Removing half a brain

20 December 2019

Interview with 

Helen Keyes, ARU; Duncan Astle, Cambridge University


Brain schematic


Katie Haylor chats to Anglia Ruskin University perceptual psychologist Helen Keyes and Cambridge University cognitive neuroscientist Duncan Astle about the neuroscience news stories that have caught their eyes this month. First up - what happens when you lose half of your brain? It sounds extreme but in severe, rare cases of epilepsy, surgeons may peform a hemispherectomy. And it turns out that the brain can adapt remarkably well to this dramatic event. Duncan told Katie more...

Duncan - So you could remove the part of the brain that you think is the origin of the seizure. Or you could chop connections that you think spread the seizure. So for example there are two halves of the brain and they're connected by a thick bundle of fibers. And sometimes you can chop the bundle of fibers to stop the spread.

Or in very extreme cases, you can perform what's called a hemispherectomy, and that's where surgeons will remove half of the brain to stop the epilepsy. And that's how you can lose half of your brain following epilepsy.

So what this paper’s doing is following six individuals who had a hemispherectomy when they were children. It’s typically done in early childhood so usually before around the age 3. These people are then seen decades later when they're adults and it might surprise you to find out that actually the cognitive and behavioural and motor outcomes are remarkably good. Normally if you met someone you probably wouldn't know that they had had it done. And of course that raises the amazing question of if they've lost half of their brain, how is it they're able to function so well? And that's what's being explored in this paper.

And so they took the individuals and they put them in a functional magnetic resonance imaging scanner or an fMRI scanner. And what that is able to do is look at which parts of the brain are active and use that to identify different brain networks. And the first finding they had is, relative to individuals who have not had this procedure, those who have, their brain networks in the preserved half of the brain are remarkably similar. So all the classic brain networks that we would see in a typical participant are present in these people who have lost half of their brain.

Katie - And just for clarity, are brain networks regions of the brain that talk to each other in particular ways?

Duncan - That's exactly what they are. So they’re different regions of the brain often in different spatial locations within the brain, but we know they're working together because they become active together.

Katie - So the half of the brain that is in use is compensating in some way then. What's  going on?

Duncan - Well that's one possible explanation. And so what the authors of this study were able to show, is that whilst the networks themselves are all much the same as they would be in somebody who's not lost half of their brain, the exchange of activity or the exchange of information between these networks is significantly greater in people who have lost half of their brain. I.e. the crosstalk between the networks seems to be enhanced in people who have lost half their brain.

And so one account of this is that what the subjects gradually develop the ability to do over many years, indeed decades, is to bring online extra networks for a particular task that wouldn't typically be used for that task in someone who hasn't lost half their brain. But because they need the additional resource or the additional activity that's recruited in order to support the performance of the network that's really being tapped by the task. So gradually over time as the brain develops, the connections between the networks become stronger.

Katie - That's amazing. Does anybody have this surgery later as an adult, when they've done some of their development? Do you think that would make a difference.

Duncan - I think it would make a difference. And so it can be done in later childhood and adolescence, that is exceptionally rare. And the reason that it's done much more rarely in those ages is because it's seen as much more drastic, and the advantage of doing it early is that it gives time for the rest of the brain to compensate.

Such is the high degree of plasticity, it means that gradually over time, functions tend to be maintained and typical development can happen. Whereas if you wait until the brain is more fully formed and then you remove half of it then the hurdle is that much greater.

Katie - Being able to survive and thrive with half a brain seems amazing to me. As a brain expert, is it surprising to you?

Duncan - Yeah it's really surprising. So we spend all our time thinking about what little pockets of neurons here, there and everywhere are doing, what their special contribution is. And then to find a group of people who have lost half of their neurons in one fell swoop is remarkable. And the very fact that they can function and live normal, happy, highly productive lives is an amazing testament to just how adaptive and plastic the human brain is.

And indeed in these cases, even though there's only six of them, what the authors are able to show is that in those people where the degree of crosstalk between the networks is greatest, their outcomes seem to be the best, which supports the idea that essentially what the brain is doing is compensating for the fact that the other half of the brain is missing.


Have you ever let something slip that, on reflection, you wish you’d kept to yourself? This month, Helen Keyes has been looking at a paper explaining why it is that we might sometimes say things we later regret...

Helen - There's lots of situations in our lives where it's wise for us to conceal information about ourselves, and we don't want to disclose personal details. So think about job interviews, or important work meetings, just situations where you might think “I'll hold back this piece of information about myself”. And the paper shows that, ironically, it's at these particular moments where we're most invested and most highly alert and trying to perform at our best, when we're most likely to over disclose or make these mistakes where we might end up being judged negatively by other people.

Katie - And by alertness, what exactly do you mean?

Helen - Well the authors call it affective arousal and when we say arousal in this context we don't mean sexual arousal, we simply mean a kind of state of readiness. Your systems are really “Go!”, you're in that high alert, very awake place.

When we talk about this affective arousal we can mean cognitively being in a heightened cognitive arousal situation or emotionally aroused or even just physiological like going for a run, that type of arousal, when your systems are really on.

Katie - Okay so it's a heightened state of whatever it may be.

Helen - That's it. So you can imagine that there's lots of situations where in exactly those moments we don't want to disclose personal information. The idea behind this is that it requires quite a lot of effort to suppress information about ourselves. If you think about the famous example of “don't think about a white bear”, and then of course it's all that you can think about, when we're going into these important, heightened situations we often “think don't say this, don't say this, don't tell this piece of information about yourself”, you're concentrating quite hard on not talking about a particular personal piece of information, that it's quite there at the front of your mind.

And unfortunately when we're in this state of heightened arousal - whether it's emotional or cognitive or physiological - the information that's most available to us is that information that's right there at the front of our mind. And that's unfortunately why it tends to come out more in these situations.

Katie - That seems really cruel because the sort of situations you could be talking about like a job interview, a date, those kinds of important events, right?

Helen - Absolutely and they actually looked at it in the context of dating. One of the studies that the authors did was they asked participants to set up a dating profile and they had to list three things about themselves on this dating profile. Now half of these participants were told that immediately following this they were going to have to do a very complicated math challenge, which had that cognitive heightened arousal, people were a bit nervous about that. And the control group weren't told this at all. And then they independently rated the things that people said about themselves in the dating profile. And they found that people who were in this heightened arousal state, so people who were thought they had to do a maths task, self disclosed a lot more and they self disclosed intimate details, potentially embarrassing details, and even incriminating details about themselves.

They also then looked at it in terms of things that you wouldn't want to tell people because you might feel a bit ashamed. So they looked at online trolling, so behaviours some of us sometimes engage in, where the whole nature of it is that it's anonymous and you would be embarrassed to tell people that you've said this in an online forum. And here they showed half of their participants affective pictures. (And by that I mean just highly emotional pictures. They could be pictures and invoking disgust or even positive emotions, just highly affective pictures). And they found that the people in the group who'd seen the affective pictures disclosed more severe transgressions of online trolling, things that they really might regret talking about it at a later date.

Then the authors wondered whether it'd be true about physiological arousal, so they ran a third study where they asked everybody to think about the most embarrassing moment, those moments that really intrude in your thoughts in the middle of the night, that still wake you up with embarrassment.Then they asked half of their participants to go jogging, and half of them didn't go jogging. When the participants came back to the lab, those that had been jogging were much more willing to then disclose to the researcher what that most embarrassing moment was.

Katie - Wow okay so you've got a trio of studies - you're making somebody physiologically aroused, going for a run; you're making someone cognitively aroused by making them to a maths problem; and then emotionally aroused by showing them these affective images. It sounds pretty scary to be honest! Do you have any advice? Would it be that perhaps trying to dampen your state of arousal by being more calm might help in these situations?

Helen - The first thing to remember is that self disclosure isn't always a negative thing. It can be really important for us in terms of bonding and building social relationships with other people that we do self disclose these embarrassing moments or these transgressions. And it can also be important your own psychological well-being and health that we feel we can talk quite openly and disclose these things. So it's not a pure negative.

But we obviously know there are situations where you have to self regulate such as a high level meeting, a job interview, or like you mentioned a potential date. So there are times when things we say could damage our reputation or even cause offence to other people. In those situations it would be best to engage in quite relaxing activities beforehand, whether that's deep breathing or meditation or whatever it is that's gonna get you into that calm state of mind. But on a more practical level, you can do simple things like avoiding drinking caffeine immediately before you're going to enter into one of these situations where you need to self regulate.




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