Numbers seem like pretty basic items, and it might be imagined that the brain has a correspondingly simple way of handling them. In reality, however, our use of numbers is extremely complex. For example, we can represent a number in a variety of ways (4, four, and IV), all of which in turn represent an underlying concept (four items: ....). Does that suggest that there's an individual, idealized representation of four in our brains? If so, how does our brain handle numbers that are unlikely to have a representation, such as 104? Do we simply interpret numbers like that as a rough magnitude difference?

Two papers and a perspective appearing in today's issue of Neuron try to determine what's going on in the brain in these situations. As with most things these days, the newest results regarding the handling of numbers come via the application of functional MRI, which tracks the activity of different areas of the brain as it's confronted with a task. Both studies start with the knowledge that numerical processing appears to require the activity of the intraparietal sulcus (IPS). They also rely on the fact that challenging an area of the brain produces a burst of activity, and then a degree of acclimation: continued challenges require less work.

In one paper, the researchers acclimated the brain to a set of similar numbers (either 16-18 or 47-50) in a single type of notation. Shifting from one cluster to the other—with the correspondingly significant change in magnitude—set of a new spike of activity and acclimation. This was true whether or not the notation was changed at the same time, leading them to suggest that the brain represents different magnitudes in different areas of the IPS. Is there any indication of symbolic specificity? They noticed a weak but significant difference when the notation was changed, but only in the left side of the IPS; the right side seemed indifferent to notation. They suggest that this might correlate with other studies that suggested that the left IPS is associated with precision (exactly 17) as opposed to magnitude (upper teens).

The second study set up similar tests to the first, but differed in the timing of the tests, the notations used, and the magnitude of the changes between the numerical values. They also did a more fine-grained analysis of signals within the IPS. In their case, they found that notation differences showed up in the right IPS, but only if the resolution of the imaging was kept extremely high. Any sort of data smoothing eliminated the effect. This seems to suggest the opposite of the other study, which found this sort of difference in the left side—what's going on?

For one, it's important to note that the tests were structured quite differently, and that the resolution of the analyses were different. As such, the disagreement may be far less important than the fact that both suggest that numbers are handled differently on the left and right sides of the brain. If these sort of results hold up, it may put an end to the notion that there is any sort of idealized representation of numbers.

As a complete aside, the perspective indicates that there seems to be yet another layer of number handling in our brain that these studies don't address: the difference between having four items revealed at once and registering them sequentially, as in a counting process. Both of these studies also saw a number of other areas in the brain involved in number perception, so all of this research indicates that our brain does a lot of work before the IPS even gets involved.

Papers now available:

A Magnitude Code Common to Numerosities and Number Symbols in Human Intraparietal Cortex

Notation-Dependent and -Independent Representations of Numbers in the Parietal Lobes