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Amygdala volume and social network size in humans

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Amygdala volume and social network size in humansAcknowledgmentsAUTHOR CONTRIBUTIONSCOMPETING FINANCIAL INTERESTSReferencesFigure 1 Amygdala volume correlates with social network size and complexity.Table 1 Linear regressions using amygdala and hippocampal volumes as independent variables and social network characteristics as dependent variables© 2010 Nature America, Inc. All rights reserved.nature neuroscience advance online publication 1B r i e f c o m m u n i c at i o n sFor many species, but particularly for primates, living in groups is a major adaptive advantage1. But living in a social group also presents its own challenges. To get along while getting ahead, it is necessary to learn who is who, who is friend and who is foe. It might be productive to form an alliance with certain group members in one context, but to outmaneuver them in another. The ‘social brain hypothesis’ sug-gests that, evolutionarily, living in larger, more complex social groups selected for larger brain regions with a greater capacity for performing relevant computations2. On the basis of its central functional role3,4 and anatomic position5 in the social brain, investigators have pro-posed that amygdala volume should be related to the size of social groups, in part because the size of a brain region is one indicator of its processing capacity6.Comparative neuroanatomical studies in nonhuman primates strongly support a link between amygdala volume and social net-work size7 and social behavior8. Species characterized by larger social groups have a larger corticobasolateral complex within the amygdala. The corticobasolateral complex conjointly expanded with evolution-arily newer cortex and the lateral geniculate nucleus, particularly the layers of the lateral geniculate nucleus that project to the ventral stream visual system7. Taken together, these comparative findings suggest that a larger amygdala provides for the increased processing demands required by a complex social life.In this study we examined whether amygdala volume varies with individual variation in the size and complexity of social groupings within a single primate species, humans. In 58 healthy adults (22 females; mean age M = 52.6, s.d. = 21.2, range = 19–83 years) with confirmed absence of DSM-IV Axis I diagnoses and normal perform-ance on cognitive testing, we examined social network size and com-plexity with two subscales of the Social Network Index (SNI9). One SNI subscale (Number of People in Social Network) measures the total number of regular contacts that a person maintains, reflecting overall network size. A second subscale (Number of Embedded Networks) measured the number of different groups these contacts belong to, reflecting network complexity. Despite the fact that the two social network variables were strongly correlated within the present sample (r = 0.86, P < 0.001), we opted to consider their separate rela-tion to amygdala and hippocampal volumes. (For more details, see Supplementary Results.)To assess amygdala (and, as a control region, hippocampal) volume, we performed quantitative morphometric analysis of T1-weighted MRI data using an automated segmentation and probabilistic region-of-interest (ROI) labeling technique (FreeSurfer, http://surfer.nmr. mgh.harvard.edu/). For methodological details, see Supplementary Methods. To adjust for differences in head size, amygdala and hippo-campal volumes were divided by total intracranial volume, as per-formed previously10,11.Linear regression analyses revealed that individuals with larger and more complex social networks had larger amygdala volumes (Fig. 1). These relationships held when controlling for the age of the partici-pant (because older individuals have, on average, smaller amygdala volumes than do younger individuals; Table 1). These relationships held when left and right amygdala volumes were analyzed separately (Table 1), indicating no lateralization of the effect.To assess discriminant validity, we performed a linear regres-sion using right and left hippocampal volumes (corrected for total intracranial volume) as independent variables and social network size and complexity as dependent variables while controlling for age (because hippocampal volume typically diminishes with age). For the whole group, these analyses showed no significant relationship Amygdala volume and social network size in humansKevin C Bickart1, Christopher I Wright2,3, Rebecca J Dautoff2,3, Bradford C Dickerson2–4 & Lisa Feldman Barrett2,3,5We found that amygdala volume correlates with the size and complexity of social networks in adult humans. An exploratory analysis of subcortical structures did not find strong evidence for similar relationships with any other structure, but there were associations between social network variables and cortical thickness in three cortical areas, two of them with amygdala connectivity. These findings indicate that the amygdala is important in social behavior.1Department of Anatomy and Neurobiology, Boston University School of Medicine, Boston, Massachusetts, USA. 2Psychiatric Neuroimaging Research Program, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts, USA. 3Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts, USA. 4Department of Neurology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA. 5Department of Psychology, Northeastern University, Boston, Massachusetts, USA. Correspondence should be addressed to L.F.B. ([email protected]).Received 6 October; accepted 24 November; published online 26 December 2010; doi:10.1038/nn.272405101520Total adjusted amygdala volume (mm3)2530354045B = 0.38B = 0.4450a bTotal number of people in social networkTotal number of embedded networks2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5012345672.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5Figure 1 Amygdala volume correlates with social network size and complexity. (a,b) Plot of social network variables (y axis) against total adjusted amygdala volume (x axis). Data points from young participants, black circles; older participants, gray triangles. A line of best fit with standardized regression coefficients (B) is also displayed for the entire sample.© 2010 Nature America, Inc. All rights reserved.2 advance online publication nature neuroscienceB r i e f c o m m u n i c at i o n sbetween


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