The effect of stimulus rate and time-separation on Volterra slices of otoacoustic emissions.

Volterra slices (VSs) of otoacoustic emissions are temporal non-linear interaction components which can be measured using the maximum length sequence technique of stimulation. Previous studies have found, but not explained, non-monotonic variations in the amplitude of VSs when stimulus rate is increased. In this study, a simple phenomenological model is investigated which provided possible insights into the effect of rate on VS amplitudes. Resulting theoretical considerations suggest that the effect of rate on VS amplitude is best examined when the time-separation parameter of the VS is held constant. To test these suggestions, data on VSs of order 2 and 3 were measured in 24 normal hearing ears in which the rate is varied while holding constant the time-separation. Under these conditions, the results reveal a monotonic reduction in the amplitude of the VSs with increasing rate. The phenomenological model offers a possible explanation of some of these results in terms of the derivatives of the input-output function of the non-linearity. In addition, measured cross-correlations between waveforms of VS of different order and slice number were obtained, revealing a complex dependence on time-separation that has not been explained.