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 language-dominant 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.

Figure 1

Syntactic comprehension test. Mean percentage correct responses for the age-matched controls, the nonagrammatic aphasics, and the agrammatic aphasics, for five different sentence types: (I) active, semantically irreversible sentences; (II) active, semantically reversible sentences; (III) simple passive sentences; (IV) sentences with an active subject-relative clause; and (V) sentences with a passive subject-relative clause. The syntactic complexity of the sentences increases from II to V.

Figure 2

Difference waveforms (obtained by subtracting the waveforms for the correct from the incorrect condition) and individual subject effect sizes at electrode site (Pz). Waveforms are averaged over participants for the group of age-matched controls (n = 12, solid line), nonagrammatic aphasics (n = 5, light dashed line), and agrammatic aphasics (n = 5, heavy dashed line). The waveforms are aligned at zero on the onset of the word that violates word-order preferences (a) or number agreement (c). (a) Word-order violation, difference waveforms. (b) Word-order violation effect sizes for individual subjects. Effect size (mean area amplitude in μV) at electrode site Pz per subject in the 600- to 1,500-ms window. (c) Agreement violation, difference waveforms. (d) Agreement violation effect sizes. Effect size (mean amplitude in μV) at electrode site Pz per subject in the 600- to 1,500-ms window. Negativity is plotted upwards.

Figure 3

The scalp distribution of the ERP effects that were obtained for the word-order violation in the normal controls, the nonagrammatic comprehenders, and the agrammatic comprehenders. Effects were based on mean amplitudes in the 600- to 1,500-ms latency window. Positive polarity effects are in red, negative-going effects are in blue. Scale values are in μV.

Figure 4

Difference waveforms (obtained by subtracting the waveforms for the correct from the incorrect condition) at a left temporo-parietal electrode site (LTP). Waveforms are averaged over participants for the group of age-matched controls (n = 12, solid line), nonagrammatic aphasics (n = 5, light dashed line), and agrammatic aphasics (n = 5, heavy dashed line). The waveforms are aligned at zero on the onset of the word that violates word-order preferences (a) or number agreement (b). Negativity is plotted upwards.