Date Published: February 22, 2018
Publisher: Public Library of Science
Author(s): Rachel O. Coats, Raymond J. Holt, Geoffrey P. Bingham, Mark A. Mon-Williams, Gavin Buckingham.
The duration of reach-to-grasp movements is influenced by the size of the contact surfaces, such that grasping objects with smaller contact surface areas takes longer. But what is the influence of asymmetric contact surfaces? In Experiment 1a, participants reached-to-lift wooden blocks off a table top, with the contact locations for the thumb and index finger varying in surface size. The time taken to lift the block was driven primarily by the thumb contact surface, which showed a larger effect size for the dependent variable of movement duration than the index finger’s contact surface. In Experiment 1b participants reached-to-grasp (but not lift) the blocks. The same effect was found with duration being largely driven by contact surface size for the thumb. Experiment 2 tested whether this finding generalised to movements towards conical frusta grasped in a different plane mounted off the table top. Experiment 2 showed that movement duration again was dictated primarily by the size of the thumb’s contact surface. The thumb contact surface was the visible surface in experiments 1 and 2 so Experiment 3 explored grasping when the index finger’s contact surface was visible (participants grasped the frusta with the index finger at the top). An interaction between thumb and finger surface size was now found to determine movement duration. These findings provide the first empirical report of the impact of asymmetric contact surfaces on prehension, and may have implications for scientists who wish to model reach-to-grasp behaviours.
A majority of neurologically intact adult humans possess the ability to reach-and-grasp objects in a highly precise manner. The skills inherent within this behaviour can be seen in the stereotypical movements of the digits as they approach an object, before the fingertips exert the forces required to interact with the item (the forces being a function of the movement goal). The stereotypical nature of the movements can be exploited by the scientist to make probabilistic predictions about the behaviour that will emerge as a function of the individual and the task. Thus, a major goal within sensorimotor research is the identification of the factors within a task that alter prehensile behaviour in a reliable manner, and thereby allow for an accurate prediction of how an individual’s behaviour will alter as a function of changing environmental constraints.
In Experiment 1b we studied whether the findings of Experiment 1a would generalise to a slightly different behaviour where participants reached-and-grasped the blocks but did not lift them off the tabletop. The use of a reach-to-grasp action meant that there were no ‘fly through’ actions so this also allowed us to investigate the duration of the thumb and index finger as we could identify the point at which they completed their approach to the target using velocity thresholds (not possible when there are fly through movements).
Experiment 2 studied whether the findings of Experiment 1a and 1b would generalise to objects grasped in a different plane. We therefore mounted conical frusta (the part of a conical solid left after cutting off the pointed end, see Fig 1) directly in front of the participants (in the sagittal plane along their midline) and asked the participants to grasp the objects with their thumb at the top and their index finger underneath (see Fig 1)–the objects could not be moved so the task had no lifting or rotating component.
To investigate the influence of the finger contact surface being visible rather than the thumb contact surface, Experiment 3 involved participants grasping the conical frusta so that their index finger was at the top of the object. All other aspects of the design were identical to Experiment 2. Again, all means not presented in Figures can be found in Table 1.
The results of the series of experiments provide the first empirical data that address the issue of the duration of prehensile movements to objects that have asymmetrical contact surfaces. The results of Experiments 1 (a and b) and 2 indicate that the total task duration was primarily a function of the thumb’s contact surface, with the index finger contact surface not having a statistically significant effect on total movement time. In Experiments 1 and 2, the contact surface for the thumb was visible whilst the contact surface for the index finger was occluded from view. In Experiment 3, the contact surface for the index finger was visible whilst the thumb’s surface was occluded. It might be expected that this arrangement would change the findings of Experiment 1 and 2, and indeed we observed an interaction between the effect of the thumb and index finger surface size on total duration.