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When tired and it was dark, I noticed that if I focused on a dim light source and moved my eyes fairly rapidly sideways, the resulting images that lingered for a short while were not smoothly blurred together, but were discrete.

I assume this is not due to the brain's inability to process the data fast enough or the retinas not repairing fast enough when light breaks part of them down, so I am lead to the conclusion that eyes (if not generally, sometimes) move in very small sudden jolts rather than smoothly.

Is this correct? What is the reason for this (is it due to the eye muscles being unable to sustain motion for a long time, or something to do with the mechanics of the eye)? Was this result anomalous?

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up vote 4 down vote accepted

There are two types of eye movement: smooth pursuit and saccadic. As the name suggests the latter movement involves quick and discontinuous movements of the eyes. Saccadic movement is used most of the time as the eyes move around analysing the current scene.

According to Wikipedia, these saccades are the fastest movements produced by the human body: peak angular speed of 900 degrees of rotation per second (this would be 5 full rotations of the eyeball per second, were such a thing possible).

The processing of information by the eye-brain system during a saccade is complex, and may be responsible for the effect that you report: it causes the phenomenon of saccadic masking explained in this quote from Wikipedia:

A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes nor any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent smearing of the image.

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I assume the advantages of smooth pursuit and saccadic movement are that the image is continuous (so no information is lost) and that the angular speed is large, respectively, or am I oversimplifying? –  Alyosha May 24 '13 at 20:06
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