The Ambiguity of Libet’s Intention Reports: Behavioral and EEG Correlates

The influential studies of Benjamin Libet and colleagues (1982,1983) showed that the pre-motor neuronal activity leading to the voluntary movement (the Readiness Potential, RP) precedes participant’s conscious awareness of the intention to perform this movement (W-time report). The observed discrepancy between the RP onset and the W-time questioned the causal role of the intention in initiating voluntary movements. Thus, Libet’s paradigm contributed significantly to the free will debate. However, the validity of Libet’s paradigm is highly disputable. It is still unclear (1) whether the W-time is an adequate behavioral marker of the intention to move; and (2) whether RP onsets are causally linked with W-time reports. The current project further investigated the validity of Libet’s paradigm. Firstly, we investigated whether W-time reports reflect the genuinely experienced intention to move or rather W-time reports are experimental artifacts. Based on the previous finding (Dominik et al., 2017; Sanford et al., 2020), we expected that naïve participants would be unable to differentiate the W-time from the time of the actual movement execution (M-time). To test this hypothesis, we split the participants to two groups, which differed only in the order of the experimental tasks. W-task required 40 W-time reports, while M-task required 40 M-time reports. Thus, W-first group started the experiment with W-task and finished with M-task, while M-first groups underwent the experimental tasks in the opposite order. As a result, we confirm the strong modulation of introspective reports by experimental procedures: W-time reports in M-first group significantly preceded W-time reports in W-first group. However, for the first time we showed that even naïve participants can differentiate W-time and M-time: participants reported significantly different W-time and M-time in both experimental groups. Summing up, our results showed that libetian interpretation of W-time report as a genuine marker of the intention to move should not be unequivocally ruled out. Secondly, we investigated the relationship between the RP and the W-time. We showed that RP onsets did not correlate with W-time reports both within and between the participants, thus replicating previous findings (Haggard & Eimer, 1999; Schlegel et al., 2013). It further questioned the validity of the direct juxtaposition between RP onsets and W-time reports. Further, we split the RP into the early-BP, the late-BP, and the N1-peak components. As a result, we showed that late-BP onsets correlate with W-time reports in both W-first and M-first groups within individuals. No correlations were found between other RP components and W-time reports. Taking everything into account, contrary to the previous studies we showed that (1) Libet’s intention reports (W-time) might reflect the intention to move; (2) late-BP onsets (but not early-BP onsets) might be considered as the neuronal correlate of the origin of the intention to move. Thus, the libetian mismatch between the onsets of electrophysiological and behavioral correlates of the intention to move is still valid for the free will debate.