Abstract
Dyslexic adolescents demonstrate deficits in word decoding, recognition, and oculomotor coordination as compared to healthy controls. Our lab recently showed intrinsic deficits in large saccades and vergence movements with a Remobi device independent from reading. This shed new light on the field of dyslexia, as it has been debated in the literature whether the deficits in eye movements are a cause or consequence of reading difficulty. The present study investigates how these oculomotor problems are compensated for or aggravated by text difficulty. A total of 46 dyslexic and 41 non-dyslexic adolescents’ eye movements were analyzed while reading L’Alouette, a dyslexia screening test, and 35 Kilos D’Espoir, a children’s book with a reading age of 10 years. While reading the more difficult text, dyslexics made more mistakes, read slower, and made more regressive saccades; moreover, they made smaller amplitude saccades with abnormal velocity profiles (e.g., higher peak velocity but lower average velocity) and significantly higher saccade disconjugacy. While reading the simpler text, these differences persisted; however, the difference in saccade disconjugacy, although present, was no longer significant, nor was there a significant difference in the percentage of regressive saccades. We propose that intrinsic eye movement abnormalities in dyslexics such as saccade disconjugacy, abnormal velocity profiles, and cognitively associated regressive saccades can be particularly exacerbated if the reading text relies heavily on word decoding to extract meaning; increased number of regressive saccades are a manifestation of reading difficulty and not a problem of eye movement per se. These interpretations are in line with the motor theory of visual attention and our previous research describing the relationship between binocular motor control, attention, and cognition that exists outside of the field of dyslexia.
Highlights
Reading depends on multiple different processes: sensory perception, eye movements, and linguistic and semantic decoding [1]
(64.97 ms vs. 44.07 ms, p = 0.001), a higher peak velocity (80.82 ◦ /ms vs. 66.30 ◦ /ms; p = 0.005) yet a lower average velocity (42.13◦ /ms vs. 64.24◦ /ms, p < 0.001), and a larger fixation disconjugacy during the saccade (0.76◦ vs. 0.61◦, p–0.037). These findings are consistent with eye movement abnormalities previously found in dyslexics independent of reading using Remobi testing [13]
Perhaps when dyslexics read a text that carries more context clues and does not rely heavily on word decoding skills, they are able to extract meaning and move more smoothly along a line of text to non-dyslexic readers. These observations are in line with our previous studies [8,9]. Given that these eye movement abnormalities in saccades and vergence exist outside of reading, we propose that the difference between the two reading texts presented here could be understood as evidence for a fragile oculomotor system that is perturbed when attempting to decode a meaningless text
Summary
Reading depends on multiple different processes: sensory perception, eye movements, and linguistic and semantic decoding [1]. It has been previously shown that, when mismatched vergence-accommodation is induced experimentally with the use of prisms or spherical lenses, reading saccades become more disconjugate, leading to residual disparities during subsequent fixation [8]. In another interesting study, binocular coordination and reading scores were studied in a group of students with demonstrated vergence problems. Once these vergence deficits were corrected, there was a decrease in the number of regressive saccades and fixation duration while reading [9]
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