Eccentric eye and head positions in darkness induce deviation from the intended path.

Head and gaze are aligned with the actual path during locomotion. Before a turn is made, gaze changes in the direction of the planned trajectory. We investigated whether eccentric horizontal head and/or eye position without vision causes deviations from the intended straight path. Twenty blindfolded healthy volunteers were asked to walk toward a previously seen target 10 m straight ahead. Various combinations of head and eye positions were tested (eye-in-head gaze straight ahead or 35 degrees left or right with head straight ahead or 70 degrees left or right). Head rotation to the left caused a gait deviation to the right (3.7 degrees ) and head rotation to the right caused a deviation to the left (2.7 degrees ; F(2,40) = 34.966; P < 0.00001). Eye position also showed a tendency to cause gait deviations opposite in direction to gaze, which was, however, not significant. Deviations from the intended straight path were largest with head rotation and eyes straight ahead (gaze 70 degrees off target) or eyes opposite to head rotation (gaze 35 degrees off target). Notably, when lateral eye deviation added to head rotation (gaze 105 degrees off target), i.e., gaze is directed backward, mean deviations decreased (2.3 degrees to the right and 1.2 degrees to the left). Thus, we show that (1) eccentric head positions induce direction-specific gait deviations that are independent of concurrent environmental visual information, and (2) that gait deviations are contraversive to eye-head gaze rather than ipsiversive as reported by others for visually controlled locomotion. The direction of deviation may reflect the compensation of an expected or perceived deviation in the direction of gaze.