Recruitment of an Area Involved in Eye Movements During Mental Arithmetic

Addition to the Right, Subtraction to the Left High-level cognitive achievements, such as writing and mathematics, have appeared relatively recently in the evolutionary record in comparison to low-level skills, such as the perception of bright-dark boundaries. The latter have been demonstrated to arise from the coding properties of neurons in visual cortical centers, but what do the former map onto? Knops et al. (p. 1583, published online 7 May) provide evidence that addition and subtraction are encoded within the same cortical region that is responsible for eye movements to the right and left, such that the neural activity associated with addition could be distinguished from that associated with subtraction by a computational classifier trained to discriminate between rightward and leftward eye movements. Addition and subtraction are encoded in the same part of the brain that is responsible for eye movements and spatial attention. Throughout the history of mathematics, concepts of number and space have been tightly intertwined. We tested the hypothesis that cortical circuits for spatial attention contribute to mental arithmetic in humans. We trained a multivariate classifier algorithm to infer the direction of an eye movement, left or right, from the brain activation measured in the posterior parietal cortex. Without further training, the classifier then generalized to an arithmetic task. Its left versus right classification could be used to sort out subtraction versus addition trials, whether performed with symbols or with sets of dots. These findings are consistent with the suggestion that mental arithmetic co-opts parietal circuitry associated with spatial coding.