Real-time semantic compensation in patients with agrammatic comprehension: Electrophysiological evidence for multiple-route plasticity

Abstract

To understand spoken language requires that the brain provides rapid access to different kinds of knowledge, including the sounds and meanings of words, and syntax. Syntax specifies constraints on combining words in a grammatically well formed manner. Agrammatic patients are deficient in their ability to use these constraints, due to a lesion in the perisylvian area of the languagedominant hemisphere. We report a study on real-time auditory sentence processing in agrammatic comprehenders, examining their ability to accommodate damage to the language system. We recorded event-related brain potentials (ERPs) in agrammatic comprehenders, nonagrammatic aphasics, and age-matched controls. When listening to sentences with grammatical violations, the agrammatic aphasics did not show the same syntax-related ERP effect as the two other subject groups. Instead, the waveforms of the agrammatic aphasics were dominated by a meaning-related ERP effect, presumably reflecting their attempts to achieve understanding by the use of semantic constraints. These data demonstrate that although agrammatic aphasics are impaired in their ability to exploit syntactic information in real time, they can reduce the consequences of a syntactic deficit by exploiting a semantic route. They thus provide evidence for the compensation of a syntactic deficit by a stronger reliance on another route in mapping sound onto meaning. This is a form of plasticity that we refer to as multiple-route plasticity. In listening to language, qualitatively different syntactic and semantic constraints are combined to derive the message from the speech signal. The basic distinction between syntax and semantics can be observed in electrophysiological recordings of brain activity, where this distinction is reflected in two qualitatively different brain signals: the P600͞SPS and the N400, respectively (1-4). In this study, we used these two languagerelated event-related brain potential (ERP) effects to investigate the language deficit of agrammatic comprehenders and the nature of possible compensatory processes resulting from brain damage.It is a well established finding in the ERP literature on language comprehension that varying the meaning relation between words affects the amplitude of a negative-going potential with an onset at Ϸ250 ms and a maximal amplitude at Ϸ400 ms after word onset. This potential is known as the N400. Although the grammatical structure of the sentence ''The pizza is too hot to drink'' is identical to that of the sentence ''The pizza is too hot to eat,'' drink elicits a larger negative-going potential than eat because of the semantic mismatch between drink and the sentence context. The amplitude of the N400 is a sensitive manifestation of the binding of word meanings into an overall interpretation of the utterance (3, 5, 6). In aphasia research, this ERP component has been used to track real-time comprehension deficits in brain-damaged patients, where it has revealed processing deficiencies in the time course of meaning activation and integr...

Thematic role assignment in patients with Broca's aphasia: Sentence–picture matching electrified

“…To explain this result, the authors proposed that semantic constraints contribute to the assignment of NPs to thematic roles in normal sentence processing, and that this influence is much stronger in Broca patients as a result of a decrease in the syntactic contribution to this process. In this respect we would like to call attention to a previous ERP study of ours (Hagoort, Wassenaar, & Brown, 2003). In that study, Broca patients with a severe syntactic comprehension impairment showed, in response to word-order violations, an N400-effect instead of a syntax-related ERP effect.…”

ERP Effects of Subject—Verb Agreement Violations in Patients with Broca's Aphasia

“…Therefore, we give preference to the account provided here because it explains these differences in degree of severity quite naturally and parsimoniously. Finally, when the same severely impaired patients were confronted with syntactic violations of word order, they did show an effect: Their waveforms were dominated by a meaning-related ERP effect (Hagoort, Wassenaar, & Brown, 2003). The N400 effect for the word order violation suggests that these sentences were processed through a semantic compensatory processing route that was not available for the agreement violation.…”

Neural correlates of syntactic transformations

“…Sentence-picture matching tasks require semantic as well as syntactic interpretation, hence semantic and syntactic processing compete for limited resources. In these tasks there is a tradeoff in favor of semantics in agrammatic patients [see also Hagoort et al, 2003]; however, semantic interpretation is not required in grammaticality judgment tasks and thus syntax can operate in isolation. There is evidence that syntactical analysis of syntactically unambiguous structures is relatively independent of semantic context or semantic violations [Hagoort, 2003], e.g., syntactic violations can be detected in semantically uninterpretable Jabberwocky sentences [Indefrey et al, 2001].…”