The Amygdala in the Human Brain & the Evolution of Sociability

I was wrong to say today was a slow day in anthropology related news, because I just found a brand new hot off the press human brain evolution paper published in the American Journal of Physical Anthropology. I found the paper through Razib’s Homo amygdala post over at his blog Gene Expression.

My thesis research is in neuroscience, so this sort of publication is exactly what I like. And if it weren’t for Razib, this paper woulda passed under my radar. To say that I’m excited is an understatement, especially because this paper relates and compares brain evolution and social cognition abilities in hominoids.

As the title of the paper, “A comparative volumetric analysis of the amygdaloid complex and basolateral division in the human and ape brain,” implies, the study was founded on comparative analysis of the amygdala from 12 ape and human specimens. Why the amygdala? Well, the amygdala is a region of the brain that functions in the formation and storage of memories associated with emotional events. It is essential in helping us read the emotions of others, and perhaps facilitated our ability to form relationships and live and work in groups.

I’m gonna give you a really brief run down on the neurobiology of the the amygdala because without it, the findings that I will summarize will be mostly mumbo jumbo. First and foremost, the amygdala the image to your right The location of the amygdala in the Human Brainshows you where the amygdala is found in the human brain. The amygdala is made up of about twelve sub-regions. To the best of my knowledge, the exact functions of each sub-region have not yet been pinpointed. But what is known is that many of them maintain connections that participate in fear and emtional reactions actively.

The primary function of one of the regions the paper talks about, the lateral nucleus, has been identified. It is the gateway of signals and information into the amygdala. Or in other words, it is through the lateral nucleus that the amygdala receives information from the outside world.

Okay with that hash job in brain anatomy and some of the intricacies of the amygdala, I think we should move to the paper. Katerina Semendeferi lead a group that measured area of the 12 amygdalas and low and behold found that the human amygdala was much larger than those of the other apes. That’s not too surprising. Our brains are the largest of the group they compared, so I would expect regions within the brain, like the amygdala to be proportionally larger as well.

Curiously, though, the lateral nucleus occupied a bigger portion of the amygdala than the other sub-regions in humans than the other primates compared. On top of having a larger lateral nucleus, more incoming connections from the temporal lobes, were noted in the human brain as well.

The temporal lobes are home to some critical social senses. The primary auditory cortex is located in the temporal lobe, and so the temporal lobes specialize in auditory processing. They are also heavily involved in speech and vision processing. This implies that as humans, the larger lateral nucleus in the human amygdala allowed more auditory and vision information to be processed. Semendeferi and team took this observation and,

“concluded that the amygdala’s lateral nucleus has enlarged relative to the rest of the structure since the human line split from the apes, and that this enlargement might reflect the “social pressures” of living in large groups. For example, Semendeferi and her colleagues note that the orangutan, which has a relatively smaller amygdala and lateral nucleus than those of the other species, is [more] solitary.”

This is interesting research to me. In a ScienceNOW news article, James Rilling suggested that the hypothesis that the prominence of the lateral nucleus is related to social cognition, is ‘reasonable’ if the sample size is extended.

“The next step, he adds, would be to use brain-imaging techniques to see whether the human lateral nucleus really does make more connections to the temporal lobe than that of other apes.”

Which, I agree. If we see the temporal lobe deliver more information or be more actively involved with the lateral nucleus of the amygdala, then that would be a solid case. I also wonder how the innervations of the temporal lobe with the lateral nucleus of the amygdala of autistic people, look and function? What I’m saying is that why not extend the comparative analysis to autistic humans and see how they compare to other humans as well as other apes. We know their amygdala’s have fewer nerve cells and their behaviors are characterized by decreased social interaction and an inability to understanding the feelings of others… but what about their lateral nucleus?

5 thoughts on “The Amygdala in the Human Brain & the Evolution of Sociability

  1. I enjoyed reading your article very much! it is interesting and fun!
    I have read that a research was done to study the brain of psychopaths and that they found out that their amygdala is smaller than most people. does that mean the apes are psychopaths? :0)

  2. This is a great study on social interest of a person. A person who has large lateral nucleus will definitely have high social interest which means they are more optimistic, altruistic and sociable in nature.

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