The stability of human eye orientation during visual fixation and imagined fixation in three dimensions

Objective: The stabilization of eye movements during voluntary fixation is believed to depend upon the visual feed back, since gaze becomes less stable in the darkness. The temporal variability of human eye orientation in both the horizontal and vertical direction is typically no more than a few minutes of arc. Ocular stability in terms of torsion has, however, been less understood. Methods: In this study, the stability of gaze in the three dimensions (horizontal, vertical, and torsional) was investigated using a video image analysis system. In ten healthy subjects, eye movements were recorded after fixating the eye on a target in the light and also in the dark (imaginary target). The standard deviations of the eye movements in the three dimensions were calculated during fixation. Results: In the light, the horizontal and vertical eye movements exhibited an alternating sequence of a slow drift and resetting microsaccades, as previously reported. The torsional eye movements, however, showed no such drifts and resetting. In the dark, the horizontal and vertical eye movements exhibited about 2–3 times larger value of the standard deviation than that in the light. In contrast to this, the difference in the standard deviation between the light and dark was not so prominent in case of the torsional eye movement as compared to those of horizontal and vertical. Conclusion: These results support the idea that information from the peripheral retina does not significantly influence the feed back system for fixating the target in the light.