Adaptation and release from it in free field listening.

Abstract

Using headphone presentation, it has been shown that detection of an interaural difference of time or level in a train of n dichotic clicks is affected by the interclick interval (ICI). With ICIs ≥10 ms, thresholds decline as the √n as predicted for optimum signal processing. With shorter ICIs, the decline is as √nk with 0≤k≤1 and k=f(ICI). Such data have been said to show ‘‘binaural adaptation,’’ a process whereby each click is less effective than the one preceding. A release from binaural adaption has been found in response to appropriate acoustic ‘‘triggers’’ such as a gap in the train or a brief burst of noise. In the present study, minimum audible angles (MAAs) were measured in the free field using two loudspeakers in the horizontal plane placed on either side of the midline. Frequency responses of the speakers were equated with digital filters. Stimuli were trains (n=1 to 16) of bandpass clicks (cf.=4 kHz) with ICIs of either 2 or 10 ms. As with headphones, shorter ICIs produced adaptation. Various potential triggers were presented in synchrony with the center of the train in order to produce a release from adaptation, and release was defined by a √2 decline in threshold relative to that for a train of length n/2, indicating that the n/2 clicks in the second half were as effective as those in the first. [Work supported by a grant from the NIDCD.]