Numerical processing in the two hemispheres: studies of a split-brain patient.

Neuroimaging and lesion studies have provided insights into the neural mechanisms underlying numerical processing, yet the roles of the right and left hemispheres have not been systematically investigated within a single study. To address this issue, we investigated subitizing and magnitude comparison abilities in a split-brain patient. The first experiment examined the two hemispheres' abilities to enumerate briefly presented sets of one to four stimuli. Both hemispheres were equally able to perform this task. The second and third experiments examined the hemispheres' abilities to make magnitude judgments about two simultaneously presented stimuli that were either identically coded (i.e., two Arabic numerals, two number words, or two arrays of dots) or differently coded (e.g., an Arabic numeral and a number word). Although the left hemisphere was more accurate than the right when the task involved number words, both hemispheres were able to make comparisons between numerical representations regardless of stimuli coding. In addition, both hemispheres exhibited a distance effect. The results are discussed in the context of Dehaene's triple-code model.