Why we use our fingers and toes to count
by Roger Highfield
The Daily Telegraph: 3rd September, 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
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.