Visual motion sensitivity in dyslexia: evidence for temporal and energy integration deficits

In addition to poor literacy skills, developmental dyslexia has been associ ated with multisensory deficits for dynamic stimulus detection. In vision t hese deficits have been suggested to result from impaired sensitivity of ce lls within the retino-cortical magnocellular pathway and extrastriate areas in the dorsal stream to which they project. One consequence of such select ively reduced sensitivity is a difficulty in extracting motion coherence fr om dynamic noise, a deficit associated with both developmental dyslexia and persons with extrastriate, dorsal stream lesions. However the precise natu re of the mechanism(s) underlying these perceptual deficits in dyslexia rem ain unknown. In this study, we obtained motion detection thresholds for 10 dyslexic and 10 control adults while varying the spatial and temporal param eters of the random dot kinematogram (RDK) stimuli. In Experiment 1 stimulu s duration was manipulated to test whether dyslexics are specifically impai red for detecting short duration. rather than longer stimuli. Dot density w as varied in Experiment 2 to examine whether dyslexics' reduced motion sens itivity was affected by the amount of motion energy present in the RDKs. Dy slexics were consistently less sensitive to coherent motion than controls i n both experiments. Increasing stimulus duration did not improve dyslexics' performance, whereas increasing dot density did. Thus increasing motion en ergy assisted the dyslexics, suggesting that their motion detectors have a lower signal to noise ratio, perhaps due to spatial undersampling. (C) 2000 Elsevier Science Ltd. All rights reserved.