Horizontal step-ramp stimuli were used to examine gaze-, eye-, and headmovement dynamics during head-unrestrained pursuit in two rhesus monkeys. In a first series of experiments, we characterized and compared head-restrained (HR) and -unrestrained (HU) pursuit responses to unpredictable, nonperiodic, constant velocity (20-80°/s) stimuli. When the head was free to move, both monkeys used a combination of eye and head motion to initially fixate and then pursue the target. The pursuit responses (i.e., gaze responses) were highly stereotyped and nearly identical among the HR and HU conditions for a given step-ramp stimulus. In the HU condition, initial eye and initial head acceleration tended to increase as a function of target velocity but did not vary systematically with initial target eccentricity. In a second series of experiments, step-ramp stimuli (40°/s) were presented, and, ∼125 ms after pursuit onset, a constant retinal velocity error (RVE) was imposed for a duration of 300 ms. In each monkey, HR and HU gaze velocity was similarly affected by stabilizing the target with respect to the monkey's fovea (i.e., RVE = 0°/s) and by moving the target with constant retinal velocity errors (i.e., RVE = ±10°/s). In the HU condition, changes in both eye and head velocity trajectories contributed to the observed gaze velocity responses to imposed RVEs. We conclude that eye and head movements are not independently controlled during HU pursuit but rather are controlled, at least in part, by a shared upstream controller within the pursuit pathways.
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