Selectivity for echo spectral interference and delay in the auditory cortex of the big brown bat Eptesicus fuscus

The acoustic environment for an echolocating bat can contain multiple objects that reflect echoes so closely separated in time that they are almost completely overlapping. This results in a single echo with a spectrum characterized by deep notches due to interference. The object of this study was to document the possible selectivity, or lack thereof, of auditory neurons to the temporal separation of biosonar signals on a coarse (ms) and fine (micros) temporal scale. We recorded single-unit activity from the auditory cortex of big brown bats while presenting four protocol designs using wideband FM signals. The protocols simulated a pair of partially overlapping echoes where the separation between the first and second echo varied between 0 and 72 micros, a pulse followed by a single echo at varying delay from 0 to 30 ms, a pulse followed at a fixed delay by a pair of partially overlapping echoes that had a varying temporal separation of 0-72 micros, and a pulse followed, with a varying delay between 0 and 30 ms, by a pair of echoes that themselves had a fixed temporal separation on a microsecond time scale. About half of the cortical units showed increased spike counts to pairs of partially overlapping echoes at particular separations (6-72 micros) compared with a baseline stimulus at 0-micros separation. For many neurons tested with a pulse followed by two overlapping echoes, we observed a sensitivity to the coarse delay between the pulse and pair of overlapping echoes and to the separation between the two echoes themselves. The sensitivity to the partial overlap between the two echoes was not tuned to a single temporal separation. For bats, this means that the absolute range to the closest reflector and range between reflectors may be jointly encoded across a small population of single units. There are several possible neuronal mechanisms for encoding the separation between two nearby echoes based on the sensitivity to spectral notches.