Muscular effort differentially mediates perception of heaviness and length via dynamic touch.

Our ability to perceive properties of handheld objects (e.g., heaviness, orientation, length, width, and shape) by wielding via dynamic touch is crucial for tooling and other forms of object manipulation-activities that are the basis of much human experience. Here, we investigated how muscular effort mediates perception of heaviness and length via dynamic touch. Twelve participants wielded nine occluded elongated objects of distinct moments of inertia and reported their perceptual judgments of heaviness and length. We measured the electromyography (EMG) activity of the participants' biceps brachii, flexor carpi radialis, and flexor carpi ulnaris muscles during wielding. Distinct single-valued functions of the eigenvalues I1 and I3 of the inertial tensor, I, closely predicted perceived heaviness and perceived length of the wielded objects. Perceived heaviness showed a direct and linear relationship with EMG activity of biceps brachii, flexor carpi radialis, and flexor carpi ulnaris. However, while perceived length showed a very weak relationship with EMG activity of biceps brachii, we found no association between perceived length and EMG activity of flexor carpi radialis and flexor carpi ulnaris. Our findings indicate that muscular effort contributes directly to perception of heaviness, but likely only serves as a medium for perception of length. While the same physical variable-i.e., the moment of inertia-provides the informational support for perception of heaviness and length, distinct psychophysiological processes underlie perception of heaviness and length via dynamic touch.