Poster D137, Monday, March 27, 8:00 – 10:00 am, Pacific Concourse
Electrocorticography reveals the neural mechanisms of the arithmetic problem-size effect
Pedro Pinheiro-Chagas1, Amy L. Daitch2, Josef Parvizi2, Stanislas Dehaene1; 1Collège de France, Paris, 2Stanford University
Both the intraparietal sulcus (IPS) and the posterior inferior temporal gyrus (pITG) are known to be involved in arithmetic thinking, but their precise roles remain poorly understood. To characterize them, we recorded electrocorticography signals from 17 neurosurgical subjects implanted with grids of electrodes, while they were asked to verify additions such as 15+3=17. Behavioral results showed a classical problem size effect: RTs increased as a function of the size of the smallest (min) operand. We next examined how high-gamma activity is modulated by problem size. We found that the total activation across the decision period increased as a function of the min operand around the IPS, but remained constant within the pITG. Electrodes within the pITG, however, showed decreasing activation during the first second after calculation onset as a function of the min operand. Importantly, when regressing out the effect of RT, modulation of the total activity in the IPS vanished, while modulation of the initial activity in the pITG remained intact. The results suggest two distinct neural mechanisms underlying mental calculation, which would have been virtually impossible to disentangle using conventional noninvasive neuroimaging methods. While the activity in IPS seems compatible with an ‘accumulation of evidence’ pattern, similarly to neurons in the monkey LIP during a motion detection task, the activity in the pITG may represent the saliency of the evidence, similarly to neurons in the monkey MT area. Thus, we propose that the neural mechanisms previously described in basic perceptual decisions also operate during more complex symbolic reasoning.
Topic Area: THINKING: Problem solving