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Six Neuronal Control Systems Keep the Eyes on Target
An Active Fixation System Keeps the Fovea on a Stationary Target
The Saccadic System Points the Fovea Toward Objects of Interest
The Smooth-Pursuit System Keeps Moving Targets on the Fovea
The Vergence System Aligns the Eyes to Look at Targets at Different Depths
The Eye Is Moved by the Six Extraocular Muscles
Eye Movements Rotate the Eye in the Orbit
The Six Extraocular Muscles Form Three Agonist–Antagonist Pairs
Movements of the Two Eyes Are Coordinated
The Extraocular Muscles Are Controlled by Three Cranial Nerves
Extraocular Motor Neurons Encode Eye Position and Velocity
The Motor Circuits for Saccades Lie in the Brain Stem
Horizontal Saccades Are Generated in the Pontine Reticular Formation
Vertical Saccades Are Generated in the Mesencephalic Reticular Formation
Brain Stem Lesions Result in Characteristic Deficits in Eye Movements
Saccades Are Controlled by the Cerebral Cortex Through the Superior Colliculus
The Superior Colliculus Integrates Visual and Motor Information into Oculomotor Signals to the Brain Stem
The Rostral Superior Colliculus Facilitates Visual Fixation
The Basal Ganglia Inhibit the Superior Colliculus
Two Regions of Cerebral Cortex Control the Superior Colliculus
The Control of Saccades Can Be Modified by Experience
Smooth Pursuit Involves the Cerebral Cortex, Cerebellum, and Pons
Some Gaze Shifts Require Coordinated Head and Eye Movements
An Overall View
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In preceding chapters we learned about the motor systems that control the movements of the body in space. In this and the next two chapters we consider the motor systems concerned with gaze, balance, and posture. As we explore the world around us, these motor systems act to stabilize our body, particularly our eyes. In examining these motor systems we shall be concerned with how these systems have resolved three biological challenges to knowing where we are in space: How do we visually explore our environment quickly and efficiently? How do we compensate for planned and unplanned movements of the head? How do we stay upright?
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The gaze system stabilizes the image of an object on the retina when the object moves in the world or the head moves and keeps the eyes still when the image remains stationary. It has two components: the oculomotor system and the head-movement system. The oculomotor system moves the eyes in the orbits; the head-movement system moves the eye sockets.
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In this chapter we describe the oculomotor system and how visual information guides eye movements. It is one of the simplest motor systems, requiring the coordination of only the 12 muscles that move the two eyes. In humans and primates the main job of the oculomotor system is to control the position of the fovea, the central, most sensitive part of the retina. The fovea is less than 1 mm in diameter and covers a tiny fraction of the visual field. When we want to examine an object, we must move its image onto the fovea.