Anisotropy in the haptic perception of force direction and magnitude.

Although force-feedback devices are already being used, the human ability to perceive forces has not been documented thoroughly. The haptic perception of force direction and magnitude has mostly been studied in discrimination tasks in the direction of gravity. In our study, the influence of physical force direction on haptic perception of force magnitude and direction was studied in the horizontal plane. Subjects estimated the direction and magnitude of a force exerted on their stationary hand. A significant anisotropy in perception of force magnitude and direction was found. Force direction data showed significant subject-dependent distortions at various physical directions. Normalized force magnitude data showed a consistent elliptical pattern, with its minor axis pointing roughly from the subject's hand to his/her shoulder. This pattern could be related to arm stiffness or manipulability patterns, which are also ellipse-shaped. These ellipses have an orientation consistent with the distortion measured in our study. So, forces in the direction of highest stiffness and lowest manipulability are perceived as being smaller. It therefore seems that humans possess a "sense of effort" rather than a "sense of force," which may be more useful in everyday life. These results could be useful in the design of haptic devices.