Control of eye-head coordination during orienting gaze shifts.

Combined eye and head displacements are routinely used to orient the visual axis rapidly (gaze). Humans can use a wide variety of head movement strategies. However, in the cat, comparatively limited eye motility forces a more routine and stereotyped use of head motion. Nevertheless, the same general principles of gaze control may be applicable to humans, rhesus monkeys and cats. The gaze control system can be modeled using a feedback system in which an internally created, instantaneous, gaze motor error signal--equivalent to the distance between the target and the gaze position at that time--is used to drive both eye and head motor circuits. The visual axis is moved until this error equals zero. Recent studies suggest that the superior colliculus of the cat provides brainstem eye and head motor circuits with the gaze motor error signal; such studies have led to speculation that information on ongoing gaze motion is fed back to the superior colliculus. It is still uncertain whether comparable collicular and brainstem neuronal mechanisms control gaze in the monkey.