Normal and impaired spelling in a connectionist dual-route architecture

This paper presents a dual-route connectionist model of spelling, in which one route maps directly from sound to spelling ( phonemes to graphemes), while in the other route the mapping is mediated by a further level of representation. The direct route is implemented as a two-layer associative network, with syllabically structured phonemic ( input) and graphemic ( output) representations, which comes to behave as a productive sound-to-spelling conversion mechanism through the exposure to a corpus of monosyllabic words. The mediated route is modelled as a frequency-sensitive lexical pathway. Nodes representing more frequent words become activated more rapidly than those of lower-frequency words. Access to both routes occurs in parallel, and the final spelling is determined by the combined output of both routes. We show that the model accounts for a wide range of data from normal spellers ( including nonword spelling, the variability in vowel spelling and the effect of surrounding phonological context, frequency effect and its interaction with spelling regularity). We also investigate the effect of a selective lesion to the lexical route in which the ceiling of lexical activation is lowered. This manipulation produces a model with surface dysgraphic characteristics, which is tested against data from two impaired subjects. As well as simulating the classic surface dysgraphic profile, including a frequency by regularity interaction, the model exhibits a phenomenon that has only recently been reported, and which provides strong evidence for the idea that multiple routes are active in parallel, and combine to produce the final spelling.