Rapid visuo-motor processes drive the leg regardless of balance constraints

When reaching for an object that unexpectedly moves, the hand is 'magnetically' drawn toward the object. This occurs even before the subject perceives object motion [1–3], possibly through sub-cortical visuo-motor processes [4]. Whether a similar process exists for the lower limb is open to question. The evolution of a rapid visuo-motor process has obvious advantages for the hand, for example for catching prey, but less obvious advantage for the foot. Rapid visual driving of the foot may even be hazardous because of the need to maintain balance: this is normally maintained during a step by a pre-step predictive throw of the body which is tightly coupled to intended future foot placement [5,6]. Visually driven, mid-step deviations of the foot would upset this coupling and threaten balance. We ask whether the foot, like the hand, is automatically and rapidly driven by vision, and if so whether the process is suppressed by the balance constraint of stepping. To investigate visual control of foot trajectory, we used a target-jumping paradigm similar to that used previously for the hand. An initial target consisted of an illuminated rectangle (14 x 21 cm), the center of which was 16 cm directly in front of the anterior border of the foot. There were initial targets for both left and right feet (28 cm between centers). One of these was lit at the start of each trial. During a 'reach' condition, six subjects (4 males, 2 females, mean age 32 yrs) reached with a foot in an attempt to place it on the initial target, while taking weight through their hands using handrails. In 1/4 of 96 trials, selected at random, the target was made to jump either medially (p = 0.125) or laterally (p = 0.125) at the point of foot-off, by simultaneously extinguishing the initial target and illuminating a new target 21 cm to the left or right. Subjects were instructed to attempt to follow the target and place their foot on it when a jump occurred. Handrails enabled a leg to be lifted indefinitely without incurring a fall, so the balance constraints normally present during upright stance were minimised. These constraints were reinstated during a separate 'step' condition when the same task was performed without handrails while stepping forward onto the initial target with the leading foot. The position of the trailing foot was not controlled, although subjects were instructed to finish the step with …