Center or side: biases in selecting grasp points on small bars

Experimental brain research, 2014

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)judging the width or the volume of the target objects. To examine 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 maximum 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 correlated poorly with the maximum grip apertures that were expected on the basis of the required precision, the actual mass, the perceived w...

Experimental Brain Research, 2014

that even simple motor acts, such as grasping, can only be understood when cognitive factors, such as the task representation, are taken into account.

2021

Whether the visuomotor coding of size in grasping obeys Weber's law is currently debated. Following up on previous work from our laboratory, here we investigated the precision associated with the maximum in-flight index-thumb aperture (MGA) in grasping small-to-medium sized objects. We report three main findings. First, grasp preparation was longer with 5 mm objects and became increasingly faster as object size increased from 10 to 20-40 mm. Second, MGA variable errors increased as sizes increased from 5 to 10-20 mm, whereas they decreased as size reached 40 mm. Third, MGA distributions were symmetrical with 5 mm objects, but became increasingly right-skewed as size increased. These results, as well as a re-analysis of previous findings, suggest that the precision of visuomotor representations varies as a function of size, consistent with the key principle underlying Weber's law. However, a fundamental constraint on precision grips (the MGA must always exceed physical size) changes the skew of the distribution and reduces the variability of MGAs as size increases from very small to medium.

Human movement science, 2018

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 i...

PLOS ONE, 2016

We use visual information to guide our grasping movements. When grasping an object with a precision grip, the two digits need to reach two different positions more or less simultaneously, but the eyes can only be directed to one position at a time. Several studies that have examined eye movements in grasping have found that people tend to direct their gaze near where their index finger will contact the object. Here we aimed at better understanding why people do so by asking participants to lift an object off a horizontal surface. They were to grasp the object with a precision grip while movements of their hand, eye and head were recorded. We confirmed that people tend to look closer to positions that a digit needs to reach more accurately. Moreover, we show that where they look as they reach for the object depends on where they were looking before, presumably because they try to minimize the time during which the eyes are moving so fast that no new visual information is acquired. Most importantly, we confirmed that people have a bias to direct gaze towards the index finger's contact point rather than towards that of the thumb. In our study, this cannot be explained by the index finger contacting the object before the thumb. Instead, it appears to be because the index finger moves to a position that is hidden behind the object that is grasped, probably making this the place at which one is most likely to encounter unexpected problems that would benefit from visual guidance. However, this cannot explain the bias that was found in previous studies, where neither contact point was hidden, so it cannot be the only explanation for the bias.

Experimental Brain Research, 1993

This study assessed the reach to grasp movement and its adaptive response to a perturbation of object size. In blocked trials, subjects (n = 12) were instructed to reach 35 cm to grasp and lift a small- (0.7 cm) or large-diameter (8 cm) cylinder. Under an unconstrained condition (condition 1), no instructions as to the type of grasp to adopt were given. Subjects thus naturally used a precision grip (PG) for the small cylinder and whole hand prehension (WHP) for the large cylinder. Under condition 2, subjects were instructed to utilize a PG for grasps of both the large and small cylinders. For condition 3, the instruction was to use WHP irrespective of object size. Kinematic organization was determined with analysis of the recordings of active markers placed on the wrist, thumb, and three fingers. For condition 1 the results showed a temporal arrangement of both components (transport and manipulation) which differed from that of conditions 2 and 3. In perturbed trials, illumination shifted from the small to large cylinder or vice versa. With condition 1, subjects automatically switched from one grasp to another with no or little increase of movement duration. This was generally achieved by an earlier temporal setting of peak wrist deceleration. For conditions 2 and 3, where a change of aperture was required, movement duration was prolonged without adaptation of earlier transport component parameters. It is concluded that the adaptive responses to a change of distal patterning also affect the organization of the proximal component. Assessment of grasps constrained by instructions may lead to interpretations of central control of the reach to grasp movement which differ from those obtained by assessing more natural prehensile patterns.

Journal of Neurophysiology, 2013

To grasp an object one needs to determine suitable positions on its surface for placing the digits and to move the digits to those positions. If the object is displaced during a reach-to-grasp movement, the digit movements are quickly adjusted. Do these fast adjustments only guide the digits to previously chosen positions on the surface of the object, or is the choice of contact points also constantly reconsidered? Subjects grasped a ball or a cube that sometimes rotated briefly when the digits started moving. The digits followed the rotation within 115 ms. When the object was a ball, subjects quickly counteracted the initial following response by reconsidering their choice of grasping points so that the digits ended at different positions on the rotated surface of the ball, and the ball was grasped with the preferred orientation of the hand. When the object was a cube, subjects sometimes counteracted the initial following response to grasp the cube by a different pair of sides. Thi...

Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale, 2010

Experimental Brain Research, 2005

When reaching out for objects, the digits' paths curve so that they approach their positions of contact moving more or less perpendicularly to the local surface orientation. This increases the accuracy of positioning the digits and ensures that any forces exerted at contact are nearly perpendicular to the surface, so that friction will prevent the digits from slipping along the surface. When lifting the object a similar force perpendicular to the surface is needed to prevent the object from slipping from one's fingers. In order to determine whether these two issues are dealt with simultaneously we let subjects pick up a cube from three different starting positions and measured the digits' movements and forces from before contact until the moment the cube started moving. The impact force was low. After impact, the digits spent about 200 ms in contact with the surface of the cube before the latter started to move. The digits first decelerated, and then they gradually built up the grip-and lift forces to move the cube upwards. We found no direct relationship between the control of the reaching movement towards the object and the force applied at the surface of the object to pick it up. We conclude that the reaching and lifting movements are quite independent.

Journal of motor behavior, 2012

To grasp an object the digits need to be placed at suitable positions on its surface. The selection of such grasping points depends on several factors. Here the authors examined whether being able to see 1 of the selected grasping points is such a factor. Subjects grasped large cylinders or oriented blocks that would normally be grasped with the thumb continuously visible and the final part of the index finger's trajectory occluded by the object in question. An opaque screen that hid the thumb's usual grasping point was used to examine whether individuals would choose a grip that was oriented differently to maintain vision of the thumb's grasping point. A transparent screen was used as a control. Occluding the thumb's grasping point made subjects move more carefully (adopting a larger grip aperture) and choose a slightly different grip orientation. However, the change in grip orientation was much too small to keep the thumb visible. The authors conclude that humans do not particularly aim for visible grasping points.