The Left Fusiform Area Is Affected by Written Frequency of Words

The recent neuroimaging literature gives conflicting evidence about whether the left fusiform gyrus (FG) might recognize words as unitary visual objects. The sensitivity of the left FG to word frequency might provide a neural basis for the orthographic input lexicon theorized by reading models [Patterson, K., Marshall, J. C., & Coltheart, M. (1985). "Surface dyslexia: Cognitive and neuropsychological studies of phonological reading." London: Lawrence Erlbaum]. The goal of this study was to investigate the time course and neural correlates of word processing in right-handed readers engaged in an orthographic decision task. Three hundred and twenty Italian words of high and low written frequency and 320 non-derived legal pseudo-words were presented for 250 ms in the central visual field. ERPs were recorded from 128 scalp sites in 10 Italian University students. Behavioural data showed a word superiority effect, with faster RTs to words than pseudo-words. Left occipito/temporal N2 (240 ms) was greater to high-frequency than low-frequency words and pseudo-words. According to the swLORETA inverse solution, the underlying neural source of this effect was located in the left fusiform gyrus of the occipital lobe (X = -29, Y = -66, Z = -10, BA19) and the right superior temporal gyrus (X = 51, Y = 6, Z = -5, BA22), which are probably involved in word recognition and semantic representation, respectively. Later frontal ERP components, LPN (300-350) and P3 (400-500), also showed strong lexical sensitivity, thus suggesting implicit semantic processes. The results shed some light on the possible neural substrate of visual reading disabilities such as developmental surface dyslexia or pure alexia.