NEURONS WITH DIFFERENT TEMPORAL FIRING PATTERNS IN THE INFERIOR COLLICULUS OF THE LITTLE BROWN BAT DIFFERENTIALLY PROCESS SINUSOIDAL AMPLITUDE-MODULATED SIGNALS

We examined how well single neurons in the inferior colliculus (IC) of an FM bat (Myotis lucifugus) processed simple tone bursts of differen t duration and sinusoidal amplitude-modulated (SAM) signals that appro ximated passively heard natural sounds. Units' responses to SAM tones, measured in terms of average spike count and firing synchrony to the modulation envelope, were plotted as a function of the modulation freq uency to construct their modulation transfer functions. These function s were classified according to their shape (e.g., band-, low-, high-, and all-pass). IC neurons having different temporal firing patterns to simple tone bursts (tonic, chopper, onset-late, and onset-immediate) exhibited different selectivities for SAM signals. All tonic and 83% o f chopper neurons responded robustly to SAM signals and displayed a va riety of spike count-based response functions. These neurons showed a decreased level of time-locking as the modulation frequency was increa sed, and thereby gave low-pass synchronization-based response function s. In contrast, 64% of onset-immediate, 37% of onset-late and 17% of c hopper units failed to respond to SAM signals at any modulation freque ncy tested (5-800 Hz). Those onset neurons that did respond to SAM sho wed poor time-locking (i.e., non-significant levels of synchronization ). We obtained evidence that the poor SAM response of some onset and c hopper neurons was due to a preference for short-duration signals. The se data suggest that tonic and most chopper neurons are better-suited for the processing of long-duration SAM signals related to passive hea ring, whereas onset neurons are better-suited for the processing of sh ort, pulsatile signals such as those used in echolocation.