Sound localization with eccentric head position.

This study investigates the influence of head-to-trunk position on auditory localization in humans. Various methods of head pointing, of two-alternative forced choice, and hand pointing were employed. Head-pointing toward actual sound sources in darkness, by using only the subjective median plane of the head as a reference, resulted in systematic underestimations of target eccentricity. The deviations of the terminal head position from the target shifted with a mean slope of approximately 0.1 degrees per degree change in head position. A corresponding shift in the localization of virtual sound sources (presented via headphones during eccentric head positions) was demonstrated by requiring forced-choice (left or right) responses with respect to the subjective median plane of the head. Head pointing toward remembered auditory targets in darkness resulted in undershoots similar to those found with actual targets. However, when a visual marker of the actual median plane of the head was additionally presented to the subject during these tasks (by a laser attached to the head that projected a spot onto a screen), sound localization was fairly accurate. Localization of eccentric auditory targets by using a swivel hand pointer also showed systematic errors similar to those found with head pointing in darkness when the head was simultaneously oriented toward the sound. When the head remained in alignment with the trunk, hand pointing resulted in overshooting responses. These results may be related to neural processes, presumably in the posterior parietal cortex, that transform auditory and visual spatial coordinates into a common, trunk-centered, frame of reference.