Dorsal stream deficits suggest hidden dyslexia among deaf poor readers: Correlated evidence from reduced perceptual speed and elevated coherent motion detection thresholds

Prelingual deafness and developmental dyslexia have confounding developmental effects on reading acquisition. Therefore, standard reading assessment methods for diagnosing dyslexia in hearing people are ineffective for use with deaf people. Recently, Samar, Parasnis, and Berent (2002) reported visual evoked potential evidence that deaf poor readers, compared to deaf good readers, have dorsal stream visual system deficits like those previously found for hearing dyslexics. Here, we report new psychometric and psychophysical evidence that deficits in dorsal stream function, likely involving extrastriate area MT, are associated with relatively poor reading comprehension in deaf adults. Poorer reading comprehension within a group of 23 prelingually deaf adults was associated with lower scores on the Symbol Digit Modality Test, a perceptual speed test commonly used to help identify dyslexia in hearing people. Furthermore, coherent dot motion detection thresholds, which reflect the functional status of area MT, correlated negatively with reading scores in each visual quadrant. Elevated motion thresholds for deaf poor readers were not due to general cognitive differences in IQ but were specifically correlated with poor perceptual speed. With IQ controlled, a highly reliable right visual field advantage for coherent motion detection was found. Additional analyses suggested that the functional status of dorsal stream motion detection mechanisms in deaf people is related to reading comprehension, but the direction and strength of lateralization of those mechanisms is independent of reading comprehension. Our results generally imply that dyslexia is a hidden contributor to relatively poor reading skill within the deaf population and that assessment of dorsal stream function may provide a diagnostic biological marker for dyslexia in deaf people.