NEURAL ENSEMBLE CODING OF TARGET IDENTITY IN ECHOLOCATING BATS

Most insectivorous bats use echolocation to determine the identity of flying insects. Among the many target features that are so extracted, the insect's wingbeat pattern and frequency appear to serve as useful cues for identification. Biosonar pulses impinging on the fluttering w ings of an insect are returned as echoes whose amplitudes vary with ti me, thus providing a characteristic signature of the insect. It has be en shown previously that neurons in the inferior colliculus, a midbrai n auditory nucleus, of the little brown bat respond to sound stimuli t hat mimic echoes from fluttering targets. To examine the manner in whi ch target identity is represented in the inferior colliculus, an ensem ble coding analysis using a filter-based approach was undertaken. The analysis indicates that a discrete subset of neurons in the inferior c olliculus, the onset units, are strongly tuned to wingbeat frequencies of targets that the bat hunts, and that ensemble response reaches a m aximum at a distinct phase of the prey capture maneuver: the late appr oach stage. On the basis of the analysis it is hypothesized that infer ior colliculus neurons may play an important role in target detection- identification processing. Although ensemble coding of temporary seque nced information has not been analyzed in the auditory system so far, this study indicates that this method of coding may provide the inform ation necessary to detect and identify targets during prey capture.