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Timing and Awareness of Movement Decisions: Does Consciousness Really Come Too Late?

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Timing and Awareness of Movement Decisions: Does Consciousness Really Come Too Late?

Adrian G Guggisberg et al. Front Hum Neurosci.

Abstract

Since Libet's seminal observation that a brain potential related to movement preparation occurs before participants report to be aware of their movement intention, it has been debated whether consciousness has causal influence on movement decisions. Here we review recent advances that provide new insights into the dynamics of human decision-making and question the validity of different markers used for determining the onset of neural and conscious events. Motor decisions involve multiple stages of goal evaluation, intention formation, and action execution. While the validity of the Bereitschaftspotential (BP) as index of neural movement preparation is controversial, improved neural markers are able to predict decision outcome even at early stages. Participants report being conscious of their decisions only at the time of final intention formation, just before the primary motor cortex starts executing the chosen action. However, accumulating evidence suggests that this is an artifact of Libet's clock method used for assessing consciousness. More refined methods suggest that intention consciousness does not appear instantaneously but builds up progressively. In this view, early neural markers of decision outcome are not unconscious but simply reflect conscious goal evaluation stages which are not final yet and therefore not reported with the clock method. Alternatives to the Libet clock are discussed that might allow for assessment of consciousness during decision making with improved sensitivity to early decision stages and with less influence from meta-conscious and perceptual inferences.

Keywords: Libet; decision-making; free will; introspection; volition.

Figures

Figure 1
Figure 1
Examples of neural predictors of movement decisions during two-option forced choice tasks. (A) The pattern of fMRI activation in the fronto-polar cortex predicts the chosen movement up to 8 s before the button press. The gray vertical line indicates the earliest time point of conscious intentions reported by the participants. Significant time points are marked with filled circles. Adapted from Soon et al. (2008) by permission from Macmillan Publishers Ltd.: Nature Neuroscience. (B) The neural firing rate of neurons in medial frontal and temporal cortex (solid line) predicts the chosen movement more than 1 s before participants report to be conscious of their intention (dashed vertical line). The dashed horizontal line represents the classifier performance upon shuffling left-right responses. From Fried et al. (2011) with permission from Cell Press. (C) High-gamma activity in the primary motor cortices of 7 participants (solid curves) discriminates between left and right movement decisions after participants report their intentions. Black squares mark the onset of an activity difference in the primary motor cortices indicating that the brain starts preparing the chosen movement. The gray stair plot shows the cumulative distribution of subjective intentions reported by the participants. The earliest high-gamma activation differences in M1 between left vs. right hand choices were observed at 194 ms (±74 ms) before movement onset, which was not significantly different from the subjective intention onset reported at 255 ± 109 ms [t(6) = 1.1, p = 0.32]. Modified from Guggisberg et al. (2011) with permission from Elsevier.
Figure 2
Figure 2
Summary of subjective (top) and neural (bottom) markers of motor decisions reported in the literature. W, subjective intention onset as determined with the Libet clock; M, subjective onset of finger movement as determined with the Libet clock; T, subjective intention onset as determined with random sampling; P, point of no return after which participants cannot stop an intended movement anymore; BP, Bereitschaftspotential; LRP, lateralized readiness potential; fMRI, predictors of movement decisions decoded from fMRI recordings; Firing rate, predictors of movement times decoded from neural firing rate in the SMA; High-gamma, significant differences in high-gamma power in the left vs. right primary motor cortex during left vs. right finger movements; EMG, electromyogram.
Figure 3
Figure 3
Spatially distinct neural substrates of primary intention (A) and introspection of intentions (B). Abbreviations: SMA, supplementary motor area; SPL, superior parietal lobule; L, left; R, right. Reproduced from Guggisberg et al. (2011) with permission from Elsevier.
Figure 4
Figure 4
The Libet clock requires a translation of the continuous experience of consciousness to a binary onset time. This may introduce delays in the reported intention onset.

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