Taylor, J. 1, 2 , Rastle, K. 3 & Davis, M. 1
1 MRC Cognition and Brain Sciences Unit
2 Newnham College, University of Cambridge
3 Royal Holloway University of London
Cognitive models of reading proposed that item-specific knowledge is necessary for irregular word reading (SEW, YACHT). Whereas item-specific knowledge is lexical (whole-word forms) in the Dual Route Cascaded model (Coltheart et al., 2001), it is semantic in the triangle model (Plaut et al., 1996). Neuroimaging contrasts are often confounded by difficulty, familiarity, and word meaning and, as such, have not discriminated between these possibilities. This fMRI experiment used an artificial language to avoid these confounds and delineate the neural systems supporting irregular and regular word learning. Twenty-two adults learned to read 24 new words written in novel symbols, whilst in an MRI scanner. Some words were regular - all symbols had one pronunciation, some words were irregular - vowel symbols were pronounced differently in different words. Regular symbols occurred in 8 (high frequency) or 4 items (low frequency). Learning involved interleaved training (see word-hear pronunciation) and testing (read words aloud) phases, and was followed by generalization to untrained words. Participants learned the trained words (regular high/low frequency-83%, irregular-73% correct) and generalized their knowledge to untrained words (70% correct). Activity in left occipitotemporal (visual-form processing) and parietal (spelling-sound processing) cortices was greater during irregular than regular word learning. However, this was also true for regular words containing low relative to high frequency symbols. In contrast, activity in left inferior frontal gyrus was greater for irregular words than either type of regular word. When confounds of difficulty, familiarity, and meaning are removed, irregular words do not engage brain regions representing visual form more than regular words. Parietal regions involved in mapping from spelling-to-sound are engaged when these mappings are either infrequent or irregular. Inferior frontal cortices are involved in resolving the phonological conflict associated with the multiple competing pronunciations available for irregular words, as embodied in both the DRC and triangle model.