Why we use our fingers and toes to count
By Roger Highfield
(Filed: 03/09/2003)

The first profound insight into the source of Carol Vorderman's number-crunching skills was reported four years ago. Prof Stanislas Dehaene of the Service Hospitalier Frederic Joliot and Prof Elizabeth Spelke of the Massachusetts Institute of Technology used a scanner to show in unprecedented detail the two brain processes involved in one of the most important of all human abilities - the use of numbers.

One mathematical process relies on language to carry out exact calculations - explaining why we recite multiplication tables. This kind of activity lit up the brain's left frontal lobe, the area known to make associations between words.

The second process that underpins our use of numbers is "analogue" ability - the mathematical intuition, or hunch, that we all use to recognise that 24+13=97 is false without calculating. We also use it to make estimates, and it gives us a sense of two-ness of eyes and five-ness of fingers. This skill relied on activity in the brain's left and right parietal lobes, responsible for visual and spatial representations.

Intriguingly, the precise region in the parietal lobes where this skill resides - the "intraparietal sulcus" - also controls finger movement. It is no accident that finger counting is an almost universal stage in the child's learning of arithmetic (though it is unclear where one ancient New Guinea culture fits in - it has a 33 base system, which includes toes, testicles and penis, said Prof Butterworth).

The activation of the parietal lobes complements earlier work showing that patients with damage to this area often suffer from "acalculia", where number skills are affected. And it dovetails with a study by one of Prof Butterworth's colleagues, Dr Elizabeth Isaacs, who has used brain scanning to investigate the poor mathematical skills of many children born prematurely.

With a technique called voxel-based morphometry, which is highly sensitive to brain structure, she compared those with poor mathematical skills with a carefully matched group of unaffected premature children. Her studies revealed a blob in the left parietal lobe where the affected children have less grey matter. Dr Isaacs said other work has linked low levels of taurine, an amino acid (a building block of the proteins that construct and operate the body) to the brain development problem.

Other insights may come from "dyscalculic" people - up to 11 per cent of the population (depending on your definition). Even though they are often well educated and intelligent, they have a profound difficulty with maths (they are often dyslexic, too), probably because of abnormality or insufficient development of the intraparietal sulcus. Prof Butterworth found that their reaction time when handling numbers is "astonishingly slow". He is now using a scanner to hunt for a structural or functional difference in their brains.