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This is the sixth in an occasional series called 'Ask a Neuroscientist.' Today, we take audience-submitted questions to Jason Castro, an assistant professor of neuroscience at Bates College in Maine, to learn more about how the brain works.

Daniel in Sauk Rapids submitted a question about people who are considered introverts and extroverts. Daniel wanted to know: Is there any structural difference in the brain that accounts for the difference?

 We know there are a few structural features in the brain that correlate with whether a person is relatively introverted, versus extraverted. But we don't have experiments that really address whether those brain differences play a causal role. We're still pretty far from having... a scientific description of personality differences at the level of cells and synapses.

 So yes, there might be a difference in the brain, but you're not sure that's the reason they have these different personalities.

 Right, we're not sure if it's the reason or just something that tends to correlate with those personality differences. One thing that people have seen quite a bit is people who identify as introverts tend to have larger and thicker brain tissue in an area of the brain called the pre-frontal cortex - a small part of the pre-frontal cortex. And the converse is also true, that that extraverts tend to have a larger area in the corresponding parts of the pre-frontal cortex, and the brain is also a little thinner there, as well.

Adam in Saint Anthony also submitted a question, which sparked an interesting conversation with Castro about free will. Adam wanted to know: What does neuroscience tell us about free will?

 There are some classic neuroscience experiments that were done by a guy named Benjamin Libet - these were about 30 years ago or so. And he found some results that have some really intriguing and, frankly, to some, some slightly unsettling applications for free will. And basically what he found is that there is brain activity that registers our intent to make a decision - a spontaneous decision - well before we've consciously decided to make a decision. 

It's sort of like we're showing up a little bit late to the party. We think that we initiate activity and our brains follow suit, but it's actually the other way around. Our brains - the activity is initiated in the brain, and it only registers after the fact.

 Does that mean brains have free will, or are they doing something very chemical and methodical, and we don't even know what's going on?

 Yeah, it's sort of a chemical mechanistic thing that's going on. We feel very much like we're these free-flowing entities that can make these spontaneous decisions, but it seems - the view that science backs - that it's more the case that we're, in a sense, going along for the ride. We're downstream of the process that's actually making decisions and initiating actions.

 Yeah, it's tough to swallow! I should add the caveat that these were done for very simple experiments - experiments where people were instructed to push a button at the time of their choosing. And what they observed was a ramp-up of electrical brain activity that came hundreds of milliseconds - maybe a quarter of a second or so - before the decision to make some action, to press the button. Whether this extrapolates to some deeper decision making, moral decisions, that's a much tougher nut to crack.

 So you're not sure if we're 'along for the ride' when we decide to break into a car, where we're not only doing an action but we are also doing wrong.

 But it sounds like you're blurring the lines a little bit between what we know as reflexes - when you hit the knee with the hammer - and free will.

 Right, in some sense what those experiments on free will are saying is basically affirmation of the idea that everything we do has to have a mechanism. Every move has a mover. Our muscles move to push a button, or take some kind of action. Our brain moves those muscles, and a part of the brain initiates the cascade that sets all those things. So, there's no ghost in the machine that's pulling the puppet strings. It seems the other way around, actually.

 I'm still trying to wrap my head around that.

 Yeah, I get a little distressed about this sometimes. But at the end of the day, these results shouldn't change the way you go about conducting your everyday business. You're still going to feel conflicted about little things, like whether you should have chosen the apple or the bag of chips for lunch. And I don't think in any practical way this would have any bearing on any big decisions with a moral element to them.

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This is the sixth in an occasional series called 'Ask a Neuroscientist.' Today, we take audience-submitted questions to Jason Castro, an assistant professor of neuroscience at Bates College in Maine, to learn more about how the brain works.


Ask a Neuroscientist: Introverts and free will

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