What are the brain mechanisms involved in elementary arithmetic? The very first brain imaging studies, using SPECT, PET and then functional MRI, quickly isolated an important region: as soon as an adult performs mental arithmetic, there is bilateral activation of the flanks of the intraparietal sulcus (see Dehaene, Piazza, Pinel, & Cohen, 2003). This region occupies a very specific position within a mosaic of sensory-motor regions involved in eye, hand and finger movements (Simon, Mangin, Cohen, Le Bihan, & Dehaene, 2002). Its activation is present whatever the calculation performed, and even when the subject is simply comparing two numbers or detecting the presence of a digit among letters. The intraparietal region seems to play an important role in the abstract representation of numbers, insofar as it is activated regardless of the notation used to present numbers (Arabic numerals, spoken or written number names), in all the cultures tested (China, Japan, USA, Israel, Europe).
Intraparietal activation is also present when numbers are presented as sets of dots (Piazza, Izard, Pinel, Le Bihan, & Dehaene, 2004). An adaptation method demonstrated the convergence of symbolic and non-symbolic representations of numbers in this region (Piazza, Pinel, Le Bihan, & Dehaene, 2007). The same method, extended to the recording of evoked potentials in 2-3-month-old infants, has shown that the intraparietal cortex is activated, particularly in the right hemisphere, as early as a few months of life to the presentation of sets of objects whose numbering varies (Izard et al., 2008). Early intraparietal lesions may also be the cause of developmental dyscalculia in some children (Molko et al., 2004).
