Eye movements and eye tracking

Eyes do not flow in a smooth fashion when engaged in visual tasks (Huey, 1908). If you were able to see your gaze on the page or on the digital screen right now, you would notice that your eyes shift from one word to the next as you are reading this sentence. Known as saccades, these “jumps” are rapid, short and repeated ballistic (i.e. jerk-like) movements which occur approximately three to four times every second. Saccades abruptly change the point of

fixations, the periods of eye immobility in which visual or semantic information is acquired

and processed (Purves, Augustine, & Fitzpatrick, 2001; D. C. Richardson & Spivey, 2004). In simple terms, individuals internalise the visual world during fixations that are executed between saccades (Bridgeman, Van der Heijden, & Velichkovsky, 1994; Simons & Rensink, 2005). Eye movements are fundamental to visual perception because visual system cannot process the huge amount of available information in the visual world at once. Thus, execution of eye movements allows us to see the world as a seamless whole, although we can only see one region at a time (Buswell, 1936; Yarbus, 1967) due to anatomical limitations (i.e., the total visual field that the human eye covers) and also, limited processing resources (Levi, Klein, & Aitsebaomo, 1985; D. C. Richardson, Dale, & Spivey, 2007).

Fixations have two elemental measures: location and duration. Both measures are highly informative of ongoing cognitive operations. We can see a stimulus clearly only when it falls into the most sensitive area of the retina (i.e., fovea) (~2o _{or 3 to 6 letter spaces), which is}

specialised for high acuity visual perception (Mast & Kosslyn, 2002; Yarbus, 1967). Thus, eye position (i.e., fixation location) gives valuable information about the location of the attentional

“spotlight” (Posner, Snyder, & Davidson, 1980). In other words, fixation location corresponds to the spatial locus of cognitive processing. On the other hand, fixation duration corresponds to the duration of cognitive processing of the material located at fixation (Irwin, 2004, p. 2).

Longer fixations suggest higher cognitive load or higher attentional processing demands required by a material or task (Irwin, 2004). The underlying idea behind the link between cognition and fixation is known as eye-mind assumption (Just & Carpenter, 1980), which

simply posits that the “direction of our eyes indicates the content of the mind” (Underwood & Everatt, 1992). Based on the location and duration of fixations, cognitive processes can be measured and evaluated objectively and precisely during the occurrence of the process in question. There is now a universal consensus on the value of eye movements and eye tracking as a methodology in the investigation of the human mind (e.g., Hyona, Radach, & Deubel, 2003; Just & Carpenter, 1980; Rayner, 1998; Rayner, Pollatsek, Ashby, & Clifton, 2012; Reichle, Pollatsek, Fisher, & Rayner, 1998; Theeuwes, Belopolsky, & Olivers, 2009; Van der Stigchel et al., 2006). Eye tracking methodology provides detailed measures with regard to the temporal order of fixations and saccades, gaze direction, pupil size and time spent on pre- defined regions of the scene.Fixation duration in a certain location relative to other locations is used as the main measure of looking behaviour in the present thesis.

Eye movements can be monitored in various different ways. A pupil corneal reflection

technique, that is based on high-speed cameras and near infrared light, is the most advanced

remote and non-intrusive eye tracking method as of today. An illuminator shines dispersed infrared light to one eye or both eyes. A high-speed video camera captures the infrared reflections coming from the pupil and cornea (i.e., the outer layer of the eye) and transforms them into high-resolution images and patterns pertaining to the position of the eye(s) at any given millisecond. Such an infrared eye tracker can record eye movements quite precisely. Precision offered by an eye tracker is indicated by temporal resolution (i.e., sampling rate) and spatial resolution. Sampling rate shows the frequency of which a tracker samples and

determines the position of the eye at a given moment. For example, the eye tracker used in the present thesis (i.e., SR EyeLink 1000) operates at a sampling rate of 1000 Hz, which means that the position of the eye is measured 1000 times every second. Put differently, it produces one sample of the eye position per one millisecond. Spatial resolution refers to the angular distance between successive samples of eye position. Thus, an eye tracker with a higher spatial resolution can detect even the smallest eye movements in a certain interest area. SR EyeLink 1000 has a spatial resolution of 0.25o _{- 0.50}o _{which means that it can detect and sample eye}

movements within an angular distance of 0.25o _{- 0.50}o_.

There generally exists a spatial difference between the calculated location of a fixation and the actual one. This difference is expressed in degrees of visual angle and reflects the accuracy of eye tracking. If you draw a straight line from the eye to the actual fixation point on the screen and another line to the computed one, the angle between these lines gives the accuracy. Thus, a smaller difference means higher accuracy. Accuracy depends on the screen size and the distance between the participant and the screen. Visual angle is also used to calculate the size of the experimental stimulus as it refers to the perceived size rather than the actual size. These measures of data quality are reported in the methods section of each experiment in accordance with the eye tracking standards and good practices in literature (Blignaut & Wium, 2014; Holmqvist, Nyström, & Mulvey, 2012; D. C. Richardson & Spivey, 2004).