Volitional control of movement: the physiology of free will

Abstract

This review deals with the physiology of the initiation of a voluntary movement and the appreciation of whether it is voluntary or not. I argue that free will is not a driving force for movement, but a conscious awareness concerning the nature of the movement. Movement initiation and the perception of willing the movement can be separately manipulated. Movement is generated subconsciously, and the conscious sense of volition comes later, but the exact time of this event is difficult to assess because of the potentially illusory nature of introspection. Neurological disorders of volition are also reviewed. The evidence suggests that movement is initiated in the frontal lobe, particularly the mesial areas, and the sense of volition arises as the result of a corollary discharge likely involving multiple areas with reciprocal connections including those in the parietal lobe and insular cortex.

Figure 1

Possible models of free will. The blocks indicate functional activities of regions of brain and the arrows indicate time.

Fig. 2

Timing of subjective events and the Bereitschaftspotential (readiness potential, RP) with data from Libet et al. (1983). RPI is the onset of the Bereitschaftspotential with ordinarily voluntary movements and RPII is the onset with movements made quickly with little forethought. W is the subjective timing of the will to move, M is the subjective timing of the onset of movement, S is the subjective timing of a shock to the finger. EMG onset or shock delivery is set at zero ms.

Fig. 3

Influences on the motor system that drive movement. If free will is one of those influences, its anatomy is unknown.

Fig. 4

Diagrammatic representation of activation of neural activity and the triggering of saccadic eye movements. Activity reaches threshold at 3 different times and saccades are initiated when threshold is reached. Modified from Schall and Thompson 1999 with permission.

Fig. 5

Regions activated with attention to intention, that is, areas activated by subjects trying to determine the onset of intending to move (W condition; called I condition in this paper) as compared with when trying to determine the movement itself (M condition). SMA is supplementary motor area, DPFC is dorsal prefrontal cortex, IPS is interparietal sulcus. Modified from Lau et al. 2004 with permission.

Fig. 6

Possible timing of subjective events in comparison to measurable events in the course of making voluntary movements. This is similar to Figure 2, but the subjective events and measurable events are plotted on separate time lines. The subjective events are plotted twice, once at the time they are ascribed to in real world time and once when they might actually have occurred. The latter is only hypothetical, but is necessarily in the right direction from the ascribed times.

Fig. 7

Mapping of free will model onto brain anatomy with some additional components. The top part is the same as the bottom model of Fig. 1. SMA is supplementary motor area, IPS is interparietal sulcus.