Unveiling the sensorimotor basis of numerical processing: A functional near-infrared spectroscopy (fNIRS) study

Interactions between number processing and hand grasping have been extensively studied; however, the specific neural mechanisms underlying these interactions remain elusive. In this study, healthy adults (N=30) performed number magnitude comparison and hand grasping, both paired with stringent control conditions (color naming and squeezing, respectively). Hemodynamic activity in the cortical networks associated with number processing and hand action was recorded using functional near-infrared spectroscopy (fNIRS). Significant hemodynamic activity was observed during both number comparison and hand grasping within the left superior posterior parietal lobule and precuneus, as well as along the border between the right precentral and postcentral gyri. These areas were not active during the control conditions. Importantly, hemodynamic activity in a sub-area of the left parietal cluster correlated with the participants' proficiency in calculation, suggesting that a larger degree of functional specificity within this region may be associated with better math skills. Calculation performance was also linked to activity in a small region within the right inferior frontal gyrus. Overall, the right parietal activity revealed differential activation in number comparison and grasping. These findings confirm both shared and task-specific neural areas involved in numerical and motor processing, and ultimately highlight the role of individual differences in shaping the tight link between numerical cognition and the sensorimotor system.