Cerebral networks for number processing: Evidence from a case of posterior callosal lesion

We report a study of number processing in a patient with a lesion selectively destroying the posterior half of her corpus callosum. This case provided an opportunity to study the cerebral distribution of numerical abilities across hemispheres, and their interhemispheric communication pathways. Tasks of interhemispheric same-different judgement with digits and sets of dots showed that exact digit identity could not be transferred between hemispheres, but that some approximate magnitude information could. With arabic numerals presented in the left visual field, reading aloud and arithmetic were severely impaired. In contrast, larger-smaller magnitude comparison was spared. With right-visual-field stimuli, the patient's performance was essentially normal in reading aloud, arithmetic and number comparison. These findings lend support to the hypothesis that both hemispheres are capable of identifying arabic digits, and of accessing and comparing the corresponding magnitudes. However, the verbal abilities which underlie overt naming and arithmetic computations are available only to the left hemisphere.