readings> the Aha! feeling 

Two psychology students staged a handbag snatch in a busy shopping precinct in Swansea recently. Deliberately going against stereotype, a girl went up and grabbed a bag from a boy. The boy shouted out: "Stop thief! That's my bag," then melted away into the crowd.

Immediately afterwards, other students quizzed passing shoppers about what they had seen. Two out of five of the witnesses were sure it had been the boy snatching the bag from the girl rather than the other way round. The experiment was designed to show how our recall of events is often tailored to meet our prejudices. But it also raises a more subtle point - how is it that we can be so certain about what we remember in the first place? What is the gut feeling we get that confirms a memory as accurate - even when it might be absolutely wrong?

This feeling of knowing that we know is fundamental to our thinking. Without it, one thought would seem as valid as another. How else could we decide whether Stockholm was the capital of Sweden or Norway, or whether we had cornflakes or croissant for breakfast? We rely on some inner twinge to tell us that one answer is right and the other wrong.

Psychology is not too good at handling inner twinges so this almost metaphysical sensation of knowing that we know has been largely ignored. However this may be about to change following the discovery that an obscure medical complaint, Capgras's syndrome, appears to result from damage to a special "sense of familiarity" pathway within the brain.

The first case of Capgras's syndrome was noted in 1923. A 53- year-old French woman started to complain that her family had been taken away and replaced by a group of identical doubles. Later, she began to feel the same about friends and neighbours and eventually felt that even she, herself, was being shadowed by a double. Since 1923, about 300 other people have been identified as suffering from the same bizarre syndrome.

Capgras patients typically believe they exist in a world of impersonators. The feeling is strongest for close family and friends but can extend to pets and places. In one recent case, a blind Edinburgh woman became convinced that her cat had been replaced by a double because it no longer felt right to her touch.

The feeling of being plunged into a world of doubles can be so alarming that it drives the Capgras sufferer to psychotic behaviour. Some years ago, a Midlands man became so certain that his father had been replaced by a robot that he slashed his throat open, trying to find the wires!

For a long time it was believed that the florid delusions of Capgras' syndrome were a form of paranoia. Psychiatrists searched for a Freudian explanation, arguing that patients were indulging in some form of elaborate denial. However researchers such as Hadyn Ellis of Cardiff University and Andrew Young of Durham University now believe Capgras' syndrome can be blamed on a relatively simple failure of normal recognition processes following brain damage from a stroke, drug overdose, or some other cause. The injury has the effect that while sufferers can still recognise people and places they know, they no longer get the physiological buzz of familiarity which normally goes with such recognition. There is an "intellectual" recognition that allows them to see that family and friends match descriptions held in memory, but sufferers no longer have the positive emotional twinge that should go along with it.

"To the patient, something doesn't seem right. Patients see someone who looks and acts just like a loved one, but they don't feel any sense of familiarity for them. To explain this anomaly, they begin to confabulate. They start to believe things like they are surrounded by wax models or robots," explains Cardiff's Ellis.

Ellis says support for the idea that the process of recognition has separate intellectual and emotional components comes from another medical condition, prosopagnosia, which happens to be almost the mirror image of Capgras' syndrome. Prosopagnosia sufferers see all faces as looking much the same and are unable to recognise family and friends. In extreme cases, sufferers cannot even recognise their own reflection. Yet measurements of their bodies' emotional responses, using indications like changes in skin conductance, show that many prosopagnosics still react subconsciously to familiar people.

This split between the emotional and intellectual sides of recognition found in both syndromes shows that emotional twinges of familiarity are a real psychological phenomenon. What makes this finding so interesting is that the familiarity feeling underpinning recognition turns up in another guise as the aha! or eureka feeling - the mental snap of the fingers we get when we have a creative insight.

When we are faced with a problem, the usual thing we do is first imagine the situation and then toy with a few possible solutions. Suddenly, in a flash of insight, one of the answers will stands out. We recognise that an answer fits and it is as if a cartoon lightbulb has gone off over our heads to tell us we are right. This confirming aha! is the same sort of feeling we get when we recognise an old friend in the street or spot a long-lost ring under a chair. The only difference is that one is an externally focused form of recognition, signalling a match of sensation to memory, and the other is internally focused, signalling a match of an answer to a problem.

This general aha!/familiarity feeling seems to offer a possible explanation for several long-standing psychological puzzles. For a start, it could explain humour. Usually what makes us laugh is the sudden recognition of something incongruous - the punchline which answers a joke with an unexpected twist or the shock of seeing social norms transgressed. Humour appears to depend on bringing about the same electric jolt of recognition that can make us laugh with delight when we have a creative insight.

Another phenomenon that the aha! feeling might explain is deja vu. This feeling of reliving a past experience appears to be produced by a misfiring of our sense of familiarity. For some reason - perhaps because the brain has tried to make a recognition match but failed to complete it - our minds are filled with a ghostly glow of familiarity and everything that happens for the following few seconds feels as though it must have happened before.

Support for this idea comes from the fact that some drugs - especially the dentists' anaesthetic, laughing gas - can spark a similar fake feeling of recognition. Laughing gas users often feel they have made incredible discoveries while high, only to find out later they had been thinking some nonsense such as that the secret of the universe was "a sort of green light" or that "a smell of petroleum prevails throughout." The gas appears to provoke such an intense aha! that any scrambled thought that occurs under anaesthetic is experienced as the profoundest revelation.

If feelings of familiarity can be triggered by drugs, it seems even more certain they have a physiological base. It would be quite a breakthrough for researchers to discover the brain structures or pathways which generate such tell-tale sensations. Again, work with Capgras and prosopagnosia sufferers could hold the key. Hadyn Ellis of Cardiff University says: "Anatomically it is very difficult to pinpoint where the damage is because it is usually quite widespread. But the new generation of brain scanners should be able to pick up the location of activity."

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