The cerebral basis of interference in comparison making
Comparing the values of the numbers 2 and 7 seems easy enough, but if the number 2 is written much larger than the number 7, we may momentarily hesitate.
To find out how variables such as physical size, luminosity and numerical values are represented in the brain, researchers at the Frédéric Joliot Hospital Service (FJHS, Orsay) have conducted a series of tests to localise the regions of the brain that are sensitive to the effort made in discriminating between two values of the same variable (number, size and luminosity) and those that are sensitive to possible interference between these variables. These tests involved recording reaction times and activated brain areas by functional MRI in volunteers given two visual stimuli and asked to state which one represented the larger number, was of larger size or was more luminous. These different comparisons were made separately in the experiment, but the numerical values, size and luminosity of the numbers were varied randomly.
The results obtained show that the estimate of the numerical values was influenced by their physical size. The luminosity did not in itself disturb these estimates, but influenced the physical size comparison. The analysis of the functional MRI profiles obtained in the course of these operations shows that precise, often common, brain regions were involved in these evaluations. Thus the representation of numbers and size seems to involve a common parietal region, whereas that of size and luminosity involved common occipital areas, suggesting a more perceptive discrimination. These studies make it possible to define the functions of different brain regions more precisely, and so help achieve the functional mapping of the human brain.