It is hard to imagine how driving performance would not be impaired if one hand was continuously off the wheel - especially if it was holding something else, like a mobile phone. But the motoring hazard or otherwise posed by hands-free phones - and by extension, ordinary conversation - is less obvious.

My husband claims that I seem incapable of driving and speaking simultaneously. Because I learnt to drive as I was entering middle age, it might be argued that I am still awaiting some rite of passage to the state of True Motorist, where one is able to move hands and feet with no conscious effort, while performing astonishing cognitive feats of speech and thought. Such insights as I have to date are spasmodic, but without doubt it is when I can relax that my driving seems better. What happens in the brain of the seasoned driver that frees them to indulge in another world, be it Mozart on the CD or a domestic quarrel, while driving home on autopilot?

At the back of our brains is a structure that has been described as just that - autopilot. It is a conspicuous cauliflower-shaped bulge that looks so much like an independent little brain that it was actually dubbed the "little" brain, the cerebellum, to distinguish it from the main brain (cerebrum). In humans its outer surface occupies an area of 50,000 square cm: hence the deep folds, as if nature had to scrunch it up to fit it into the cramped area of our skulls. It is an area that is discernible in a vast range of animal brains and consists of cookie-cutter-like modules, all with the same repeating circuitry, that correspond in turn to each part of the body, in a distorted map.

In humans, damage to this structure results in severe impairments in coordinating movements ("ataxia"). The central problem arising from a disrupted cerebellum is of orchestrating a host of movements with the information from the senses flooding into the brain. Accordingly, the performance of complex skills, such as driving or playing the piano, is difficult. Cases of damage to the cerebellum have revealed that this region is critical for generating the type of movements guided moment to moment by the senses.

This type of reactive movement is different from those affected in other motor disorders, such as Parkinson's disease. At the nuts and bolts level of brain-cell operations, the cerebellum can be seen more than any other part of the brain as a kind of machine - whatever is fed in from the senses elicits a corresponding response - in movement. It is perhaps easy to see then why intellectual activities such as chat might not conflict with the chain of senses and movement of driving. So long as things are proceeding to the programmed plan, the cerebellum frees us from the tyranny of thinking about controlling our muscles.

Even in this most automated of brain systems, awareness can be important if it involves the short-term prediction of the consequences of a movement. It has been claimed that one can improve one's tennis by simply imagining often enough and vividly enough that one is engaged in a real game. Perhaps my motoring would look up if I let my cerebellum take me on extensive virtual drives.