The responses of single units in the ventral cochlear nucleus of the guinea pig to damped and ramped sinusoids

Human listeners hear an asymmetry in the perception of damped and ramped si nusoids; the partial loudness of the envelope component is greater than the partial loudness of the carrier component for damped sinusoids. Here we sh ow that an asymmetry also occurs in the physiological responses of most uni ts in the ventral cochlear nucleus to these same sounds. The activity elici ted by damped sinusoids is mainly restricted to the beginning of each envel ope period, which is not the case for ramped sinusoids. This can be quantif ied by computing the ratio of the tallest bin of the modulation period hist ogram to the total number of spikes (the peak-to-total ratio, p/t). Damped sinusoids produce a higher p/t than ramped sinusoids, which demonstrates ph ysiological temporal asymmetry. it is also the case that ramped sinusoids t ypically elicit more spikes than damped sinusoids. The physiological asymme try occurs where the perceptual asymmetry is present. It is maximal at modu lation half-lives of 4 and 16 ms, greatly reduced at 1 ms and absent at 64 ms. Different unit types exhibit differing degrees of temporal asymmetry. O nset units produce the greatest p/t asymmetry, primary-like units produce t he least asymmetry and chopper units are in-between. With regard to total s pike count, the maximal asymmetry occurs with chopper units. If primary-lik e units are assumed to reflect the activity in primary auditory nerve fibre s, then there is enhancement of temporal asymmetry in the ventral cochlear nucleus by both onset and chopper units. (C) 2000 Elsevier Science B.V. All rights reserved.