| |
Abstract:
Previous imaging studies of dyslexic individuals have
reported reduced activation of the posterior inferior region of the
left temporal lobe (BA37) with orthographic stimuli (Shaywitz et
al, 1998; Brunswick et al, 1999). In the normal population this
area is engaged by a range of word retrieval tasks. By employing
picture naming, the present study aimed to evaluate whether
abnormal activation would be observed in dyslexics during
phonological retrieval in the absence of orthographic decoding.
Eight dyslexics and ten control participants, matched for age and
performance IQ, were scanned using positron emission tomography
(PET) during 3 conditions: reading words; naming pictures; saying
'yes' or 'okay yes' to nonsense visual shapes. Relative to
baseline, both groups activated a common system for word reading
and picture naming. However, the dyslexic readers showed abnormal
activation in the left posterior inferior temporal cortex (BA37),
left frontal operculum and right cerebellum for both word reading
and picture naming. In BA37 dyslexics showed attenuated responses
to picture naming but reversed responses during reading (i.e. less
activation for words relative to baseline). The results indicate a
generalised deficit in the neural system underlying lexical
retrieval, which evokes abnormal cortical circuits particularly
during reading.
|
