Rate effects in interaural and sequential level difference perception

Abstract

The relative weighting of post-onset interaural level difference (ILD) cues decreases with increasing modulation rate of a signal. It is unclear, however, whether overall ILD sensitivity decreases with increasing rate, particularly if stimulus duration and loudness are kept constant. Moreover, it is unclear if the rate effect arises also in monaural sequential level discrimination. In experiment 1, ILD-based lateralization discrimination thresholds and sequential level difference (SLD) discrimination thresholds were measured using bandpass-filtered pulse trains (4 kHz) with rates of 100, 400, and 800 pulses/s. From 100 to 400 pulses/s, both ILD and SLD thresholds decreased, contrary to a rate limitation. From 400 to 800 pulses/s ILD thresholds increased while SLD thresholds remained constant. The latter result is consistent with a high-rate limitation being specific to binaural hearing. Experiment 2 evaluated whether this rate limitation is due to the loss of transmitted modulation at high rates. The ILD thresholds for an unmodulated 4-kHz pure tone were indeed higher than those for pulse-trains. An auditory nerve model (Zilany et al., 2014, JASA 135, 283–286) combined with an interaural discharge rate comparison stage qualitatively predicted the nonmonotonic pattern of ILD thresholds. Overall, the results suggest that modulation can be beneficial for ILD perception.