Gaze (eye-in-space) velocity-duration and velocity-amplitude curves were prepared for head-fixed and head-free gaze shifts in the rhesus monkey with an emphasis on large amplitudes. These plots revealed the presence of two distinct gaze reorientation mechanisms, one used when the gaze shift was small (less than 20 degrees) and the other utilized for large coordinated gaze shifts when the head was free. When head-free and head-fixed saccadic gaze shifts were compared in the same animal, no differences in the metrics were found for amplitudes less than 20 degrees. However, for large gaze shifts where contribution of the head to the change in gaze angle was considerable, head-free saccades were found to exhibit lower peak gaze velocities and greater durations than those recorded with the head-fixed paradigm. In order to differentiate between the eye saccades and combined saccadic eye-head gaze shifts, the latter have been termed gaze saccades. Change in head position and change in eye position were both measured during the actual gaze shift and were plotted against the gaze-shift amplitude to determine whether the head movement contributed significantly to the change in gaze angle. The results indicate that below 20 degrees the gaze shift is accomplished almost exclusively with the eyes and the head moves very little; however, for larger saccades, the head contributes approximately 80% of the total change in gaze angle with the eyes contributing only approximately 20%. Large saccadic eye-head gaze shifts do not exhibit 'bell-shaped' velocity profiles as do smaller head-fixed saccades; instead, gaze accelerates to reach a peak velocity after approximately 30-40 ms. This velocity is then maintained for the duration of the gaze shift. Close scrutiny of the fine structure of the velocity profiles of the eye, head, and gaze channels indicates that during gaze saccades, the eye and head movement motor programs interact to maintain gaze velocity nearly constant, unaffected by changes in head velocity. Previous authors had stated that when velocity-duration plots are obtained for oblique saccades of constant amplitude, the resulting points could be fitted with a hyperbolic function. These results were confirmed for head-free gaze saccades and extended to larger amplitudes. When an oblique saccade is made, the smaller component is stretched in duration to match the duration of the larger component. However, as the gaze shift becomes large (greater than 40 degrees), the relationship becomes more complex.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanisms of eye-head coordination were studied in two alert juvenile rhesus monkeys. Animals were trained to follow a target light to obtain a water reward and the combined eye-head gaze shifts in response to target steps with a variably sized horizontal components were studied. During a certain random portion of the gaze shifts, a torque motor was used to perturb the head to investigate the operational state of the vestibuloocular reflex (VOR) during the saccadic gaze shift. The effects of perturbing the head were assessed during five different conditions: horizontal target steps ranging from 10 to 80 degrees in amplitude; oblique target steps where the vertical component was larger than the horizontal component; purely vertical target steps 10-40 degrees in amplitude; both horizontal and oblique target steps delivered while the animals' saccades had been slowed by the use of diazepam; and large spontaneous gaze shifts in response to both sounds and visual stimuli. Comparison of perturbed and unperturbed large-amplitude (greater than 40 degrees) gaze shifts indicate that the VOR is turned off for most of the duration of the movement. Nonetheless, there is an apparent interaction between the saccadic eye movement and the head movement, thus, as the head velocity increases, the eye velocity decreases so that gaze velocity remains nearly constant throughout the gaze shift. Since the VOR is turned off when this interaction occurs, it must represent an interaction between the actual eye and head movement motor programs themselves. Although the results were not quite as clear for small saccades (less than 20 degrees), experiments on animals whose saccades had been slowed either by the use of diazepam or by combining a small horizontal component with a large vertical component indicate that the VOR is left on during these smaller gaze shifts. During quite small gaze shifts (less than 10 degrees), the VOR is clearly functioning; however, as the size of the gaze shift is increased, this becomes less clear, and there appears to be a region where the VOR operates with a gain substantially less than normal before it enters the large gaze shift region where the VOR is turned off entirely.(ABSTRACT TRUNCATED AT 400 WORDS)