Tests of interhemispheric disconnection including visual, somesthetic, kinesthetic, auditory, and complex motor functions were performed on seven patients with chronic epilepsy who had undergone partial surgical section of the corpus callosum, verified by magnetic resonance imaging. Two patients with only one third of the splenium remaining demonstrated disconnection syndromes involving all modalities except vision, which was completely intact. Five patients had lesions involving the rostrum and the anterior two thirds to four fifths of the body of the callosum, with the splenium spared. They demonstrated little evidence of disconnection in the modalities indicated, except for left ear suppression on a dichotic listening task and partial somatosensory disruption in some cases. These results emphasize the importance of the posterior corpus callosum for interhemispheric sensory and sensorimotor transfer, although some discrepancies between current behavioral data and previous anatomic findings remain.

The performance of 20 aphasic patients on a verbal list-learning task was examined in relation to site of lesion, as documented by CT scan. Patients with lesions of the inferior frontal lobe and/or the basal ganglia were severely impaired in both acquisition and long-term retention of the list, while the performance of patients with posterior temporoparietal involvement was nearly normal. These results contrasted sharply with scores on short-term memory tests by the same patient groups which showed an opposite trend. The findings support a functional and neuroanatomical dissociation of short and long-term memory systems and suggest that neural connections of the inferior frontal lobe and the basal ganglia may be crucial for initiating the retrieval process.

• A 58-year-old, right-handed man became aphasic following a left-hemisphere stroke. Despite the massive size of the lesion, the aphasia was transient. An apraxia and agraphia did, however, persist three years after onset. The pattern of language impairment and subsequent recovery is believed to represent the rarely documented condition of mixed cerebral dominance for language.

Verbal and nonverbal recall of a simple tactual stimulus was assessed in patients receiving left intracarotid injections of sodium amytal. During the period of dominant hemisphere anesthetization, a common object was presented to the left hand-right hemisphere but was kept out of view. Following recovery from the drug, six of the eight patients were unable to recall the object verbally, but recognized it immediately in a visual multiple-choice array. These results demonstrate a dissociation of verbal and nonverbal memory mechanisms which suggests that delayed interhemispheric transfer of a functional sensory message is highly improbable.

Verbal and nonverbal recall of a simple tactual stimulus was assessed in patients receiving left intracarotid injections of sodim amytal. During the period of dominant hemisphere anesthetization, a common object was presented to the left hand-right hemisphere but was kept out of view. Following recovery from the drug, six of the eight patients were unable to recall the object verbally, but recognized it immediately in a visual multiple-choice array. These results demonstrate a dissociation of verbal and nonverbal memory mechanisms which suggests that delayed interhemispheric transfer of a functional sensory message is highly improbable.

The anterior commissure, which has been presumed to play a minor role in interhemispheric communication, was tested for the transfer of visual, auditory, and olfactory information in patients with complete sections of the corpus callosum. Four of five patients tested with presumed intact anterior commissures demonstrated interhemispheric transfer of verbal and pictorial stimuli presented visually to a single hemisphere. Evidence was also obtained for auditory and olfactory transfer, although successful interhemispheric communication in all three modalities was not found for any one patient. The data suggest that the human anterior commissure is capable of mediating multisensory, interhemispheric messages of a complex nature and provide evidence of functional plasticity in a phylogenetically early cerebral structure.

Cognitive functioning was extensively evaluated in a recent callosum-sectioned patient. A number of standardized and specialized experimental memory tests, as well as a sophisticated hypothesis testing task, were administered both before and after operation. No post-operative deficits were obtained. In fact, this patient showed marked improvement on almost every measure utilized. These results demonstrate that the cognitive processing of complex information is not necessarily dependent on the integrity of the corpus callosum, but rather suggest that cognitive functioning is largely an intrahemispheric process.

Preliminary psychologic testing was carried out on four patients who had undergone surgical sectioning of a portion or all of the corpus callosum and anterior commissure as a treatment for uncontrollable seizures. Results confirm earlier findings indicating the importance of the forebrain commissures in the interhemispheric exchange of a variety of sensory and motor information, and demonstrate that particular portions of the commissural system are responsible for transferring the information of specific sensory modalities. The patients also showed surprising abilities in performing complex tasks assumed to require integration of information from both hemispheres.