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Abstract:
Bloomfield Centre, McGill University, Montreal, Canada Brain
lesions in the left posterior temporal lobe often result in
semantic memory deficits. Functional neuroimaging in normal
subjects using various semantic tasks, however, has generally
demonstrated predominant activity paradoxically in the inferior
frontal lobe region. We have developed a paradigm that provides
evidence for the left temporal region as the critical neural
correlate of semantic activation. By increasing picture-naming
difficulty with less familiar stimuli during a previous PET study,
a significant increase in CBF to this region was demonstrated.
Regression analyses also correlated CBF in this region to
individuals naming accuracy. With the same paradigm, the present
fMRI study aimed to determine if the overall activation pattern
could be replicated, and to examine the extent of intersubject
consistency. Our group results revealed that with fMRI there was
activation in the same left posterior temporal region as with PET.
In fact, there was a greater volume of activation at a higher level
of significance than found in PET. The centroids of activation,
however, differed by 7 +/- 5mm between the modalities, with the
fMRI centroid location posterior to that for PET. This was due in
part to variability in single-subject activation, with a subgroup
having more posterior and dorsal activation versus another subgroup
having more anterior and ventral activation. Therefore, fMRI
results replicated PET, and showed the individual variation in the
precise locus of functional activation.
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