Target motion direction influence on tracking performance and head tracking strategies in head-unrestrained conditions.

We examined subjects' behavior when they tracked periodic oscillating targets moving along a randomly oriented ramp with the head free to move. This study focuses on the effect of target motion direction on pursuit performance and on head tracking strategies used by human subjects to coordinate eye and head movements. Our analyses revealed that the gaze tracking gain was modulated by both target oscillation frequency and target motion direction. We found that pursuit gain was modulated by the target motion direction: vertical pursuit being less accurate than horizontal pursuit. While gaze tracking was sensitive to target frequency and orientation, head behavior was less modulated by a change of target frequency than by a change of target motion direction. Additionally, subjects had two main strategies for moving their head: They oriented their head to favor rotations around either the head dorsoventral (target motion directions <20 deg) or mediolateral axis (target motion directions >70 deg). In between, the subjects did not choose a consistent rotation axis for identical target motion directions.