Imagine flying a kite, walking on stilts, or eating with a knife and fork. Easy, isn't it? We can do that because the human brain is able to perform imaginary gymnastics with a mental image of the body. This so-called body image is sophisticated and malleable, and can be twisted and rotated in the three dimensions of space. We need it to do many of the things that we humans do, such as playing football, driving a car or eating with chopsticks.
But for a person to be really good at playing football or eating with chopsticks, the brain has to incorporate the ball or the chopsticks into its body image. The difference between a first-time user of chopsticks and an expert is that in the expert's mental self- representation, the chopsticks are treated as an extension of the arm.
The recent discovery that a female New Caledonian crow called Betty can both make and manipulate a simple tool made from garden wire to get at its food suggests that this idea of a mental body image may not be unique to humans. And it's not just Betty. Research on chimpanzees also suggests that the concept of body image may also exist to some extent among other animals.
Human consciousness is intimately linked with the idea of body image. Without this "silent sense" we would be unable to function. Yet, because it is always with us we don't really notice it until it disappears. This happens in weird and devastating impairments such as an amputee's sensation of a "phantom limb" - the feeling that the missing leg is still there. Or the case described by the neurologist Oliver Sacks in which a man complained that his own leg was the leg of a corpse that some prankster had put in his bed.
When it is working properly, the body image shows an extraordinary fluidity, adapting to the smallest physical demand while allowing the individual to continue to differentiate himself from the external world. "Even subtle changes, like an injury that weakens one muscle of the arm, will throw off the calibration and require the system to re-adjust," says Michael Graziano, a neuropsychologist at Princeton University in New Jersey. "Thus, the system retains a high degree of plasticity throughout life. In a sense, we have no problem using a club with spatial accuracy because the brain treats it like an altered arm," he says.
It is this ability to recognise self even under changing circumstances that scientists believe formed the foundation of symbolic thought, abstract reasoning and, ultimately, language - hence the interest in how and where it is encoded in the brain. And although chimps may possess certain fuzzy elements of it, it was generally regarded as the exclusive domain of humans.
So, when the cognitive neurobiologist Atsushi Iriki, of Tokyo Medical and Dental University in Japan, reported last year that monkeys can also recognise themselves on a video screen and become dextrous users of tools, he caused something of a stir. In a remarkable series of experiments, Iriki and colleagues trained macaques to manipulate a hand-held rake to retrieve pieces of fruit that were placed just out of reach. During the two weeks that it took them to become skilful swipers, the researchers recorded the electrical activity of cells in the parietal cortex - towards the back of the monkeys' brains - through electrodes.
As long ago as 1911, the parietal cortex was implicated as the seat of body image when neurologists Henry Head and Gordon Holmes observed cases of parietal lobe damage in which patients rejected their own body parts, and speculated that the feather in the hat of an Edwardian lady became integrated into her so-called body schema.
Iriki found that the monkey's parietal cortex contains cells that respond to both visual and touch input, and he suggests that these neurons integrate the two types of information into a coherent body image. Over the course of …