Brain Mechanisms of Arithmetic: A Crucial Role for Ventral Temporal Cortex

Elementary arithmetic requires a complex interplay between several brain regions. The classical view, arising from functional magnetic resonance imaging (fMRI), is that the intraparietal sulculs (IPS) and the superior parietal lobe (SPL) are the main hubs for arithmetic calculations. However, recent studies using intracranial electroencephalography (iEEG) have discovered a specific site, within the posterior inferior temporal cortex (pITG), that activates during visual perception of numerals, with widespread adjacent responses when numerals are used in calculation. Here, we re-examined the contribution of the IPS, SPL and pITG to arithmetic by recording iEEG signals while subjects solved addition problems. Behavioral results showed a classical problem-size effect: RTs increased with the size of the operands. We then examined how high-frequency broadband (HFB) activity is modulated by problem size. As expected from previous fMRI findings, we showed that the total high-frequency broadband (HFB) activity in IPS and SPL site increased with problem size. More surprisingly, pITG sites showed an initial burst of HFB activity that decreased as the operands got larger, yet with a constant integral over the whole trial, thus making these signals invisible to slow fMRI. While parietal sites appear to have a more sustained function in arithmetic computations, the pITG may have a role of early identification of the problem difficulty, beyond merely digit recognition. Our results ask for a re-evaluation of the current models of numerical cognition and reveal that the ventral temporal cortex contains regions specifically engaged in mathematical processing.