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.

Keywords: brain–computer interface; free choice; point of no return; readiness potential; veto.

Fig. 1.

Experimental design and possible trial outcomes. (A) The experiment consisted of three consecutive stages. During stage I, the stop signals were random. After stage I, a classifier was trained on button presses from stage I and the BCI predictor was activated. In the subsequent stages II and III, stop signals were elicited in real time by the BCI predictor. After stage II, subjects were informed about the predictor and instructed to try and move unpredictably. (B) Possible trial outcomes (see main text).

Fig. 2.

Mean readiness potential (RP), EMG activity, and button press distribution. The top panel shows the average squared EMG potential recorded at the right calf, averaged over all stages and subjects. The Inset on the Right shows the button press distribution relative to EMG onset, pooled across stages and subjects. The three colored lines in the bottom panel show the grand average RP at channel Cz, during individual stages of the experiment. For stage I missed button press trials were used, for stages II and III silent trials were used because these were not interrupted by the BCI (see text for details on silent trials). Individual RPs were averaged across subjects (colored shadings indicate SEM). The scalp topographies show the EEG potential of all recorded channels, averaged over three time intervals indicated by the shaded regions: [−550 −400] ms, [−150 0] ms, and [250 400] ms. There was no significant difference between RPs of the three stages [F_(2,18) = 0.02, P = 0.97; F_(2,18) = 0.12, P = 0.89; and F_(2,18) = 0.20, P = 0.82, respectively].

Fig. 3.

Percentage of trial outcomes across stages for the four trial categories (as in Fig. 1B). All trial categories in one stage (bars of same color) add up to 100%. Shown is the average across subjects (error bars indicate SEM).

Fig. 4.

Distribution of BCI predictions time-locked to EMG onset (vertical line). The three panels show the distribution of stop signals timings in predicted button press trials (A, red) and in aborted button press trials (B, green). C (red and green) shows their joint distribution. The black distribution superimposed as outline in all three panels shows the stop signal distribution in silent trials adjusted to account for the imbalanced probability of a trial being silent (40%) or not (60%). All bins comprised intervals of 100 ms, and counts were pooled across stages II and III of all subjects. Please note that, in silent trials, the distributions refer to the first stop signals that would have been emitted.

Fig. 5.

Summary model of results (see text for details). Abbreviations: BP, button press; EMG, electromyogram; ERD, event-related desynchronization; RP, readiness potential; SSRT, stop signal reaction time.