The Role of the Left Hemisphere in Verbal and Spatial Reasoning Tasks

Laterality of reasoning processes have long been a source of investigation. Differing formats of verbal and spatial reasoning tasks have meant it has not been possible to extricate true performance level from artefacts of input and output modalities. The Verbal and Spatial Reasoning Test (VESPAR) offers this opportunity, by virtue of matched sets of verbal and spatial inductive reasoning problems. Two series of 40 patients with unilateral left and right hemisphere lesions were tested on two verbal and two spatial subtests of the VESPAR, together with a battery of baseline tests. The performance of the left and right hemisphere lesion cases was compared with a normal standardisation sample. Whereas only the left hemisphere group failed the verbal sections, both left and right hemisphere groups failed the spatial sections. The influence of aphasia on spatial reasoning was considered to be an incomplete explanation for the failure of the left hemisphere group on the spatial sections. It is concluded that this investigation provides firmer evidence of a crucial role for the left hemisphere in both verbal and spatial abstract reasoning processes.