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, 28 (11), 2344-51

The Timing of the Conscious Intention to Move

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The Timing of the Conscious Intention to Move

Masao Matsuhashi et al. Eur J Neurosci.

Abstract

The foundation of modern neuroscience and psychology about intention for action was laid by B. Libet et al. [(1983) Brain 106, 623-642]. They reported the time of awareness of wanting to move to be about 0.2 s before voluntary movement onset. However, despite repeated confirmation of the result, their method has been criticised for its dependence on self-reported timing and subjective memory, and the interpretation has been widely debated without general consensus. Here, we show that the mean time of the conscious intention to move was 1.42 s before movement, estimated based on the subject's real-time decision of whether or not there was a thought to move when a tone occurred. This event is after the onset of the bereitschaftspotential, an electroencephalographic activity preceding voluntary movement, but about 1 s earlier than the timing of intention reported previously based on the subject's recall. Our result solves some problems of the conventional method, thus giving a clearer answer to the controversies. The difference between the conventional result and our result suggests that the perception of intention rises through multiple levels of awareness, starting just after the brain initiates movement.

Figures

Figure 1
Figure 1
Schematic diagrams of mental and physiological procedures proposed in this study. The three diagrams are shown in approximately the same time scale. A: Illustration showing the T (onset of thought to move) and P (point of no return) time relationship in a single movement. The subjects were told to perform self-paced finger extension while tones were administered randomly. The solid horizontal line with a burst represents sample surface EMG waveform. Period 1: The subject was waiting. Period 2: The subject was thinking of and initiating the forthcoming movement. If there was a tone in this period the process was vetoed and no movement followed. Period 3: The subject could not stop his/her movement if he/she heard the tone in this period. Period 1’: The subject was again waiting and ignored any tones heard in this period. B: Box-and-whisker plot of estimated times in all subjects. RT (simple reaction time) sign is shown as its negative. The boxes are aligned to the first and third quartile, and whiskers extend out to the most extreme data which is no more than 1.5 times the interquartile range from the box. The thick vertical line in the box denotes the median. An outlier is shown by an empty circle. BP1: Early BP onset. BP2: Late BP onset. C: Proposed summary showing the relationship of the physiologically-determined movement genesis and the behaviorally-identified development of intention. The latter was described in the framework by Smallwood and Schooler (2006) with approximate W time by Libet et al (1983) added.
Figure 2
Figure 2
The tone distribution, estimated density curves and BP waveforms of subject 6 (A) and subject 10 (B) in the Table 1. Upper panels: thin squares represent histogram of tone distribution relative to the movement onset. Vertical axis is scaled to number of tone events per second. Thick solid and dotted lines are fitted sigmoid curves for T and P time estimation, respectively. The estimates are shown by thin dotted vertical lines and numbers on the top. Middle panels: Short vertical lines at the bottom represent tone distribution. Thick solid and dotted lines are smoothed tone distribution functions for T and P time, respectively, calculated by variable bandwidth kernel density estimation. Thin dotted curves are upper and lower limit of 99% variability distribution. T and P are defined as the time upper 99% line crosses baseline level (thin horizontal line) and are shown by thin dotted vertical lines and numbers on the top. Lower panels: averaged BP waveforms recorded at C3 and CZ electrodes are superimposed. BP1 and BP2 are obtained as intersections among baseline and liner regression lines for early BP and late BP periods. Values at C3 and CZ electrodes are averaged and shown by thin dotted vertical lines and numbers on the top.

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