Semantic activation within and across the cerebral hemispheres: what's left isn't right.

This study examined differences in the spread of semantic activation within and between the cerebral hemispheres. A lateralised lexical decision task using indirect priming was presented to 58 undergraduates with primes and targets separated by 215 or 750 milliseconds (ms). Prime and target words were presented to the same or opposite visual fields and were either directly related (book-read), indirectly related (lion-[tiger]-stripes), or unrelated (cup-street). At 215 ms participants exhibited significant priming effects to directly related words in all conditions except when primes and targets were both presented to the right hemisphere (RH). In contrast, priming to indirectly related words was effective only when primes and targets were presented to opposite hemispheres. At 750 ms, significant priming occurred for directly related words in all conditions, and for indirectly related words when primes were presented to the RH. Results suggest that priming for directly and indirectly related concepts occurs unilaterally in each hemisphere before 215 ms. Both prime types activate semantic networks in the RH within 750 ms, whereas the LH processes information in a more focused manner. This suggests that activation spreads contralaterally from each hemisphere first to directly and then to indirectly related concepts, indicating the importance of incorporating contralateral priming contrasts in lexical decision tasks.