Functional magnetic resonance imaging of neural activity related to orthographic, phonological, and lexico-semantic judgments of visually presented characters and words

Functional magnetic resonance imaging was used to investigate neural activi ty during the judgment of visual stimuli in two groups of experiments using seven and five normal subjects. The subjects were given tasks designed dif ferentially to involve orthographic (more generally, visual form), phonolog ical, and lexico-semantic processes. These tasks included the judgments of whether a line was horizontal, whether a pseudocharacter or pseudocharacter string included a horizontal line, whether a Japanese katakana (phonogram) character or character string included a certain vowel, or whether a chara cter string was meaningful (noun or verb) or meaningless. Neural activity r elated to the visual form process was commonly observed during judgments of both single real-characters and single pseudocharacters in lateral extrast riate visual cortex, the posterior ventral or medial occipito-temporal area , and the posterior inferior temporal area of both hemispheres. In contrast , left-lateralized activation was observed in the latter two areas during j udgments of real- and pseudo-character strings. These results show that the re is no katakana "word form center" whose activity is specific to real wor ds. Activation related to the phonological process was observed, in Broca's area, the insula, the supramarginal gyrus, and the posterior superior temp oral area, with greater activation in the left hemisphere. These activation foci for visual form and phonological processes of katakana also were repo rted for the English alphabet in previous studies. The present activation s howed no additional areas for contrasts of noun judgment with other conditi ons and was similar between noun and verb judgment tasks, suggesting two po ssibilities: no strong semantic activation was produced, or the semantic pr ocess shared activation foci with the phonological process. (C) 1999 Wiley- Liss Inc.