Psychologist Sheldon Cohen and his colleagues at Carnegie Mellon University, Pennsylvania, also found that the positive thinkers who do develop symptoms complain about them less.

The team studied over 300 initially healthy volunteers. First, each person was interviewed over two weeks to gauge his or her emotional state. This involved being scored in both positive categories – happy, pleased, relaxed – and negative categories -anxious, hostile and depressed.

Next the researchers squirted rhinovirus, the germ that causes colds, into each subject’s nose. Follow-up interviews questioned them daily for five days about any developing symptoms.

The people scoring in the bottom third for positive emotions were three times more likely to catch a cold that those scoring in the top third. ”People who express more positive emotions are less susceptible to upper respiratory tract infections than people with a negative emotional style,” says Cohen. Intriguingly, the scores for negative emotions showed no correlation with infections.

Low stress

One possible explanation for the protective effect of positive emotions is that happy people may lead healthier lifestyles than unhappy people, says Cohen. Happier volunteers were found to have lower blood levels of stress-related hormones such as cortisol, which influences high blood pressure.

”There is accumulating evidence that stress, and certainly emotional responses indicative of stress, influence the immune response,” says Ron Eccles, head of the Common Cold Centre in Cardiff University, UK. “Mind-body interactions are very interesting, this is an example of how the mind may influence body symptoms.”

Cohen’s team also assessed how those people who did catch a cold coped with the symptoms. This involved collecting the tissues used by volunteers to blow their noses and weighing them, to calculate the amount of mucus each person produced.

“Even when symptoms were physically the same, participants with a negative emotional style complained more,” Cohen told New ĐÓ°ÉÔ­´´.

The new research is published in July issue of Psychosomatic Medicine

As well as qualifying the teary “she hit me harder” playground argument and explaining why we can’t tickle ourselves, the discovery may provide insight into some self-delusional symptoms of schizophrenia.

To test the notion that the brain downplays sensations generated by body movements because it can predict what will happen, Daniel Wolpert and colleagues at University College London in the UK engaged six pairs of adult volunteers in “tit-for-tat” experiments.

Each volunteer took turns to squash their partner’s left index finger below a lever recording the amount of force applied. Their partner was instructed to reply to each push with equal force. The researchers quickly discovered that both volunteers pushed harder and harder on the lever. Afterwards the volunteers were surprised to learn that everyone had been given the same instructions and that their partners had not been asked to increase the force used.

Equal force

In a second experiment, subjects were asked to apply pressure to one of their own fingers with force equal to that generating by a mechanical device. Repeatedly they over- judged the force required, leading the team to conclude that the brain perceives self-exerted forces as weaker than they actually are.

“From the brain’s point of view events in the outside world and those generated by yourself get mixed together,” Wolpert told New ĐÓ°ÉÔ­´´. “The brain separates them using prediction.”

When the body makes a movement a message is sent to the brain warning it what to expect. In this way, the brain predicts and already knows the outcome of an action before it is performed. This explains why you cannot tickle yourself – your brain already knows what to expect and de-sensitises the signal from your skin.

Wolpert says this may help to explain schizophrenic symptoms involving delusion of control. “Some schizophrenic people generate a movement but can’t predict what is going to happen. They might claim the movements are generated by other beings, such as aliens, although they are made by themselves,” he explains.

Journal Reference: Science (vol 301, p187)

ĐÓ°ÉÔ­´´s have long suspected that microwave radar from satellites could “see” below the surface of very dry ground. Many were startled when images from a shuttle mission in the 1980s revealed what appeared to be ancient river drainage patterns below the eastern Sahara desert. Since then there have been other intriguing finds, including ring structures buried under Antarctic ice that look like meteorite craters or the remains of subglacial volcanic eruptions.

But until now there has been no proof that these images really do show underground objects. Researchers were unsure how radar is affected by underground features, and no one had ever used radar sensing to detect objects they knew were there. So although the images looked convincing, it was possible that they simply showed varying soil properties or surface slopes.

Dan Blumberg and Julian Daniels of the Ben Gurion University of the Negev in Israel decided to test the idea. They buried flat squares of aluminium at different depths under the sand in the Negev desert, then flew an aircraft over the area to carry out radar sensing of the layers beneath the surface.

By comparing the radar results with the squares’ known positions, the researchers showed that the patterns detected by the radar really did show the buried pieces of metal. “Now we have systematic proof,” Blumberg told New ĐÓ°ÉÔ­´´. “Buried objects can be detected from airborne systems.”

Buried deeper

That suggests the satellite images show real structures, too. Blumberg says the result confirms that ancient river routes do lie hidden beneath centuries of Saharan sand. Their location ties in with that of the oases and temporary lakes around which desert peoples build their communities. “Mapping river channels buried in sandy areas can improve our understanding of the geological and climatic history of the region,” says Daniels.

So far, the researchers have only looked for objects buried up to 40 centimetres deep. But now that they have proved the technique works, they are planning studies with different types of object, buried deeper.

For their experiments, they use microwaves of the longest possible wavelength, called P-band (see Graphic). At the moment, satellites generally use microwaves with shorter wavelengths because the resolution is better. But P-band radiation can penetrate farther underground, so Blumberg hopes that adding it to satellite sensors will allow them to probe deeper, perhaps up to 9 metres down.

“Using the P-band is quite new,” says Andrew Wilson, a remote-sensing expert for Britain’s Natural Environment Research Council. “It can penetrate the ground farther, so it would be good at revealing archaeological objects.”

Mass graves

Blumberg hopes that as well as archaeological remains, the method will in time be used to find fossils and geological structures. There could also be military or humanitarian applications. The resolution would probably be too low to pick up individual landmines, but it could show underground buildings or pipes, or perhaps even mass graves.

The main snag is that radar can only penetrate the ground in very dry areas, because liquid water tends to absorb the radiation. But Blumberg says that 15 per cent of the Earth’s surface is dry enough for the method to work, including the Antarctic and deserts. The surfaces of some planets and moons fit the bill too, he points out. He hopes P-band radar might be used to reveal structures such as water channels below the dry, frozen surface of Mars.