Haptic processing by the left hemisphere in a split-brain patient.

We report the case of a patient who suffered an ischemic accident resulting in damage to the anterior part of the corpus callosum and to the white matter in the posterior right hemisphere. Recognition of two-dimensional haptic stimuli explored with the right hand was severely impaired. The deficit was not specific to the type of stimuli, since letters, digits and geometrical shapes were not correctly recognized. Poor performance was not due to a specific mode of haptic exploration, since deficits were also observed without active manipulation of the stimuli. In contrast, the patient correctly named visual letters presented in the right visual hemifield (left hemisphere), and recognized three-dimensional common objects palpated with the right hand. Comparable results were observed in a surgical split-brain patient tested as a control. We conclude that (i) the construction of spatial representations of haptic stimuli, such as two-dimensional stimuli or three-dimensional block letters, cannot be fully realized in the intact left hemisphere, this ability requiring the contribution of both hemispheres, and (ii) tests for correct naming of common objects do not provide sufficient evidence to establish the integrity of the system involved in the identification of haptic information processed by the right hand of split-brain patients.