Morphometrics of the Amygdala
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Hover over keys for definitions:Recent data indicate that the human amygdala is distinct from that of our closest living relatives, the great apes, in both size and neuron number (Barger, et al., 2007; 2012). The absolute size of the amygdala is approximately four times greater than that of great apes, but does not exceed allometric predictions, while human neuron numbers fall within the great ape range.
However, evidence for changes in the intrinsic organization of the amygdala over the course of human evolution, i.e., evolutionary reorganization, indicate that it may have become specialized to process a heightened amount of neocortical information. Specifically, in allometric analyses, the nucleus of the amygdala which receives the greatest amount of input from the neocortex, the lateral nucleus, is nearly 40% larger than predicted for a great ape of human brain size and contains 69% more neurons than predicted for a primate with a similar number of total amygdala neurons. In contrast, the nucleus which has the greatest influence on brainstem activity, is over 3 time smaller than predicted (but contains only around 10% fewer neurons than predicted).
The amygdala is a subcortical structure which has long been highlighted as a major constituent of the neural circuits processing emotional and social behavior (Maclean, 1949; Adolphs, 2010). Anatomically and functionally, it is composed of 13 distinct, interconnected nuclei, some which receive with brainstem and hypothalamic structures that regulate emotional and "fight or flight" behaviors (Freese and Amaral, 2009). As such, the amygdala is situated in a position to modulate emotional responses to external stimuli and has been characterized as a salience (Adolphs, 2010), value (Morrison, 2010), and threat (Amaral, 2003) detector.
The comparative evidence (Barger, et al., 2007;2012) suggests that the human amygdala has undergone evolutionary reorganization of its component parts, especially emphasizing the lateral nucleus, the primary gateway for neocortical information arriving in the amygdala. This distinguishes the human amygdala even from that of great apes, pointing to potential evolutionary changes in the manner in which emotion processing may occur in human brains.
Universal
The proximate mechanisms responsible for the difference are not explicitly known. Variation in amygdala size has been linked to social behavior, but the influence of ontogenetic change and neural plasticy on these differences in unclear. Little information on the development of individual amygdaloid nuclei is available for any species of primate other than the rhesus macaque. The lateral nucleus has been indicated to be influenced in 2 neurodevelopmental disorders associated with social behavior (Schumann and Amaral, 2006; Galaburda and Bellugi, 2001), but, again, these studies have focused largely on adults and have not addressed developmental change.
The size of the amygdala appears to be most linked to measures of social complexity. Within species, increased amygdala volume has been linked to measures of social group size within species (Bickart, et al., 2011; Kanai and Bahrami, 2012; Sallet, et al., 2011). Across primate species, increases in social group size are positvely related to increases in the size of the portion of the amygdala most connected with neocortex (Barton, 2000).
Several researchers have pointed to the behavioral importance of changes in emotional behaviors to human-specific social adaptations and have theorized that these changes would be reflected in human brains (e.g., Hare, 2007; Vilensky, et al., 1982). The neural data suggest that the human amygdala distributes more neural resources to nuclei that process neocortical input and evaluate information early in the processing scheme than those that directly interact with brainstem nuclei to initiate autonomic responses to stimuli.
The amygdala is present in all mammals. Homologues are present in birds and reptiles (Jarvis, 2009).
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