Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control.
Libet et al. (1983) developed a method to compare the onset time of a readiness potential (RP) with the onset time of the corresponding intention to perform a spontaneous voluntary motor act. In relation to the onset of the RP, the time of conscious intention to move followed 350 msec later. From these results Libet (1985) concluded that the cerebral initiation of a spontaneous voluntary act begins unconsciously. We investigated the alternative interpretation that with the instruction to pay attention to feelings of 'wanting to move,' automatic and normally unconscious motor acts might have been brought to a level of conscious awareness. Therefore we conducted 3 kinds of experiment. In the first, RPs were measured from subjects performing unconscious movements. The second experiment was a replication of Libet's study while the third was a resting condition in which subjects looked for intentions to move introspectively. The results showed that RPs beginning approximately 500 msec before movement onset can be obtained with unconsciously as well as consciously performed spontaneous motor acts. The different scalp distributions of the two types of RP indicate that unconscious movements can be attributed to the activation of a contralateral process (lateral premotor system (LPS), primary motor cortex), whereas voluntary spontaneous motor acts seemed to be predominated by the medial premotor system (MPS). It is proposed that in the Libet situation focused attention on internal events led to the conscious detection of a normally unconscious process. This resulted in the activation of the MPS, especially the supplementary motor area (SMA), which released the starting signal for the execution of the motor act. We believe that the activation of the SMA and the urge to move occurred at the same time.