Connectionsit modelling of surface dyslexia based on foveal splitting: Impaired pronunciation after only two hlaf pints

Connectionist modelling of surface dyslexia based on foveal splitting: Impaired pronunciation after only two half pints Richard Shillcock † Padraic Monaghan rcs/pmon@cogsci.ed.ac.uk Institute for Adaptive and Neural Computation, Division of Informatics † Department of Psychology University of Edinburgh, 2 Buccleuch Place, Edinburgh, EH8 9LW, UK Abstract In cases of surface dyslexia and phonological dyslexia there is a dissociation between the reading of irregular words and nonwords. This dissociation has been cap- tured in connectionist models of dyslexia in terms of im- pairments to the models’ phonological representations. We report a series of connectionist simulations based on an alternative neuro-anatomically motivated theory that dyslexia is at least partly caused by hemispheric desyn- chronisation. Problems of interhemispheric transfer af- fect the mapping between orthography and phonology be- cause the human fovea is precisely vertically split: a fix- ated word is initially split and the two parts contralaterally projected to the two hemispheres of the brain. Much of lexical processing can be reconstrued as the integration of this information (Shillcock, Ellison & Monaghan, 2000). We demonstrate that the dissociation between the read- ing of irregular words and nonwords can be understood in terms of a failure to integrate the initially split input. Introduction The word pint is pronounced differently from its ortho- graphic neighbours dint, hint, lint, mint, tint. In this sense its pronunciation is irregular. Because of the presence of such words in its lexicon, English is said to have a deep orthography. Irregular pronunciations have been observed to cause processing problems both for normal readers and for certain impaired readers: in the former a low frequency irregular word such as pint may take measurably longer to pronounce than a matched regu- lar word, and in the latter such a word might be er- roneously regularised (pint pronounced to rhyme with mint). For these reasons irregular words have been used in a large number of studies of how readers translate the orthographic representation of a word into its phonolog- ical specification. In this paper we approach the pro- nunciation of irregular words from the perspective of a new model of visual word recognition based on the neuro-anatomical observation of foveal splitting (Shill- cock, Ellison & Monaghan, 2000). We will argue that this model provides a principled motivation for some of the problems associated with irregular pronunciations in both normal and impaired readers. The measurable processing problems that normal adult readers have with low frequency irregular words (pint) do not extend to high frequency irregular words such as have (whose pronunciation cannot be predicted from cave, rave, pave, knave) (Paap & Noel, 1991; Seiden- berg, Waters, Barnes & Tanenhaus, 1984; Taraban & Mc- Clelland, 1987). Perhaps the most productive approach to modelling this interaction between regularity and fre- quency has been that developed by Seidenberg and Mc- Clelland in their (1989) connectionist model of reading and by Plaut and others subsequently (e.g. Plaut & Mc- Clelland, 1993; Plaut, McClelland, Seidenberg & Pat- terson, 1996; Harm & Seidenberg, 1999). Seidenberg and McClelland’s original model was a simple feedfor- ward model; later models have contained recurrent con- nections and more structured input and output layers. In general, in these models the apparently rule-based be- haviour that generates regular pronunciations is recast in statistical terms, so that a low frequency irregular pro- nunciation (pint) is militated against by the more fre- quently occurring regular pronunciation of the relevant vowel in similar contexts. Thus, when the irregular pro- nunciation is itself high frequency (have), that particular mapping between orthography and phonology is suffi- ciently emphasised to co-exist with the regular mapping. The superpositional storage that characterises these mod- els means that a minority mapping such as that required by pint is disproportionately demanding in terms of com- putational resources compared with the efficient general- isation represented by a regular pronunciation. This fact is also demonstrated in Plaut and McClelland’s (1993) model in which attractors behave componentially when the input and output representations of words are split into onset, nucleus and coda. In their model, pronunci- ation of the vowel in pint involves the connections from all three input slots (onset, nucleus and coda), whereas the regular, default pronunciation of the i involves only the nucleus. Irregular words pose particular problems for surface dyslexics, who are liable to produce regularisation er- rors (Patterson, Coltheart & Marshall, 1985; Manis et al., 1996). In connectionist models, the learning and re- tention of irregular pronunciations are generally vulner- able. For instance, Seidenberg and McClelland showed that restricting the number of hidden units impairs learn- ing low frequency irregular words, and Harm and Sei- denberg (1999) produced a similar effect by lowering the learning rate overall. Plaut et al. (1996) and others ex- plore the idea that a division of labour occurs between the direct orthography-phonology mapping and the same