Human gaze stability in the horizontal, vertical and torsional direction during voluntary head movements, evaluated with a three-dimensional scleral induction coil technique.

The stability of gaze in three dimensions (horizontal, vertical and torsion) was measured with a new type of scleral search coil in eight emmetropic observers. Subjects held the head still or oscillated it at 0.16-0.67 Hz (amplitude about 10 deg) in the horizontal, vertical or torsional plane while fixating a point target at optical infinity. Veridical gaze and head coordinates were calculated with full correction for non-linear goniometric relations and for cross-coupling artifacts due to misalignments of the coil on the eye. The amount of gaze instability in the horizontal and vertical direction was virtually identical. With the head still, in either of these directions the mean standard deviation of gaze position (inclusive saccades) was about 7 min arc; mean non-saccadic retinal image speeds were 20-30 min arc/sec. During head oscillation these values increased to about 16 min arc and 1 deg/sec; a mean of about 2.5% of the head motion remained uncorrected by the compensatory eye movements. These findings agree well with our earlier results for the horizontal plane; the effect of the corrections was relatively small because the adventitious cross-coupling of horizontal and vertical to torsional head movements proved to be usually smaller than 10%. However, the corrections were important when head torsion was deliberately produced. Gaze stability in the torsional plane was considerably inferior to that in the horizontal and vertical plane. With the head held still, the mean SD of torsional gaze position was about 17 min arc; mean torsional non-saccadic retinal image speed was about 46 min arc/sec. Gain of the torsional compensatory eye movements was frequency dependent and rose from about 0.26 in static conditions (0 Hz) to about 0.42 at 0.16 Hz and 0.64 at 0.67 Hz. Accordingly, position instability and speed of the retinal image in torsion were about an order of magnitude larger than in the horizontal and vertical direction.