The stability of human eye orientation during visual fixation.

Using the magnetic search coil technique, gaze stability in the horizontal, vertical and torsional planes was measured binocularly in human subjects during visual fixation. Horizontal and vertical eye rotations exhibited a mixture of slow drifts and resetting microsaccades yielding an average standard deviation of 0.11 and 0.10 deg, respectively. In contrast, torsional rotations showed unsystematic smooth drifts with fewer saccades yielding an average standard deviation of 0.18 deg. The lower precision of gaze control in the torsional plane may reflect (i) a discrepancy between the encoding of retinal images in two dimensions but of ocular motor control signals in three dimensions, and (ii) the visual consequences of ocular drifts in the torsional plane, which differ from those in the horizontal and vertical planes.