Voluntary control of smooth eye movements and their velocity.

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IT has been shown recently that the eyes can make sustained smooth eye movements indistinguishable from normal pursuit movements in the absence of a moving visual stimulus if an after-image is tracked1–3. This experimental procedure avoids the objections which can be raised against previous demonstrations of smooth eye movements without moving targets which have involved either alternative sources of information about movement4, or closing the eyes5, or changes in subjects' level of arousal6. But it has not yet been shown that smooth eye movements can be made voluntarily, and it is possible that they can be executed by alert, open-eyed subjects only if the saccadic system is inhibited by a visual target. This is a report on one subject, a 20-yr-old girl undergraduate, C. G., who can make smooth eye movements at will in the dark and can change the velocity of these movements on demand. These eye movements are indistinguishable from normal pursuit, do not resemble the eye movements made by the same subject when tracking an after-image, and seem, therefore, to represent specific voluntary control of the smooth pursuit eye movement system.