3.3 Eye Movements

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 (this is why stereograms as described in Humphreys & Bruce (1989, p. 39) can be viewed), but is normally the result of a moving stimulus.

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 (Jacob 1995).

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. It takes about 100-300ms to initiate a saccade, i.e. from the time a stimulus is presented till the eye starts moving, and another 30-120ms to complete the saccade, depending on-among other things-the visual angle traversed. Saccades can be initiated voluntarily, but are ballistic: that is, once they are initiated, their path of motion and destination cannot be changed-which must be taken as an indication that visual attention in the peripheral area selects the next location for the eyes to move to. During saccades-actually from about 50ms before saccades are initiated (Barber & Legge 1976, p. 58)-processing of the visual image is suppressed (but not entirely inhibited), possibly due to a detection mechanism of large-scale movements of the entire retinal mosaic (Bruce & Green (1990, p. 170) cite Stevens et al. (1976)). Thus, processing of the retinal image takes place mainly between the saccades, during the so-called fixations, that last for about 200-600ms.

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.

Nystagmus

is a pattern of eye movements that occur 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.

Drift and microsaccades

occur during fixations and consist of slow drifts followed by very small saccades (microsaccades) that apparently have a drift-correcting function (although, Barber & Legge (1976, p. 56) cite Steinman et al. (1973) for suggesting that microsaccades do not have any function). These movements are involuntary.

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. If an image is artificially fixed on the retina it disappears, but physiological nystagmus causes every point of the retinal image to move approximately the distance between two adjacent foveal cones in 0.1 seconds. Physiological nystagmus actually occurs during a fixation period, is involuntary and generally moves the eye less than 1°.

From this it is seen that any tracking of the eyes will result in tracking data consisting of superimposed movements of different origins. Whereas the saccades and fixations originate from attentional processes, the drift, microsaccades and physiological nystagmus are physiologically determined. This implies that the attention-pertaining data somehow needs to be extracted from the "raw" eye tracking data, and that there is a lower limit on the precision with which one can measure what the person is attending to.