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The current study focused on the time course of the effects of the rod-and-frame illusion (RFI) on the kinematics of targeted forearm rotations. Participants were asked to reproduce perceived rod orientations by propelling a hand-held cylinder forward while rotating it to the target orientation. Rod and frame orientations were systematically varied, and cylinder rotations were normalized to time. Average realized cylinder orientations confirmed that when the frame orientation deviated from the vertical, a reproduction error occurred in the direction opposite to the direction of the frame tilt. In contrast, the perceived orientation of the stimulus rod was exaggerated relative to the vertical (i.e., reproduction errors were in the direction of the rod tilt). Furthermore, linear regression analyses for every normalized time sample showed that the rod and frame effects start simultaneously, but they reach their maximum effect at different points in time. We discuss the implications of our findings for current views on the effects of visual illusions on motor control.

The concept of canonical representations within the motor system has been both supported and refuted using a variety of behavioral studies. Here, based upon neurophysiological data, I discuss the relationship amongst those neuronal substrates of action and the behavioral components of a movement. A novel view of reaching and grasping has been proposed which predicts that movements with similar kinematic and dynamic properties have a similar representation within the nervous system (Smeets & Brenner, 1999). However this is broadly inconsistent with a variety of neurophysiological findings that emphasize the independence amongst representations of action.

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information about the object's position and orientation guides the hand to the object, while information about the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object—on the basis of its shape, surface roughness, and so on—and then moving one's thumb and fingers more or less independently to these positions. We modeled this description using a minimum-jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variables such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behavior.