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The neurophysiology of gambling

If you read my previous post on the neurobiology of risk, you may recall that the ventral striatum regulates risk and rewards. In most people, thinking about winning money increases dopamine in the ventral striatum, and thinking about losing money decreases it. This is where gambling comes in.

This excellent post on the neuroscience of gambling describes a series of experiments done on monkeys that show that dopamine neurons learn when to expect rewards. Dopamine increases when the reward comes, decreases when it’s supposed to come but doesn’t, and wildly increases with unexpected rewards. And unexpected rewards are the principal attraction of gambling.

Instead of getting bored by the haphazard payouts, our dopamine neurons become obsessed. When we pull the lever and get a reward, we experience a rush of pleasurable dopamine precisely because the reward was so unexpected. (The clanging coins are like a surprising squirt of juice [for a monkey trained to expect juice as a reward]. It’s operant conditioning gone berserk.) Because our dopamine neurons can’t figure out the pattern, they can’t adapt to the pattern. The end result is that we are transfixed by the slot machine, riveted by the fickle nature of its payouts.

Thus, the unexpected reward essentially makes the ventral striatum very happy, and it can’t figure out how to become very happy again except by continuing the same behavior that led to it before. And there you are, two hours later, out of money at the blackjack table because you were waiting for that rush.

However, most of us have self-regulating systems in the brain that will eventually tell us that logically, we can’t spend all our money chasing the hope of an unexpected reward, that this isn’t enough of a reward overall, and we get into our cars or onto our flights from Vegas and leave the gambling table and its ventral-striatum-enticing allure behind.

But in pathological gamblers, the ventral striatum doesn’t react the way it’s supposed to (perhaps because the unexpected reward becomes expected?). Riba, Kramer, Heldmann, Richter, and Munte (2008, PLoS ONE) gave volunteers dopamine-increasing drugs and found that not only did the subjects make riskier choices, but parts of the basal ganglia and midbrain, which are important parts of the reward system in the brain, showed decreased activity after unexpected rewards.

As it happens, the dopamine-increasing drugs Riba et al. used were intended to treat Parkinson’s disease. It’s known that such dopamine agonists, as they’re called, can trigger pathological gambling behavior. Riba et al. suggest that pathological gambling–at least in Parkinson’s patients–comes from a need to overcome a dulled response in the reward systems of the brain.

This is similar to the concept of drug tolerance, where regular drug users must use more drug than they previously did in order to get the same effect because their system has become less sensitive to the drug. Essentially, gambling addicts are like any other addict: they don’t get the normal feelings that non-addicts do when pursuing their behavior of choice, so they do it longer, harder, and more in order to achieve the reward they crave.

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