Interhemispheric transfer without forebrain commissures

Surgical section of the corpus callosum (CC) usually produces disconnection deficits. The absence of disconnection symptoms in individuals born without a CC has been attributed to the use of residual commissures, such as the anterior commissure (AC). The aim of the present study was to evaluate interhemispheric transfer of sensory and cognitive information in a patient who lacks both the CC and the AC. First, to assess transfer of sensory information, interhemispheric transmission times (ITTs) to simple visual, auditory and somesthetic stimuli were measured using Poffenberger's paradigm. Second, a visual letter discrimination task was used to evaluate interhemispheric transfer of cognitive information. Subjects made a same/different judgment based on the physical or the lexical characteristics of the stimuli by pressing one of two response keys. Results of the sensory tasks showed that the visual and somesthetic ITTs of the agenetic subject were significantly longer than those of controls, whereas his auditory ITT was normal. In the cognitive task, the agenetic subject was able to discriminate letters intra- and interhemispherically according to their physical features. However, he was unable to perform a correct lexical analysis except when the letters were presented to the left hemisphere. The results indicate that sensory information may be transferred via subcortical commissures, but that transfer of information of a more cognitive nature requires the integrity of a cortical commissure.