Do dual-route models accurately predict reading and spelling performance in individuals with acquired alexia and agraphia?
Introduction
Dual-route models are scientific hypotheses about the cognitive architecture of the information-processing system used for reading and spelling (Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001; Houghton & Zorzi, 2003; Jackson & Coltheart, 2001). According to these models, written language processing is accomplished by two distinct but interactive procedures that are referred to as the lexical and non-lexical routes (Fig. 1).1 Reading and spelling by the lexical route relies on the activation of word-specific orthographic and phonological memory representations. Although spoken and written words also automatically activate the corresponding conceptual representations in the semantic system, access to word meanings is not considered critical for accurate oral reading or spelling to dictation. The lexical route can process all familiar words, regardless of whether they are regular or irregular in terms of their letter–sound relationships, but it fails with unfamiliar words or non-words because these items do not have lexical representations. In contrast to the whole-word retrieval process employed by the lexical route, the non-lexical route utilizes a subword-level procedure based on sound-spelling correspondence rules. The non-lexical route can succeed with non-words (e.g., plunt) and also with regular words that strictly obey English phoneme–grapheme conversion rules (e.g., must), but it cannot produce a correct response to irregular words that violate these rules (e.g., choir). Attempts to read or spell irregular words by the non-lexical route result in regularization errors (e.g., have read to rhyme with save, or tomb spelled as toom). It should be noted that although dual-route models contain functional components that are unique to either the lexical route (e.g., orthographic lexicon) or the non-lexical route (e.g., phoneme–grapheme conversion module), the two procedures are not considered to be completely independent. For instance, the two routes share processing components at the phoneme and letter levels (Fig. 1). Furthermore, it is assumed that all written and spoken input is processed obligatorily by both routes in parallel, with cooperative or competitive interactions taking place at the phoneme (reading) or letter (spelling) output stage (Coltheart et al., 2001; Houghton & Zorzi, 2003). However, dual-route theory maintains that only the lexical route can deliver a correct response to irregular words, whereas the integrity of the non-lexical route is essential for accurate reading/spelling of non-words.
Dual-route models have provided a powerful theoretical framework for interpreting the written language performance of individuals with acquired alexia/agraphia. In particular, by specifying the functional architecture of the written language processing system it becomes possible to use the impaired and preserved reading/spelling abilities of neurological patients to identify the damaged or dysfunctional cognitive module. For instance, damage to the lexical route gives rise to surface dyslexia/dysgraphia, characterized by a disproportionate difficulty in reading/spelling irregular words (Beauvois & Dérouesné, 1981; Patterson, Marshall, & Coltheart, 1985; Rapcsak & Beeson, 2004; Roeltgen & Heilman, 1984). Reading/spelling of regular words and non-words is relatively spared, however, as these items can be processed successfully by the intact non-lexical route. By contrast, damage to the non-lexical route results in phonological dyslexia/dysgraphia, characterized by poor reading/spelling of non-words (Beauvois & Dérouesné, 1979; Coltheart, 1996; Henry, Beeson, Stark, & Rapcsak, 2007; Roeltgen, Sevush, & Heilman, 1983; Shallice, 1981). Reading/spelling of familiar regular and irregular words is relatively unimpaired because, for these items, patients can rely on the preserved lexical route.
In addition to explaining different subtypes of acquired alexia/agraphia by reference to a cognitive model of normal written language processing, dual-route theory can also be used to generate quantitative predictions about reading/spelling performance. For instance, Coltheart and co-workers (Castles, Bates, & Coltheart, 2006; Coltheart et al., 2001, Coltheart, 2006a) have suggested that one can predict regular word reading accuracy by knowing how well individuals perform on lists of irregular words and non-words. Recall that according to dual-route models irregular words can only be read correctly by a lexical strategy, whereas non-words can only be read correctly by a non-lexical strategy. Therefore, the proportion of irregular words, or p(IRREG), and the proportion of non-words, or p(NWD), that a person can accurately read provide relatively pure estimates of the competency of the lexical and non-lexical routes. Because dual-route theory posits that either route can process regular words, reading accuracy for these items, or p(REG), should be predictable from p(IRREG) and p(NWD) by the following formula:
To illustrate the logic behind the equation, let us assume that a patient obtains reading scores of 60% correct for irregular words and 40% correct for non-words. Because irregular word scores estimate the competency of the lexical route, we can predict that this individual should be able to read 60% of regular words by a lexical strategy. Reading accuracy for the remaining 40% of regular words will be determined by the functional capacity of the non-lexical route, as reflected by non-word reading scores. This means that our patient should be able to read an additional 16% of regular words by a non-lexical strategy (i.e., 40% of the remaining 40% regular word items). Therefore, the dual-route equation predicts that by relying on the combined residual capacity of the lexical and non-lexical routes this person should obtain a reading score of 76% (i.e., 60 + 16%) correct on a list of regular words.
Coltheart and co-workers (Castles, Bates, & Coltheart, 2006; Coltheart et al., 2001) have applied the prediction equation to nine sets of reading data obtained from three different subject populations: young normal readers (n = 2136), children with developmental dyslexia (n = 93), and children with brain damage due to stroke (n = 17). These investigators documented a high correlation between predicted and observed regular word reading scores in all three groups (range: +.825 to +.980) and concluded that the findings provided strong support for dual-route models of reading.
The purpose of our study was to determine whether the dual-route equation does an equally good job with reading data obtained from adult neurological patients with acquired alexia and to explore whether the formula can be used to predict spelling performance in these individuals. We also examined whether a multiple regression model based on dual-route theory successfully predicts the reading and spelling performance of our patients.
Section snippets
Participants
The data reported here were collected from 33 neurological patients who were administered a comprehensive reading/spelling battery as part of an ongoing investigation of the cognitive mechanisms and neural substrates of written language processing (e.g., Henry et al., 2007; Rapcsak & Beeson, 2004). All subjects gave informed consent prior to participation and the study was approved by the IRB of the University of Arizona. All participants were native English speakers and none had a history of
Results
Observed accuracy for reading/spelling regular words plotted against predicted accuracy from the dual-route equation (Castles, Bates, & Coltheart, 2006; Coltheart et al., 2001) is shown in Fig. 2. It is apparent that the equation performs remarkably well in predicting written language performance in neurological patients with alexia/agraphia. In particular, simple regression analyses indicated that the dual-route equation explains 88.8% of the variance in regular word reading scores (F1,30 =
Discussion
Support for dual-route theories of written language processing comes from a variety of sources, including observations in normal readers/spellers, neuropsychological data from individuals with developmental or acquired alexia/agraphia, computational modeling, functional neuroimaging, and genetic research (for reviews, see Bates et al., 2007; Castles, Bates, Coltheart, Luciano, & Martin, 2006; Coltheart, 2006b, Coltheart et al., 2001; Houghton & Zorzi, 2003; Jackson & Coltheart, 2001; Jobard,
Acknowledgments
The work reported in this paper was supported by grants DC008286 and DC007646 from the National Institute on Deafness and Other Communication Disorders and by grant P30AG19610 from the National Institute on Aging.
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