Reward system atrophy relates to heightened feelings of physical closeness in Alzheimer's disease.

The experience of interpersonal space may be altered in Alzheimer's disease (AD) due to atrophy in brain systems that support emotion and reward processing. Seventy participants (AD = 36, healthy controls = 34) underwent structural neuroimaging and completed a modified stop-distance paradigm, an established interpersonal distance task, in which they indicated at which distance they preferred to have a conversation with the experimenter. After estimating the distance between themselves and the experimenter, participants rated their emotional experience. Feelings of physical closeness were calculated as the discrepancy between the perceived and actual distance between the participant and experimenter. Participants also estimated their distance from the wall, which served as a non-social control task. Unlike the healthy controls, the participants with AD felt physically closer to the experimenter than they were (despite no differences in objective interpersonal distance) and reported greater positive emotional experience. Voxel-based morphometry analyses revealed greater feelings of physical closeness related to smaller gray matter volume in the right ventral striatum and right medial orbitofrontal cortex (pFWE<.05). These results suggest that reward system atrophy influences how people with AD experience interpersonal space. Individuals with AD may feel physically closer to others than they are and find social proximity more enjoyable than healthy controls.