-frequency registration seems inadequate by itself to account for the temporal acuity exhibited by the bat.