It all began 30 years ago when Scientific American magazine published an article describing what happens if you cut the connection between the two halves of the brain, so they no longer communicate with each other. Language and problem solving were found to be the job of the left side, while visual and spatial tasks were carried out by the speechless right hemisphere. Hence the popular idea that our brains housed this neurological odd couple.
But "dichotomania", as such over-simplification has been dubbed, obscures a much more interesting debate about how the left and right brains actually interact in daily life. "Far from being the silent one, lost in a creative haze," says Robert Ornstein, professor of cognitive neuroscience at Dartmouth College in New Hampshire and author of a new book on the two hemispheres, "the right brain turns out to be vital for understanding the really interesting aspects of language, like jokes, lies and metaphors."
However, one of the authors of the original Scientific American article, Professor Michael Gazzaniga, formerly of Stanford University in California, disagrees. He has written an update for the magazine, published last month, which declares that "the left brain's consciousness far surpasses that of the right".
Gazzaniga's claim is rooted in one of the most striking of the early findings - that the left brain is the ultimate story-teller. It spins yarns to explain what is going on, with absolutely no concern for the truth. For instance, Gazzaniga would write out a request, such as ``pick up that shovel'' and reveal it only to the left eye of a split-brain patient, so that the information is processed by the brain's right hemisphere (visual signals from the left eye are sent to the right side of the brain).
Then he would ask the patient: "why did you pick up that shovel?" The reply would come from the left hemisphere, the only one that can speak. Although there was no access to the real reason, because the link to the right brain was cut, the patient would give some plausible explanation about tidying up or putting it in a safe place.
Subsequent research suggests that this "interpreter mechanism", as Gazzaniga called it, may lie behind the phenomenon of false memory. If you ask a normal person about something they have seen, they will often include details that were not actually there - a well known problem for eye-witness reports. Studies of split-brain patients show that the left brain is far more likely to make these errors than the right. Brain scans have shown that when a true memory is being recalled only the right brain lights up, while both hemispheres are active during a false one.
Mostly, however, the "interpreter mechanism" is reliable. In fact, Gazzaniga believes the left hemisphere is what gives us our sense of identity. One of the brain's many extraordinary feats is taking information from hundreds of modules - for speech, for movement, for vision and so on - and integrating them into a whole, so we can say: "I did this, I saw that." Gazzaniga suggests that this could be done by the left hemisphere, endlessly trying to explain why things happen. "The inventive and interpreting left hemisphere," he concludes, "has a conscious experience very different from that of the truthful, literal right brain."
Ornstein's interpretation could not be more different. For him the right brain is the one that gives us an overall view of the world and enables us to understand where we are. He was one of the researchers involved in the debate from the beginning. It was his best-selling book, The Psychology of Consciousness, that helped to popularise the original idea of the division of labour between the two hemispheres. When he returned to the subject 30 years later, he found over 40,000 scientific papers on the subject.
Particularly revealing were a series of studies into left and right brain function using that valuable neurological research tool - the joke. "Most of the time we don't think about jokes, they are either funny or they aren't," says Ornstein. "But this research shows that understanding a joke requires some quite complex mental processing, much of which goes on in the right brain." In his new book, The Right Mind: Making Sense of the Hemispheres (Harcourt Brace), he gives an example: "A woman wants to cook a rabbit stew but the hares hanging at the butcher's are quite large, so she says to the butcher, `I'd like to make some rabbit stew but these things are too big. Could you cut one in two for me?' Then comes the punch line `Sorry ma'am, we don't split hares here'.'' Not a rip roarer but a nice little pun, a play on hare/hair. However, research by Professor Howard Gardner at Harvard University has found that patients with right brain damage do not get jokes like this.
"To understand a joke you have to be able to follow the story and try to guess at what is going to happen," Ornstein says. "You know the punch line will not be what you expected but, and this is the important part, it will fit in with the story in a surprising way. The problem that right- brain patients have is that they can't hold a situation in their mind - and then relate it to the word play that links the two."
It seems that the right brain is what we use to understand the context, which is why these patients also find it difficult to spot sarcasm. Take this situation. The boss asks a secretary to send off some letters quickly. Some hours later he finds her making a social phone call to a friend, the letters unposted. "You have been working hard," the boss says. Ask a right brain patient why he said that and you are likely to get a fantastical explanation of why he might want to praise the secretary. Here, Gazzaniga's "interpreter mechanism" is hard at work without the right brain to keep it in check.
"There's a pattern emerging from this sort of research that shows the right brain handling ambiguous, metaphorical information where you need to be able to see the broad picture," says Ornstein. "The left brain does better at activities that are sequential and precise, which is why it is used for language and fine muscular movements."
Apart from this ability to see the big picture, the right brain is also in charge of an apparently mixed bag of other functions with no obvious link, such as negative emotions, the control of large muscle groups and musical perception. But why?
Ornstein's other suggestion is that the link originates in the womb. Because the right hemisphere develops slightly faster, it gets to handle the low-frequency sounds heard inside the mother's body. Immediately after birth, it deals with the equally broad-brush visual information about the mother's face.
Other important early functions like responding to unpleasant emotions and controlling the movements of large muscles are routed to the right brain for the same reason. This early experience with the broad, fuzzy picture, Ornstein believes, could form the basis for handling ambiguous information later.
It is all a long way from the logical left and the free-spirited right. Thirty years on, the left looks more like a salesman, the right a corporate strategist, but you need both every minute of the day.