Gaze displacement and inter-segmental coordination during large whole body voluntary rotations.

Displacements of the visual axis and multi-segmental (eye-to-foot) coordination in the yaw plane were studied in ten human subjects (Ss) during voluntary reorientations to illuminated targets of eccentricities up to 180 degrees . We also investigated how knowledge of target location modifies the movement pattern. Eccentric targets (outbound trials) elicited eye, head, trunk and foot movements at latencies ca. 0.5, 0.6, 0.7 and 1.1 s, respectively. Knowledge of target location (return trials) reduced latencies for foot and trunk (but not eye and head) thus eye, head and trunk moved more en bloc. In most trials, the initial gaze shift fell short of the target and more than 50% of the visual angle was covered by the sum of vestibular nystagmic fast phases and head-in-space displacement, until target fixation. This indicates that during large gaze shifts the 'anticompensatory' role of the vestibulo-ocular reflex in target acquisition is prominent. During some predictable trials Ss acquired targets with a single large gaze shift, shortening target acquisition time by more than 200 ms. In these, gaze velocity (trunk-in-space + head-on-trunk + eye-in-orbit) remained often fairly constant for durations of up to 500 ms, suggesting that gaze velocity is a controlled parameter. Such pattern occurred during trunk mobilization, thus eye velocity co-varied with head-in-space rather than head-on-trunk velocity. Foot rotations were stereotyped and of constant frequency, suggesting they are generated by locomotor pattern generators. However, knowledge of target location reduced foot latencies indicating that local and supraspinal mechanisms interact for foot control. We propose that a single controller is responsible for the coupling of the multiple body segments and gaze velocity control during gaze shifts.