Article

Brain preparation before a voluntary action: Evidence against unconscious movement initiation

Dunedin School of Medicine, University of Otago, New Zealand; Department of Psychology, University of Otago, New Zealand
Consciousness and Cognition DOI:10.1016/j.concog.2009.08.006 pp.447-456

ABSTRACT Benjamin Libet has argued that electrophysiological signs of cortical movement preparation are present before people report having made a conscious decision to move, and that these signs constitute evidence that voluntary movements are initiated unconsciously. This controversial conclusion depends critically on the assumption that the electrophysiological signs recorded by Libet, Gleason, Wright, and Pearl (1983) are associated only with preparation for movement. We tested that assumption by comparing the electrophysiological signs before a decision to move with signs present before a decision not to move. There was no evidence of stronger electrophysiological signs before a decision to move than before a decision not to move, so these signs clearly are not specific to movement preparation. We conclude that Libet’s results do not provide evidence that voluntary movements are initiated unconsciously.

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    Article: An accumulator model for spontaneous neural activity prior to self-initiated movement.
    Aaron Schurger, Jacobo D Sitt, Stanislas Dehaene
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    ABSTRACT: A gradual buildup of neuronal activity known as the "readiness potential" reliably precedes voluntary self-initiated movements, in the average time locked to movement onset. This buildup is presumed to reflect the final stages of planning and preparation for movement. Here we present a different interpretation of the premovement buildup. We used a leaky stochastic accumulator to model the neural decision of "when" to move in a task where there is no specific temporal cue, but only a general imperative to produce a movement after an unspecified delay on the order of several seconds. According to our model, when the imperative to produce a movement is weak, the precise moment at which the decision threshold is crossed leading to movement is largely determined by spontaneous subthreshold fluctuations in neuronal activity. Time locking to movement onset ensures that these fluctuations appear in the average as a gradual exponential-looking increase in neuronal activity. Our model accounts for the behavioral and electroencephalography data recorded from human subjects performing the task and also makes a specific prediction that we confirmed in a second electroencephalography experiment: Fast responses to temporally unpredictable interruptions should be preceded by a slow negative-going voltage deflection beginning well before the interruption itself, even when the subject was not preparing to move at that particular moment.
    Proceedings of the National Academy of Sciences 08/2012; 109(42):E2904-13. · 9.68 Impact Factor
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Keywords

Benjamin Libet
 
conscious decision
 
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cortical movement preparation
 
electrophysiological signs
 
Gleason
 
Libet
 
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movement preparation
 
Pearl
 
people report
 
signs
 
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stronger electrophysiological signs
 
unconsciously
 
voluntary movements
 
Wright