Figure 1.

Panel A shows the succession of experimental conditions, which each subject was assigned. While the order of the first two conditions was obligatory, the order of the succeeding four conditions was randomized; however, in I and IM pressing conditions, I-perturbation conditions always preceded R-perturbation conditions. Data from the shaded condition (I-pressing) are not presented here. Panel B shows a schematic of the Inverse Piano device, from the top (with the front of the device at the bottom of the figure), and from the front. Only the sensors (and drive shafts during perturbations) protrude through the table top; the actuators are mounted under the table.