Amygdala evolution and cortical-subcortical integration

I finally had a chance to take a more careful look at this paper by

Chareyron, L. J., Banta Lavenex, P., Amaral, D. G., & Lavenex, P. (2011). Stereological analysis of the rat and monkey amygdala. Journal of Comparative Neurology, 519(16), 3218-3239.
I think the figure here summarizes a major point of the paper. Although the scale bar is not the same for the 3 species, it is evident that the lateral amygdala (red) is disproportionately represented in the human case. To the contrary, the central nucleus is less represented. The basolateral amygdala has extensive connectivity with cortex, whereas the central nucleus is more “autonomic”. One can speculate that the increase in relative size of the basolateral amygdala paralleled increases in cortical representations. In fact, this could be an example of the proposal by Harvey and Barton that brain structures with major anatomical and functional links evolve together (independently of evolutionary changes in other unrelated structures).
I completely agree with the paper’s suggestion that their results are consistent with the “hypothesis of a higher convergence and integration of information in the primate amygdala.”
On the other hand, I don’t agree with their conclusion that “although the fundamental function of the amygdala, to regulate fear and emotional learning, is conserved across species, amygdala function might be under greater influence of cortical activity in primates, and therefore integrate additional contextual information that influences the regulation of more complex behaviors such as social interactions.” I believe the statement is still too attached to the traditional view of the amygdala as a simple “alarm system”. Such view neglects the amygdala’s sophisticated involvement in a host of perceptual and cognitive functions (see this paper) and underestimates the potential for altered connectivity to change the functional repertoire of the amygdala.
Left: Rat (top), Macaque (middle), and Human (bottom) amygdala. Right: schematic illustration of cortical-subcortical connectivity.

Left: Rat (top), Macaque (middle), and Human (bottom) amygdala. Right: schematic illustration of cortical-subcortical connectivity with the amygdala. From Chareyron et al. (2011).

 

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