Physiology of free will

Abstract

Free will is a perception that people have that they choose to make their movements. This perception includes a sense of willing the movement and self-agency that they are responsible for the movement. If there is a "free will force" that plays a role in movement selection, it should precede movement. There is no evidence for a driving force, and the perception of willing is not fully processed until after the movement. The perceptions of free will likely arise from an interaction between frontal and parietal areas. Free will might be considered to exist if a person's brain is functioning normally without coercion. Ann Neurol 2016;80:5-12.

Fig. 1

Diagrammatic set-up for the experiments of Libet et al. and Vinding et al. Subjects sit in front of a “clock” with a ball that moves around the circumference every 3 seconds. They make movements whenever they want, and then afterwards report the “time” of distal intention (thinking about making the movement sometime in future), proximal intention (willing of the movement, W), or the movement itself (M). In the Libet et al. experiment, the EEG is backaveraged from the movement; in the Vinding et al. experiment, the EEG is backaveraged from the time of distal intention.

Fig. 2

Diagrammatic set-up for the experiment of Matsuhashi and Hallett. Subjects made movements at freely chosen times as soon as they thought about doing so (time T). They also listened to tones delivered randomly. If they heard a tone after deciding to move, then they should veto the movement. Hence, looking at the timing of tones with respect to movements, in period 1, tones will occur since T had no happened yet, in period 2, there should not be any tones since the movement will be vetoed, in period 3, tones will occur again since the point of no return (P) will have been reached and the movement cannot be vetoed. Diagram is modified from the original paper.

Fig. 3

Diagrammatic set-up for the experiment of Schneider et al. In the first part of the experiment, A, subjects made movements freely and when they moved a light went on. EEG was analyzed during these movements and a prediction algorithm was calculated. In the second part of the experiment, B, the situation for the subject was the same, but if the algorithm detected that the subject was going to make a movement, then the computer turned on the light. After that the subject sometimes did and sometimes did not move. Following the light and possible move, we asked the subject to report what they were thinking when the light went on.

Fig. 4

Time line for events related to movement initiation and execution with associated subjective perceptions. BP= Bereitschaftspotential. T=time of thinking about making a movement. W=time of willing a movement. M=time that a movement is thought to be initiated. W*=end of the time when the perception of W can be influenced. Note that the time of W is referred backward in time even though its processing is not completed until after the movement in already initiated.

Fig. 5

Diagrammatic set-up for intentional binding experiments. Subjects use a “Libet clock” for subjective timing. First, A, the timing of button pressing is noted (similar to Libet’s M) and, B, the timing of a randomly occurring tone. Then, C, the tone is yoked to the button press at varying intervals, and again subjects are asked to specify when the button press and tones occurred. In the setting of C, the button press subjectively occurred later and the tone occurred earlier. Hence, the two events are bound together by the intentional act.

Fig. 6

Block diagram of different parts of the brain and how they connect to produce movement and the perceptions of willing and agency. Movement intention forms in the premotor areas under internal influences of limbic system, basal ganglia, hypothalamus and frontal lobe and external influences from parietal lobe. Premotor cortex sends a command to motor cortex that produces a movement. Feedforward signals come from premotor and motor cortices to parietal area and such processing can give rise to the sense of willing. Comparing feedforward signals with feedback from the movement generated looks for mismatch. If no mismatch, then a perception of agency can arise. Note that it appears that the perceptions of willing and agency both employ a network between parietal and frontal areas.