EEG correlates of Fitts's law during preparation for action.

Humans' inability to move fast and accurately at the same time is expressed in Fitts's law. It states that the movement time between targets depends on the index of difficulty, which is a function of the target width and the inter-target distance. The present study investigated the electrophysiological correlates of Fitts's law during action planning using high-density electroencephalography. Movement times were scaled according to Fitts's law, indicating that participants could not overcome the speed-accuracy trade-off during a 1-s preparation period. Importantly, the index of difficulty of the planned movement correlated linearly with the amplitudes of the cognitive N2 and P3b components, which developed during the planning period over parieto-occipital areas. These results suggest that the difficulty of a movement during action planning is represented at a level where perceptual information about the difficulty of the ensuing action is linked to motor programming of the required movement.