Event-related functional magnetic resonance imaging study of the extrastriate cortex response to a categorically ambiguous stimulus primed by lettersand familiar geometric figures

Functional neuroimaging studies have suggested a specific role of the extra striate cortex in letter string and visual word form processing. However, t his region has been shown to be involved in object recognition and its spec ificity for the processing of linguistic stimuli may be questioned. The aut hors used an event-related functional magnetic resonance imaging design wit h category priming to record the response elicited by the passive viewing o f single letters, geometric figures, and of the categorically ambiguous sti mulus "O" that pertains to both sets of familiar symbols. Bilateral activat ions in the extrastriate cortex were found, with a left predominance partic ularly pronounced for the ambiguous stimulus. Individual analysis of spatia l extent and signal intensity showed a priming x stimulus x hemisphere inte raction. When primed by the congruous categoric set, a bilateral decrease i n activation was observed for letters and geometric figures. The ambiguous stimulus behaved as a letter for the left hemisphere, with decreased activa tion when primed by letters, whereas in the right hemisphere, an adaptation effect occurred when primed by geometric figures. These priming effects su ggest that, for the ambiguous stimulus, letter processing was systematicall y involved in the left extrastriate cortex. The current results support the existence of a neural substrate for the abstract category of letters.