On the response function and range dependence of manual estimation

Manual estimates without vision of the hand are thought to constitute a form of cross-modal matching between stimulus size and finger opening. However, few investigations have systematically looked at how manual estimates relate a perceived size to the response across different ranges of stimuli. In two experiments (N = 18 and N = 14), we sought to map out the response properties for (1) manual estimates of visually presented stimuli as well as (2) visual estimates of proprioceptive stimuli, and to test whether these properties depend on the range of stimuli. We also looked at whether scalar variability is present in manual estimates, as predicted by Weber's Law for perceptual tasks. We found that manual estimates scale linearly and with a slope of close to 1 with object sizes up to 90 mm, before participants' hand size limited their responses. In contrast, we found a shallower response slope of about 0.7 when participants performed the inverse task, adjusting the size of a visual object to match a not actively chosen, induced finger opening. Our results were mixed with regards to scalar variability in large objects. We saw some indication of a plateau, but no evidence for an effect of mechanical constraints in the range studied (up to 90 mm). Participants also showed a clear tendency to overestimate small differences when a set of objects differed little in size, but not when stimulus differences were more pronounced.