Abstract
Reach-to-grasp movements change quantitatively in a lawful (i.e. predictable) manner with changes in object properties. We explored whether altering object texture would produce qualitative changes in the form of the precontact movement patterns. Twelve participants reached to lift objects from a tabletop. Nine objects were produced, each with one of three grip surface textures (high-friction, medium-friction and low-friction) and one of three widths (50 mm, 70 mm and 90 mm). Each object was placed at three distances (100 mm, 300 mm and 500 mm), representing a total of 27 trial conditions. We observed two distinct movement patterns across all trials—participants either: (i) brought their arm to a stop, secured the object and lifted it from the tabletop; or (ii) grasped the object ‘on-the-fly’, so it was secured in the hand while the arm was moving. A majority of grasps were on-the-fly when the texture was high-friction and none when the object was low-friction, with medium-friction producing an intermediate proportion. Previous research has shown that the probability of on-the-fly behaviour is a function of grasp surface accuracy constraints. A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the ‘functional grasp surface size’) increased with surface friction coefficient. Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour.
Highlights
Most humans demonstrate an exquisite ability to manipulate objects with their hands
The present study explored whether we might find similar invariant patterns in the qualitative form of reach-to-grasp movements
We hypothesised that the proportion of these different movement patterns would be affected by the surface texture of the objects being grasped
Summary
Most humans demonstrate an exquisite ability to manipulate objects with their hands. Expert manual interaction with an object requires the actor to move their hand to the object of interest (the precontact phase) and apply the appropriate fingertip forces in order to manipulate the object (the contact phase). The notion of multiple inverse models suggests that a small environmental change (e.g. a different surface texture) might be sufficient to trigger a different higher-order inverse model and elicit a qualitatively different action - despite the task appearing to require the same class of movement. If the safety margins of the task decrease (e.g. by making the object wider and closer to the maximum grasp aperture) the proportion of on-the-fly movements decreases This observation suggests that the probability of observing a particular movement pattern is affected by the margins of safety. In order to explore the manner in which humans interact with objects of different textural properties, we asked participants to reach-to-grasp and lift objects from a tabletop while experimentally manipulating object width, distance and surface texture. Mon-Williams and Bingham’s [8] findings led us to predict that decreasing the surface friction would decrease the proportion of on-the-fly movements
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