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Free will decoded

Researchers gave smokers real or placebo cigarettes to understand the brain’s reward system. Image credit: Pixabay/pakura

We may like to think that we are uniquely rational creatures, but some of our reactions may be more ingrained than we like to think. 

Professor Wolfram Schultz at Cambridge University in the UK has zeroed in on how the brain measures risk – down to the single neuron – and his work is helping to explain how we make economic decisions.

To measure risk, he puts animals into situations where they have to choose between the risk of getting a larger reward and a safe outcome. Say that one button certainly gives five millilitres of apple juice and another gives either none or 10 millilitres. The average reward would be the same at five millilitres. But a risk-seeker might demand a higher average reward.

‘We wanted to see a signal for risk ... a short change in activity of specific neurons of specific brain structures, like dopamine neurons,’ Prof. Schultz explained. ‘The higher the risk, even if the reward mean stays the same, the stronger the signal.’

Identifying the neurons that detect and code risk is enabling them to understand what happens when a person makes a risk assessment.

‘Imagine you have a lighthouse by the sea, and it flashes occasionally, and then suddenly it flashes much faster, you realise that this is a different signal now ... that is what the neurons are doing. So the individual neurons will flash faster when the risk is higher, although the mean of the reward is the same,’ Prof. Schultz said.

‘You have a physiological marker which is independent of what people say in terms of desire or repugnance.’

Dr Carmelo Vicario, Bangor University, Wales

The ability to measure risk has an application in economics, which examines choice by measuring the value people set on receiving goods, called utility.

Big rewards

Economists talk about utility as an internal mental currency, abbreviated as utils. People and animals are utility-maximisers – they go after as big a reward as they can get.

‘Your neurons in the brain that are supposedly coding the subjective value, are actually coding these utils. That’s the first utility signal ever found in neurons,’ Prof. Schultz said.

The work is part of a project called RISKY REWARDS, funded by the EU’s European Research Council, and the team is also doing certain behavioural experiments with animals to prove that they are indeed utility maximisers and that their neuronal coding is responsible.

So far, it turns out that the neural coding of risk matches what economic theoreticians had suspected all along. ‘The mathematics is not just artificial, it is founded in real life, in brains, and economics the same way,’ said Prof. Schultz. ‘It is absolutely stunning, it works very well.’

Researchers at the Bangor University in Wales are also looking at the physical manifestations of rewards, such as financial gains or the taste of chocolate, and have found that they trigger a response on the tongue, even when the person isn’t aware of it.

Researchers placed an electrode on a subject’s tongue and showed them unpleasant items to test the tongue’s response. Image courtesy of REWARDING THE TONGUE

As part of the EU-funded REWARDING THE TONGUE project, Dr Carmelo Vicario studied the reward and dislike responses in humans using transcranial magnetic stimulation, a non-invasive technique which can be used to measure the excitement of a certain area in the brain.

His results showed that there is a negative relation between stimulus to the part of the brain that deals with dislike, and measurable changes in the electrical activity of the tongue’s motor neurons, known as motor-evoked potential amplitude.

Essentially, your tongue neurons show a reduced physical activation to processing dislike, regardless of whether you’re conscious of it or not.

‘My prediction was that any change in the activity rewards system may be mapped in terms of difference in the motor-evoked potential amplitude recorded from the tongue,’ said Dr Vicario. ‘You have a physiological marker which is independent of what people say in terms of desire or repugnance.’

Smokers

That gave him the idea to look at withdrawal symptoms in smokers. He recorded the tongues of around 20 smokers during craving periods and when they were given a real or placebo cigarette. What he found showed a clear difference in signals, with higher motor excitability on the tongue motor neurons when people were suffering withdrawal symptoms, and normalised excitability when they were rewarded with a smoke, real or otherwise.

That could someday be refined to be able to gauge the true degrees of withdrawal patients are experiencing.

Taken the other way around, it might even be possible to alter reward-related behaviour through the tongue.

‘If we can use the motor tongue area, not only for monitoring the rewards system, but also for modulating or changing it, this type of protocol may have direct implications for treatments of several disorders,’ Dr Vicario said.

That could help conditions where the brain’s reward system is malfunctioning, such as addiction, post-traumatic stress, or obsessive-compulsive disorder.

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