Sound duration selectivity in the pallid bat inferior colliculus.

Neurons selective for sound duration have been reported in the auditory midbrain and cortex of several specialized vertebrate species that process behaviorally relevant signals of stereotypic duration. This study examines duration selectivity in the inferior colliculus (IC) of the pallid bat to determine if this selectivity is limited to regions that serve echolocation, or if it extends to low-frequency regions that serve passive listening. It also focuses on the temporal response properties of duration-selective neurons to elucidate mechanisms that may underlie the creation of this selectivity. Of 140 neurons tested, 36% were selective for short durations of </=7 ms, and acted as short-pass or bandpass duration filters. Sixteen percent, termed long duration neurons, differed in that they required minimum sound durations of 5-50 ms before responding, and all acted as long-pass duration filters. Short duration neurons were equally common in the high-frequency region serving echolocation and the lateral low-frequency region that serves passive listening, indicating that selectivity for short duration sounds was not associated only with the specialized function of echolocation. Long duration neurons were most common in the medial low-frequency region IC. Selectivity for short and long duration sounds was therefore not uniformly distributed across the IC. Analyses of the temporal response properties of short duration neurons, and the application of bicuculline to block gamma-aminobutyric acid-A receptors, were used to infer the synaptic interactions that underlie the creation of duration selectivity, the role of inhibition in its creation, and whether a coincidence mechanism proposed by Casseday et al. (Science 264 (1994) 847-850) is consistent with the behavior of the duration-selective neurons recorded in the pallid bat IC. Present results suggest that while some neurons do behave in a manner that is consistent with the coincidence mechanism, the behaviors of others suggest that more than one mechanism may create a selectivity for short duration sounds.