Emergence and perceptual guidance of prehensile action
Successful coordination of prehensile action depends upon the selection and control of appropriate reach and grasp movements. This thesis explores how prehensile actions are shaped and regulated by perceptual information. According to ecological psychology, behaviour is achieved through the detection of information specifying the opportunities the environment affords for action. A review of the literature identified that as this information evolves over time, a comprehensive understanding of prehension has to consider how affordance perception and continual guidance of action come together in the pursuit of goal-directed action. In a series of interlinking studies the initiation, hand transport and grasp components of prehension were investigated in order to determine how affordances are manifested in the emergence of, and guidance within, prehensile actions. Study 1 explored the effect of information specifying affordances on the time taken to initiate and perform a ball-posting action. Results indicated that affordance perception was reflected in initiation time, whilst affordance actualisation was reflected in movement time, demonstrating that effects of affordance perception extend beyond action preparation to directly influence the emergence of action. Study 2 investigated the selection and regulation of perceptual information during the guidance of hand transport. General tau theory (Lee, 1998) was applied to test i) whether the ratio of coupling between hand and object motion is held constant throughout the reach, and ii) whether this ratio, k, describes the kinematics of handobject contact under varying spatiotemporal task constraints. Results indicated a constant tau ratio during the middle, but not the end phase of the movement; moreover, although the summary ratio k was not sensitive to task manipulations, the time-dependent counterpart, K(t), did exhibit effects of task constraints. This indicates that the guidance of hand transport was a continuous process, where, dependent upon the task goals, the regulation of perceptual information changed throughout the action. The final study, Study 3, examined digit coordination during the grasp. Focusing on the transition from 2-digit to 3-digit grip configurations, the study addressed whether grip selection is made before or during the action. Results showed the transition between 2- and 3-digit grips occurs at a within trial level. The grip configuration utilised could only be distinguished as 2- or 3-digit during the second half of the movement, indicating that grip selection emerges online during the unfolding action. Together these studies provide evidence of continual guidance of prehensile actions and offer support for the consideration of prehensile action as a set of nested task goals. It was concluded that affordance perception and movement guidance are interrelated and evolve continuously throughout the unfolding prehensile action. In light of these findings, issues relating to action selection emerging online from the organism-environment interaction are discussed.