Like written and spoken language, human numerical abilities are quite astonishing for how they are organised in the brain.
After brain injury, various maths or numerical abilities can be shown to ‘doubly dissociate‘, meaning that parts of the ability can be independently damaged and so it can be inferred that they rely on independent (but, of course, interacting) brain systems.
The surprise comes from the fact that as a species, abilities like complex language, writing and maths are relatively recent cultural innovations.
While some of the core abilities may be inherited, there must be some aspects of the more complex skills which become tied up with the development of brain structure as we grow to account for the way in which they break down in very selective ways after brain damage.
Dehaene is one of the key researchers in understanding the neuropsychology of numerical ability and what he calls ‘number sense’ – a more general intuitive perception of quantity and number.
It has been suggested that this is also linked to other ways of perceiving the world, as can be seen from some strange interactions between number and space that can be seen in experiments:
But the brain is the product of evolution‚Äîa messy, random process‚Äîand though the number sense may be lodged in a particular bit of the cerebral cortex, its circuitry seems to be intermingled with the wiring for other mental functions. A few years ago, while analyzing an experiment on number comparisons, Dehaene noticed that subjects performed better with large numbers if they held the response key in their right hand but did better with small numbers if they held the response key in their left hand.
Strangely, if the subjects were made to cross their hands, the effect was reversed. The actual hand used to make the response was, it seemed, irrelevant; it was space itself that the subjects unconsciously associated with larger or smaller numbers. Dehaene hypothesizes that the neural circuitry for number and the circuitry for location overlap. He even suspects that this may be why travellers get disoriented entering Terminal 2 of Paris‚Äôs Charles de Gaulle Airport, where small-numbered gates are on the right and large-numbered gates are on the left. ‚ÄúIt‚Äôs become a whole industry now to see how we associate number to space and space to number,‚Äù Dehaene said. ‚ÄúAnd we‚Äôre finding the association goes very, very deep in the brain.‚Äù
The article is a great read and a useful introduction to some of the key findings in the field, as well as containing a whole load of eye-opening findings about number and the brain.
Link to New Yorker article ‘Numbers Guy’.