Reaching beyond grasp

A skill fundamental to human behaviour is our ability to interact with objects in the environment. One of such skills which has received considerable attention from researchers is grasping behaviour. The investigation of grasping has been largely stimulated by the work of Marc Jeannerod (1981; 1984). Jeannerod's pioneering work led to a surge in research on human prehension, some of it looking at the relationship between grasping parameters and object's properties including size and position, but much of it dedicated to the investigation of other properties, including fragility, size of the contact surface, texture and weight. Of interest for the present thesis is that an important object's feature such as shape, and how this feature influence prehension, has so far received little attention. A possible reason behind such lack of interest may lay in the nature of the measure chiefly utilized for describing grasping kinematics, i.e. the maximum distance between the index finger and thumb. Although the use of this measure simplifies the study of a complex biomechanical system as the hand making its behavior more easily quantifiable, it also prevents the investigation of factors to which the hand "reacts" only through such complexity. The shape of the to-be-grasped object might be one of these factors. Support to this idea comes from recent evidence showing that, when looking at all digits rather than two-digits kinematics, the effect of object shape emerges, i.e., hand shaping phenomenon (Santello and Soechting, 1998). With this in mind, Chapter 1 provides the theoretical background for the experimental work included within the present thesis. Specifically it includes a critical discussion concerned with the comparison between the "multi" and the "two-digits" approach and it highlights how the investigation of the relationship between object shape and hand shaping has the potential to shed further light on how hands prepare to grasping. The following chapter (Chapter 2) provides a description of the methods and the technical aspects which are common to all the experiments included within the present thesis. Then, the description of the experimental work initiates in Chapter 3. Here I report on an experiment aimed at investigating whether and how a sudden and unexpected change in object shape affects hand posture while reaching towards an object. The results of this study suggest that the object's shape perturbation does not disrupt the hand shaping phenomenon. The experiment reported in Chapter 4 considers the processes of selection-for-action by looking at the effects that distractor objects, of a similar or a different shape than the target object, may have on hand shaping. Regardless the shape of the distractor object, its presence affects hand shaping in terms of fingers' abduction angles, but not fingers' angular excursions. The grasping action is often followed by another action (e.g. pass the object to another person, placing it on a shelf). The effects that the implicit demands embedded in a "second" action on the kinematics of the "first" action are reported in two experiments described in Chapters 5 and 6. The first of these two experiments was aimed to test whether the hand posture might modulate accordingly to the accuracy constraints dictated by the task to be performed after the grasping of the very same object (Chapter 5). Results from this experiment show that the hand does not strictly mirrors the shape of the to-be-grasped object, but it is sensitive to the accuracy requirements imposed by the end-goal. The experiment described in Chapter 6 goes a step forward by manipulating not simply the accuracy requirements of the task following grasping, but its functional nature. In this experiment subjects were requested to reach towards and grasp the same object as to perform different actions. The invariance of the shape of the target object did not lead to an invariable hand posture while reaching towards it. The hand was shaped according to the functional goal of the action. The experiment reported in Chapter 7 was designed to address whether the time intervening between the reach-to-grasp phase and the subsequent phase, leading to the action goal, plays a role in action planning and execution. Overall the results indicated that the presence of an interruption between object grasping and the following action led the central nervous system to plan and control hand movement as if no other actions had to be performed. Finally, in Chapter 8, I attempt to provide a critical discussion of the reported results. These results are discussed in light of current theories proposed to explain how the central nervous system controls a complex motor behaviour such as prehension and how contextual information may influence such control.