Chapter 1. Introduction and Background
pronounce the word in the corresponding phonological lexicon. The second route on the other hand, uses a set of rules to map from the orthographic to the phonological form, without having to refer to the lexicon at all (see Fig. 1.1). The rules are used to convert the letters of the written word to the associated phonetic sounds.
From this, it is clear that exception words can only be read using the lexical route, since their spelling-to-sound correspondence is highly irregular. Since exception words do not obey any rules for mapping orthography to phonology, they must be looked up in the lexicon. If the non-lexical route would be used for these words, the result would be regularisation errors, where the word would be read according to the rules for regular words. On the other hand, new words that have never been seen before, and non- words, can only be read using the non-lexical route, since they are not present in the lexicon. These words are read according to the rules for mapping regular words from orthography to phonology. Defendants of the Dual-Route theory often maintain that it is not possible for any single route model to correctly read both words and non-words (or novel words).
One of the tests of this model that is often cited are the two main forms of dyslexia. In surface dyslexia, patients have problems reading exception words, whereas perfor- mance on non-words is normal (see for example Coltheart et al. (1983)). On the other hand, phonological dyslexia is characterised by difficulties in non-word reading but normal exception word reading (see for example Howard and Best (1996)). In the Dual Route model, surface dyslexia can be explained as being caused by an impair- ment of the lexical route, whereas phonological dyslexia is caused by an impairment of the rule-based route. A number of studies have been done with dyslexic patients to test the dual-route model (Castles and Coltheart, 1993; Coltheart et al., 1993; Weekes and Coltheart, 1996).