The pleasure principle

What do lovers and drug addicts have in common? Scientists now think that the pathways in the brain that govern arousal are also linked to addiction. Jerome Burne weighs the implications

Considering our cultural pre-occupation with sex, it is rather surprising that the first brain-scan studies of sexual arousal have only just been published. Mario Beauregard of the Neurological Institute of Montreal recorded the brain activity of men and women while they watched erotic videos. Unsurprisingly, he found that the regions devoted to processing visual data were busy but so also were circuits in the amygdala, linked with emotions, and a region in the orbitofrontal cortex (OFC) – just behind the eye sockets – that handles judgements.

This is just one of hundreds of research projects currently underway aimed at pinpointing the areas of the brain involved with controlling and responding to behaviour. It's a search that dates back to the last century when neurologists would gather reports of patients who had lost some ability due to damage to a certain area of the brain.

In 1861, for instance, the Frenchman Paul Broca reported his finding that a small area on the left side of the brain is vital for speech. Just over a hundred years later, the neurologist and writer Oliver Sacks revealed in The Man Who Mistook His Wife for a Hat, the remarkable effect of the destruction of part of the parietal lobe – you lose the ability to comprehend the meaning of shapes.

For the last 20 years, however, neurologists haven't had to wait for cases to come to them, they can peer into the fearsomely complex workings of the brain using scanning techniques known as fMRi (functional magnetic resonance imaging). Using very sophisticated calculations, this allows researchers to see which brain regions are using up more oxygen when you are performing some task – such as watching erotic movies. The assumption is that the more active areas are involved with the task at hand. The long-term hope would be to develop drugs to treat regions that are malfunctioning.

The erotic-movies research might sound frivolous – one eminent journal rejected it on the curious grounds that it "was not of general interest". But, in fact, it is part of a project to understand addiction better. Drugs such as cocaine and heroin work by hijacking pleasure pathways in the brain that evolved hundreds of millions of years ago to make sure that animals were powerfully interested in sex.

Some researchers believe that a better understanding of these sexual pathways might throw light on the workings of addiction. Over the last decade, studies on a strain of monogamous vole have found that one brain area – the ventral pallidum – is active when a male desires either his partner or a drug. Then at the beginning of this year, Annarose Childress of the University of Pennsylvania reported that a pathway involving the amygdala and the OFC were critical in cocaine addiction, the same pathway that Beauregard reported was active in his sexually aroused subjects.

So the question then becomes: why do cocaine addicts generally find it so much harder to control their desires than jilted lovers do? When Beauregard asked his subjects to turn off their erotic feelings, the level of activity in the limbic system disappeared totally, while the OFC remained active. This makes sense because other studies have shown that the OFC is associated with putting on the brakes, with looking at the consequences of an action.

Meanwhile, Childress found that cocaine addicts had less gray matter in their OFC than non-addicts, but whether that was the cause or the result of addiction isn't yet clear.

These findings on sex and addiction tie in with research that suggests that, as far as brain wiring is concerned, sex and violence go together, too. A number of studies have found that murderers often have damage to their pre-frontal cortex – where the OFC is located. A dysfunction of the orbital-medial prefrontal cortex has been found in people with impulsive aggression. Scans of more than 500 people concluded that violent people have diminished activity in the prefrontal cortex, while activity in the amygdala is increased.

Not far below the surface of many discussions of brain scanning lurks the issue of responsibility. If I've got weaker controls on my murderous impulses or desire for cocaine than other people, does that in some way make me less culpable?

Dr Dorothy Lewis, a psychiatrist at Yale University, who has studied hundreds of murderers, doesn't think so. "Most brain damaged people are non-violent," she says "and most people with a serious mental illness are not violent either." However, just being a male may be enough to make you less controlled.

Psychologist Ruben Gur scanned the brains of 57 men and 59 women and found that women had a significantly larger OFC than men. The implication is that angry women are neurologically better equipped to put the brakes on. His next study is to investigate whether those with larger OFCs actually are more controlled.

It is at this stage that we enter a legal, moral and scientific minefield. There is growing concern that findings in neuroscience may soon be taking us into a "posthuman" world, as the historian Francis Fukuyama has dubbed it.

Two conferences were held last year, at the Royal Institution in London and the University of California in San Francisco, to discuss the various ways that neuroscience was about to "transform our understanding of human nature". What will happen when "our technology provides access to the full spectrum of psychological states underlying behaviour"? How will our "bolstered sense of the brain as a deterministic machine undermine our notion of free will?"

There were echoes of such concerns in a recent report from the Nuffield Council on Bioethics that concluded that judges should consider reducing the sentences of convicted offenders "if scientists prove that their crimes were influenced by their genes".

But such concerns may well be very premature. We are still a long way from such sci-fi feats as spotting potential criminals just by scanning their patterns of brain activity. It's not just that the technology is not far enough advanced; a few dissenting voices claim there are fundamental flaws in the whole venture. One of these is Nikos Logothetis of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany. He has raised hackles in the neuroscience community by publishing results that suggest that brain scans don't show us what we think they do.

When he recorded from the individual brain cells of monkeys using implanted electrodes at the same time as running an fMRI scan, he found that the activity revealed by the scan was only a small fraction of what else was going on. He concluded: "For every area that glows there might be a whole network of vital regions working away in the background that don't show up because the technique isn't sensitive enough." What's more, an area may show up as active simply because of an unusual positioning of a blood vessel.

Ultimately, however, Logothetis is optimistic, claiming that his new methods will fairly soon be able to reveal "the whole chemical and molecular underpinning of the recovery process active in strokes and other sorts of brain damage".

But there may be a more fundamental problem. Over 20 million mice are used every year to uncover the links between genes and the brain states, with the aim of finding new drugs to treat disorders such as Parkinson's and Alzheimer's. But research reported in Science just before Christmas raised questions about how informative this work is. Films taken of lab mice in their cages at night show them exhibiting a range of stereotyped behaviour, the sort that is usually a sign of severe mental distress. Some were running round in circles for hours, while others were plucking all the fur from their faces. Just how useful are behavioural and brain studies done on animals in this state?

A version of this article appears in the February edition of the newsletter 'Medicine Today'; www.medicine-today.co.uk