Hemispheric dissociations in controlled lexical-semantic processing

Cognitive mechanisms of semantic priming in individuals with intact cerebral hemispheres were studied using the visual half-field method and lexical-decision tasks. In Experiment 1, unidirectionally associated word pairs were presented in a forward direction (e.g., BEAVER-TAIL) to isolate priming attributable to automatic activation or expectancy-based processing. Forward priming was restricted to the right visual field-left hemisphere, and it occurred only when expectancy-based processing was encouraged in the instructions. Experiments 2 and 3 found backward priming (e.g., TAIL-BEAVER) only in the left visual field, indicating that the right hemisphere contributes to retrospective semantic matching of the target back to the prime. The results suggest that the 2 hemispheres have different roles in controlled processing of semantic relations.