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Cortical Movement Preparation and Conscious Decisions: Averaging Artifacts and Timing Biases

The main goal of our article (Trevena & Miller, 2002a) was to elucidate the smearing artifact that arises in event-related potential (ERP) research and the consequences of this artifact for studies examining the relative timing of brain activity and conscious awareness. We believe that our article substantiates four main conclusions, and we consider the implications of various points raised by the commentators with respect to each of these conclusions. (1) A smearing artifact can make the onset of an ERP component appear much earlier in average ERP than it actually occurs, on average, in individual trials. For example, smearing makes the readiness potential (RP) and lateralized readiness potential (LRP) appear to start earlier than they actually do. Thus, smearing could potentially explain the finding that these types of brain activity appear to precede conscious decisions to move (e.g., Libet, Gleason, Wright, & Pearl, 1983). That is, conscious decisions might always precede RP and LRP onset in individual trials, even if the onsets of these smeared components of brain activity start before the average time of the conscious decision. A proper interpretation of Libet et al.’s (1983) effect thus requires consideration of the effects of smearing. (2) One plausible way to correct for the smearing artifact is to compare the onsets of RP and LRP against the earliest times at which conscious decisions to move are made. If these onsets precede even the earliest conscious decisions, then the smearing artifact is not wholely responsible for their early onsets. (3) After using this correction in the paradigm of Libet et al. (1983), the onset of RP precedes conscious decisions by too large an amount to be explained solely by the smearing artifact. On the other hand, RP onset also precedes conscious decisions and motor responses by such a long time that it is unlikely to represent subconscious motor preparation for the spontaneous response. It could instead represent a general preparation to ‘‘do something in the not-too-distant future,’’ an intention that the participant must consciously hold given the instructions of the experimenter. (4) After using this same correction, the onset of the LRP does not precede conscious decisions. Thus, the conscious decision may always precede LRP onset.