Abstract
Two groups of male university students who had been diagnosed as dyslexic
when younger, and two groups of control subjects of similar age and
IQ to the dyslexics, were scanned whilst reading aloud and during
a task where reading was implicit. The dyslexics performed less well
than their peers on a range of literacy tasks and were strikingly
impaired on phonological tasks. In the reading aloud experiment,
simple words and pseudowords were presented at a slow pace so that
reading accuracy was equal for dyslexics and controls. Relative to
rest, both normal and dyslexic groups activated the same peri- and
extra-sylvian regions of the left hemisphere that are known to be
involved in reading. However, the dyslexic readers showed less activation
than controls in the left posterior inferior temporal cortex Brodmann
area (BA) 37, or Wernicke's Wortschatz, left cerebellum, left thalamus
and medial extrastriate cortex. In the implicit reading experiment,
word and pseudoword processing was contrasted to visually matched
false fonts while subjects performed a feature detection paradigm.
The dyslexic readers showed reduced activation in BA 37 relative
to normals suggesting that this group difference, seen in both experiments,
resides in highly automated aspects of the reading process. Since
BA 37 has been implicated previously in modality-independent naming,
the reduced activation may indicate a specific impairment in lexical
retrieval. Interestingly, during the reading aloud experiment only,
there was increased activation for the dyslexics relative to the
controls in a pre-motor region of Broca's area (BA 6/44). We attribute
this result to the enforced use of an effortful compensatory strategy
involving sublexical assembly of articulatory routines. The results
confirm previous findings that dyslexic readers process written stimuli
atypically, based on abnormal functioning of the left hemisphere
reading system. More specifically, we localize this deficit to the
neural system underlying lexical retrieval.
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