PROCESSING OF SEMANTIC ANOMALY BY RIGHT AND LEFT HEMISPHERES OF COMMISSUROTOMY PATIENTS: EVIDENCE FROM EVENT-RELATED BRAIN POTENTIALS

Abstract

The ability of 5 commissurotomized patients to appreciate semantic anomalies presented to their right and left hemispheres was tested using both electrophysiological and behavioural measures. In all cases, the patients heard sentence fragments that were completed either by semantically congruous or incongruous words briefly flashed to the left visual field, right visual field or to both fields simultaneously. A dissociation between behavioural and event-related brain potential (ERP) measures was observed. All 5 patients were able to indicate by a pointing response with greater than chance accuracy whether the terminal word of a sentence made sense (i.e., appropriate for the context) or was nonsensical. This was true regardless of the hemisphere receiving the terminal word. Likewise, all the patients responded to right visual field anomalies with a cerebral potential (N400) that was typically elicited by such words in control subjects. In contrast, only those 2 patients who developed an overt speech capability under the control of the right hemisphere produced N400 waves in response to left visual field anomalies. These findings were interpreted as suggesting possible relationships within language generation and semantic priming.