Eye movements

Eye movements have been of interest to mankind for a long time. Already the Greek philosopher Aristotle was concerned with the coordinated movement of our eyes. Nevertheless, it took until the end of the 19th century before eye movement research really started to gain speed with the introduction of more quantitative eye tracking methods (earlier studies had used afterimages or auditory methods).

In general, three larger classes of eye movements are distinguished based on their functions:

1. Compensatory eye movements reduce the influence of head and body movements on vision by canceling self-induced motion blur. The VOR is driven by the vestibular system and induces short-latency eye movements in the opposite direction to the head movement. For longer, slowly-accelerating movements, the optokinetic nystagmus (OKN) is induced by the resulting retinal image motion (e.g. when sitting in a train and looking out of the window). The OKN has a ten times longer latency than the VOR, since it “computes” self-motion from the movement of the image on the retina.
2. Goal-directed eye movements move and keep an important detail of the retinal image on the fovea. This spot has the highest resolution in the retina but covers only a tiny fraction of our visual field – in humans roughly one degree, which corresponds to the size of a thumbnail at the outstretched arm. Goal-directed eye movements enable us to sample flexibly our whole visual environment with this high spatial resolution. They can be further separated in fast (saccades) and slow (pursuit) movements.
3. Fixational eye movements like ocular drift and microsaccades occur during fixation. Although the eyes seem stationary, very small movements are still at work. Especially microsaccades seem to be important for the avoidance of image fading during sustained fixation and can indicate the orientation of covert attention shifts. Although their actual function is highly debated, it seems that there is only little difference in microsaccades and their larger counterparts. Microsaccades elicit similar perceptual changes, have equally stereotyped kinematics and compensate for inequalities in foveal resolution, just as saccades do for the retina.

The interested reader is referred to chapter 3.3.3 and 3.3.4 in the review "Neuro-ophthalmology - A short primer" by U. Schwarz.