This paper reexamines neurophysiological results from echolocating big brown bats to propose a new perspective on FM biosonar processing in the auditory system. Individual auditory neurons are frequency-tuned and respond to brief, 2-10 ms FM sweeps with an average of one spike per sound to register their tuned frequencies, to detect echo arrival, or to register a local null in the echo spectrum. When initiated by the broadcast, these responses comprise a cascade of single spikes distributed across time in neurons tuned to different frequencies that persists for 30-50 ms, long after the sound has ended. Their progress mirrors the broadcast's propagation away from the bat and the return of echoes for distances out to 5-8 m. Each returning echo evokes a similar pattern of single spikes that coincide with ongoing responses to the broadcast to register the target's distance and shape. The hypothesis advanced here is that this flow of responses over time acts as an internal model of sonar acoustics that the bat executes using neuronal computations distributed across many neurons to accumulate a dynamic image of the bat's surroundings.
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