Efference copy is an internal copy of a motor innervation. In the oculomotor system it provides the only extraretinal signal about eye position that is available without delay, and it is shown to be the most important extraretinal source of information for perceptual localization and motor activity. Efference copy accompanies all voluntary eye movements and some involuntary ones, including pursuits, saccades, and the fast phases of vestibular and optokinetic nystagmus. Not all eye movements are accompanied by an efference copy; its presence is determined by a movement's function, not it dynamics. Because the gain of the efference copy mechanism is less than 1, and it does not take account of oculomotor delays and kinematics, it is supplemented by other mechanisms in achieving space constancy. It functions differently for perception and for visually guided behavior. There is only one efference copy for both eyes, reflecting Hering's law, and it is subject to adaptation.

Adaptation, Physiological↗

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Direction constancy in rapidly refreshed video displays.

We studied spatial orientation across saccadic eye movements by adding a disturbance to the system that achieves space constancy. A small target was flickered between 33 and 960 Hz, and it was displaced during saccadic eye movements. The threshold for detecting its displacement was measured. Sensitivity to displacement was almost twice as great when the target was jumped in the direction opposite the eye movement as when it was jumped in the same direction. This would be expected from a partial breakdown of space constancy: The world should seem to move in the direction opposite to an eye movement (when the eyes move to the right, for example, the world would appear to move to the left). Sensitivity to displacement was greater at lower flicker rates. The results imply that both masking and extraretinal signals are important in suppressing the detectability of target displacements during saccades, and that flicker on video display terminal may distort space perception. These and other results lead to a recommendation that video terminals be refreshed at least 120 times/s.

Computer Terminals↗