A glimpse at the evolution of the fearing and trusting brain

Together with other mechanisms, the amygdala is involveld in a complex neural circuitry that transforms photons hitting your eyes into the feeling that "Mom is mad at me because I break her favorite vase". Often referred to as the fear center, the amygdala is more like an online supervisory system that sets levels of alert. Many of its activities are of a social nature. Explicit and implicit distrust of faces elicits amygdala activation (Winston et al., 2002), while trust is increased with amygdala impairment (Adolphs et al., 1998). Moreover, the trust enhancing effect of oxytocin is mediated by amygdalar modulation: oxytocin reduces fear and hence allows trusting. In a nutshell, emotional memorization, learning and modulation performed by the amygdala obeys the following flowchart:

(from Schumann 1998)

A subpart of the amygdala, the lateral nucleus, processes information about social stimuli (such as facial expression). Autistic individuals tend to have impaired lateral nucleus, which makes sense if this nucleus is an important social-cognitive device (autistic subjects perform poorly in task that involves mental states attribution or other social inferences).According to Emery and Amaral (2000), inputs form the visual neocortex cortex enters the amygdala through the lateral nucleus, where its "emotional meaning" is attributed (I know, it is simplification...); the basal nucleus adds information about the social context. Hence this nucleus acts as a sensory integrator (LeDoux, 2000).

In a new paper in American Journal of Physical Anthropology, Barger et al. studied the relative size of different nuclei of the amygdala in different primates (humans, chimpanzee, bonobo, gorilla, etc.). The study revealed that the human lateral nucleus represents a larger fraction of the amygdala:

The authors conclude:

The large size of the human L [lateral nuclei] may reflect the proliferation of the temporal lobe over the course of hominid evolution, while the inverse may be true of the gorilla. The smaller size of the orangutan AC [amygdaloid complex] and BLD [Baso-lateral division] may be related to the diminished importance of interconnected limbic structures in this species. Further, there is some evidence that the orangutan, which exhibits one of the smallest group sizes on the contin- uum of primate sociality, may also be distinguished neuroanatomically from the other great apes, suggesting that social pressures may play a role in the development of the AC in association with other limbic regions.

Living in large groups thus may have shaped the evolution of emotional processing capacities of our brains. In the economy of nature, negotiating our way in a complex social world requires accute and specialized cognitive capacities in order to cooperate, trust, reciprocate, etc. This research show the potentials of evolutionary cognitive neuroscience (see this post).


References

• Adolphs R, Tranel D, Damasio AR (1998) The human amygdala in social judgment. Nature 393: 470-474.
• Barger, N. Stefanacci, L., & Semendeferi, K. (2007) A comparative volumetric analysis of the amygdaloid complex and basolateral division in the human and ape brain. American Journal of Physical Anthropology.
• Emery and Amaral, 2000 N.J. Emery and D.G. Amaral, The role of amygdala in primate social cognition. In: R.D. Lane and L. Nadel, Editors, Cognitive Neuroscience of Emotion, Oxford Univ. Press, New York (2000), pp. 156–191.
• LeDoux JE (2000) Emotion circuits in the brain. Annu Rev Neurosci 23: 155-184
• Schumann, J.A. 1998. Language Learning. Vol. 48 Issue s1 Page ix-326
• Winslow JT, Insel TR (2004) Neuroendocrine basis of social recognition. Curr Opin Neurobiol 14: 248-253
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