Dissociating Word Reading and Lexical Decision in Neglect Dyslexia: A Connectionist Account

Dissociating Word Reading and Lexical Decision in Neglect Dyslexia: A Connectionist Account Giovanni Pagliuca (gp517@york.ac.uk) Padraic Monaghan (pjm21@york.ac.uk) Department of Psychology, University of York York, YO10 5DD, UK Abstract Neglect dyslexia is a reading impairment acquired as a consequence of brain injury characterized by failures to read verbal material on the left side of a text or at the beginning of words. Neglect dyslexia patients make many errors when naming isolated words, whereas they perform nearly normally when required to make a lexical decision judgment. This behavior has been interpreted in terms of a dual route model where a preserved lexical route is used to perform the lexical decision task, and an impaired non-lexical phonological route is used for naming. We trained a connectionist model to read single words and then lesioned it in order to simulate neglect dyslexia. We show that the damage to the model contributes to the same naming/lexical decision dissociation in a model with a single route for reading. Introduction Neglect dyslexia is a reading impairment usually associated with right brain damage and unilateral visuospatial neglect. Patients with neglect dyslexia may fail to read verbal material on the left side of an open book, or the beginning words of a line of text, or more often the beginning letters of a single word (Bisiach & Vallar, 1988; Hillis & Caramazza, Neglect dyslexia has been traditionally described according to a general framework of word representation proposed by Caramazza and Hillis (1990). In this model the process of word recognition has been hypothesized to be hierarchically organized through three stages (referred to as retinocentric , stimulus-centered and word-centered ), and neglect dyslexia has been shown to arise independently at each of these levels with different patterns of reading errors. With reference to the retinocentric level of representation, neglect dyslexia has been interpreted as an impairment of selective attention to the visual hemifield contralateral to the lesion in reference to a spatial coordinate system centred on the eye fixation point. Damage at this level results in the classic failure to correctly report the initial letters of a centrally presented word. In the last few years a mounting body of evidence has suggested that partial information from the contralesional side is accessible at many levels of processing in these patients. Vallar, Guariglia, Nico, & Tabossi (1996) documented a patient with severe neglect dyslexia who was impaired when asked to read aloud compound words such as girasole (sunflower), made up of two words: gira (turn) and sole (sun), but was able to produce appropriate associations to the compound as a whole (e.g. rose). This relative sparing of the associative task suggests that information from the neglected side of the space is still accessible to the lexical processing system to some degree. Relatedly, some patients with mild neglect dyslexia impairments make less errors in reading words than nonwords (Ladavas, Shallice, & Zanella, 1997a). This lexical effect also suggests that the lexical representation of a word might be partially accessed. Further evidence for preserved processing of some aspect of lexical processing from the contralesional portion of the word comes from a series of investigations by Ladavas, Umilta’ and Mapelli (1997b). The authors found that Italian patients who could not read words or nonwords aloud were nevertheless able to perform correctly on a lexical decision task and a semantic categorization task (living-nonliving) 1 . This dissociation was found for the same stimuli. Furthermore, Arduino, Burani and Vallar (2003) showed that, for neglect patients, lexical decision was influenced by the same morpho-lexical variables that influence lexical decision in control participants. The authors asked 6 Italian neglect dyslexic patients to perform a reading task on a list of words and nonwords and then tested them on the same stimuli on a lexical decision task. Patients made fewer errors in lexical decision for a written stimulus than in reading the word aloud. Arduino et al. (2003) also showed that lexical decision was influenced by the frequency of the word (high frequency words more likely to be accepted as lexical entries than low frequency words) and by the presence of a high frequency neighbor for the nonwords (a nonword with a high frequency neighbor was less likely to be rejected). Neglect dyslexia therefore provides insight into the interaction and relative sparing of levels of lexical processing in the brain. In the next section we review models of reading that have been posited to account for the fine-grained behavioral data in neglect dyslexics. Neglect dyslexia and the Reading System The dissociation between reading and lexical decision has been interpreted by Ladavas et al. (1997a, 1997b) in terms of an interaction between the attentional system and the two main routes proposed by the dual route model of reading (Coltheart, 1978; Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001). According to this theory, there are two different procedures for producing a phonological code and Patient data comes from untimed responses. Performance under such circumstances is almost 100% correct for reading and lexical decision in matched controls.