The following is a summary of “Grasping follows Weber’s law: How to use response variability as a proxy for JND” published in the November 2022 issue of Vision by Bhatia, et al.

A key idea in psychophysics is Weber’s law. Since the just noticeable difference (JND) between stimuli grows with stimulus size, larger stimuli should be evaluated with greater variability, according to the theory. However, visually guided grasping goes against this prediction: The variability was comparable to repeatedly holding little items while doing so with huge objects. 

The result led to the common conclusion that clutching was against Weber’s law. The astounding discovery sparked a frenzy of studies that produced inconsistent findings and had potentially significant ramifications for theories about the brain’s functional organization. Researchers demonstrated that nonlinearities in the scaling of the grasping response were disregarded in earlier investigations.  These nonlinearities, for instance, were the result of the finger span being constrained so that the opening of the fingers meets a ceiling for heavy things. They provided a mathematical method for accounting for these nonlinearities and demonstrated the method using the data from three significant research on the subject as well as their own. 

They discovered that JNDs increased with object size in all four datasets when they were accurately calculated, as predicted by Weber’s law. They concluded that, like nearly other sensory dimensions, gripping obeys Weber’s law.