Abnormalities in social and emotional behavior are the major diagnostic criteria for behavioral variant frontotemporal dementia (bvFTD). Investigators have attributed their behavioral disturbances to disease in mesial prefrontal and related networks, such as the salience network. This study examined the main neural correlates of informant-reported socioemotional dysfunction among patients with bvFTD compared to those with early-onset (before age 65) Alzheimer's Disease (EOAD).

Participants included 13 patients with bvFTD and their caregivers and 18 patients with EOAD and their caregivers. The caregivers consisted of a spouse, family member, or other informant who resided with the patient. They completed the informant-based Socioemotional Dysfunction Scale (SDS), a 40-item scale which rates common disturbances in social and emotional behavior on a five-point Likert scale (1-5). The patients underwent magnetic resonance imaging (MRI) with tensor-based morphometry (TBM) analysis of the 3D T1-weighted MRI scans. Computations of mean Jacobian values within select regions of interest (ROIs) in frontal and temporal lobes generated numerical summaries of regional volumes, and voxel-wise regressions created 3D statistical maps of the association between tissue volume and SDS total scores. Statistical analyses included independent samples t-tests group differences in ROIs and SDS scores, Pearson correlations between SDS scores and brain volumes, and multiple regression of ROIs with SDS scores and group as predictor variables.

Compared to the EOAD group, the bvFTD group had significantly higher SDS scores (p < .001; d = 2.24), smaller frontal lobe volumes (specifically dorsolateral-prefrontal cortex, p = .003; d = 1.24), and larger temporal lobe volumes (specifically hippocampus, p = .014; d = 0.979). Within the bvFTD group, higher SDS scores were associated with a smaller right anterior temporal lobe (ATL; p = .005; r = -.729), especially the lateral ATLs (p = .002; r = -.776), and a smaller bilateral orbitofrontal cortex (OFC; p = .016; r = -.650). In contrast, within the EOAD group, higher SDS scores were associated with a smaller right parietal cortex (p = .030; r = .542). In the entire sample (both bvFTD and EOAD), higher SDS scores was associated with a smaller lateral ATL volumes (p = .019; r = -.431). Regression analyses confirmed that SDS score predicted lateral ATL volume (p = .041; b = -.262) after controlling for diagnosis (p < .001; b = -.692).

These findings are consistent with greater socioemotional dysfunction, smaller frontal, and larger mesial temporal regions in bvFTD, when compared to EOAD. The findings, however, suggest that positively disturbed socioemotional behavior in bvFTD, as reported by caregivers, results from involvement of the right temporal lobe and the lateral temporal region, with further contribution from disease in OFC. The association of SDS scores and ATL volume across diagnostic groups suggests that this region is instrumental in socioemotional functioning and that the SDS may have diagnostic value in distinguishing the "right-temporal variant" of bvFTD.