Influence of postural constraints on eye and head latency during voluntary rotations.

Redirecting gaze towards new targets often requires not only eye movements, but also synergistic rotations of the head, trunk and feet. This study investigates the influence of postural constraints on eye and head latency during voluntary refixations in the horizontal plane in 14 normal subjects. Three postural conditions were presented, (1) sitting in a chair using only eye and head movements, (2) standing without feet movements and (3) standing with feet movement. Head-eye reorientations towards eccentric un-predictable locations were performed towards ±45° and ±90° targets and back towards a central, spatially predictable target. Results showed that postural constraints affected eye latency but only when subjects knew the future location of the target (recentering "return" trials). Specifically, relatively longer eye latencies were observed when subjects had to turn their feet back towards the predictable central target. These findings suggest that the additional CNS processing required to reduce degrees of freedom during predictive motion introduces delays to the eye movement in order to efficiently assemble the components of a new motor synergy.