Reading requires three-dimensional motor control: saccades bring the eyes from left to right, fixating word after word; and oblique saccades bring the eyes to the next line of the text. The angle of vergence of the two optic axes should be adjusted to the depth of the book or screen and - most importantly - should be maintained in a sustained manner during saccades and fixations. Maintenance of vergence is important as it is a prerequisite for a single clear image of each word to be projected onto the fovea of the eyes. Deficits in the binocular control of saccades and of vergence in dyslexics have been reported previously but only for tasks using single targets. This study examines saccades and vergence control during real text reading. Thirteen dyslexic and seven non-dyslexic children read the French text “L'Allouette” in two viewing distances (40 cm vs. 100 cm), while binocular eye movements were measured with the Chronos Eye-tracking system. We found that the binocular yoking of reading saccades was poor in dyslexic children (relative to non-dyslexics) resulting in vergence errors; their disconjugate drift during fixations was not correlated with the disconjugacy during their saccades, causing considerable variability of vergence angle from fixation to fixation. Due to such poor oculomotor adjustments during reading, the overall fixation disparity was larger for dyslexic children, putting larger demand on their sensory fusion processes. Moreover, for dyslexics the standard deviation of fixation disparity was larger particularly when reading at near distance. We conclude that besides documented phoneme processing disorders, visual/ocular motor imperfections may exist in dyslexics that lead to fixation instability and thus, to instability of the letters or words during reading; such instability may perturb fusional processes and might – in part - complicate letter/word identification.

Reading, an essential skill for successful function in today's society, is a complex psychological process involving vision, memory, and language comprehension. Variability in fixation durations during reading reflects the ease of text comprehension, and increased word frequency results in reduced fixation times. Critically, readers not only process the fixated foveal word but also preprocess the parafoveal word to its right, thereby facilitating subsequent foveal processing. Typically, text is presented binocularly, and the oculomotor control system precisely coordinates the two frontally positioned eyes online. Binocular, compared to monocular, visual processing typically leads to superior performance, termed the "binocular advantage"; few studies have investigated the binocular advantage in reading. We used saccade-contingent display change methodology to demonstrate the benefit of binocular relative to monocular text presentation for both parafoveal and foveal lexical processing during reading. Our results demonstrate that denial of a unified visual signal derived from binocular inputs provides a cost to the efficiency of reading, particularly in relation to high-frequency words. Our findings fit neatly with current computational models of eye movement control during reading, wherein successful word identification is a primary determinant of saccade initiation.

The eye produces saccadic eye movements whose reaction times are perhaps the shortest in humans. Saccade latencies reflect ongoing cortical processing and, generally, shorter latencies are supposed to reflect advanced motor preparation. The dilation of the eye’s pupil is reported to reflect cortical processing as well. Eight participants made saccades in a gap and overlap paradigm (in pure and mixed blocks), which we used in order to produce a variety of different saccade latencies. Saccades and pupil size were measured with the EyeLink II. The pattern in pupil dilation resembled that of a gap effect: for gap blocks, pupil dilations were larger compared to overlap blocks; mixing gap and overlap trials reduced the pupil dilation for gap trials thereby inducing a switching cost. Furthermore, saccade latencies across all tasks predicted the magnitude of pupil dilations post hoc: the longer the saccade latency the smaller the pupil dilation before the eye actually began to move. In accordance with observations for manual responses, we conclude that pupil dilations prior to saccade execution reflect advanced motor preparations and therefore provide valid indicator qualities for ongoing cortical processes.