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There is a strong interaction between visual and motor processes. Not only does vision serve the motor function, but the motor processes themselves are essential to vision. Without an ability to control our eyes, to shift them left or right, up or down, there would be no sight and no visual information. You direct your attention to a certain part of the environment, and so you orient your eyes.

Activated in some complex, poorly understood way, the eye muscles contract and relax appropriately and the eyes rotate and the body moves in some manner. Mysteriously, each of us views the world from the center of our personal virtual reality, even as it moves around. This is an awesome display of neural computing power! Never mind the fact that we can see at all, that we are conscious! The why of all this is the biggest mystery yet!

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Rotation of each eye is controlled by three pairs of muscles, each pair acting like reins on a horse. With ends attached to the outer surface of the eye, each pair rotates the eye about a unique direction, the rotation occurring by contracting one or the other of the muscles of a pair.

Two of the muscles (called the medial and lateral recti) rotate the eye sideways, i.e., to the left or right. The second pair (called the superior and inferior recti) move the eye upwards or downwards. And the other muscles, viz., the superior and inferior obliques, tend to twist the eye around its line of sight. Somehow we get cues from the rotations. It's possible we keep internal records of them, i.e., that we remember them and use the data to construct a model of the environment. Several other movements in the visual system may also provide information about the world.

You may be aware that your eyes dart around, jumping from here to there. These movements are the so-called saccadic movements of the eye, or saccades. They are rapid ballistic movements, not unlike the shot of a rocket or a bullet, or of a tennis ball, for that matter.

In a ballistic movement, in a given physical environment, the trajectory is determined by:

1. The force applied at the point of propulsion.
2. The direction in which the force acts.

Similarly, the path of a saccade is determined by a mechanism that issues commands to the muscles to swing the eye to a new orientation. Presumably there are no intermediate forces.

Saccadic movements are short-duration or very fast movements once initiated, though they take a bit of time to get started. From R. N. Haber (Information Processing Approaches to Visual Perception), their latency period is on the order of a fifth of a second -- it takes that long just to get started. But once activated, they need only about a tenth of a second to rotate the eye forty degrees. That's about one hundred degrees per second. The angular speeds are even greater for bigger angles, approaching 1000 degrees per second for a ninety-degree shift!

Saccades are common in perception. They occur when something on the periphery of your field of vision catches your attention and you flick your eyes to the object to get a better look. They are occurring now, in fact -- as you complete each line of print on this page, you skip to the beginning of the next line (assuming that's your reading pattern). They can also be involved when, for example, you fall behind visually while tracking a tennis ball or when the ball takes an unexpected bounce; to reposition your line of sight on the ball, you might execute a saccade.

Accommodation is a movement of the eye that provides a cue to depth. As a movement in place, accommodation defines your ability to shift your gaze along your line of sight from an object at one distance to an object at another distance. You focus by changing the curvature of the lens of your eyes, using the ciliary muscles surrounding the eyes.

The reverse occurs when the ciliary muscle is relaxed. The ligament is stretched and tension on the lens is increased. This flattens the lens and increases its focal length, bringing farther objects more into focus. In the so-called relaxed state of accommodation, then, the ciliary muscle is relaxed but the lens is under the greatest tension.

If you focus on an object, the image of the object will be etched on your retinas and the image will be quite sharp, assuming you have fairly normal eyes. However, should the object move and go slightly out of focus, blur circles form around the image and accommodation takes place to restore the focus.

According to Lloyd Kaufman (Sight and Mind, an Introduction to Visual Perception), it is the blur circles that trigger the occurrence of the accommodation of the lens. But they do the job only if your attention is on the object. How this happens is clouded in mystery. It seems, however, that accommodation is only indirectly voluntary. Although you need to give the image your attention, the process is ultimately initiated by events going on in the cerebral cortex. As we just learned, too, the eye can accommodate in two directions: one by relaxing the ciliary muscle, the other by contracting it. So how, in any given case, would the muscle know which action to take?

There are only two possible radial motions: toward and away from the eye. But there are an infinite number of motions at right angles to them, like the many possible spokes of a wheel when looking down the axle. For our purposes, though, we only need to consider the spokes on the horizontal plane, so we have just two motions to contend with, viz. right and left. Since the eyes happen to be tied to one another functionally, the effect is to create four movement pairs. Two of the pairs are called disjunctive, or vengeance, movements. And the other two are called conjugate movements.

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If both eyes are allowed to view the ball, there occurs another source of depth information, stemming from the fact that each eye sees the ball from a slightly different direction, or perspective.

Attention to an object has a way of inducing motor fusion. That is, attention changes the vengeance angle so that:

1. The lines of sight from the eyes intersect at the object.
2. The images are brought into approximate coincidence, producing visual disparity, or parallax.
3. Somehow the two images are fused into one, producing what we recognize as a single object.

This fusion can occur even when accommodation isn't possible, so apparently the vergence movements in the two cases must not be the same.

If the visual field now contains two objects, at different distances from the eyes, changing vergence to eliminate disparity of one may still leave the other one disparate. That is, swinging the line-of-sight from the two eyes to intersect on one of the objects and thus to bring the disparate images of that object into coincidence, may leave the images of the other object separate. There could still be a difference in the horizontal angular separation of the two images in one eye and that in the other eye.

If the fixation point happens to be intermediate to the two objects, both of them might remain disparate, though not to the same degree as in the first situation. The total disparity is, in fact, the same, regardless of the fixation. It is, as Kaufman indicates, sufficient to denote the depth relations of the two objects. This depth response due to relative disparity, as opposed to convergence per se, is known as binocular stereopsis, or just stereopsis.

To emphasize the point, stereopsis is the experience of depth, and not the mere occurrence of an intersection of lines-of-sight or the coincidence of images. We are able to fuse the images from our two eyes into a single percept and use the differences between the images to compute the distance to objects in our field and thus experience depth.

Computational accuracy requires considerable fineness in the neuronal linkages from the eyes to the cortex. The neurons from each eye have to be arranged in such a way that only those pairs from corresponding points in the visual field reach common cortical cells.

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