Movement-Preceding Neural Activity under Parametrically Varying Levels of Time Pressure

Self-initiated movements are known to be preceded by the readiness potential or RP, a gradual increase in surface-negativity of cortical potentials that can begin up to 1 second or more before movement onset. The RP has been extensively studied for decades, and yet we still lack a clear understanding of its functional role. Attempts to model the RP as an accumulation-to-bound process suggest that this signal is a by-product of time-locking to crests in neural noise rather than the outcome of a pre-conscious decision to initiate a movement. One parameter of the model accounts for the imperative to move now, with cued movements having a strong imperative and purely spontaneous movements having no imperative. Two different variants of the model have been proposed, and both predict a decrease in the (negative) amplitude of the early RP as the imperative grows stronger. In order to test this empirically, we conducted an experiment where subjects produced self-initiated movements under varying levels of time pressure, and we investigated the amplitude, shape, and latency of the RP as a function of the imperative to move, operationalised as a time limit. We identified distinct changes in the amplitude of the early RP that grew non-linearly as the time limit grew shorter. Thus these data did not support the prediction made by the model. In addition, our results confirm that the shape of the RP is not stereotypically negative, being either positive or absent in about half of the subjects.