The contribution of microsaccades and drifts in the maintenance of binocular steady fixation.

BACKGROUND: The eye performs three types of eye movements during fixation: fast microsaccades are interrupted by slow drift movements, and tremor is superimposed on the drifts. The contribution of the microsaccades and drifts in maintaining fixation has been discussed since the late 1950s. Initially, microsaccades were thought to correct the misalignment from the optimal fixation locus induced by the drift movements, a theory still postulated in more recent work. The present study aimed to uncover to what extent each fixation movement contributes to maintain steady binocular fixation.
METHOD: Binocular fixation during a 40-s fixation task was recorded using an infrared recording technique for ten normal test persons. Start and end point of each microsaccade and drift were superimposed on a fixation map, and the distance to the preferred retinal location of fixation (PRL) was measured.
RESULTS: It was found that 32.6% of the microsaccades corrected the previous drift movement towards the PRL, whereas 53.1% of the drifts corrected the endpoint of the previous microsaccade towards the PRL. The overall mean post-microsaccadic and mean post-drift distance to the PRL for the ten normal test persons were 0.46 degrees and 0.41 degrees , respectively; the difference was not statistically significant. Interindividually, the mean post-microsaccadic distance to the PRL ranged between 0.21 degrees and 0.91 degrees and the mean post-drift distance to the PRL ranged between 0.20 degrees and 0.72 degrees .
CONCLUSION: Neither the endpoints of the microsaccades nor the drifts bring the visual line to coincide with the centre of the PRL. Consequently, it must be the eye movements performed during the drifts ("slow control") that keep the visual line in the centre of the foveola.