Manual estimations of functionally graspable target objects adhere to Weber’s law

The manual estimation task requires that participants separate the distance between their thumb and forefinger until they perceive it to match the size of a target object. Ganel and colleagues (Curr Biol 18:R599–R601, 2008a) demonstrated that manual estimations yield just-noticeable-difference (JND) scores that linearly increased with increasing target object size; that is, responses adhered to Weber’s law and thus evince response mediation via relative and perception-based visual information. In turn, more recent work has reported that the size of a target object influences whether JNDs provide a reliable metric for evaluating the nature of the visual information supporting manual estimations. In particular, Bruno et al. (Neuropsychologia 91:327–334, 2016) reported that JNDs for ‘large’ target objects (i.e., 80 and 120 mm) violate Weber’s law due to biomechanical limits in aperture opening. It is, however, important to recognize that the absolute size of the ‘large’ target objects employed by Bruno et al. may have exceeded some participants’ functional aperture separation and resulted in a biomechanical strategy serving as the only viable response mode. Hence, the present investigation employed a manual estimation task wherein target object sizes were proportionately matched to decile increments (i.e., 10, 20, …, 70 and 80%) of individual participants’ maximal aperture separation. Results showed that JNDs increased linearly with increasing target object size. Accordingly, we propose that manual estimations of target objects within a functionally ‘graspable’ range adhere to Weber’s law and are mediated via relative and perception-based visual information.