Experiment 1. Identifying amygdala-based intrinsic connectivity networks
As predicted, using the three cortical ROIs as seed regions, the connectional analysis revealed three voxel clusters within the amygdala (). As predicted, we found the strongest connectivity between the lOFC and the ventrolateral amygdala, the vmPFC and the medial amygdala, and the cACC with the dorsal amygdala. These connectionally-defined amygdala subregions strongly resemble the subregions of the amygdala depicted in that we predicted based on cytoarchitectonically-defined nuclear groups of the amygdala ().
Figure 4 Connectionally-defined amygdala subregions from this study and cytoarchitectonically-defined amygdala subregions from the Juelich Histological Atlas Connectionally-defined amygdala subregions from our study (a) and cytoarchitectonically-defined amygdala (more ...)
Using spherical seed ROIs placed within each amygdala subregion (), we next delineated partially distinct large-scale intrinsic functional connectivity maps () for a network supporting social perception, a network supporting social affiliation, and a network supporting social aversion; these networks largely resemble the hypothesized networks derived from animal tract-tracing studies and human task-related fMRI studies (). As predicted, the ventrolateral amygdala subregion showed strongest connectivity with areas important for perceptual processes including the fusiform gyrus and neighboring areas of the ventromedial temporal cortex extending to the pole, as well as the rostral STS, and caudal, medial, and lateral OFC. The medial amygdala subregion showed strongest connectivity with limbic areas important for affiliative behaviors including the vmPFC and neighboring subgenual and rostral ACC, the ventromedial striatum localized largely in the nucleus accumbens, and the ventromedial hypothalamus. Finally, the dorsal amygdala subregion showed strongest connectivity with areas important for aversive behaviors including the cACC, the insula and somatosensory operculum, the ventrolateral striatum localized in the putamen, the caudolateral hypothalamus, and regions in the thalamus and brainstem.
Each amygdala subregion anchors one of three distinct large-scale corticolimbic networks.
We replicated all three amygdala subregions and networks in our independent replication sample. The three initial cortical seed regions (lOFC, vmPFC, cACC) identified clusters of voxels within the amygdala that were highly similar to those identified within our discovery sample (). Seeds within these three subregions produced large-scale connectivity maps in the replication sample that were reliable with those identified within the discovery sample (). The eta2 coefficients were 0.88, 0.82, and 0.86 for lateral, medial, and dorsal amygdala seeds, respectively.
Replication of amygdala subregions and networks in an independent sample (N=83)
Experiment 2. Amygdala connectivity and social network size
As hypothesized, individual differences in the strength of connectivity within the networks supporting social perception and affiliation predicted social network size over and above variations in amygdala volume (which predicted 15% of the variance in social network size). The results demonstrated that participants with stronger amygdala connectivity within these networks had larger social networks relative to those individuals with weaker connectivity within these networks (). Individual differences in the strength of the amygdala’s connectivity within the network supporting social aversion did not predict differences in social network size (), however. Furthermore, using a multiple linear regression analysis, we found that stronger amygdala connectivity within the networks supporting social perception and affiliation each contributed independently to larger social network size (along with amygdala volume), predicting a total of 41% of its variance (see ). We found similar patterns of results for individual differences in the complexity participants’ social networks (i.e., the number of groups in which participants have at least four network members) and the diversity of their networks (i.e., the number of roles participants play within their networks); this is not surprising given that both social network complexity and diversity were strongly correlated with social network size ( and ).
A larger social network is predicted by stronger connectivity between amygdala subregions and corticolimbic regions important for perception and affiliative behavior.
Larger amygdala volume and stronger intrinsic connectivity of the ventrolateral and medial amygdala subregions each contributed independently to larger social network size in our brain-behavior sample (N=29)
Larger amygdala volumes and stronger intrinsic connectivity for the ventrolateral and medial amygdala subregions each contributed independently to more complex social networks in our brain-behavior sample (N=29)
Larger amygdala volumes and stronger intrinsic functional connectivity for the ventrolateral amygdala subregion each contributed independently to more diverse social networks in our brain-behavior sample (N=29)
We next explored the specific regions within the networks supporting social perception and affiliation that were driving the relationship between intrinsic connectivity and social network size. As seen in , people with larger social networks had stronger connectivity between the ventrolateral amygdala and the STS and fusiform gyrus within the network supporting social perception. They also had stronger connectivity between the medial amygdala and the vmPFC within the network supporting social affiliation. See for MNI coordinates of these and additional regions that demonstrated correlations with social network size.
Exploratory analyses revealed that the connectivity between the amygdala and specific regions within the networks supporting social perception and affiliation are the best predictors of social network size.
Regions within the ventrolateral and medial amygdala’s intrinsic connectivity networks (defined in the discovery sample) that correlated with social network size in an exploratory analysis in our brain-behavior sample (N=29)
A final set of analyses characterized the anatomical and behavioral specificity of the results above. Intrinsic connectivity within two networks important for social cognition, but not involving the amygdala – the mentalizing and mirror networks – was not related to either social network size or complexity (r = 0.01-0.02; p > 0.3). Intrinsic amygdala connectivity was not related to other self-report measures of sociality, perceived social support or life satisfaction (r = −0.28-0.16; p > 0.15).