Eye Movement Explained

I found this article yesterday on Kevin Koch's website. He explains in scientific detail (pretty heavy stuff) about the various movements of the eye and why they move the way the do. Here is an excerpt of definitions from the article and you can read the whole piece here:

Eye Movements 3 - Saccades and fixations


Eye movements can be classified into seven different types:

Convergence

is a motion of both eyes relative to each other that ensures that an object is still foveated by both eyes when its distance from the observer is changed. The closer the object is, the more the eyes point towards each other. This movement can be voluntarily controlled, but is normally the result of a moving stimulus.

Saccades

are the principal method for moving the eyes to a different part of the visual scene, and are sudden, rapid movements of the eyes. Saccades can be initiated voluntarily, but are ballistic: that is, once they are initiated, their path of motion and destination cannot be changed. Visual input is suppressed during a saccade.

Pursuit motion

is a much smoother, slower movement than a saccade; it acts to keep a moving object foveated. It cannot be induced voluntarily, but requires a moving object in the visual field. One frequent failing of thoughtless animation is having the eyes demonstrate pursuit motion when there is no object being followed by the character’s eyes.

Nystagmus

is a saw-toothed pattern of eye movements that occurs as a response to the turning of the head (acceleration detected by the inner ear) or the viewing of a moving, repetitive pattern (the train window phenomenon). It consists of smooth `pursuit’ motion in one direction to follow a position in the scene, followed by a fast motion in the opposite direction to select a new position. This is an eye movement that has probably never been animated, and if it was, it would probably get rejected by the supervisor or director because it would look so odd.

Drift and microsaccades

occur during fixations, and consist of slow drifts followed by very small saccades (microsaccades) that apparently have a drift-correcting function. These movements are involuntary, and their function is in question.

Physiological nystagmus

is a high-frequency oscillation of the eye (tremor) that serves to continuously shift the image on the retina, thus calling fresh retinal receptors into operation. Physiological nystagmus actually occurs during a fixation period, is involuntary, and generally moves the eye less than 1°. As with microsaccades, you’d need to be in an extreme close-up for this to register, but it’s another reason the human eye looks ‘alive’ in live-action extreme close-ups.

Rolling

of the eyes is a rotational motion around an axis passing through the fovea and pupil. It is involuntary, and is influenced by among other things the angle of the neck. Although this is also something we can safely ignore as animators, some rigs will automatically provide this when using the ‘eye-target’ controller.