LEXICAL-SEMANTIC EVENT-RELATED POTENTIAL EFFECTS IN PATIENTS WITH LEFT-HEMISPHERE LESIONS AND APHASIA, AND PATIENTS WITH RIGHT-HEMISPHERE LESIONS WITHOUT APHASIA

Lexical-semantic processing impairments in aphasic patients with left hemisphere lesions and non-aphasic patients with right hemisphere lesi ons were investigated by recording event-related brain potentials (ERP s) while subjects listened to auditorily presented word pairs. The wor d pairs consisted of unrelated words, or words that were related in me aning. The related words were either associatively related, e.g. 'brea d-butter', or were members of the same semantic category without being associatively related e.g. 'church-villa'. The latter relationships a re assumed to be move distant than the former ones. The most relevant ERP component in this study is the N400. In elderly control subjects, the N400 amplitude to associatively and semantically related word targ ets is reduced relative to the N400 elicited by unrelated targets. Com pared with this normal N400 effect, the different patient groups showe d the following pattern of results. aphasic patients with only minor c omprehension deficits (high comprehenders) showed N400 effects of a si milar size as the control subjects. In aphasic patients with more seve re comprehension deficits (low comprehenders) a clear reduction in the N400 effects was obtained, both for the associative and the semantic word pairs. The patients with right hemisphere lesions showed a normal N400 effect for the associatively related targets, but a trend reward s a reduced N400 effect for the semantically related word pairs. A dis sociation between the N400 results in the word pair paradigm and P300 results in a classical tone oddball task indicated that the N400 effec ts were not an aspecific consequence of brain lesion, but were related to the nature of the language comprehension impairment. The conclusio ns drawn from the ERP results are that comprehension deficits in the a phasic patients are due to an impairment in integrating individual wor d meanings into an overall meaning representation. Right hemisphere pa tients are more specifically impaired in the processing of semanticall y move distant relationships, suggesting the involvement of the right hemisphere in semantically coarse coding.