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 saccad ic shifts in gaze. The reasons for this variation are unknown. Because comb ined eye-head movements are associated with a recentering of the eyes in th e orbits, humans who are "head movers" tend to maintain the eyes within a n arrower range than do non-head movers. We explored the possibility that var iations in the ability to control eye movements at eccentric positions lead to variations in customary ocular motor range and, by extension, explain t he variations in head-movement tendencies. We studied ten normal adults. In each, we measured the full-scale ocular motor range and customary ocular m otor range (the eccentricity range within which the eye was found at the co nclusion of eye- or eye-head saccades). We also determined the eye-only ran ge, the orbital range within which the probability of a head movement accom panying a gaze shift was low. Customary, eye-only, and full-scale ranges sp anned (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 severa l ocular motor behaviors as functions of eye eccentricity. The stable fixat ion range, defined by the range over which drift velocities were below 1 de grees /s, spanned 81.1+/-11.2 degrees in the light and 69.5+/-22.5 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 eccentricit y spanned 66.3+/-7.1 degrees and 69.0+/-10.0 degrees respectively. Small ce ntrifugal saccades (5-10 degrees) tended to become either slowed or hypomet ric 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 initi al saccade placed the fovea upon the target averaged 35.5+/-10.7 degrees fo r 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 rang es were either unrelated or inconsistently related to full-scale range, ind icating that the deterioration of eye movements with increasing ocular ecce ntricity is not a simple consequence of the eyes encountering the limits of their excursion. None of the kinematic ranges correlated positively with c ustomary or eye-only range. Thus, while head movements may be orchestrated so as to maintain the eyes within a desired range, that range land thus hea d movement tendencies) is not predicated upon the range of ocular eccentric ity over which eye movements are accurately controlled.