Object Representations Used in Action and Perception

The remarkable skill of the human hand in object manipulation tasks is not the result of rapid sensorimotor processes or powerful effector mechanisms. Rather, the secret lies in the way that the nervous system organizes and controls manipulation tasks. Skilled and dexterous object manipulation requires the ability to tailor motor commands, predictively, to the goals of the task and the physical properties of manipulated objects (Johansson and Flanagan 2009 ; see also Frey, this volume). Thus, the ability to estimate these properties, based on sensory cues and memory, is essential for dexterous performance. Skilled manipulation also requires the ability to predict the sensory outcomes of motor commands. By comparing predicted and actual sensory feedback, the sensorimotor system can monitor task progress and take appropriate goal-directed corrective actions if a mismatch occurs. In addition, the sensorimotor system can update information about the physical properties of objects, as well as the mechanical interaction between objects and the body, so as to reduce future mis-matches. The fi rst part of this chapter focuses on predictive control mechanisms in object manipulation tasks and the way the sensorimotor system represents the physical properties of objects. In addition to manipulating objects, we use our hands to perceive object properties, including weight. Judgments about weight are comparative in nature rather than absolute (Ross 1969 ; Ellis and Lederman 1998 ; Flanagan et al. 2008). That is, weight judgments are biased by expectations about weight. Various weight illusions reveal this bias, including the size–weight and material–weight illusions (Ross 1969 ; Ellis and Lederman 1999 ; Flanagan et al. 2008). Thus, a small cube is judged heavier than an equally weighted large cube, and a cube covered in foam is judged heavier than a cube covered in metal, because the small and foam-covered cubes are