Abstract
Gaze data of 31 participants of a memory recall experiment was analyzed and the I-DT dispersion based algorithm of Salvucci and Goldberg (2000) was used to identify fixations. It was found that individuals differ considerably with regard to the stability of eye gaze and that fixational eye movements affect the accuracy of fixation identification and the optimum dispersion threshold. It was also found that fixation radius and the distance between the points in a fixation that are the furthest apart are the most reliable metrics for a dispersion-based fixation identification algorithm. Finally, it is argued that the correct setting of dispersion threshold is of utmost importance, especially if the participants are not homogeneous with regard to gaze stability.
Highlights
Eye movement is characterized by a series of quick jumps or high velocity movements, known as saccades, followed by fixations, which are periods of time in which the eye is stabilized and remains relatively still
For some of the participants, the average point-to-point distances were significantly more than could be attributed to equipment limitations and can only be declared in terms of fixational eye movements: tremor, drift and/or microsaccades: Tremor occurs with a typical frequency of about 90 Hz (Martinez-Conde et al, 2004), meaning that the eyetracker that was used in this study (Tobii 1750, 50Hz) would be unable to pick up individual oscillations
The minimum of the average Levenshtein distance (LD) and corresponding threshold values are given in Table 1 along with a range of threshold values where the LD is below 20%
Summary
Eye movement is characterized by a series of quick jumps or high velocity movements, known as saccades, followed by fixations, which are periods of time in which the eye is stabilized and remains relatively still. Drifts occur simultaneously with tremor and are slow motions of the eye (0.1-0.5 deg/s) (Martinez-Conde et al, 2004). Fixational microsaccades are small (~0.3 ), fast (~10 deg/s) eye movements that occur involuntarily they can be voluntarily suppressed (Fiorentini and Ercoles, 1966; Winterson and Collewijn, 1976). They occur intermittently once or twice in a fixation of 300 ms (Martinez-Conde et al, 2004). A recent review by Collewijn & Kowler (2008) reopens the 50 year old debate about the role of microsaccades by concluding that that microsaccades are neither essential to maintain a stable line of sight, nor for keeping foveal images visible
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