# Eye Movement Parameters in Children with Reading Difficulties
## 2 Method
### Saccadic Eye Movement Evaluation  
Saccadic eye movement evaluation started with the calibration (5 points) and validation (4 points) of the eye tracker. If the calibration accuracy did not reach 0.5 degrees, a repeated calibration and validation process was performed. The calibration and validation stimulus and background color settings were identical to the experimental settings--black stimuli (5 cd/m\({}^{2}\); RGB (0, 0, 0)) demonstrated on a light grey background (90 cd/m\({}^{2}\); RGB (179, 179, 179)) [43]. After the calibration and validation, a fixation cross appeared at the center of the screen (3 s), after which the saccadic eye movement stimulus appeared in one of four positions (up, down, left, or right) relative to the central fixation object. Saccadic eye movement stimuli were either a circle or a star (depending on the task) 0.7 degrees in size. The distance from the center of the fixation cross to the center of the saccadic eye movement target was 11.3 degrees in the vertical positions and 14.0 degrees in the horizontal positions. Two saccadic eye movement tasks were presented to the participants--(1) a reflexive saccade task and (2) an anti-saccade task (Figure 1). In the reflexive saccade task, participants were instructed to shift their gaze (make a saccade) from the fixation cross to the stimulus (circle) presented in the periphery as soon as soon as the stimulus appeared in either of the positions (left, right, up or down from the fixation cross). After the saccadic stimulus disappeared (5 s), the participants were instructed to return to the fixation stimulus. Note that during the presentation of the saccadic eye movement stimulus, the fixation cross was no longer visible. In the anti-saccade task, the participants were instructed to make a gaze transfer (anti-saccade) in the opposite direction from the saccadic eye movement stimulus (star), but with the same amplitude as the distance of the presented stimulus. When the anti-saccade stimulus disappeared, the participants were instructed to return to the fixation stimulus. Saccadic eye movement data were extracted and analyzed using Tobii Pro Lab software, working in accordance with the Tobii Pro Fusion eye tracker. Tobii I-VT and ClearView filters were applied to determine fixations, saccades, and blinks [44]. Only monocular data for the dominant eye were used in further data analysis. The data for saccadic eye movement analysis included 352 (171 boys and 181 girls) participants who managed to record at least one saccade or more with the Tobii Pro Fusion device. Of all participants whose saccadic eye movements were analyzed, 90 had reading difficulties (RD), and 256 children were included in the control group (CD). In order to determine the minimum sample size required for the study, power analysis (G*Power version 3.1.9.7) was conducted ([PERSON] et al., 2007 [45]). The results of the power analysis indicated that the required sample size to achieve 80% power for detecting a medium effect (f = 0.25), at a significance criterion of \(\alpha\) = 0.05, was N = 60 for ANOVA: repeated-measures within-between interaction. Thus, the obtained sample size of N = 346 is sufficient to test the study hypothesis. Data analysis was performed using the Microsoft Excel, RStudio, and SPSS programs. To obtain information about the first saccade towards the target object, the Tobii Pro Lab function for AOI (Area of Interest) analysis was applied. The area of interest (an area invisible to the observer) was placed in the center of the screen, where the fixation stimulus was located before the presentation of saccadic eye movement stimulus. In order to obtain the necessary saccadic eye movement parameters, the peak velocity of the exit saccade (saccade from the AOI towards the target) was determined. The peak velocity of saccadic  
Figure 1: Saccadic eye movement stimulus for analysis of reflexive saccades (**A**) and anti-saccades (**B**). eye movements was determined using a single-parameter I-VT filter with a peak velocity threshold of 30\({}^{\circ}\)/s [44].