The what, which, when, why and who of Off responses in the auditory system.

For decades, the Off responses noted in the auditory system from the cochlear nucleus up to the secondary auditory cortex have been largely ignored. Over the last few years, several studies have reinvigorated interest in Off responses in the auditory system and described their cellular mechanisms and the plasticity they display during various behaviours. While it is possible to design tasks where Off responses are necessary for an animal's behavioural performance, we posit that it is the interplay of On and Off responses that the brain uses more generally to enable sound feature perception, including sound duration and sound offset detection. Moreover, based upon close examination of the latencies and durations of published Off responses, we propose here that two different types of Off responses should be considered. We suggest that short latency-short duration responses participate in the neural coding of acoustic parameters whose estimates cannot be obtained on the sole basis of onset features. Meanwhile, long latency-long duration responses potentially reflect more cognitive variables associated with the learned use of a particular stimulus in a behavioural context. Proposing two varieties of Off responses draws attention to the need to better characterize post-stimulus firing in future studies to more definitively distinguish divergent mechanisms.