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 required only during self-motion, there are several other relevant senses which could provide supplementary and, in some cases, necessary information to make accurate and precise judgments of the direction of self-motion. We assessed the contributions of several of these senses using tasks chosen to reflect the reference system used by each sensory modality. Head-pointing and rod-pointing tasks were performed in which subjects 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 supravestibular thresholds down a linear track. The motor-kinesthetic system was stimulated by having subjects actively walk along the track. A helmet-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 optical flow field. Subjects could be positioned at any orientation relative to the heading, and heading judgments were obtained using unimodal visual, vestibular, or walking cues, or combined visual-vestibular and visual-walking cues. Vision alone resulted in reasonably precise and accurate head-pointing judgments (0.3° constant errors, 2.9° variable errors), but not rod-pointing judgments (3.5° constant errors, 5.9° variable errors). Concordant visual-walking stimulation slightly decreased the variable errors and reduced constant pointing errors to close to zero, while head-pointing errors were unaffected. Concordant visual-vestibular stimulation did not facilitate either 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 comparable to those obtained in the visual condition. During active self-motion, subjects made large headpointing undershoots when visual information 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 toward or move in the direction of self-motion.