Two types of rapid eye movements, saccades and quick phases (QP), share the same neural circuits, but have different purposes. Quick phases reset the eye in the orbit, avoiding an attempt to move the eyes beyond the oculomotor range. In general, QP move the eyes into the direction one is turning. This is useful because it brings the world toward which one is turning into view more quickly. Algorithms for deciding when to begin a QP, and how large it should be, move the eyes, on average, into the direction of turning. This makes it reasonable to assume that the afoveate QP system evolved to provide commands in a head-coordinate system. The saccadic system evolved in foveate animals on top of this system, and thus it is reasonable to assume it also provides commands in head coordinates.
Keywords: Algorithm; Nystagmus; Quick phase; Vestibulo-ocular reflex.
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