Curious oddments of auditory-nerve studies

Three interesting theoretical issues are presented to illustrate how certain isolated observations on auditory-nerve activity can be puzzling until other, seemingly unrelated phenomena are documented. The issues are (1) disinhibition; (2) 'peak-splitting'; and (3) independence of spike generation in primary neurons innervating the same inner-hair cell. (1) The issue of disinhibition is important for theories of lateral inhibition. For auditory-nerve fibers, the question can he phrased, 'If the rate of discharge to a tone at the characteristic frequency (CF) of a unit can he reduced by adding a second tone off the CF, is it possible to suppress this reduction by adding a third tone, even further off the CF?' The data are insufficient to conclude that disinhibition is found for auditory-nerve fibers and other explanations are available to account for the results of three-tone experiments. (2) Normally, only a single peak in the histogram of responses to low tones is phase-locked, but at high stimulus levels, the histograms will show two, or even three, peaks per stimulus cycle ('peak-splitting'). At still higher levels, the histograms again show only a single peak, but it is phase-shifted from the original peak for low stimulus levels. This complex sequence of events can be accounted for by simple models. (3) Although simultaneous recordings from pairs of auditory-nerve fibers have failed to show non-stimulus related correlations between spike trains, it has not been directly demonstrated that any two recorded fibers innervate the same hair cell. However, an indirect argument is offered to support the idea that fibers innervating a single inner-hair cell must have independent spike generators.