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Thursday, March 27, 2003

NATURAL SELECTIONS

Gambling on dopamine


Paul Newman's character in the 1967 movie "Cool Hand Luke" earns his eponymous nickname by bluffing wildly in a poker game, winning with a hand that amounts to nothing. "Yeah, well," he mumbles, "Sometimes nuthin' can be a real cool hand."

Gambling certainly appeals to the emotions. But there must be more to it than that. Why does gambling attract so many of us, so many putatively "normal" people?

Most of us are aware that the odds are stacked against us. Yet there is something exciting about taking a risk for the chance of a reward.

The chances of winning the jackpot prize of the national lottery in Britain are approximately 1 in 14 million, yet millions of punters buy tickets every week. Gambling on horse racing and playing pachinko are common activities in Japan, and while the odds of winning are better than winning the jackpot in the lottery, the payoff is lower and the old rule still applies: The house always wins in the end.

Given this, how can gambling, a patently maladaptive behavior, be explained by natural selection? It has been argued that gamblers misperceive the odds and think they are more likely to win than they really are. While such arguments apply to people with mild brain disorders, they do not work for the vast majority of us.

But evidence is emerging that dopamine, the feel-good brain chemical, is involved in situations when it is important to pay attention, when the brain is learning. How this might be linked with the appeal of gambling, and other types of risk-taking, will become clear.

Dopamine is a neurotransmitter that is widely active in the brain, and misfiring dopamine neurons are linked to Parkinson's disease, schizophrenia and drug addiction. It is also well-known for being the stuff that neurologist Oliver Sacks used to cure, temporarily, patients with sleeping sickness. (Robin Williams played Sacks in "Awakenings," the screen adaptation of that story.)

Dopamine is released when a reward is anticipated. It is the neurotransmitter which made Pavlov's famous dogs salivate at the sound of a bell that they'd come to associate with dinner time.

Now Christopher Fiorillo of the Institute of Physiology at the University of Fribourg in Switzerland, and colleagues at the University of Cambridge in England, have performed experiments that suggest that dopamine has a role during times of uncertainty, as well as anticipation.

Fiorillo and colleagues trained monkeys in a similar way to how Pavlov trained his dogs, but they used visual stimuli instead of a bell. Moreover, the scientists delivered a drop of syrup to the monkeys after the visual stimulus, but not always: They varied the probability of the liquid reward being delivered. In other words, the scientists introduced an element of uncertainty into the experiment.

A different visual stimulus (a pattern on a computer monitor) indicated a 100, 75, 50, 25 or zero percent chance of the monkey getting the reward (which followed seconds after the visual stimulus). The monkeys would lick their lips at different rates on seeing each stimulus, indicating that they distinguished between them.

But there was a surprise result. The researchers found that when uncertainty was at a maximum (when there was only a 25 percent chance of getting a syrup drop) almost 30 percent of dopamine neurons showed increased activity. And when the "payoff" to the monkeys was increased (when the scientists increased the volume of syrup) so too did the strength of firing of the neurons.

Some neurons released more dopamine as delivery of the reward became less certain. Fiorillo and coworkers suggest that this aspect of brain activity encourages attention and learning in the face of uncertainty.

And they speculate that the increase in dopamine contributes to the rewarding aspects of gambling. The suggestion is not far-fetched, since it is known that dopamine is involved in drug addiction. Gambling, too, becomes addictive for many people.

Of course, the human brain did not evolve in an environment dominated by casinos (certain residents of Las Vegas notwithstanding). Surrounded by pachinko parlors, roulette wheels and slot machines (and now tempted by Internet gambling and virtual racing), our brain chemistry may tip us into behaviors that are not beneficial.

"Although risk-taking behavior may be maladaptive in a laboratory or casino," Fiorillo and colleagues write, "where the probabilities are fixed and there is nothing useful to learn, it could be advantageous in natural settings, where it would be expected to promote learning of stimuli or actions that are accurate predictors of reward."

In the wild, risk-taking may help animals that face uncertainty to learn about their environment and how to predict the outcome of certain events.

If the researchers are correct, they will have identified the mechanism in the brain that is the ultimate source of income for the owners of casinos in Las Vegas and pachinko parlors across Japan: All of them bet on dopamine to make a fortune.

Rowan Hooper welcomes comments at rowan.hooper@tcd.ie


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