Elsevier

Human Movement Science

Volume 80, December 2021, 102879
Human Movement Science

Moving objects by imagination? Amount of finger movement and pendulum length determine success in the Chevreul pendulum illusion

https://doi.org/10.1016/j.humov.2021.102879Get rights and content

Abstract

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 have not been performed. In this study, we examined how participants produce these unconscious oscillations using a motion capture system. As expected, the Chevreul pendulum illusion was produced when the fingers holding the pendulum generated an oscillating frequency close to the resonant frequency of the pendulum, where very small driving movements of the arm are sufficient to produce relatively large pendulum motion. We found that pendulum length significantly affected the ability to produce the illusion - participants were much more successful with a 40 cm compared to an 80 cm pendulum. Further, we found that participants that tended to move their fingers more were more successful in producing the illusion but did not find a connection between inter-joint coordination and ability to generate the illusion.

Introduction

Can humans cause movements in external objects without perceiving that they are generating the movement? Are there motor actions that result from thoughts or mental images and are potentially instantiated independently of conscious engagement? Once attributed to external spirits, non-conscious motions of the hand-held pendulum have been attributed to ideomotor phenomenon. Ideomotor theory posits that actions are represented by their perceivable effects (Shin, Proctor, & Capaldi, 2010). This theory has been used as a way of explaining how voluntary movements can occur when one is not consciously aware of making movements. A classic example of this is when a hand-held pendulum will start moving without the holder feeling like they are performing any movement. Is it possible that participants can make movements and yet not realize that they are making them?

In 1808, Antoine-Claude Gerboin from Strasbourg School of Medicine described his observation of how the hand-held pendulum would move mysteriously when the person held it over certain substances. In the 1830s, a French scientist, Michel Eugene Chevreul studied the movement of such a hand-held pendulum in several situations (Chevreul, 1854; Jastrow, 1962), verifying that the movement decreased when the arm was being supported at the hand in contrast to the arm being externally supported at the shoulder, and that the oscillations were sight dependent. Chevreul postulated that imperceptible muscle activations were responsible for the pendulum's first oscillations, which increased (but were still imperceptible) under the influence of visual feedback (Chevreul, 1854). Although nearly 200 years have passed since this and similar examples of ideomotor behavior, such as Ouija board spelling were explained (Spitz, 1997), people are still being deceived by modern versions of ideomotor behavior, including tragically in facilitated communication (Burgess et al., 1998).

There have been several attempts to better understand the Chevreul pendulum illusion. In the 1970s, Easton and Shor used photogrammetry (Easton & Shor, 1975; Easton & Shor, 1976) and confirmed that sight is an important factor in determining when the pendulum will oscillate (Easton & Shor, 1975), and that as more attention is directed toward the pendulum, the more it oscillates (Easton & Shor, 1976). They also confirmed that restraining the arm at the wrist decreased the pendulum oscillation and demonstrated that out of 75 participants, only 60 were able to create the illusion (Easton & Shor, 1976). Hypnosis research uses Chevreul's pendulum illusion as a tool for testing a patient's response to the technique, with patients that are not able to move the pendulum generally being unresponsive to hypnosis (Karlin, Hill, & Messer, 2007). The studies of Easton and Shor, however, were not sensitive enough to describe how the participants generate the pendulum motion. Despite the early research and ongoing public fascination with these phenomena, there is a relative paucity of research examining the mechanistic accounts of ‘automatic’ pendulum oscillations.

A pendulum has a resonant frequency that is primarily dependent on its length. The maximum oscillation amplitude for a hand-held pendulum will be achieved if the driving frequency (i.e. the frequency of oscillations of the hand) is equal to the resonant frequency (Newburgh, 2004). Thus, in order to make the pendulum oscillate significantly, the participants are required to oscillate the pendulum-holding fingers at a frequency close to the resonant frequency of the pendulum. In this exploratory study, we set out to examine the relationship between generated arm movement and success in producing pendulum motion. We purposely chose pendulums that had natural frequencies of approximately 0.7, 1.05 and 2.0 Hz, in order that higher-frequency physiological tremor, e.g. 3–5 Hz for the elbow and 8–12 Hz for the finger (Hallett, 1998), would not produce significant pendulum motion. In this way, any significant pendulum motion must be a result of the participant moving their arm at an appropriate frequency. Using motion capture equipment, we aim to investigate how the total amount of movement of the fingers, and the coordination between joints of the arm affect successful performance of the illusion. Further, we wish to examine at which frequencies participants are able to generate pendulum oscillations, and how the different joints contribute to the movement of the pendulum.

Section snippets

Participants

Thirteen right-hand dominant participants (9 females), with normal or corrected vision took part in the study. The sample size was based on a pilot experiment (Cantergi & Friedman, 2018), where significant effects were observed in a sample size of 10 participants. The experimental protocol received ethical approval from the Tel Aviv University Institutional Research Board, and participants signed an informed consent form before starting the experiment. All methods were carried out in accordance

Results

Some of the participants were successful in achieving the pendulum illusion. The success was greater when using the 40 cm pendulum with vision (62%). Lower success rates were achieved with the longer pendulum (46%), although the difference was not statistically significant (χ2(1) = 0.286, p = 0.59). A significantly lower success rate was shown with shorter pendulums (8%; (χ2(1) = 5.444, p = 0.02)). When no visual feedback was allowed (no vision condition), the success rate was lower (31%), but

Discussion

Hand-held pendulums can seemingly oscillate on their own, without perceived conscious control. This phenomenon, known as the Chevreul pendulum illusion, is likely a result of subtle muscle movements caused by thinking of the generated movement. In this study, we examined how a hand-held pendulum, at three different resonant frequencies, results in significant oscillations that drive the pendulum movement without conscious causal agency. We demonstrated that the movement of the pendulum is

Data availability

The raw data and analysis software are available for download from figshare: doi: https://doi.org/10.6084/m9.figshare.14708883

Author contributions

JF and BA developed the study concept. Data collection were performed by DC. JF performed the data analysis. DC drafted the manuscript, and JF and BA provided critical revisions. All authors approved the final version of the manuscript for submission.

Competing interests

The authors declare no competing interests.

Acknowledgements

We thank Alison Kabo for help in data collection.

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