Eye Movement Axes
A Brief Taxonomy of Eye Movements
Types of Eye Movements
Much of the time the two eyes move as a unit, through equal angles in the same direction. Such eye movements constitute conjugate, or version eye displacements. Other eye movements involve coordinated, but unequal, degrees of movement of the eyes, and are variously referred to as non-conjugate, vergence, or disjunctive eye movements.
Vergence movements result in pupils which move together or apart:
Holding eye movements (sitting still)
Catching eye movements (saccades)
Sustaining eye movements (microsaccades)
Compared with either the "holding" or "catching" movements discussed above, the defining characteristic of "sustaining" eye movements is their amplitude: they are extremely small. As illustrated by inspecting the figure below, normal observers generally make, even while fixating a stationary object, small eye movements of which they are unaware, which are a few minutes of arc in extent. Foveal cones are spaced approximately 0.5 minutes apart, so that these eye movements impose a temporal pattern on the photoreceptor's output. Indeed, it turns out that vision is entirely dependent upon the small-scale irregular motion of the retinal image which is produced by these movements. Hence, they actually "sustain" visual perception.
Tracking Eye Movements: Optokinetic Nystagmus and Smooth Pursuit
Tracking eye movements are those which are made under visual guidance for the purpose of holding an object of regard and/or following it when it moves. They are rather less reflexive than vestibulo-ocular movements, for they are capable of being modified by acts of will. However, few subjects can generate them in the absence of an appropriate stimulus, and so they are rather less voluntary than saccades or vergence eye movements.
Optokinetic Nystagmus (OKN): Easiest to demonstrate by viewing a rotating, striped drum that fills a substantial portion of the visual field. For moderate rotational velocities, the subject's eyes will follow a particular stripe as it moves (slow phase). As the gaze is carried further from the primary position, a quick anticompensatory flick is made to bring the point of fixation back onto some new feature of the drum (fast phase).
OKN depends upon subject's attitude, and instructions: "looking" and "gazing" at a rotating striped drum produce different kinds of OKN. "Looking" instructions produce less frequent movements of larger amplitude; "gazing" instructions produce more frequent, lower amplitude eye movements (see figure below). The slow phase of OKN can follow stimulus velocities up to approximately 100o/sec, beyond which the system breaks down, and inappropriate eye movements, or none at all, are made to moving targets.
Smooth Pursuit: Voluntary smooth eye movements which track the location of objects of interest. Although voluntary, smooth pursuit requires a stimulus to track; they cannot be executed in the absence of some environmental stimulus (try it with a partner if you don't believe me).
Saccadic Eye Movements
Saccades are the voluntary movements of the eyes which serve to bring a new part of the visual field into the foveal region. While essentially voluntary, they share many characteristics with the quick phases of vestibular and optokinetic nystagmus and microsaccades. Saccades are stereotyped movements. They are so fast that there is little time for visual guidance: saccadic eye movements are therefore said to be ballistic. The figure below illustrates a number of saccades of different amplitudes. A notable feature of such records is the velocities attained by the eye during large amplitude saccades: in excess of 400o/sec.
To execute a saccade, the visual system must convert retinal distance (for example, between current position, and a desired new position) into signals controlling the extraocular muscles. The complexity of these calculations is reflected in the rather long latencies associated with the initiation of saccadic eye movements. A typical stimulus arrangement is for the subject to fixate a small light. This light is extinguished while another light, at a variable distance from the first, is turned on: the saccade the subject makes to the new light typically does not begin before 200 ms.
During saccades, visual sensitivity is suppressed (see figure below), though it is easily demonstrated that it is not completely eliminated. In general, however, one cannot "see" ones own saccadic eye movements in a mirror.
Saccadic eye movements can reveal global aspects of perception, such as the scan patterns and fixation locations of subjects inspecting human faces (see figure below).
Miniature Eye Movements
In humans there are three different types of miniature eye movements: tremor, drift, and microsaccades. The effect of these miniature movements is to move the retinal image about, over the fovea, in a pattern considerably larger than a foveal cone photoreceptor (see figure below).
Tremor: The amplitude of tremor is smallest, about 5-10 seconds of arc. Binocular measures of tremor show that it is uncorrelated between the two eyes, suggesting a peripheral origin.
Drift: Drift movements are relatively large and slow, possessing velocities of 1 min/sec, and median amplitudes of around 2-5 minutes. Each drift movement is terminated by a microsaccade, and drift movements appear to be uncorrelated between the eyes.
Microsaccades: Microsaccades serve the same function as their larger cousins, ordinary gross saccades, which is to bring a visual target into register with the fovea. They probably share the same control mechanism as saccades. Microsaccades are primarily corrective in nature.