Hemispheric differences in relational reasoning: Novel insights based on an old technique

Relational reasoning, or the ability to integrate multiple mental relations to arrive at a logical conclusion, is a critical component of higher cognition. A bilateral brain network involv ing lateral prefrontal and parietal cortices has been consistently implicated in relational reasoning. Some data suggest a preferential role for the left hemisphere in this form of reasoning, whereas others suggest that the two hemispheres make important contributions. To test for a hemispheric asymmetry in relational reasoning, we made use of an old technique known as visual half-field stimulus presentation to manipulate whether stimuli were presented briefly to one hemisphere or the other. Across two experiments, 54 neu- rologically healthy young adults performed a visuospatial transitive inference task. Pairs of colored shapes were presented rapidly in either the left or right visual hemifield as par-ticipants maintained central fixation, thereby isolating initial encoding to the contralateral hemisphere. We observed a left-hemisphere advantage for encoding a series of ordered. visuospatial relations, but both hemispheres contributed equally to task performance when the relations were presented out of order.To our knowledge, this is the first study to reveal hemispheric differences in relational encoding in the intact brain.We discuss these findings in the context of a rich literature on hemispheric asymmetries in cognition.