Retinal micromovements, the visual line, and Donders' Law

PURPOSE: To report the relationship of the retinal micromovements to the vi sual line and to confirm the validity of Donders' Law, METHODS: Two video cameras suspended from a headband were used to record ey e (video-oculography) and head movements, Eye positions in held gaze and fo llowing various trajectories to a target were recorded in five normal, youn g subjects. The videotapes were analyzed off-line using a computer algorith m. RESULTS: Retinal micromovements cause the visual line to trace a zigzag pat hway across the foveola, which has an approximate diameter of 350 mu m (abo ut 2 degrees). The mean micromovement was about 10 mu m in 33.3 msec, The c umulative effect of successive micromovements may move the visual line acro ss the foveola from edge to edge depending on the elapsed time, When the vi sual line reaches the edge of the foveola it changes its direction. When th e eye resets to the same target by different trajectories, the visual line may alight up to about 350 mu m from its original location anywhere within the foveola, CONCLUSIONS: Donders' Law is upheld because for each direction of gaze, and regardless of the trajectory used to reach that direction of gaze, the ret ina has a constant orientation to an index head plane at any given moment i n time. Failure to consider that the micromovements cause a shift in the po sition of the visual line within the foveola may account for the exceptions to Donders' Law found by contemporary researchers using invasive recording techniques. (Am J Ophthalmol 2000;129:224-234, (C) 2000 by Elsevier Scienc e Inc, All rights reserved.)