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Table 1 A taxonomy relating commonly used eye-tracking metrics and their respective units to perceptual and cognitive processes of interest to researchers

From: A review of eye tracking for understanding and improving diagnostic interpretation

Measure Units Description
Fixation count Frequency count The number of times the eye fixates in a particular region of interest, related to at least: the salience of the area, the informational value of the area, how much information is available in a single fixation, or the processing difficulty of the information (Findlay & Gilchrist, 2008; Henderson & Hollingworth, 1998; Henderson, Malcolm, & Schandl, 2009)
Regressive fixation count Frequency count Re-fixating a previously fixated region, to resolve ambiguity or other processing difficulties (Spivey & Tanenhaus, 1998; Underwood & Radach, 1998)
Fixation duration Milliseconds How long the eye fixates on a region prior to a saccade, related to the difficulty in processing the information in that region, the value of information available in that region, the time needed to plan the next saccade, and the predicted value of information available following the next saccade (Findlay & Gilchrist, 2008; Rayner, 1998; Sumner, 2011)
Amplitude Degrees The magnitude of a saccade, influenced by how much information can be processed in the area of a single fixation, and the distance to the next planned fixation target (Rayner, 1998)
Saccade peak velocity Degrees/second The maximum speed achieved within a saccade, related to physiological arousal, mental workload, or the predicted value of information available at the subsequent fixation (Di Stasi, Catena, Cañas, Macknik, & Martinez-Conde, 2013; Montagnini & Chelazzi, 2005; Xu-Wilson et al., 2009)
Blink rate or inter-blink interval Frequency count/time or milliseconds The number of eye blinks detected by an eye tracker’s algorithms, inversely related to physiological arousal, wakefulness, processing difficulty, motivation, and mental workload (Holmqvist et al., 2011; Siegle, Ichikawa, & Steinhauer, 2008)
Blink amplitude and blink duration Milliseconds The extent and duration of an eye blink (temporary closure) event, inversely related to physiological arousal, wakefulness, processing difficulty, motivation, and mental workload (Holmqvist et al., 2011; Ingre, Åkerstedt, Peters, Anund, & Kecklund, 2006).
Phasic pupil diameter Millimeter diameter Rapid and dramatic pupil diameter changes related to processing task- and goal-relevant information, and exploiting that information to perform a task (Beatty, 1982; Laeng, Sirois, & Gredeback, 2012)
Tonic pupil diameter Millimeter diameter Sustained pupil diameter changes that establish a new baseline diameter from which phasic responses deviate, related to sustained cognitive processing, task difficulty, cognitive effort, arousal, and vigilance (Laeng et al., 2012; Siegle et al., 2008).