Conscious decisions to move and cortical movement preparation: which comes first?

Abstract

The idea that conscious decisions determine our actions has been challenged by a report which suggested that the brain starts preparing for a movement before the conscious decision to move (Libet et al., 1983). Since they found that cortical movement preparation started before the conscious decision to move, Libet et al. argued that movements are initiated unconsciously, and not as a result of our conscious decisions. In this thesis, we described the study on which these claims were based, summarised several criticisms of Li bet et al. 's conclusions, and presented four experiments addressing two main alternative explanations ofLibet et al. 's results. One of these was based on a smearing artifact that occurs during EEG averaging, and the other questioned Libet et al. 's assumption that the EEG present before the decision to move reflected the processes underlying movement initiation.

The main aim of the first two experiments was to test an explanation ofLibet et al. 's results based on an artifact of EEG averaging. In these two experiments, temporal order judgment methodology was used to look at the distribution in time of the reported times of the decision to move. Specifically, we tested whether the earliest reported decision-time could have occurred before the onset of cortical movement preparation (which Libet et al. reported at 500 msec before the movement). The results of Experiment 2 (but not Experiment 1) suggested that the earliest decisions may have been reported more than 500 msec before the movement Thus (since the earliest decisions occurred before the RP onset) there was some support for the possibility that the decision may have been before the start of the RP on each individual trial, despite Libet et al.'s averaged results.

A second possible explanation for Libet et al. 's results was based on a review of different measures of cortical movement preparation (summarised in Chapter 3), which suggested that the Readiness Potential (the RP, which Libet et al. used to measure movement preparation) may not be specific to movement initiation, but may instead reflect a number of processes associated with the anticipation of a future movement. We also argued that the Lateralised Readiness Potential (LRP), which measures hand-specific movement preparation, seems to reflect the processes necessary for a movement to be initiated immediately.

The main aim of Experiments 3 and 4 was to compare participants' reported decision-times with both the RP and the LRP. The results of these studies suggest that reported decision-times were always after the start of the RP (this was partly because the RP recorded here started longer before the movement than that reported by Libet et al., 1983). In contrast, decisions were often reported before the start of the LRP. We concluded that although there may be some anticipation of a future movement (represented by the RP) more than a second before the movement occurs, the specific processes necessary for the movement to be initiated (represented by the LRP) may not start until after the decision to move.