Sensitivity to binaural cues beyond threshold as revealed by eye movements

Abstract

Binaural cues are paramount for sound localization along the azimuth. Studies on the perception of the critical binaural cues, interaural time and level differences (ITDs and ILDs, respectively), typically measure sensitivity at threshold using N-interval, forced choice paradigms. This approach gives little or no information regarding perceptual abilities beyond threshold. In this study, an anticipatory eye movement (AEM) paradigm (cf. McMurray, 2004) was used as a novel measure to study binaural cue sensitivity throughout a wide perceptual range. This paradigm is sensitive to gradient (rather than all-or-none) perception of auditory cues. Adults with normal hearing visually tracked the location of a ball that becomes hidden on a computer screen, and anticipated its motion and reappearance via eye movements guided by binaural cues. The auditory stimuli were 4.8-s 1/3-octave narrowband noises centered at different frequencies, and contained an ILD or ITD change, indicating the impending motion of the ball. Greater cue levels elicited systematically quicker and more accurate saccades. ILD-driven AEMs were elicited for all noise center frequencies, with greater sensitivity for higher frequency noises, consistent with ecological significance. Eye movements in this study suggest variation in binaural cue perception beyond threshold that is gradient in terms of latency and accuracy.