For as long as they have existed, students have embraced the role of obliging guinea-pigs, on hand to take part in all manner of intrusive, humiliating and bizarre experiments dreamed up by their supervisors. Nonetheless, one of Michel Cabanac's experiments must have raised eyebrows. "I offered them money to feel pain," says the physiologist at Laval University in Québec. "It can be quite dangerous, because what if a student has just destroyed his parents' car? He's going to need money really badly."

Undeterred, Cabanac lined his students up against a wall. It was going to be bad, but not as bad as they might have thought. He got them to sit, as if perched on an imaginary stool, a position which forced their weight onto their knees. "Try it," he says. "The pain soon becomes unsufferable." Cabanac then promised the students increasingly large lumps of cash to endure the pain. The more he offered, the longer they suffered. The longest lasted for eight minutes 20 seconds.

Ironically, Cabanac's experiment was part of a broader investigation into the science of pleasure. His aim was to find out what, if anything, was the point of pleasure. His conclusions, and those of other scientists working in the field, suggest that not only is pleasure good for our health, but it is at the root of our ability to make sense of the complex world in which we live.

Cabanac's proof that people will suffer pain for payment will come as no surprise to the millions who do things they hate in return for a monthly pay cheque. But in a follow-up experiment, Cabanac showed there was a more fundamental point to make about what influences our behaviour.

Before the second test, Cabanac asked people to rate the pleasure they got from playing a video game. They were then sat in a temperature-controlled room and Cabanac, while cooling it down, asked them to rate how unpleasant the feeling was. He then combined the two experiments. "We cooled the room down, and every time, the same thing happened. As soon as it was cold enough for their displeasure rating to just outweigh the pleasure of playing the game, they stopped the experiment," he says.

According to Cabanac, the tests show that, while it might not be obvious all the time, each of our decisions is ultimately driven by pleasure-seeking. "Pleasure is the common currency that allows us to make any, and I mean any, decision in our lives," he says. "Any decision is made according to the trend to maximise pleasure."

Pleasure-seeking certainly makes evolutionary sense. As organisms developed, the emergence of pleasure as a sensation will have helped reinforce healthy behaviour, such as eating certain foods, having sex and keeping warm. But while Cabanac's theory might make evolutionary sense, that doesn't make it correct. It doesn't take long to think of examples where a decision looks entirely unpleasurable. What about a decision that ultimately leads to a person's own death? How could that choice come out as the most pleasurable path to take?

In 1969, Jan Palach, a Czech student, set himself on fire in protest at the Soviet invasion of his country. He died from his injuries three days later. "That's an atrocious death, yet he did it by choice," says Cabanac, who assumes Palach was not mentally ill. "The fact that he was suffering hell by dying by fire was compensated by an overwhelming joy of telling the Russians 'Look. Look what we are able to do against you. You do not win.'"

At the Neurosciences Research Institute at the State University of New York, director George Stefano believes that pleasure is not only the driver for every decision we make, but is a crucial component for making sense of the world. "As human beings, we always pride ourselves in being rational, but if we were 100% rational, we would have to weigh up every single possible action we might take at any time. Imagine how time-consuming that would be. Even a cognitive organism doesn't have time to be truly rational," he says. Pleasure, says Stefano, is our brain's way of short-cutting the rational process by subconsciously and continuously ranking what is most important to us from the vast number of options we are faced with. Stefano's phrase for it is likely to make dedicated hedonists smile: "Pleasure leads to pure rationality," he says.

As with the majority of neuroscience, some of the most reliable evidence comes from studying people who were born with, or have later suffered, damage to specific parts of the brain. At the University of Iowa, a team lead by neuroscientist Hanna Damasio has been studying people with lesions in a region of the cortex associated with pleasure. They found that although the patients had no intellectual impairment, in a simple gambling test, they made hopeless decisions. "They are oblivious to the consequences of their actions," the team noted in a paper published in the journal, Brain.

Despite decades of effort, scientists are still teasing out the precise neural circuitry that allows us to experience pleasure. In the 1980s, many scientists believed there was just one major brain circuit that governed pleasure. Triggered by the neurochemical dopamine, it excited the cerebral cortex and other areas of the brain such as the amygdala and nucleus accumbens. But more recent research has cast doubt on the role of dopamine. It now seems the chemical plays a subtly different role - making us feel desire rather than pleasure.

Scientists now know that another brain circuit, triggered by chemicals called opiods, does play a key role in pleasure sensations. Injecting drops of opiod into a part of the brain called the ventral pallidum heightens the enjoyment of sweet tastes, they found, suggesting it boosts the natural pleasure sensation. Meanwhile, at Oxford University, a team lead by neuroscientist Edmund Rolls has discovered that a region of the brain called the orbitofrontal cortex (OFC), which lies just behind the eyes, contains bundles of cells that are triggered by different types of pleasurable experience. Signals from the OFC are then thought to feed into the dopamine and opiod circuitry.

According to Cabanac, pleasure can only be a transient sensation, the feeling of warming up when cold, or of eating when hungry. He believes that the lack of these gaps between how we feel and how we want to feel explains a lot of misery in modern society. "We're not hungry, we're not cold, we have everything," he says. The result, he says, is that we are tempted to seek pleasure in other ways, by taking drugs, or over-indulging in pleasurable activities. Extremely hedonistic lifestyles may be caused by compulsive behaviour leading to an endless craving for pleasurable sensations, or subtle damage to the underlying brain circuitry, he adds.

Of course, it's possible to have too much of a good thing, and pleasure can easily become pain. This flipping of pleasure into pain has been investigated using brain scans focusing on the OFC and have captured the fine line that is the difference between the two states. Marilyn Jones-Gotman of the Montreal Neurological Institute at McGill University recruited self-confessed chocoholics and fed them lumps of chocolate while monitoring their brain activity using a brain-scanning technique called positron emission tomography. After each chunk, the person was asked to rate how much they wanted another piece. "We fed them until they absolutely could not face another bite," she says.

Jones-Gotman found that as the experience of eating chocolate flipped from being intensely pleasurable to downright repulsive, activity in the orbitofrontal cortex shifted from the centre to nearer the outside. They had captured the exact point where pleasure became pain. But is this the change that tells us when we've had too much of a good thing? Jones-Gotman doesn't think so. "If you're over-eating, expecially in cases like Christmas dinner when you're eating food that people like very much and associate it with the good feeling of previous Christmases, you probably won't stop until it's actually painful," she says.

While pleasure may have evolved as a way to encourage creatures to indulge in healthy behaviour and avoid more harmful pursuits, Stefano believes there is another benefit. Inside brain neurons, and also other tissues in the body, is a chemical called proenkephalin. He says that when we experience pleasure, proenkephalin is broken down, producing a substance that causes a feel good sensation. But the same enzymes involved in that process also release another chemical called enketylin, a strong antibacterial agent. "Just think of the beauty of that: when you're feeling good, you protect yourself," says Stefano.

Though scientists are slowly teasing out the secrets of pleasure, they have a long way to go. One problem is that little funding goes into looking at why things go right in humans. Instead, money pours into researching disease and disorders. "It's time that changed a little," says Stefano. "Feeling good is healthy. Why don't we look into it?"

Further reading

www.nel.edu/25_4/NEL250404R01_Esch-Stefano_v4_2p.pdf

The neurobiology of pleasure and its effects on health

http://wings.buffalo.edu/aru/ARUreport01.htm

A review of pleasure systems in the brain

www.thebrain.mcgill.ca

Graphics of the brain and neural circuitry

Affective Neuroscience: The foundations of human and animal emotions, Jaak Panksepp, OUP, 2004

Descartes' Error: Emotion, reason, and the human brain, Antonio Damasio, OUP, 1994