Furtive incursions into bicameral minds. Integrative and coordinating role of subcortical structures

Section of forebrain commissures results in a pervasive disconnection syndrome characterized by the incapacity of the two cerebral hemispheres to exchange their respective information content. Yet commissurotomized patients are capable of unified behaviour in their daily activities and, in laboratory situations, are able to produce a single response as a function of the information received by both hemispheres. This study further investigated the nature of the information that can be communicated between the disconnected hemispheres in 3 commissurotomized patients. The first part comprised 7 experiments in which the patients were required to compare 2 digits simultaneously, 1 projected to each hemisphere. While the stimulus and experimental conditions were the same across experiments, the information content on which to base the comparison was manipulated. The level of performance varied as a function of the information content, such that the patients were unable to decide whether the 2 digits were the same or different, performed better when required to compare the digits in terms of their odd-even category, and were highly accurate at deciding which digit was the higher. In addition, when instructed to focus on the numerical value of the digits to decide whether the 2 digits were the same or different, they performed significantly above chance. In the second part, they were requested to report verbally information about emotional faces and familiar faces projected to the right hemisphere. While they were unable to access the visual characteristics and the identity of the faces, they could provide various correct items of semantic information about the individuals. Taken together, the results indicate that the disconnection syndrome is limited to specific categories of information and does not extend to all the knowledge that may be derived from a given object. They also suggest that subcortical structures do not serve simply as pathways linking the two hemispheres but play an essential coordinating role in the integration of hemisphere activity.