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

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 N1 component of the peripheral compound action potential (CAP). It should therefore be possible to predict click DLs from N1 potentials. Here, a Signal Detection model, using a series expansion, was used to derive equations in N1 for the level-dependence of the DL. The first-order equation predicts a dependence on the standard deviation of N1, and an inverse dependence on the rate-of-growth of the mean N1. The second-order equation is more complicated. Both approximations were applied to N1s 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 N1, not the standard deviation of N1, determines the hump in the empirical DL.