Reduced neural specificity for a romantic partner in the nucleus accumbens over relationship duration.

Neural processes distinguishing romantic love from opposite-sex friendships remain a key challenge in neuroscience. Research on monogamous prairie voles has revealed that the nucleus accumbens (NAcc) is pivotal for partner-specific processing through plastic changes. However, it remains unclear in humans whether the NAcc differentiates a partner from opposite-sex friends, and how partner-related processing changes as the relationship matures. In a sample of 47 heterosexual male participants, we investigated the neural representations of a female partner, a female friend, and a male friend, in the NAcc, caudate nucleus and putamen. We collected fMRI data from participants during a social incentive delay task designed to elicit neural responses in anticipation of social approval from each of them. Classifier-based multivoxel pattern analysis (MVPA) demonstrated that neural activity patterns in all three regions distinguished the female partner from the female friend. Importantly, similarity-based MVPA revealed that, in the NAcc, the female friend was represented closer to the male friend than to the partner. Furthermore, exploratory analyses indicated that individuals in longer romantic relationships presented less distinguishable neural responses between the partner and the female friend in the NAcc. These findings suggest partner-specific processing in the NAcc, with this specificity diminishing as the relationship matures.