Non-phase locked auditory cells and ‘envelope’ detection

Many primary auditory neurons do not follow the temporal fine structure of waveforms within their passband; these are termed non-phase locked cells. Such cells do not encode the sound pressure waveform directly but rather some nonlinear functional of the waveform such as the envelope. But which definition of the envelope — or more generally which nonlinear functional — does a particular cell implement? This difficulty is one example of a general problem: How do we characterize information transfer in a cell which codes the output of some unknown nontrivial computation on the sensory stimulus? Here we suggest an approach in which one chooses from a set of possible nonlinear functionals by finding that functional of the stimulus which can be estimated most accurately from observation of the spike train. We apply these ideas to non-phase locked auditory afferents to measure the tuning of these cells for complex stimuli.