Hemispheric asymmetries in activation and integration of categorical information

Hemispheric asymmetries in activation and integration of semantic information were studied in the normal brain. In Experiment 1, categorically related primes and targets (HAND-NOSE) were presented to the left visual field (LVF) or to the right visual field (RVF) in a primed lexical decision task. The ratio of nonword targets in relation to all unrelated targets in the stimulus lists was manipulated. It was assumed that at the low nonword ratio the priming effects would primarily tap automatic semantic processing. At the higher ratio, priming would be generated predominantly by postlexical meaning integration or semantic matching. The results revealed automatic priming in the RVF/left hemisphere and postlexical priming in the LVF/right hemisphere. Experiment 2 measured automatic categorical activation with the lower nonword ratio and by presenting primes at the centre of the field. Bilateral priming was observed. It is suggested that the left hemisphere automatically activates categorically related word meanings in both hemispheres. The right hemisphere contributes by maintaining the meanings active and by retrospectively integrating them to the context.