Changes in the group-mean power spectrum across index of difficulty (ID) and visual feedback conditions.

Each data point within each spectrum represents the across-participant average of log10 power within a given log10 frequency bin. The negative of the slope (−b) of the linear regression equation—y = bx + a—fit to each group-mean power spectrum was equivalent to the exponent (β) of a power function—y = 1/xβ—that is, if b = −1, then β = 1. β was the primary index of movement amplitude time-series structure. The amount of spectral power within each frequency bin represents the magnitude (size) of the time-series oscillations or variability within that frequency bin. In turn, the elevation of each spectrum represents the amount of overall movement amplitude variability across the range of frequencies.