Abnormal functional activation during a simple word repetition task: A PETstudy of adult dyslexics

Eight dyslexic subjects, impaired on a range of tasks requiring phonologica l processing, were matched for age and general ability with six control sub jects. Participants were scanned using positron emission tomography (PET) d uring three conditions: repeating real words, repeating pseudowords, and re st. In both groups, speech repetition relative to rest elicited widespread bilateral activation in areas associated with auditory processing of speech ; there were no significant differences between words and pseudowords. Howe ver, irrespective of word type, the dyslexic group showed less activation t han the control group in the right superior temporal and right post-central gyri and also in the left cerebellum. Notably, the right anterior superior temporal cortex (Brodmann's area 22 [BA 22]) was less activated in each of the eight dyslexic subjects, compared to each of the six control subjects. This deficit appears co be specific to auditory repetition as it was not d etected in a previous study of reading which used the same sets of stimuli (Brunswick, N., McCrory, E., Price, C., Frith, C.D., & Frith, U. [1999]. Ex plicit and implicit processing of words and pseudowords by adult developmen tal dyslexics: A search for Wernicke's Wortschatz? Brain, 122, 1901-1917). This implies that the observed neural manifestation of developmental dyslex ia is task-specific (i.e., functional rather than structural). Other studie s of normal subjects indicate that attending to the phonetic structure of s peech leads to a decrease in right-hemisphere processing. Lower right hemis phere activation in the dyslexic group may therefore indicate less processi ng of non-phonetic aspects of speech, allowing greater salience to be accor ded to phonological aspects of attended speech.