Connectionist Modeling of the Breakdown and Recovery of Reading via Meaning

Abstract

At least two processing routes in the brain are involved in pronouncing written words: a semantic route that derives the pronunciation via meaning, and a phonological route that derives it via spelling-sound correspondences. Simulations involving partial damage to an isolated semantic route (Plaut & Shallice, 1993) provide a comprehensive account of the rather peculiar combination of symptoms exhibited by patients with deep dyslexia, including the occurrence of semantic errors (e.g., reading RIVER as "ocean"), their co-occurrence with visual errors, and influences of imageability or concreteness on correct and error performance. Furthermore, when a version of the model is retrained after damage (Plaut, 1996), the degree and variability of its recovery and generalization are qualitatively similar to the results of some cognitive rehabilitation studies. The results challenge traditional assumptions about the nature of the mechanisms subserving word reading, and illustrate the value of explicit computational simulations of normal and impaired cognitive processes. They also suggest that connectionist modeling can provide a framework for generating specific hypotheses about strategies for rehabilitation.Cognitive neuropsychology attempts to relate the patterns of impaired and preserved abilities of brain-injured patients to models of normal cognitive functioning, with the goals of explaining the behavior of the patients in terms of the effects of damage in the model, and of informing the model based on the observed behavior of patients (Coltheart, 1985;Ellis & Young, 1988;Shallice, 1988). A major motivation for many researchers is that a more detailed analysis of the normal mechanism, and the way it is impaired in particular patients, should lead to the design of more effective therapy to remediate these impairments (Howard & Hatfield, 1987; Riddoch & Humphreys, 1994; Seron & Deloche, 1989). Moreover, the patterns of recovery exhibited by patients place additional constraints on models of normal and impaired cognitive processing. The purpose of this chapter is to illustrate in a particular domain-reading via meaning-how computational principles from connectionist or parallel distributed processing (PDP) research can provide insight into the nature of normal cognitive processes, how they can break down following brain damage, how they can recover, and how to design therapy to maximize this recovery.Perhaps the most detailed attempts at relating the behavior of damaged connectionist networks to that of brain-injured patients has been in the domain of acquired reading disorders (see, e