Kinematic redundancy and variance of eye, head and trunk displacements during large horizontal gaze reorientations in standing humans.

Shifting the direction of the line of sight in everyday life often involves rotations not only of the eyes and head but also of the trunk. Here, we investigated covariation patterns of eye-in-orbit, head-on-trunk and trunk-in-space angular horizontal displacements during whole-body rotations to targets of up to 180 degrees eccentricity performed by standing healthy human subjects. The spatial covariation was quantified statistically across various behavioral task conditions (unpredictable, memory driven predictable, visual feedback) and constraints (accuracy) by principal components (PC) analysis. Overall, the combined movement was stereotyped such that the first two PCs accounted for essentially the whole data variance of combined gaze transfers up to about 400 ms, suggesting that the three mechanical degrees of freedom under consideration are reduced to two kinematic degrees of freedom. Moreover, quantification of segment velocity variability across repetitions showed that velocities of eye-in-space and head-in-space (i.e. 'end-point' velocity) were less variable than those of the elemental variables composing them. In contrast, three statistically significant PCs accounted for the covariation of the three segments during presumably vestibularly mediated nystagmic transfers, suggesting control by a separate driving circuit. We conclude that progression of the line of sight is initially stereotypic and fulfills criteria defining a motor synergy.