HEADING JUDGMENTS DURING ACTIVE AND PASSIVE SELF-MOTION

Previous studies have generally considered heading perception to be a visual task. However, since judgments of heading direction are require d only during self-motion, there are several other relevant senses whi ch could provide supplementary and, in some cases, necessary informati on to make accurate and precise judgments of the direction of self-mot ion. We assessed the contributions of several of these senses using ta sks chosen to reflect the reference system used by each sensory modali ty. Head-pointing and rod-pointing tasks were performed in which subje cts aligned either the head or an unseen pointer with the direction of motion during whole body linear motion. Passive visual and vestibular stimulation was generated by accelerating subjects at sub- or supra-v estibular thresholds down a linear track. The motor-kinesthetic system was stimulated by having subjects actively walk along the track. A he lmet-mounted optical system, fixed either on the cart used to provide passive visual or vestibular information or on the walker used in the active walking conditions, provided a stereoscopic display of an optic al flow field. Subjects could be positioned at any orientation relativ e to the heading, and heading judgments were obtained using unimodal v isual, vestibular, or walking cues, or combined visual-vestibular and visual-walking cues. Vision alone resulted in reasonably precise and a ccurate head-pointing judgments (0.3 degrees constant errors, 2.9 degr ees variable errors), but not rod-pointing judgments (3.5 degrees cons tant errors, 5.9 degrees variable errors). Concordant visual-walking s timulation slightly decreased the variable errors and reduced constant pointing errors to close to zero, while head-pointing errors were una ffected. Concordant visual-vestibular stimulation did not facilitate e ither response. Stimulation of the vestibular system in the absence of vision produced imprecise rod-pointing responses, while variable and constant pointing errors in the active walking condition were comparab le to those obtained in the visual condition. During active self-motio n, subjects made large head-pointing undershoots when visual informati on was not available. These results suggest that while vision provides sufficient information to identify the heading direction, it cannot, in isolation, be used to guide the motor response required to point to ward or move in the direction of self-motion.