NEUROLOGISTS have traditionally analysed extraocular motility in terms
of deficiencies of directional gaze or of disordered ocular reflexes. It should
be apparent that an analysis of the movements themselves might also be
of value in understanding the mechanisms regulating extraocular behaviour.
In 1903, Dodge defined the two basic ways in which the eyes move. The
first is by discrete and rapid jumps (up to 600°/sec.) called saccades or
version movements; the second is by a slower but continuous progression
(up to 45°/sec.) called following or tracking movements. The two types of
eye movements are employed for different purposes. Saccades serve to
bring the fovea promptly on to an eccentric point in the visual field, whereas
smooth movements enable the fovea to be maintained on a continuously
moving object. Thus, saccades can be considered as a response to position,
and following movements as a response to velocity (Westheimer, 1954;
Rashbass, 1961). Evidence from a wide variety of studies in normal
individuals indicates that there may be a distinct neural mechanism regulating
each of these eye movements and that normal ocular motility depends
upon the integration of the two systems (Dodge, Travis and Fox, 1930;
Rashbass, 1961; Young and Stark, 1963; Robinson, 1965).