Impulsivity and addiction
Why do some people get addicted to drugs, whilst others don't? A study from Cambridge University, published in February 2019, shed some light on this question. Katie Haylor spoke to study author Camilla Nord...
Camilla - What neuroscience research tells us is that the processes that underlie your potential to try a substance are actually quite distinct from those that underlie getting addicted to something.
Katie - That's Cambridge University neuroscientist Camilla Nord and Camilla's a Cambridge University neuroscientist. And she told me about the work of another researcher in Cambridge called Karen Ursch
Camilla - And she tested the first degree relatives. So that's like sister, brother, parent or child, of people with drug addictions. And she found that they all showed this heightened impulsivity which is a behavioural trait that means you're likely to make decisions hastily, not think things through, maybe things that would be better to pause before you make that particular decision. And they all sort of showed this trait.
But what was different about the people who'd actually gone on to develop an addiction, not just their relatives, were that they also showed this trait called sensation seeking. That means you really are going after those things that are fun in life. So really what she showed is that you need a combination of these two behavioural traits, impulsivity and sensation seeking, to show a propensity or maybe a likelihood of developing a drug addiction.
Katie - Right. So that could be the difference between smoking a cigarette behind the bike sheds when you're 13 and going on to become a chain smoker as an adult, or taking illegal drugs less frequently compared to those who develop a dependence. Back in February 2019 Camilla published a study shining light on the question of who gets addicted to drugs, specifically it looked at a trait called impulsivity, but there are a whole host of other factors that could potentially contribute to someone's risk of developing an addiction and the risk profile might vary between individuals.
Camilla - For some people their high impulsivity and high sensation seeking might be all it takes for them to develop a drug addiction. But for other people perhaps they need those traits in combination with other kinds of more social or environmental factors. So I think the risks vary between people because it's so complex.
Katie - Camilla wanted to see if there are any neural markers or indicators in the brain predicting those who might be more likely to develop an addiction later in life.
Camilla - So the only time you can really look at this is in adolescence before they've been exposed to any kind of substance. So that's what we did. We took 99 young people with no history of substance abuse and put them in a brain scanner. There's a potential for early intervention if we find an early marker before the initiation of any kind of substance abuse. The second reason is a more scientific one. In the past when you put people with a drug addiction in a brain scanner you can't really disentangle “is that particular effect on the brain because they've been taking a drug for decades” or “is it because they have some kind of underlying brain difference that might make them predisposed to take drugs”. Who knows? But if you take adolescents who haven't yet developed any kind of substance addiction we can find out what are those underlying mechanisms without dealing with the effects of a drug.
Katie - The team scanned ninety nine youngsters using an MRI scanner to image how much myelin was in that brain cells. Myelin’s a fatty protein layer that forms around nerves and facilitates fast electrical communication. They zoomed in on a brain area with this MRI scan called the putamen. Camilla explained that addiction studies in animals have shown the putamen behaves differently in those who are more likely to go on to become addicts. And they actually use the same kind of behavioural task with these teens that they put in the scanner as with the rodents in previous work.
Camilla - So in rodents you can measure how impulsive a particular rodent is by whether they're likely to poke their nose into a little box before they know which box to poke their nose into. So they're trained that once a light flashes, they poke their nose in that box they get a reward, they get a treat usually. But if they do this poke preemptively, before they've even been told where to poke, then they get no reward it's maladaptive and that's considered an impulsive response.
So in our experiment we used a human version of this task, where humans have to sort of resist pressing a particular box before they know which box to press. But it's actually quite tempting to release what you're holding preemptively to go to press the box in anticipation of getting some extra points on the task. And this is our behavioural measure of impulsivity in humans.
Katie - While we couldn't replicate this exact experiment, I was curious to know how I might fare on an impulsivity test. So Camilla obliged.
Camilla - I've brought you one of the most classic tests of impulsivity and this task has to do with a form of decision making impulsivity which is maybe a little bit different than the form of behaviour impulsivity I was telling you about.
Katie - I was tasked with deciding “do I want a small amount of money now, or a larger amount of money later”? Simple enough …
Would I prefer 54 pounds today or 55 in 117 days? I'm going to have to go with today... Would I prefer fifty five pounds a day or 75 in 61 days? Okay. So that's a couple of months. No I'm gonna go for 55 pounds today... Would you prefer 19 pounds today or 25 in 53 days? Again I'm going to go for 19…. Lastly would I prefer 31 pounds a day or 85 in seven days? Okay. Seven days only a week. So I'm gonna go for seven days.
Now I've only done a couple of questions but I feel like I'm quite impulsive.
Camilla - What we can do with your answers there is essentially fit you to a curve, at which point will you choose the future rewards versus the present rewards? And yes probably from your first couple of questions you would seem a little bit impulsive, but it's not always good to wait for rewards. It's not always good to wait two years for one more pound in these particular questions. So there's a benefit to being a bit impulsive, in fact like most traits there's a reason we've all evolved to have them.
But there are of course disadvantages to being very impulsive in that it might actually be a good thing to wait a month for an extra 20 pounds. And we tend to see it relates to other kinds of impulsivity as well.
Katie - Okay enough indulging in my own habits, back to the study.
Camilla - We found that the kids who are most impulsive showed lower levels of myelination in the putamen. And the reason why this is exciting is because none of them have ever had a substance addiction. But they have this trait that makes them potentially predisposed to developing a substance addiction in the future and they already show and neural difference that maybe is predictive of developing a substance addiction. But we won't know that until much longer term studies are carried out.
Katie - So what does all of this mean for youngsters who might be at risk of developing a substance addiction later in life? I asked Camilla to put the pieces of this puzzle together.
Camilla - I think “less myelination in the brain relates to impulsivity” could mean a lot of different things. And one could be a causal link between this neural correlate and future development of a disorder. But we don't know that yet.
I think there is probably a causal link between behavioural impulsivity and development of a disorder because we see that very strongly in animal studies. So animals who perform like this with greater impulsivity on the same task, they're the ones who get much more addicted to cocaine in animal studies of drug addiction than other animals.
So I think we can say that causal link with a little bit more certainty than the causal link between mild nation and impulsivity, I think of it more as they are both manifestations of the current state of that person. One is in the behaviour and one is in the brain but they're both representing what that person's individual risk factors are.
There a couple of reasons why this research could end up being useful. The first of course, is if we were to come up with any kind of behavioural intervention - that would normally be some kind of psychological intervention - that could help prevent future substance addictions.
I think the other potential use is in understanding the development of addiction in the first place. So if we understand the time course. So first you get changes in this part of the brain, that might initiate this sort of behaviour, tt gives us a much fuller picture over risk and then what could mitigate that risk. So it could be in a follow up study, some of the people who look high risk in our sample don't go on and develop addiction. And what is special about those people? That's what our kind of neural marker of impulsivity could help us follow up.