A theory is presented of how a driver might visually control his braking. A mathematical analysis of the changing optic array at the driver's eye indicates that the simplest type of visual information, which would be sufficient for controlling braking and would also be likely to be easily picked up by the driver, is information about time-to-collision, rather than information about distance, speed, or acceleration/deceleration. It is shown how the driver could, in principle, use visual information about time-to-collision in registering when he is on a collision course, in judging when to start braking, and in controlling his ongoing braking. Implications of the theory for safe speeds and safe following distances are discussed, taking into account visual angular velocity detection thresholds, and some suggestions are made as to how safety on the roads might be improved.

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Human infants learning to stand use visual proprioceptive infonnation about body sway in order to maintain stable posture. Moreover, the visual proprioceptive infonnation is more potent than the nonvisual. This is shown by an experiment in which infants were caused to sway and even fall forward 01" backward in response to appropriate visual stimulation.According to the classical view formulated by Sherrington (1906), the receptor systems of the body may be classified as exteroceptors, proprioceptors, or interoceptors. It is assumed that each receptor system subserves a unique function, exteroceptive, proprioceptive, or interoceptive.J. J. Gibson (1966) has questioned this assumption. He argues that if exteroception is defined to be the obtaining of information about events extrinsic to the organism, and proprioception the obtaining of information by the organism about its own actions, then vision, in particular, is not only exteroceptive, as is classically assumed, but is also proprioceptive.The proprioceptive function of vision seems apparent enough in driving a vehicle; the driver clearly uses visual information about his and the vehicle's movement to guide the vehicle. Does vision also function proprioceptively in the control of more basic activities? The activity we chose to study is standing.Standing involves continuous compensatory adjustments of the musculature. It is a process of feedback control. Any sway of the body away from the vertical has to be registered and compensatory muscular adjustments made else balance is lost.The classical view is that the information about body sway that is used in standing comes from receptors ("proprioceptors," in classical terminology) in the vestibular canals and in the joints and muscles, particularly of the ankles and hips (Eldred, 1960). We may refer to these as mechanoreceptors, since they are responsive to mechanical force.The question of whether vision too functions proprioceptively in standing does not seem to have been considered. There is, however, some suggestive evidence in the literature. For example, Wood (1895) reported that people standing on a stationary "haunted swing" while the surrounding room was rotated about a

The way in which gait is regulated to meet the demands of the terrain was investigated by analyzing the movements of skilled long jumpers during their run-up to the takeoff board. The analysis revealed that the run-up consists of two phases: (a) an initial accelerative phase, ending about 6 m from the board, during which athletes attempt to produce a stereotyped stride pattern; and (b) a zeroing-in phase, during which they adjust their stride pattern to eliminate error that has accrued. Further analysis revealed that the athletes were regulating a single gait parameter-the vertical impulse, or lift, of their steps. During the stereotyped approach phase they tried to maintain a constant impulse, thereby keeping flight and swing-through time constant. During the zeroing-in phase, they adjusted their flight times (and hence their stride lengths) by regulating the impulse of their steps. The essence of their skill thus appears to lie in the precise adjustment of the impulse toward the end of the run-up. The nature of the visual information that might be used to make the adjustments is discussed.Although research has increased our understanding of the biomechanics of locomotion and of some of the underlying neurophysiological mechanisms (Alexander &

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