Weight Illusion Caused by Sinusoidal Vibration Correlates with Grip Force Adjustment

Our research team previously identified a weight illusion in which a lifted object feels heavy when it continuously presents a sinusoidal vibration to the fingertips. However, the mechanism underlying this illusion remains unknown. We thus hypothesized that the autonomous grip force adjustment against a vibrating object would be one of the factors underlying the weight illusion. The autonomous grip force adjustment increases the motor outputs of a human hand system, subsequently raising the sense of effort to keep holding the lifted object. The grip forces and perceived heaviness were evaluated using vibratory stimuli with five different frequencies (30 Hz, 60 Hz, 100 Hz, 200 Hz, and 300 Hz) and three different amplitudes (156 mu m, 177 mu m, and 203 mu m). The results showed that the stimuli at lower frequencies or large amplitudes increased the grip forces more and felt heavier than the stimuli at higher frequencies or small amplitudes. Specifically, the 30 Hz stimuli felt the heaviest and increased the grip force the most. An increase in the grip force was positively correlated with the perceived heaviness. These results indicate that vibratory stimuli influence both the grip force and weight perception. Our findings can contribute to developing haptic displays to present virtual heaviness.

Automated summary

Our research team previously discovered a weight illusion where a continuously vibrating object felt heavy to the fingertips. We hypothesized that the grip force adjustment would be a factor underlying the illusion. The study evaluated grip forces and perceived heaviness using vibratory stimuli with different frequencies and amplitudes, and found that lower frequencies or larger amplitudes increased grip forces and perceived heaviness.