Moving by thoughts alone? Amount of finger movement and pendulum length determine success in the Chevreul Pendulum Illusion

Hand-held pendulums can seemingly oscillate on their own, without perceived conscious control. This illusion, named after Chevreul, is likely a result of ideomotor movements. While this phenomenon was originally assumed to have a supernatural basis, it has been accepted for over 150 years that the movements are self-generated. However, until now, recordings of the small movements that create these oscillations has not been performed. In this study, we examined the mechanism that produces these significant involuntary oscillations using a motion capture system. We determined that the Chevreul pendulum illusion is produced when the fingers holding the pendulum generate an oscillating frequency close to the resonant frequency of the pendulum. At an appropriate frequency, very small driving movements of the arm are sufficient to produce relatively large pendulum motion. Further, subjects that tended to move their fingers more were more successful in producing the illusion.