Reckoning the moment of reckoning in spontaneous voluntary movement

Abstract

In everyday life, movements are sometimes triggered by external sensory stimuli, like when the traffic light ahead turns from green to red and you move your foot from the gas pedal to the brake pedal in response. But many of our voluntary movements are spontaneous, meaning that they are not tied to any recent stimulus in the current sensory environment. This capacity affords us what Gold and Shadlen refer to as “freedom from immediacy” (1). Interest in so-called “self-initiated” movements has grown considerably since the discovery of the “Bereitschaftspotential,” or readiness potential (RP), by Kornhuber and Deecke in 1965 (2), a slow buildup of neural activity in motor areas leading up to movement onset. This same slow buildup has since been observed at the single-neuron level in both humans and other animals (3–7). The RP was presumed to reflect the covert inner trigger for self-initiated movements: the process of “planning and preparation for movement” (8). One question that naturally arises is: When, if at all, along the time course of the RP does the brain make the final commitment to initiate movement? Is there a point of no return after which the sequence of action potentials becomes “ballistic” and movement, although not yet happening, can no longer be aborted? This is the question that Schultze-Kraft et al. (9) ask through a clever experiment involving a direct brain–computer interface (BCI). On-line detection of the RP allowed them to present a stop signal when the probability of an impending movement was high. This process afforded the authors a unique perspective on the inhibition of voluntary, uncued actions.