Fractal fluctuations in muscular activity contribute to judgments of length but not heaviness via dynamic touch.

The applied muscular effort to wield, hold, or balance an object shapes the medium by which action-relevant perceptual judgments (e.g., heaviness, length, width, and shape) are derived. Strikingly, the integrity of these judgments is retained over a range of exploratory conditions, a phenomenon known as perceptual invariance. For instance, judgments of length do not vary with the speed of rotation, despite the greater muscular effort required to wield objects at higher speeds. If not the amount of muscular effort alone, then what features of the neuromuscular activity implicated while wielding objects contribute to perception via dynamic touch? In the present study, we investigated how muscular activity mediates perception of heaviness and length of objects via dynamic touch. We measured EMG activity in biceps brachii and flexor carpi radialis as participants wielded objects of different moments of inertia. We found that variation in the amount of muscular effort (literally, root-mean-square values of EMG activity) predicted variations in judgments of heaviness but not length. In contrast, fluctuations in the activity of biceps brachii and flexor carpi radialis were fractal, and variation in the degree of fractality in the two muscles predicted variation in judgments of length. These findings reflect the distinct implications of dynamic touch for perception of heaviness and length. Perceptions of length can be derived from minimal effort, and muscular effort only shapes the medium from which judgments of length are derived. We discuss our findings in the context of the body as a multifractal tensegrity system, wherein perceptual judgments of length by wielding implicate, at least in part, rapidly diffusing mechanotransduction perturbations cascading across the whole body.