- 08:11 pm
This paper set out to explore digit kinematics during prehension. Traditionally prehension has been defined as coordination of reaching and grasping components (Jeannerod 1981) and a number of studies have explored prehension within this definition. However Smeets and Brenner (1999) suggested prehension was the independent movement of the contributing digits to their respective planned end positions, that is the coordination of two individual pointing fingers contacting an object. To test this, this study investigated whether disturbing the target that one pointing finger is reaching for, would disturb the second finger in its trajectory too. For if they are truly two independent pointers, as Smeets and Benner proposed, then they should work independently of each other in relation to the changing target. Eleven participants were asked to repeatedly reach for a non-changing target object as swiftly and accurately as possible. After over 80 reaches to this un-changing target, the target object was able to rapidly change its size via compressed air either becoming longer or shorter at one end, while the person was within reach, therefore perturbing the reach pattern that had already begun, and affecting the positioning of the two digits in achieving accurate end grasp position.
The main finding was that changing the future end position of one of the digits not only had an effect on this digit’s own kinematics but also on the kinematics of the opposing digit. This is in direct conflict with the hypothesis of Smeets and Brenner that prehension is really two digits moving independently to their respective end positions. The authors suggest that if this hypothesis were true, the kinematics of the digit for which nothing had changed in terms of future end position should have remained unchanged as well; this was clearly not the case. It is proposed therefore that the thumb and index finger might have different roles in prehension. The thumb movement might represent the reaching component, which would mean that the index-finger movement should be seen relative to the thumb movement, and this relative movement would represent the grasping component. The authors rejected Smeets and Brenner’s double pointing hypothesis and support the “traditional” definition proposed by Jeannerod that prehension has both reaching and grasping components. The paper also discusses the intrinsic and extrinsic properties of objects that we reach and grasp for, and how those properties influence our knowledge of the object and our behaviour and interaction with it.