The point of no return in vetoing self-initiated movements

Abstract

In humans, spontaneous movements are often preceded by early brain signals. One such signal is the readiness potential (RP) that gradually arises within the last second preceding a movement. An important question is whether people are able to cancel movements after the elicitation of such RPs, and if so until which point in time. Here, subjects played a game where they tried to press a button to earn points in a challenge with a brain-computer interface (BCI) that had been trained to detect their RPs in real time and to emit stop signals. Our data suggest that subjects can still veto a movement even after the onset of the RP. Cancellation of movements was possible if stop signals occurred earlier than 200 ms before movement onset, thus constituting a point of no return.free choice | readiness potential | brain-computer interface | point of no return | veto I t has been repeatedly shown that spontaneous movements are preceded by early brain signals (1-8). As early as a second before a simple voluntary movement, a so-called readiness potential (RP) is observed over motor-related brain regions (1-3, 5). The RP was found to precede the self-reported time of the "'decision' to act" (ref. 3, p. 623). Similar preparatory signals have been observed using invasive electrophysiology (8, 9) and functional MRI (7, 10), and have been demonstrated also for choices between multiple-response options (6,7,10), for abstract decisions (10), for perceptual choices (11), and for value-based decisions (12). To date, the exact nature and causal role of such early signals in decision making is debated (12)(13)(14)(15)(16)(17)(18)(19)(20).One important question is whether a person can still exert a veto by inhibiting the movement after onset of the RP (13,18,21,22). One possibility is that the onset of the RP triggers a causal chain of events that unfolds in time and cannot be cancelled. The onset of the RP in this case would be akin to tipping the first stone in a row of dominoes. If there is no chance of intervening, the dominoes will gradually fall one-by-one until the last one is reached. This has been coined a ballistic stage of processing (23,24). A different possibility is that participants can still terminate the process, akin to taking out a domino at some later stage in the chain and thus preventing the process from completing. Here, we directly tested this in a real-time experiment that required subjects to terminate their decision to move once a RP had been detected by a brain-computer interface (BCI) (25-31). ResultsSubjects were confronted with a floor-mounted button and a light presented on a computer screen. Once the light turned green ("go signal"), subjects waited for a short, self-paced period of about 2 s after which they were allowed to press the button with their right foot at any time. They could earn points if they pressed while the light was green, but lose points if they pressed after the light had turned red ("stop signal"). The experiment had three consecutive stages (Fig. 1A). In stage I, stop signals were e...

Reckoning the moment of reckoning in spontaneous voluntary movement

“…The study by Schultze-Kraft et al (9) clearly shows that a neural commitment to act cannot be identified with the onset of the RP, over 1,000 ms before EMG onset in this study, because EMG can still be prevented up to 200 ms before EMG onset. Additionally, the peak of the RP cannot be associated with such a commitment either.…”

“…He tried to soften the consequences for proponents of conscious volition by suggesting that we can veto an impending movement during the final 200 ms before movement onset, which is when the decision to move goes from being preconscious to being conscious. However, according to Schultze-Kraft et al (9), the final 200 ms before movement onset is precisely when one can no longer veto the movement. Therefore, their data argue against the idea of the last 200 ms being a window of opportunity for acts of "free won't," and instead suggest that the subjectively reported time of the urge to move, although imprecise, is an accurate estimate of the time of the neural commitment to move.…”

“…The results of Schultze-Kraft et al (9) show that the movement goal-the button press-can still be withheld even when the first sparks of EMG activity have 9). Time t = 0 is the onset of the EMG signal.…”

“…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.…”

“…Schultze-Kraft et al (9) have found two sequential points of no return: one with respect to preventing an EMG signal, situated around 200 ms before EMG, and one related to preventing the button press, roughly 50 ms after EMG onset. The presence of dual points of no return is likely specific to their task, which required first lifting the foot (EMG onset) to then depress the button.…”