A Brain of Two Halves

WOULD you consider yourself to be logical and analytical or creative and empathic? According to popular psychology you're one or the other, and it's all down to which half of your brain you use the most: the rational and calculating left or the intuitive, artistic right.

It's a myth, of course, but like all good ones it contains a grain of truth. Your cerebral cortex - the outer layer of your brain that deals with higher functions - is indeed split into two halves. They are connected by a flat bundle of nerve fibres called the corpus callosum, but work in subtly different ways - and these differences occasionally flicker into your conscious awareness.

The left-brain/right-brain myth arose from experiments done in the early 1970s on people who had had their corpus callosum cut as a last-ditch treatment for epilepsy. These "split-brain" patients showed some strikingly odd responses to information that was preferentially sent to one side of the brain or the other by presenting it to the extreme left or right of their visual field. This works because the right visual field is monitored by the right eye, which routes straight into the left brain, and vice versa.

For example, when a word or picture is presented to their right brain, split-brain patients are often unable to read or recognise it. This and similar experiments led to the idea that the left side of the brain deals with logic and facts while the right side is more intuitive and interpretive. We now know that this dichotomy is too simplistic, but its essence holds true. The latest view is that the two hemispheres have subtly different styles of information processing: the left has a bias towards detail, the right a more holistic outlook. You can watch a video of a split-brain experiment:



Most people, of course, have a functional corpus callosum that shunts information between the hemispheres. Even so, subtle left-right differences exist. One task where the hemispheres operate differently is face recognition. When most of us see a face, our right cerebral hemisphere does the lion's share of the work recognising its gender and decoding its expression. And because the right hemisphere is fed by the left visual field, that means we have a notable left-sided bias in our judgement of faces.

Look at this pair of faces (left). Which appears happier? Chances are you chose the bottom one. The two faces are, however, identical apart from being mirror images of one another. The picture is called a chimeric face and is made by taking two pictures of the same face, one with a neutral expression and the other smiling, chopping the pictures in half and joining the two mismatched pieces. Our general bias towards the left side of the face (as we look at it) makes us see the faces as different even though they are essentially equivalent.

It isn't just visual processing that is lateralised. There is some evidence that emotion is too, with the right side of the brain more specialised for negative emotions and the left for positive ones. Amazingly, simply activating one or other hemisphere by moving parts of your body can noticeably change your emotional state.

You can experience this by repeating an experiment first done in 1989 by Bernard Schiff and Mary Lamon of the University of Toronto in Canada (Neuropsychologia, vol 27, p 923). They asked 12 volunteers to perform a "half smile", lifting one corner of their mouths and holding it for a minute. Left-smilers reported feeling sadder afterwards, while right-smilers felt more positive.

Other researchers have reproduced the effect simply by getting people to contract the muscles of their left or right hand a few times. More recent research has suggested that motivation is similarly affected: people who performed right-sided muscle contractions became more assertive and spent longer trying to crack an impossible maths puzzle.

Unsurprisingly, these claims are controversial, with some teams failing to replicate the results. Last year, however, Eddie Harmon-Jones of Texas A&M University in College Station used EEG to confirm that flexing the hand muscles produces changes in emotion, but only when it is preceded by activation of the opposite cortex (Psychophysiology, vol 43, p 598). The left-brain/right-brain legend, it appears, is alive and well.