Changes in the group-mean <i>β</i> as a function of the index of difficulty (ID) and visual feedback condition.

β is the exponent of a power function—y = 1/xβ. A β value was calculated for each participant’s power spectrum at each ID level and visual feedback condition. Then, each group-mean β value was calculated by averaging across the individual-participant βs at a given ID and visual feedback condition. The error bar extending above each group-mean bar represents 1 SEM.