Amplitude of human head movements associated with horizontal saccades

Human saccades may or may not be associated with head movements. To date, l ittle attention has been devoted to the mechanisms determining head movemen t recruitment and scaling. Normal human subjects made horizontal, centrifug al saccades along an encircling array of light-emitting diodes. Measurement s of gaze, bead, and eye-in-head angle were made at the conclusion of the h ead movement (or at the end of the eye movement in eye-only saccades). We f ound that head movement amplitude (Delta H) related in a simple fashion to the eye eccentricity that would have resulted if the gaze shift had been pe rformed without a head movement. Plots of Delta H vs this predicted eye ecc entricity (E-PRED) had a central flat region in which gaze shifts were unac companied by head movements (the eye-only range) and two flanking lobes in which Delta H was a linear function of E-PRED (the eye-head ranges). Delta H correlated with E-PRED better than with gaze shift amplitude, as would be expected if head movements were controlled so as to keep eye eccentricity within a particular range. Head movement tendencies were quantified by the width of the eye-only range, the slope of the eye-head range, and the width of the region within which the eye was likely to be found at the conclusio n of the completed gaze-shifting behavior (the customary ocular motor range ). The measures ranged widely in these normal subjects: 35.8+/-31.9 degrees for the eye-only range (mean+/-SD), 0.77+/-0.16 for the slope of the eye-h ead range, and 44.0+/-23.8 degrees for the customary ocular motor range. Ye t for a given subject, the measurements were reproducible across experiment al sessions, with the customary ocular motor range being the most consisten t measure of the three. The form of the Delta H vs E-PRED plots suggests th at the neural circuitry underlying eye-head coordination carries out two di stinct functions - gating the head movement and scaling the head movement. The reason for the large intersubject variability of head movement tendenci es is unknown. It does not parallel intersubject differences in full-scale eye (in orbit) range or full-scale neck range.