Retinal micromovements, the visual line, and Donders' law

To report the relationship of the retinal micromovements to the visual line and to confirm the validity of Donders' Law.

Two video cameras suspended from a headband were used to record eye (video-oculography) and head movements. Eye positions in held gaze and following various trajectories to a target were recorded in five normal, young subjects. The videotapes were analyzed off-line using a computer algorithm.

Retinal micromovements cause the visual line to trace a zigzag pathway across the foveola, which has an approximate diameter of 350 microm (about 2 degrees). The mean micromovement was about 10 microm in 33.3 msec. The cumulative effect of successive micromovements may move the visual line across the foveola from edge to edge depending on the elapsed time. When the visual line reaches the edge of the foveola it changes its direction. When the eye resets to the same target by different trajectories, the visual line may alight up to about 350 microm from its original location anywhere within the foveola.

Donders' Law is upheld because for each direction of gaze, and regardless of the trajectory used to reach that direction of gaze, the retina has a constant orientation to an index head plane at any given moment in time. Failure to consider that the micromovements cause a shift in the position of the visual line within the foveola may account for the exceptions to Donders' Law found by contemporary researchers using invasive recording techniques.