The target as an obstacle: Grasping an object at different heights

Abstract

Humans use a stereotypical movement pattern to grasp a target object. What is the cause of this stereotypical pattern? One of the possible factors is that the target object is considered an obstacle at positions other than the envisioned goal positions for the digits: while each digit aims for a goal position on the target object, they avoid other positions on the target object even if these positions do not obstruct the movement. According to this hypothesis, the maximum grip aperture will be higher if the risk of colliding with the target object is larger. Based on this hypothesis, we made a set of two unique predictions for grasping a vertically oriented cuboid at its sides at different heights. For cuboids of the same height, the maximum grip aperture will be smaller when grasped higher. For cuboids whose height varies with grip height, the maximum grip aperture will be larger when grasped higher. Both predicted relations were experimentally confirmed. This result supports the idea that considering the target object as an obstacle at positions other than the envisioned goal positions for the digits is underlying the stereotypical movement patterns in grasping. The goal positions of the digits thus influence the maximum grip aperture even if the distance between the goal positions on the target object does not change.

A review of grasping as the movements of digits in space

“…5B). This was indeed found to be the case in a recent experiment (Verheij and Smeets 2018). The increases with distance traveled along the surface are considerable: 0.4-mmlarger maximum grip aperture and 7-ms-longer movement time for each additional centimeter along the object (Fig.…”

“…The increases with distance traveled along the surface are considerable: 0.4-mmlarger maximum grip aperture and 7-ms-longer movement time for each additional centimeter along the object (Fig. 5 of Verheij and Smeets 2018). The effects of the size of the contact surface on the speed and grip aperture, therefore, depend on the digits' paths relative to the object.…”

An Electro-Oculogram Based Vision System for Grasp Assistive Devices—A Proof of Concept Study

“…Prior to initializing the hand movement for grasping the target object, we first visually assess the object to determine its shape, size, and orientation [ 1 , 2 ] which will be further used for manipulating the grasp type, grasp aperture and wrist orientation accordingly. The sensory information is then processed by the Central Nervous System (CNS) for motor planning in the cortex and implementation using the muscular system [ 3 , 4 ]. On reaching the object, the wrist and fingers of the hand move accordingly to hold the object; the gaze fixates on the target until the completion of the whole task [ 5 ].…”