The aim of this thesis is to examine the role of visual information for the planning, execution and control of grasping movements. In several behavioral studies I investigated the changes in grasping kinematics resulting from a change of the underlying visual information. Thus, the intention was, beyond understanding the processes of grasping in more detail, to use grasping movements as a tool to learn about the processes of perception.
In the first project, it was tested in which way the amount of visual information influences the execution of goal directed grasping movements. Theoretical background of this study is the proposition of the real time view of action (Westwood &
Goodale, 2003; Westwood, Heath, &
Roy, 2003; Goodale, Westwood, &
Milner, 2003; Goodale, Kroliczak, &
Westwood, 2005) stating that movements directed to visible and remembered objects are controlled by different processing mechanisms (dorsal vs. ventral pathway). We tested this prediction by examining grasping movements executed under full vision and after three different delay durations. Results indicate that changes of grasping kinematics are due to an exponential decay of visuomotor information and not due to a change of the representation used, therewith contradicting the real time view of action.
The second study dealt with another prediction of the real time view of action. It has been argued that movements to visible targets are calculated in real time and are not influenced by perceptual memory or any earlier movement programming (e.g., Cant, Westwood, Valyear, &
Goodale, 2005; Garofeanu, Kroliczak, Goodale, &
Humphrey, 2004). This hypothesis was tested by visually presenting a distractor object of a certain orientation and measuring grip orientation when grasping a target object subsequently. Results showed that the kinematics of visually guided grasping movements are affected by the properties of the previously shown distractor object. The study provides evidence that perception and memory are involved in the execution of visually guided movements. This finding also contradicts the real time view of action.
The third project was concerned with the effects of size perturbations on the grasping movement. The aim was to investigate the adjustment of the grip under different conditions. Results indicate that vision of the hand is not necessary to correct the grip successfully during movement execution. Consequently, these experiments suggest that feed forward mechanisms play a major role in adjusting a planned motor program.
Taken together, the findings obtained in all projects provide evidence that action and perception interact strongly. This is also supported by recent neuroimaging studies showing that the cortical activation during perception and action tasks largely overlaps (e.g., Faillenot et al., 1997, 1999). Furthermore, our experiments show how grasping movements are influenced by different object properties and task demands.
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