When an observer with normal eyes is instructed to shift his gaze suddenly from one fixated point in the visual field to a second, the time characteristics of the movements of the eyes depend upon the positions of the two points within the field. Dodge1 pointed out that if the two points are confined to the horizontal plane which includes the centers of rotation of the two eyes then two basic types of movements can be differentiated according to the velocity of the movement:
If the two points are confined approximately to an objective fronto-parallel plane, then the movement is very rapid (Fig. 1). The velocity of this movement may be higher than 400 degrees per second and increases as the separation of the fixated points increases. Such movements are called saccadic movements. A detailed analysis of some of their characteristics has been recently made by Westheimer.2
ALPERN M, WOLTER JR. The Relation of Horizontal Saccadic and Vergence Movements. AMA Arch Ophthalmol. 1956;56(5):685–690. doi:10.1001/archopht.1956.00930040693006
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