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We used to think having a distinct left and right brain was something unique to humans. But brain lateralization has now been found in everything from chickens to spiders! Does this change our theories for why some brains work that way?


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Sources:

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3859130/
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IMAGE SOURCES:
https://www.istockphoto.com/photo/spider-on-the-web-in-the-sunshine-gm625756668-110258201
https://www.videoblocks.com/video/woman-talking-closeup-on-mouth-super-slow-motion-shot-at-240fps-ttudeph
https://www.videoblocks.com/video/portrait-of-happy-elderly-man-painting-in-a-public-square-in-madrid-spain-and-looking-at-camera-creative-spanish-artist-drawing-with-pencil-on-the-street-and-smiling-stbwdsaysjxashb2u
https://www.videoblocks.com/video/temple-monkey-eating-angkor-wat-cambodia-noejbryklijuu7vvg
https://www.videoblocks.com/video/clever-monkey-snatches-food-from-girls-hands-niyojfokxik4894tk
https://www.videoblocks.com/video/monkeys-eating-fruit-on-ledge-kb6bymk
https://www.istockphoto.com/photo/baby-plays-with-toys-gm184364803-17247349
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https://www.istockphoto.com/photo/neurons-with-myelin-sheaths-produced-by-oligodendrocytes-gm1134932102-301750415
Thanks to Dashlane for sponsoring this episode of SciShow Psych.

Go to dashlane.com/scishowpsych to learn more about Dashlane’s internet privacy and security features. {♫Intro♫}. Despite what you might have seen on the internets, there’s no such thing as a right-brained or left-brained person.

But your brain is split down the middle, and the two halves have different specializations and are even structured differently at the neuronal level. This is what’s known in neuroscience as brain lateralization. We used to think this was something unique to us, which led researchers to think that we evolved asymmetry because of our unique cognitive abilities.

But lateralization has now been found in everything from chickens to spiders—and that’s allowed us to come up with some other, more interesting ideas for why and how our hemispheres evolved. You see, despite superficial similarities between your brain’s right and left hemisphere, they’re quite distinct. There are more miniature columns of neurons in the left hemisphere than the right, for example.

Also, neurons in the left hemisphere tend to have more myelination—a fatty coating that speeds up signal transmission. And we’ve long known that our hemispheres are functionally different, too. Most of your ability to process language is associated with brain activation in the left hemisphere, while things like spatial processing or facial recognition activate the right.

How specialized these hemispheres are does vary between people. Still, we’re all at least somewhat lateralized, and at first, we thought this was because humans were special. Classic.

But once we started to find asymmetry in birds and other animals, we had to come up with other ideas. One of those ideas is that the selective pressure for lateralization came from the need to perform skilled tasks that require just one limb. You see, studies have found that the more lateralized an animal’s brain is, the more likely it is to have a preference for one side—what we often call “handedness”.

Let’s, say, a monkey is slightly better at grabbing for food with its right hand. It benefits most by always using that hand instead of sometimes trying to use the other. So monkeys with more lateralized brains might outcompete less lateralized ones.

Research has-- found that when chimpanzees fish for termites with just one hand, they get more of them per minute. And we might see the benefit of handedness in people, too. Like, back in 1970, researchers timed 219 kids aged 3 - 15 to see how quickly the kids could move some pegs around a pegboard using only one hand.

And those who were faster at moving the peg also had a clear preference for one hand on other tasks, like cutting paper and throwing. But, it hasn’t been shown that this kind of task performance translates to real differences in evolutionary success. Another potential explanation for lateralization is more basic: to speed up thinking by shortening the connections between neurons.

In humans, neural connections between the hemispheres have to pass through the corpus callosum. That can cause a signal transmission delay of over 25 milliseconds—which is a long time when we’re talking neuronal chit-chat. And the bigger a brain is, the longer those signal delays can be— especially if the neurons that need to communicate end up on opposite sides of the brain.

So, all animals, but especially humans and larger mammals, could have benefited from consolidating tasks to one side or the other. But again, this is more of a convenient explanation than an evolutionary smoking gun. Lateralization could have evolved because grouping certain neurons together is just a better way to build a big brain, or it could have to do with performing skills better.

Finally, there’s a chance things are even more complicated than that. Like, maybe lateralization developed in most animals for one thing, but human evolution took that one step further. In animals, scientists have seen that lateralization helps with something called parallel processing — basically, being able to do two things at once.

That may explain why it’s so common. For example, we know that chicks mostly use their left hemispheres when hunting for grain among pebbles, but use their right hemispheres to monitor for predators. And the more lateralized they are, the better they are at keeping an eye out for trouble while searching for food.

It’s easy to see how that would be good from an evolutionary perspective, but there’s not much evidence that this happens in humans. Studies have found that people with more asymmetry tend to have better verbal intelligence and visuospatial skills, though. So some researchers have proposed that, in us, lateralization helps with a different kind of parallel processing: thinking about the world in two different ways.

The idea here is that there is no ideal kind of neuron or neuron structure that works best for everything we might want our brains to do. If you eat some red berries and then get sick, for example, you might need to come up with a quick explanation for what happened. That’s where the faster-firing, discreetly organized neurons in the left hemisphere might jump in.

Woah! They were poisonous and you should avoid them. At some point, though, it’s important to know that you’re right—like if you’re lost in the woods and those berries are the only thing you can find to eat.

That’s when the right hemisphere, with more overlapping neurons and more holistic processing, can step up to detect if any of your explanations conflict. Like, maybe you realize you’ve eaten those berries other times and didn’t get sick, so they’re probably not poisonous. In support of this idea, there does seem to be some evidence that the left hemisphere focuses on creating explanations and drawing inferences, while the right hemisphere inhibits your responses while checking for conflicts.

But that evidence is not super clear cut, and it’s hard to test the evolutionary relevance of having different modes of reasoning separated into two hemispheres. So ultimately, we’ll need to do more research to fully unlock the mystery of why brains in humans and other animals are lateralized. These massive computers in our heads are just really hard to crack.

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