Neural analysis of visual information during locomotion.

For most vertebrate species, vision is critical during locomotion. In this review, we survey what is known about neural mechanisms that might be involved in this visual analysis. Problems that such mechanisms are likely to solve include: (1) determination of heading (the discrepancy between an observer's direction of motion and direction of gaze); (2) detection of course changes; (3) setting of courses using goals and landmarks; (4) obstacle avoidance; (5) accurate foot placement. To understand how the visual system handles these tasks, we must first consider what neurons 'see' during locomotion: this is determined both by an observer's motion and by his gaze. We then review response properties of neurons in the cortical 'motion pathways' of the monkey and cat as they relate to problems encountered during locomotion. The most studied areas are the medial superior temporal area in the monkey, which has been linked to heading determination, and the lateral suprasylvian area in the cat, where many cells are sensitive to motion in depth, and some are selective for optic flow patterns generated during locomotion. A few subcortical populations have also been linked to visual analysis during locomotion. Most notable are cells in the pigeon's nucleus rotundus: these respond selectively to looming stimuli, some firing at a specific time before the stimulus collides with the bird. Another intriguing population is in the cat's visual pontine nucleus, where cells respond to large displays suggestive of optic flow during locomotion.