Eye-head coordination and the variation of eye-movement accuracy with orbital eccentricity.

Different humans vary widely in the tendency to move the head during saccadic shifts in gaze. The reasons for this variation are unknown. Because combined eye-head movements are associated with a recentering of the eyes in the orbits, humans who are "head movers" tend to maintain the eyes within a narrower range than do non-head movers. We explored the possibility that variations in the ability to control eye movements at eccentric positions lead to variations in customary ocular motor range and, by extension, explain the variations in head-movement tendencies. We studied ten normal adults. In each, we measured the full-scale ocular motor range and customary ocular motor range (the eccentricity range within which the eye was found at the conclusion of eye- or eye-head saccades). We also determined the eye-only range, the orbital range within which the probability of a head movement accompanying a gaze shift was low. Customary, eye-only, and full-scale ranges spanned (mean +/-SD) 41.1+/-16.9 degrees, 30.2+/-18.8 degrees, and 92.8+/- 9.1 degrees, respectively. We then assessed variations in kinematics of several ocular motor behaviors as functions of eye eccentricity. The stable fixation range, defined by the range over which drift velocities were below 1 degree/s, spanned 81.1+/-11.2 degrees in the light and 69.5+/-21.5 degrees in the dark. The range over which the gains of the vestibulo-ocular reflex in the light and smooth pursuit approached their values at zero eccentricity spanned 66.3+/-7.1 degrees and 69.0+/-10.0 degrees, respectively. Small centrifugal saccades (5-10 degrees) tended to become either slowed or hypometric with increasing eccentricity. Sensitive to both slowing and hypometria, the ratio of peak gaze velocity to target shift amplitude was flat over a range spanning 65.7+/-14.9 degrees. Finally, the ranges over which the initial saccade placed the fovea upon the target averaged 35.5+/-10.7 degrees for eye-only saccades and 36.6+/-15.0 degrees for eye-head saccades. With the exception of the range of stable fixation in the light, the kinematic ranges were either unrelated or inconsistently related to full-scale range, indicating that the deterioration of eye movements with increasing ocular eccentricity is not a simple consequence of the eyes encountering the limits of their excursion. None of the kinematic ranges correlated positively with customary or eye-only range. Thus, while head movements may be orchestrated so as to maintain the eyes within a desired range, that range (and thus head movement tendencies) is not predicated upon the range of ocular eccentricity over which eye movements are accurately controlled.