Figure 1.

Experimental design and task performance. A, Participants were asked to imagine grasping the middle third of a tilted black-white cylinder (“stimulus”), which could be in 1 of 15 different orientations. As soon as they selected the movement, they indicated with a verbal response (“black” or “white”) whether their thumb was on the cylinder's black or white part. ITI, Intertrial interval. B, While participants selected how to grasp the tilted cylinder, oscillations in the sensorimotor cortex ipsilateral or contralateral to the grasping hand were independently modulated with short blocks (∼1 min) of tACS at either alpha (10 Hz) or beta (20 Hz) frequency. Stimulation was only applied during right hand trials to allow for wash-out of aftereffects of the intervention. There were also right hand trials that did not involve stimulation (“no stim”), which were used for comparison with the stimulation conditions. C, Percentage of trials in which participants reported to have imagined grasping the cylinder with their thumb on the white part, as a function of cylinder orientation, plotted separately for trials involving the left and the right hand. Dotted/dashed segments of the performance curves indicate cylinder orientations that afforded a single grasping configuration (e.g., 135 degrees for right hand trials, low-demand regime) or multiple grasping configurations (e.g., 45 degrees for right hand trials, high-demand regime), respectively. Error bars indicate ± 1 SE. For the line plots, data points were interpolated every 12 degrees and smoothed over 3 consecutive orientations. It can be seen that the preferred manner in which the cylinder was grasped depended on its orientation and followed the biomechanical constraints of the body. D, Duration of action selection (z-scores) as a function of cylinder orientation, plotted separately for trials involving the left and the right hand (all trials). Movement selection took 990 ± 390 ms (left hand; mean ± SE) and 975 ± 400 ms (right hand). Movement selection took longer (110 ± 20 ms, left hand; 105 ± 20 ms, right hand) when the cylinder afforded two different grasping configurations (high-demand trials) than when the cylinder could be grasped in a single manner (low-demand trials). The lines of individual subjects have been realigned based on the individual high demand orientations, before averaging. Other conventions as in C.