1 3 Exp Brain Res (2014) 232:3569–3578 DOI 10.1007/s00221-014-4046-2 RESEARCH ARTICLE The influence of target object shape on maximum grip aperture in human grasping movements Rebekka Verheij · Eli Brenner · Jeroen B. J. Smeets Received: 10 July 2013 / Accepted: 15 July 2014 / Published online: 29 July 2014 © Springer-Verlag Berlin Heidelberg 2014 Introduction Humans have the capacity to grasp a target object in many different ways. Yet, they show characteristic movement pat- terns. Differences between movement patterns under dif- ferent circumstances can reveal how various variables are taken into account when planning a grasping movement. One of these variables is target object shape, the focus of the present paper. It has repeatedly been shown that target object shape influences the maximum grip aperture (MGA) for a certain distance between the final positions of the dig- its (Zaal and Bootsma 1993; Cuijpers et al. 2004; Eloka and Franz 2011; Hu et al. 1999; Borchers et al. 2014). In this study, we test several explanations for why MGA depends on target object shape: because the shape influ- ences the desired precision of the digits’ final positions (‘desired precision,’ Smeets and Brenner 1999), because humans have the objective to avoid collisions between the digits and other parts of the target object than the goal posi- tions (‘collision avoidance,’ Cuijpers et al. 2004; Borchers et al. 2014; Verheij et al. 2012), because the grip aperture scales with target object mass (Eastough and Edwards 2007), or because, if a target object is grasped along its width, MGA is not scaled to the actual width of the target object but to the perceived width (‘perceived width,’ Franz 2001; Franz et al. 2000, 2001) or to the perceived volume (‘perceived volume,’ Borchers et al. 2014). In the follow- ing paragraphs, we will address these five explanations and their explanatory value. Desired precision Planning how to grasp a target object starts by selecting suitable positions on the target object’s surface, after which the digits are moved toward these goal positions (Smeets Abstract The shape of a target object could influence maximum grip aperture in human grasping movements in several different ways. Maximum grip aperture could be influenced by the required precision of digit placement, by the aim to avoid colliding with the wrong parts of the target objects, by the mass of the target objects, or by (mis)judg- ing the width or the volume of the target objects. To exam- ine the influence of these five factors, we asked subjects to grasp five differently shaped target objects with the same maximal width, height and depth and compared their max- imum grip aperture with what one would expect for each of the five factors. The five target objects, a cube, a three- dimensional plus sign, a rectangular block, a cylinder and a sphere, were all grasped with the same final grip aperture. The experimentally observed maximum grip apertures cor- related poorly with the maximum grip apertures that were expected on the basis of the required precision, the actual mass, the perceived width and the perceived volume. They correlated much better with the maximum grip apertures that were expected on the basis of avoiding unintended col- lisions with the target object. We propose that the influence of target object shape on maximum grip aperture might pri- marily be the result of the need to avoid colliding with the wrong parts of the target object. Keywords Prehension · Obstacle · Limb movements · Visuomotor behavior · Movement control R. Verheij (*) · E. Brenner · J. B. J. Smeets Faculty of Human Movement Sciences, MOVE Research Institute Amsterdam, VU University Amsterdam, Amsterdam, The Netherlands e-mail: r.verheij@vu.nl