In the year of 1970, a shameful experience was carried out on a psychiatric patient in New Orleans. He is only known as ''Patient B-19.''
The patient had a drug problem and he had been expelled from the military for showing homosexual tendencies. His psychiatrist, Robert Heath, as a part of therapy and attempting to ''cure'' him of being gay, hooked electrodes into his brain. The electrodes were attached to what were thought to be the ''pleasure centres'' of his brain.
With the electrodes on, B-19 had the liberty to turn them on by pressing a button. And what he did was – he kept pressing it, time after time – over 1000 times a session, according to a BBC article authored by David Edmonds, presenter of 'The Big Idea' on the BBC World Service.
Kent Berridge, professor of biopsychology and neuroscience at the University of Michigan, later explained that it made B-19 very sexually aroused.
He felt a compulsion to masturbate and found both men and women sexually attractive and whenever the electrodes were removed, he strongly protested.
The psychiatrist Robert Heath noted something odd when he asked B-19 to describe how the electrode made him feel.
He expected adjectives like "fantastic", "amazing", or "wonderful". But, B-19 didn't use any of them. In fact, it turned out that he didn't seem to enjoy the experience at all.
So why then he kept pressing the button and why again did he protest when the electrodes were removed?
Kent Berridge says we have to start by recognising that although B-19 didn't enjoy the sensations produced by the electrodes, he nonetheless wanted to turn the electrodes on.
But, isn't that a contradictory thing to do?
For many years psychologists and neuroscientists assumed that there was no real difference between liking something and wanting it. "Liking" and "wanting" were thought to be synonymous.
Alongside this assumption - that wanting equates to liking - there was another.
It was widely believed that there was a system in the brain, involving the hormone dopamine that drove both wanting and liking, writes David Edmonds.
There also seemed to be compelling evidence that dopamine was essential for pleasure.
Rats, like humans, love sugary stuff, but when dopamine was removed from their brains and sweet substances were placed in their cages, they ceased to seek these foods out. Cut off the dopamine, it was thought, and you cut out the pleasure.
But was this right? Kent Berridge found another way to investigate the link between dopamine and pleasure.
After removing dopamine from rats' brains, he fed the rats a sugary substance. "And to our surprise, the rats still liked the taste normally. The pleasure was still there!"
In another experiment at his lab, the dopamine levels were raised in rats, leading to a huge increase in eating - but no apparent increase in liking.
One may wonder if it can at all be sure if a rodent is enjoying itself. Well, the answer is that rats have facial expressions rather like humans. When they eat a sweet substance, they lick their lips and when it's something bitter, they open their mouths and shake their head.
So what's going on? Why do rats still like a food they no longer seem to want?
Kent Berridge had a hypothesis, but it was so wild that even he didn't really believe it for a long time.
Was it possible that wanting a thing, and liking it, corresponded to distinct systems in the brain? And was it possible that dopamine didn't affect liking - it was all about wanting?
For many years, the scientific community remained sceptical. But now the theory has become widely accepted. Dopamine increases temptation.
Dopamine intensifies the temptation for food if you're hungry, and makes the smoker crave a cigarette.
The most startling evidence that the dopamine system fires wanting, and not liking, comes once again from the unfortunate laboratory rat. In one experiment, Kent Berridge attached a little metal stick to the rat cage that, when touched, delivered a minor electric shock. A normal rat learns, after one or two touches, to stay well away from the stick.
But by activating the rat's dopamine system, Berridge was able to make the rodent become engrossed by the stick. It would approach it, sniff it, nuzzle it, and touch it with its paw or nose. And even after the minor shock was received, it would return time after time within a five- or 10-minute period, before the experiment was stopped.
He argues that wanting is more fundamental than liking.
Ultimately, it doesn't really matter for the preservation of our genes whether we like sex, or like food. Far more important is whether we want to have sex, and whether we seek out food.
The single most important implication of the wanting-liking distinction is the insight it offers us into addiction - be it to drugs, alcohol, gambling, and perhaps even to food.
For the addict, wanting becomes detached from liking.
The dopamine system learns that certain cues - such as the sight of a coffee machine - can bring rewards.
Somehow, in ways that are not fully understood, the dopamine system for the addict becomes sensitised. The wanting is triggered by numerous cues. Drug addicts may find their urge to take drugs sparked by a syringe, a spoon, even a party, or being on a street corner.
But the wanting never ceases to go away - or not for a very long time. That makes drug addicts extremely vulnerable to relapse. They want to take the drugs again, even if the drugs give them little or no pleasure.
For rats, the dopamine sensitisation can last half a lifetime. The task now for researchers is to find whether they can reverse this sensitisation - in rats, and then hopefully, in humans.