Hierarchical Stimuli and Hemispheric Specialization: Two Case Studies

Local versus global visual processing was examined in two patients with massive unilateral left hemisphere lesions using a directed attention task involving hierarchical stimuli. Previous studies found an impressive global advantage in patients with posterior left hemisphere lesions on similar tasks. In addition, whereas patients with left inferior parietal lobule (IPL) lesions showed the global interference on local processing that is typically observed in normals, patients with lesions centered on the superior temporal gyrus (STG) demonstrated no interference. Paradoxically, our two patients who had complete destruction of both the left IPL and STG regions showed an overall local advantage due to local interference on global processing. We propose that following extensive left hemisphere damage, the isolated right hemisphere may be able to perform efficiently the type of processing usually ascribed to the left hemisphere (i.e., local). However, at least under certain conditions, this apparent functional plasticity seems to occur at the expense of the type of processing normally associated with the right hemisphere (i.e., global).