Lateralization biases for narrowband stimuli in listeners with typical and symmetrical hearing thresholds

Abstract

In listeners with normal audiometric thresholds, it is assumed that stimuli presented over headphones with zero interaural differences are perceived to be centered in the head. In this study, intracranial lateralization was measured by having ten young adults with normal and interaurally symmetric thresholds (within 5 dB) point to perceived sound locations along a visual reference scale. The stimuli were either tones or 50-Hz-bandwidth narrowband noises, at frequencies of 300, 400, 500, 600, and 700 Hz. A range of interaural time differences (ITDs) was applied while the interaural level difference was always zero. To diminish any transducer-based effects, the headphones were reset and reversed between each of ten blocks of trials. Lateralization curves for five of the ten listeners showed a substantial bias to one ear at a subset or all frequencies tested; the worst individual case demonstrated a perceived 60% lateralization bias (from the midline to the left ear) for a zero ITD. This result challenges a long-standing assumption held in binaural hearing and, while similar results have been observed previously, the basis of the phenomenon remains largely unexplored.