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Neuroimaging study. We used functional magnetic resonance imaging (fMRI) to study the degree to which the representation of shapes, and especially letters and digits, is localized in left occipitotemporal cortex.
Five normal right-handed subjects participated in 8 fMRI runs (5 minutes 20 seconds each) in which they passively viewed blocks of letter strings, blocks of digit strings, blocks of geometric shape strings, and blocks of fixation points (baseline). Each run contained two blocks of each of the four stimulus types in counterbalanced orders. The blocks lasted 40 seconds each and contained 40 trials in which stimuli were displayed for 150msec, one per second. A procedure was applied to compensate for motion and data were analyzed using a variant of the general linear model approach proposed by Worsley & Friston (1995).
Statistically significant segregation was observed in individual subjects. In four of the five subjects an area in or near the left fusiform gyrus of occipitotemporal cortex (Brodmann area 37) responded significantly more to letters than digits or any of the other stimuli. One of these subjects was scanned again six weeks later and showed activation in exactly the same two areas both times. No digit-specific areas were observed in any subjects in this region. Some occipital areas were significantly activated by all three stimulus types compared with fixation (p<0.05, corrected).
These results provide direct evidence for the existence of a letter-specific area in the visual system of some literate adults: Letters were represented in areas that do not represent simple geometric shapes or even digits in four of the five subjects.
These results were just presented at the 1996 Annual Meeting of the Society for Neuroscience (Farah, Polk, Stallcup, Aguirre, Alsop, D'Esposito, Detre, & Zarahn, 1996).
Neuroimaging | Theory | Cognitive Modeling | Behavioral Studies | Discussion | Papers