Hemispheric asymmetries in hierarchical stimulus processing are modulated by stimulus categories and their predictability.

Hemispheric dominance has been behaviourally documented for the local (left hemisphere, LH) or global (right hemisphere, RH) processing of hierarchical letters. However, Fink et al. (1997) indicated that stimulus category modulates this hemispheric asymmetry. The purpose of this study was to investigate the influence of the category (letters versus objects) on hemispheric specialisation for global and local processing using a visual half-field presentation in a task where participants ignored whether the target appeared at the global or local level. In Experiment 1 we replicated the classic hemispheric asymmetry for global/local processing of hierarchical letters. In Experiment 2, which consisted of hierarchical object processing, a RH dominance for the local level was observed. In Experiment 3 a within-participant design was used where anticipation about the stimulus category was precluded, resulting in the classic RH and LH specialisations for global and local processing for both letter-based and object-based stimuli. Taken together, these results suggest that the highly demanding local processing stage engages one hemisphere more than the other, according to the lateralisation of cerebral networks specialised for stimulus category. In addition, the direction of lateralisation for the local level was also modulated by the predictability of the stimulus category.