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.