The similarities between the brains of humans and other speciesby Marc Wilson
Humans are different from other species, but not as much as we’ve assumed.
Brains are rubbery things, and, probably to gasps of my colleagues who work more closely with them, I think it’s safe to say we’ve not got a complete handle on how they work.
Which isn’t to say we don’t know quite a lot. We know that different bits are associated with different faculties. For example, the amygdala, buried deep and just behind the eyes, is associated with processing emotions. The prefrontal cortex (PFC) is the bit up front and that’s important for complex behaviours like decision-making.
We share a lot, but not all, of this gear with other species. Monkeys also have amygdalas. Since at least the 1930s, we’ve known that monkeys without amygdalas behave quite differently from their intact peers. For instance, they don’t shy away from snakes and other dangerous animals. Instead, they play with them. Animals without amygdalas lose their sensible sense of fear.
The same thing applies to us. One particularly relevant case is that of “SM” (brain folk give their cases initials like this). SM has an unusual condition that means she has no amygdala. If the amygdala is useful for generating fear, and fear is a sign that there is danger, it’s no surprise that SM finds herself in trouble without realising it. In one account, she would walk through the same park where she was mugged only the day before.
For a long time, it’s been thought that not all animals have as extensive frontal lobes as us. Ours are generally large relative to our total brain volume, but not our body size. Some lap dogs and fish do better on the latter measure than we do. Although it’s still generally the case, there’s less reason for us to feel superior to other species on the basis of large frontal lobes alone – our PFCs are about as proportional to the size of other brain structures as they are in other primates.
For a long time, in our hubris, we also thought that we were the only species to exhibit cerebral asymmetry – two hemispheres of the brain that do different things. The classic example is language, the heavy lifting for which is done by one hemisphere or the other. For the majority of people, it is done by the left side of the ol’ burger.
Another example is handedness. The muscles on each side of our body are controlled by the opposite hemisphere, so a right-handed person is typically left-hemisphere dominant. There is now abundant evidence of lateralisation across a good number of other animal species.
This brings me to bird brains. Many people would say that we should share more in common with primates than, say, the crow that harasses my dog from the garage roof. But there’s some fascinating research that shows that crows are also lateralised.
Caledonian crows dominate these discussions because they make and use tools to pry bugs out of crevices and they show a preference for which side they hold and use their tools.
However, crows and other species that show lateralisation are typically about 50:50 left or right dominant. So, rather than focusing on how we’re specially different from other species, questions tend to centre on why we have a hemispheric dominance that results in most of the population being right-handed.
This article was first published in the October 27, 2018 issue of the New Zealand Listener.
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