Arithmetic and the brain: a direct cortical electrostimulation study on two patients with intractable epilepsy.
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In the last decades, much research has been done on the neural basis of number processing and arithmetic. Most studies have used fMRI, a technique that does not differentiate between brain areas which are essential for calculation and areas which are only involved. A technique that does not possess this disadvantage is direct cortical electrostimulation. In the current study direct cortical electrostimulation has been used to investigate the neural basis of arithmetic by testing two patients with intractable epilepsy, who had been selected for brain surgery. Clarifying the neural substrate of calculation contributes to the preserving of arithmetic abilities during neurosurgical interventions in these patients. The preoperative and postoperative results of the patients have been compared and the performances of the patients have been compared to those of a healthy control group as well. A new digital arithmetic task has been developed to test the arithmetic abilities by measuring reaction time in addition to accuracy, which has barely been done in the past. Although the results showed nearly no differences in accuracy, many differences in reaction time were found. Moreover, stimulation of certain sites of the cortex resulted in an increased reaction time, although it did not interfere with accuracy. It has become clear that reaction time is a more sensitive measure than accuracy and that it is therefore important to take it into account when assessing arithmetic during direct cortical electrostimulation in patients with intractable epilepsy. This may prevent permanent postoperative calculation problems.