1. Saccades: Move the fovea from one object of interest to another. They 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. 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 -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. Thus, processing of the retinal image takes place mainly between the saccades, during the so-called fixations, that last for about 200-600ms. (The word saccade is borrowed from the French. It is derived from the Old French sachier meaning to shake. In horse riding, a saccade is the bruque shaking given to the reins of a horse as a signal to the horse. Also called saccadic movement.)

2. Drift and Microsaccades: They occur during fixations and consist of slow drifts followed by very small saccades (microsaccades) that apparently have a drift-correcting function for suggesting that microsaccades do not have any function). These movements are involuntary.

3. Smooth pursuit: Keeps the fovea on a moving target, it is a much smoother, slower movement than a saccade. It cannot be induced voluntarily, but requires a moving object in the visual field.

4.(Con) vergence: Adjusts the individual angles of both eyes relative to each other so that it 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.

5. Rolling of the eyes: 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.

6. Nystagmus: Keep the eye still in space when the head moves. 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.

7. Physiological Nystagmus: Keep the eye still in space when the head moves. 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°.

8. Fixation: Keeps the fovea still on the target, a completed pause from the eye on a single interesting element.

The Hot Spot's above are a cummulation of groups of subjects and shows through heat spots, of variable intensity: the duration and the numerosity of the fixations, the concentration of vision of a group.

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.