Asymmetric representation of symmetric semantic information in the human brain

Specific pairs of semantic entities have symmetric relationships, such as word pairs with opposite meanings (e.g., "intelligent" and "stupid"; "human" and "mechanical"). Such semantic symmetry is a key feature of semantic information. However, the representation of symmetric semantic information in the brain is not yet understood. For example, it remains unclear whether symmetric pairs of semantic information are represented in overlapping or distinct brain regions. We addressed this question in a data-driven manner by using the voxelwise modeling of movie-evoked cortical response measured by functional magnetic resonance imaging. In this modeling, response in each voxel was predicted from semantic labels designated for each movie scene. The semantic labels consisted of 30 different concepts, including 15 pairs of semantically symmetric concepts. Each concept was manually evaluated using a 5-point scale. By localizing the semantic representation associated with each concept based on the voxelwise accuracy of brain-response predictions, we found that semantic representations of symmetric concept pairs are broadly distributed but with little overlap in the cortex. Additionally, the weight of voxelwise models revealed highly complex, various patterns of cortical representations for each concept pair. These results suggest that symmetric semantic information has rather asymmetric and heterogeneous representations in the human brain.