Intensity-difference limens predicted from the click-evoked peripheral N1: The mid-level hump and its implications for intensity encoding

Abstract

The intensity-difference limen (DL) for an acoustic click rises at moderate click levels, a feature called the ‘mid-level hump’. It has long been hypothesized that, because a click does not evoke sustained firing in any primary afferent, the DL must therefore originate from the initial burst of synchronized spikes in the eighth nerve. That burst causes the N 1 component of the peripheral compound action potential (CAP). It should therefore be possible to predict click DLs from N 1 potentials. Here, a Signal Detection model, using a series expansion, was used to derive equations in N 1 for the level-dependence of the DL. The first-order equation predicts a dependence on the standard deviation of N 1, and an inverse dependence on the rate-of-growth of the mean N 1. The second-order equation is more complicated. Both approximations were applied to N 1s from the cat. Both produced a mid-level hump; at its peak, the DLs from the second-order approximation were the smaller ones, and were of the same order of magnitude as the empirical DLs. Overall, the computations show that the rate-of-growth of the mean N 1, not the standard deviation of N 1, determines the hump in the empirical DL.