Neurophysiological mechanisms of optimized graphomotor performance in biscriptuals

Many literate adults master two writing systems: a phenomenon termed biscriptuality. Biscriptuals do not just juggle two scripts-their graphomotor coordination also generally outperforms that of monoscriptuals, suggesting that some components of their motor system have been optimized. To uncover the neurocognitive foundations of this biscriptual advantage, we synchronized a loop-tracing task on a digitizing tablet with electroencephalographical (EEG) recordings. Theta (4-7 Hz) and beta (13-30 Hz) oscillatory dynamics were analyzed to test whether biscriptuals display better optimized planning and monitoring, and/or better sensorimotor control. At the behavioral level, biscriptuals displayed a robust advantage in tracing frequency and stability compared to monoscriptuals. At the neural level, biscriptuals showed lower frontal theta power than monoscriptuals, with frontal theta power positively correlated with behavioral variability. Optimization of predictive inference mediated by midfrontal theta oscillations thus stands out as a hallmark of the biscriptual advantage. Biscriptuals also displayed a lower degree of beta synchronization over parieto-occipital electrodes than monoscriptuals, with beta power being less correlated with kinematic dynamics. Similar effects in the beta band were observed for all participants when task difficulty decreased. These effects suggest higher reliance on sensory feedback when graphomotor execution is more demanding. Overall, these results call for a model of handwriting control where prefrontal and sensorimotor components contribute to implementing internal predictive models, whose accuracy and reliability depend on the type of expertise acquired when learning to write.