Temporal Modulations of Variability and Skewness in Grip Aperture Suggest the Involvement of Biomechanical...

Some reports suggested that the variability of finger apertures in grasping violates Weber's law, but such a violation is currently debated by several theoretical accounts. Here, data from 3 studies, yielding 54 participants, were analyzed to assess which account is more plausible. The temporal unfolding of grasps directed at various object sizes was investigated, tracking the mean aperture, variability (standard deviation), and distributional asymmetry (skewness) of the finger aperture over normalized time. Overall, the three studies converged in revealing that: (a) the mean apertures increased at early stages of the movement and then decreased at late stages, (b) variability increased at early stages but decreased at late stages, and (c) skewness revealed a more complex pattern, starting positive at early stages, switching to negative around 50% of movement time, and returning to positive at late stages. Crucially, variability increased as a function of object size, in accord with Weber's law, whereas skewness decreased. These findings suggest that biomechanical factors constrain variability differently as grasping unfolds: maximum apertures are constrained by the finger's maximum span, whereas objects' size constrains final apertures. This work provides evidence that the variability of finger apertures in grasping obeys Weber's law but is progressively constrained by biomechanics at late control stages.