A number of studies have reported that grasps and manual estimations of differently sized target objects (e.g., 20 through 70 mm) violate and adhere to Weber's law, respectively (e.g., Ganel et al. 2008a, Curr Biol 18:R599-R601)-a result interpreted as evidence that separate visual codes support actions (i.e., absolute) and perceptions (i.e., relative). More recent work employing a broader range of target objects (i.e., 5 through 120 mm) has laid question to this claim and proposed that grasps for 'larger' target objects (i.e., >20 mm) elicit an inverse relationship to Weber's law and that manual estimations for target objects greater than 40 mm violate the law (Bruno et al. 2016, Neuropsychologia 91:327-334). In accounting for this finding, it was proposed that biomechanical limits in aperture shaping preclude the application of Weber's law for larger target objects. It is, however, important to note that the work supporting a biomechanical account may have employed target objects that approached -or were beyond-some participants' maximal aperture separation. The present investigation examined whether grasps and manual estimations differentially adhere to Weber's law across a continuous range of functionally 'graspable' target objects (i.e., 10,…,80% of participant-specific maximal aperture separation). In addition, we employed a method of adjustment task to examine whether manual estimation provides a valid proxy for a traditional measure of perceptual judgment. Manual estimation and method of adjustment tasks demonstrated adherence to Weber's law across the continuous range of target objects used here, whereas grasps violated the law. Thus, results evince that grasps and manual estimations of graspable target objects are, respectively, mediated via absolute and relative visual information.
Keywords: Grasping; Manual estimation; Method of adjustment; Vision; Weber’s law.