Patients 'unafraid' to gamble show role of amygdala in decision-making
9 February 2010
Researchers at the California Institute of Technology studied a phenomenon known as 'loss aversion' in two patients with lesions to the amygdala, a region deep within the brain involved in emotions and decision-making. The results of the study, part-funded by the Wellcome Trust, are published today in the journal 'Proceedings of the National Academy of Sciences'.
Loss aversion describes the avoidance of choices which can lead to losses, even when accompanied by equal or much larger gains. Examples in the everyday life include how we make a decision on whether to proceed with an operation: the more serious the potential complications from the operation - even if the risk is low compared to the chances of success - the less likely we would be to proceed. It even has implications on organ donation rates - if people are required to 'opt-in' to a system, they are less likely to move away from the default option.
Dr Benedetto De Martino, a Sir Henry Wellcome Trust Postdoctoral Fellow and first author of the report, explains: "Imagine you're on 'Who Wants to Be a Millionaire'. You've just answered the £500 000 question correctly and have moved on to the final question. You're down to your 50:50 lifeline but don't know the answer. If you get it right, you'll win £1 million; if you get it wrong, you'll drop back to £32 000. The vast majority of people would take the 'loss averse option' and walk away with £500 000."
This new study has explored whether loss aversion is mediated by the amygdala, as is currently hypothesised. The researchers studied two patients affected by a rare genetic condition which has led to the formation of lesions to the amygdala. These lesions prevent the patients from perceiving, recognising or feeling fear. For example, the patients can recognise all other emotions in a person's face, but if shown a fearful face they cannot say what emotion that person is experiencing.
Each patient - together with twelve 'healthy' controls - took part in a task designed to test whether the chance of losing money affected people's likelihood to gamble
At the beginning of the experiment, each participant was given $50 with which to gamble on the outcome of flipping a coin, which carries a fifty-fifty chance of winning (or losing). However, each time, the amount that the volunteers could win or lose varied. For example, one time they might stand to win $50 or lose $20 depending on the outcome. The second time, they might stand to win $30 or lose $40.
The researchers found that, as expected, the healthy individuals were less likely to gamble when the difference between the potential winnings and potential losses was smaller - for example, while they might gamble if they stood to win $50 but lose only $10, they would be less likely to gamble if they stood to win only $20 but lose $15. When the potential losses outweighed the potential gains, the controls would not gamble.
However, the two patients with impaired amygdala activity were much less affected by the disparity between potential gains and losses; occasionally, even when the potential losses outweighed the potential gains they would choose to gamble, showing a lack of loss aversion.
"A fully-functioning amygdala appears to make us more cautious," explains Ralph Adolphs, the Bren Professor of Psychology and Neuroscience. "We already know that the amygdala is involved in processing fear, and it also appears to make us 'afraid' to risk losing money."
"It may be that the amygdala controls a very general biological mechanism for inhibiting risky behaviour when outcomes are potentially negative, such as the monetary loss aversion which shapes our everyday financial decisions," comments Dr De Martino, a visiting researcher from University College London.
"Loss aversion has been observed in many economic studies, from monkeys trading tokens for food to people on high-stakes game shows," adds Colin Camerer, the Robert Kirby Professor of Behavioral Economics, "but this is the first clear evidence of a special brain structure which is responsible for fear of those losses."
Dr De Martino and colleagues also investigated whether, as well as being 'loss averse', the patients were also 'risk averse'. Risk aversion and loss aversion are two similar, but not identical, processes and as such can be easily confused. People who are 'risk averse' are less likely to take chances even when they do not stand to lose anything.
The volunteers were again asked to make a decision based on the outcome of a coin toss. However, in this situation, the options were either to take a set amount without gambling (for example, $5), or gamble with a chance of winning $10 or receiving nothing but not losses were involved. In this experiment, both patients and controls showed little difference in their decisions, suggesting that the amygdala goes not control this aspect of risk taking.
The research was supported by the Gordon and Betty Moore Foundation, the Human Frontier Science Program, the Wellcome Trust, the National Institutes of Health, the Simons Foundation, and a global Center of Excellence grant from the Japanese government.
Image: Superior view of the human brain. Credit: Heidi Cartwright, Wellcome Images
De Martino B et al. Amygdala damages eliminates monetary loss aversion. E-pub in advance, Proc Natl Acad Sci U S A 2010. [Epub ahead of print].
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