Meg Ryan's demonstration of a faked orgasm in the film When Harry Met Sally showed how easy it is to deceive a man in bed.

But neuroscientists have now found that a brain scan can detect when women are merely going through the motions - it's when they think too much.

Dutch researchers persuaded 13 couples to have sex while either the man or the woman had their head encased in a PET brain scanner at the University of Groningen in the Netherlands. The women were also asked to fake an orgasm so images could be compared with the real thing.

The results give a picture of how brains react during sex. Contrary to the popular assumption that men's brains "switch off" during sex while women need to be more mentally engaged to gain maximum pleasure, the images show that both sexes experience a major neurological shutdown when they orgasm.

Gurs Holstege and his team asked either the man or the woman in each couple to lie down with their head strapped into the scanner, while their partner manually stimulated them. The partners then swapped places, while the scientists looked on and used the PET scanner to monitor minute changes in brain activity as sex progressed.

Professor Holstege admitted that the study hit a few difficulties at first, with the laboratory conditions meaning that only half the couples felt able to rise to the occasion. The participants also complained of cold feet in the scanning room, prompting researchers to give them socks.

By the end of the study, the lowered lighting and socks meant that 80 per cent of the couples were able to have sex.

The scientists found that during stimulation, a part of the brain called the amygdala, linked to emotions such as fear and anxiety, was deactivated in men and women. The insula, a primitive part of the brain that scientists still do not understand fully but is also linked to emotion, was also deactivated in men, along with the SII region, related to the interpretation of stimulation. The hippocampus of the women's brains, an area linked to memory, was also switched off.

At the moment of orgasm, there was even greater deactivation in both men and women. Professor Holstege said: "The deactivation was enormous in men but it was also very pronounced in women. We see extreme deactivation in large parts of the brain in women, and especially in the emotional brain. It seems that the key factor in having an orgasm is forgetting everything around you, letting go, not being anxious or fearful or alert to your surroundings."

When the women were asked to fake orgasms, the scans showed that this deactivation did not occur, and more brain activity was shown. Professor Holstege said: "It was very clear when the women were faking orgasms. While they made the same movements as when they had a real orgasm, the part of the brain that controls motor activity was activated, showing they were thinking about what they were doing, while when they were really climaxing, it was an unconscious movement."

His team is planning to study couples in their post-coital state to look at changes in brain activity following sex.