Eye Movements: Behavioral

Movements of the eyes are crucial for being able to see a clear and sharp image of the world. Understanding why this is the case requires considering some facts about vision. An optical image of the visual environment is formed on the retina of each eye, much like the image in a digital photographic camera. Because of properties of both the retinal receptors and the brain, the quality of human vision is inhomogeneous across space. Visual resolution is best in a small, central portion of the retina (the fovea), and falls steeply toward the periphery. Movements of the eyes are used to bring images of interesting objects to the fovea, where they can be seen with optimal clarity. Movements of the eye are also responsible for ensuring that the selected images do not leave the fovea, or move across the retina too rapidly, each time the head moves.

These functions are carried out by three main types of eye movements:

• 1.
  Compensatory eye movements. Humans are always moving their heads. The head is never completely stationary and moves to some extent even when we try to hold as still as possible. Compensatory eye movements, approximately equal in size and opposite in direction to the head movements, are generated continuously to counteract these head movements and keep the image relatively stationary on the retina. Compensatory eye movements are largely automatic reflexes, controlled by evolutionary old systems. These movements are functionally comparable to the image stabilization systems, built into many modern cameras, that enable sharp snapshots despite unsteady hands.
• 2.
  Saccadic eye movements. Saccades are rapid, stepwise rotations of the eye that take the point of gaze from one region of interest to the next. Saccades are the principal way of bringing images of selected objects to the fovea, where visual acuity is best. In most everyday life tasks, people make saccades in sequence from one selected object to another, allowing gaze to pause briefly at different portions of a visual scene. These brief pauses between saccades are sometimes referred to as fixations. Figure 1 shows an example of a sequence of saccadic eye movements made while counting a set of dots. The upper portion of the figure shows the position of the line of sight superimposed on the display of dots. The lower portion shows the identical pattern of eye movements in a different format, namely, as horizontal and vertical eye positions as a function of time. The rapid shifts in eye position are the saccades. Notice that the pauses between the saccades last only about one half to one third of a second.
• 3.
  Vergence eye movements. The visual fields of the two eyes overlap, providing a basis for binocular and stereoscopic (three-dimensional [3-D]) vision. Vergence eye movements are movements of [Page 422]the eyes relative to each other that control the spatial registration (correspondence) of the two retinal images. Static vergence keeps the two eyes aligned, and dynamic vergence adjusts the angle between the lines of sight during binocular shifts of gaze between objects at different distances.

Figure 1 Sequence of Saccadic Eye Moments

Source: Wilder, J. D., Kowler, E., Schnitzer, B. S. Gersch, T. M., & Dosher, B. A. (in press). Attention during active visual tasks: Counting, pointing or simply looking. Vision Research.

Notes: (Top) Eye movements while counting dots. The line of sight began near the center of the display. The first saccade was made down and to the right. The next five saccades stepped across to the left, followed by four more saccades in the upper portion of the display. (Bottom) Horizontal and vertical eye positions over time. The rapid shifts in position are the saccades.

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