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* To whom correspondence should be addressed. E-mail: bhess{at}neurol.unizh.ch.
We have examined the spatiotemporal characteristics of postrotatory eye velocity following roll and pitch off-vertical axis rotations (OVAR). Three rhesus monkeys were placed in one of two orientations on a three-dimensional (3D) turntable: upright (90° roll or pitch OVAR), 45° nose-up (45° roll OVAR), and 45° left ear-down (45° pitch OVAR). Subjects were then rotated at ±60°/s around the naso-occipital or interaural axis and stopped after ten turns, in one of seven final head orientations, each separated by 30°. We found that postrotatory eye velocity showed horizontal-vertical components after roll OVAR and horizontal-torsional components after pitch OVAR that varied systematically as a function of final head orientation. The quantitative analysis suggests that, in contrast to the analogous yaw OVAR paradigm, a system of up to three real, gravity-dependent eigenvectors and eigenvalues determines the spatiotemporal characteristics of the residual eye velocities after roll and pitch OVAR. One of these eigenvectors closely aligned with gravity whereas the other two determined the orientation of the earth horizontal plane. We propose that the spatial characteristics of eye velocity after roll and pitch OVAR follow the physical constraints of stationary orientation in a gravitational field and reflect the brain's best estimate of head-in-space orientation within an internal representation of 3D space.
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