Evidence for ocular motor deficits in developmental dyslexia: application of the double-step paradigm.

PURPOSE. Dyslexia is a language-based learning disability characterized by difficulties with reading, spelling, and writing. Persons with dyslexia often have deficits in processing rapid temporal sensory information. There is also evidence of sensorimotor deficits in persons with dyslexia. Whether these deficits include ocular motor problems is still an open question. Some previous studies have shown an increased saccadic latency in dyslexics, whereas others have not reproduced this finding. The purpose of the present study was to investigate saccadic latency in young adults with dyslexia during the double-step paradigm, a task that requires rapid sequential visual information processing and saccade generation. The study hypothesis was that dyslexics have a longer saccadic latency in the second orthogonal saccade, a task that nondyslexics parallel process and perform rapidly. METHODS. Eight students with dyslexia and eight age-matched control subjects participated in the study. Their eye movements were monitored with the scleral search coil technique in simple saccade trials and in the double-step paradigm. The second saccade was either orthogonal or colinear to the first. Intersaccadic interval and latency were calculated for the second saccade. RESULTS. No difference in saccadic latency was found for colinear second saccades; however, dyslexics had significantly longer latencies for orthogonal second saccades. This included a subset of subjects who had longer latencies for orthogonal than for colinear saccades. CONCLUSIONS. The findings indicate that under certain conditions, when the demand for rapid visual information processing is high and a rapid saccade sequence is required, some persons with dyslexia show ocular motor deficits manifested by longer saccadic latencies.