During this delay, the position of the target with respect to the fovea is computed (that is, how far the eye has to move), and the difference between the initial and intended position, or “ motor error” (see Chapter 19), is converted into a motor command that activates the extraocular muscles to move the eyes the correct distance in the appropriate direction. After the onset of a target for a saccade (in this example, the stimulus was the movement of an already fixated target), it takes about 200 ms for eye movement to begin. The time course of a saccadic eye movement is shown in Figure 20.4. The rapid eye movements that occur during an important phase of sleep (see Chapter 28) are also saccades. Saccades can be elicited voluntarily, but occur reflexively whenever the eyes are open, even when fixated on a target (see Box A). They range in amplitude from the small movements made while reading, for example, to the much larger movements made while gazing around a room. Saccades are rapid, ballistic movements of the eyes that abruptly change the point of fixation.

The functions of each type of eye movement are introduced here in subsequent sections, the neural circuitry responsible for three of these types of movements is presented in more detail (see Chapters 14 and 19 for further discussion of neural circuitry underlying vestibulo-ocular movements). There are four basic types of eye movements: saccades, smooth pursuit movements, vergence movements, and vestibulo-ocular movements.

The body keeps the score target