Fixations were defined temporally and spatially using a pre-set minimum fixation duration of 80 milliseconds (ms) and a maximum dispersion value of 100 pixels.
We wondered if the standard deviation of fixation disparity, i.e., the variability in fixation disparity during fixations, was dependent on the position within the text.
Further, we plotted the standard deviation of fixation disparity as function of horizontal fixation position (see Figure 6).
In addition to larger mean fixation disparities in dyslexics, our study clearly showed, for the first time, that the standard deviation of fixation disparity during reading fixations were increased in dyslexic children.
Regarding the sensory driven, fine tuning of vergence adjustment during fixations, the overall fixation disparity showed a tendency for slightly larger fixation disparities for dyslexic children and a general tendency for larger fixation disparities for reading at near distance for all children.
In addition, we found a significant difference between non-dyslexic and dyslexic children in the stability of the fixation disparity during reading fixations (F1,18 = 11.60, p<0.01).
Moreover, for dyslexics the standard deviation of fixation disparity was larger particularly when reading at near distance.
Some studies [5], [25], [27] suggest a correlation with fixation disparity and its stability but this remains controversial.
Interestingly, analyzing the horizontal fixation disparity showed a tendency for an increased fixation disparity for close reading (F1,18 = 3.43, p = 0.08) and a clear tendency for an overall larger fixation disparity for dyslexic children (F1,18 = 3.68, p = 0.07), while these effects did not interact (F-value <1).
More generally, fixation disparity might be related to the resting state of the vergence system and/or the coupling of accommodation and vergence [20], [23].
