The relationship between social network size and brain structure in older adults living with HIV.

The social brain hypothesis suggests that primate brains evolved to manage social group complexities. While chronic HIV infection is associated with both structural brain changes and social exclusion, the possibility that social experience may contribute to brain changes has not been studied in this population. Here, we aimed to estimate the direction and strength of the relationship between gray matter volume and social network size in older people living with HIV in Canada. Fifty-eight HIV + participants (3 women) from the Positive Brain Health Now cohort underwent structural brain imaging and reported the size of their social network. We tested the relationship between social network size and gray matter volume in key brain regions previously identified in healthy older adults. Negative correlations were observed between social network size and gray matter volume in all regions of interest, adjusting for age, education, and total intracranial volume. The strongest correlation was in the left anterior cingulate cortex. We found evidence that social network size is related to gray matter volume in brain regions involved in social behavior among older people, mostly men, with longstanding HIV infection. However, the direction of this effect was opposite to that predicted. This echoes some previous work in healthy male samples. These findings suggest the need to consider social as well as biological variables in studying the brain impacts of living with HIV. Further work is needed to clarify which social variables have the greatest influence, and how they affect the brain.