Impairments in verb morphology after brain injury: A connectionist model

Abstract

The formation of the past tense of verbs in English has been the focus of the debate concerning connectionist vs. symbolic accounts of language. Brain-injured patients differ with respect to whether they are more impaired in generating irregular past tenses (TAKE-TOOK) or past tenses for nonce verbs (WUG-WUGGED). Such dissociations have been taken as evidence for distinct ''rule'' and ''associative'' memory systems in morphology and against the connectionist approach in which a single system is used for all forms. We describe a simulation model in which these impairments arise from damage to phonological or semantic information, which have different effects on generalization and irregular forms, respectively. The results provide an account of the bases of impairments in verb morphology and show that these impairments can be explained within connectionist models that do not use rules or a separate mechanism for exceptions.

Accessing Words from the Mental Lexicon

“…The ERPs reflected those morphological priming effects in the second session, but were less clear in the first session (Kaczer et al, in preparation Others have denied a psycholinguistic basis for morphological representations. Rather, morphological effects are supposed to emerge as the result of semantic and word-form processing as well as from their interaction (Joanisse and Seidenberg 1999;2005;Plaut and Gonnerman 2000). However, if semantic processes influenced the facilitation effects reported by Koester and Schiller (2008;201), semantic transparency should have resulted in a difference between transparent and opaque conditions.…”

The relationship between phonological and morphological deficits in Broca?s aphasia: Further evidence from errors in verb inflection

“…The degree of semantic impairment for these same target verbs was assessed using a synonym judgement test. As predicted by the Joanisse and Seidenberg (1999) model, the patients had essentially normal performance in generating and recognising the past-tense forms of novel and regular verbs, but substantially reduced accuracy on the irregular past tense that was modulated by the frequency of the words and, critically, by the degree of the patients' semantic impairment. Bird, Lambon Ralph, Seidenberg, McClelland, and Patterson (2003) investigated the hypothesised link between poor regular verb performance and phonological impairment in 10 patients with Broca's aphasia.…”

“…The double dissociation is attributed to (a) the greater reliance on word meaning for irregular verbs (Joanisse & Seidenberg, 1999) and (b) the greater phonological complexity of the regular past tense, following from the fact that morphological regularity correlates inversely with phonological regularity (Burzio, 2002;McClelland & Patterson, 2002). This single mechanism account predicts that poor performance with irregular verbs, especially for lower frequency items, should be associated with semantic impairment, while the relative deficit for regular verbs reported in anterior aphasic patients should be associated with phonological impairment.…”

“…According to an alternative view, given the pervasive consistencies that can be found amongst the irregular verbs, the contrast between regular and irregular verbs should not be viewed in absolute terms but as a more graded difference (Joanisse & Seidenberg, 1999;McClelland & Patterson, 2002;Rumelhart & McClelland, 1986). This view proposes a single system that draws on semantic and phonological knowledge, and processes all verb types.…”

“…By this account, the double dissociation found across different patient groups is assumed to reflect phonological and semantic influences on a single process (see Joanisse & Seidenberg, 1999 for a computational instantiation of this theory). The double dissociation is attributed to (a) the greater reliance on word meaning for irregular verbs (Joanisse & Seidenberg, 1999) and (b) the greater phonological complexity of the regular past tense, following from the fact that morphological regularity correlates inversely with phonological regularity (Burzio, 2002;McClelland & Patterson, 2002).…”