Action, perception and postural planning when reaching for tools.

The dorsal and ventral streams model of action and perception suggests that reaching to grasp a tool for use involves integrated operation of the two streams. Few attempts have been made to test the limits of this integration in normal subjects. Twenty normal subjects reached for tools or geometric objects which were rotated rapidly during reaching or immediately beforehand. In a first experiment it was shown that reaching for an inverted tool was slower than reaching for objects which required hand inversion due to proximity to a physical barrier. Also, for the right hand, tool rotation during reaching provoked a higher incidence of hand rotation in the wrong direction than did rotation of objects. In a second similar experiment, hand inversion when grasping objects was induced by the need to plan a future action rather than by proximity of a physical barrier. Despite this balancing of complexity of postural planning for tools and objects, hand rotation errors for both hands were more common for tools than objects. This was consistent with the two-stream model in suggesting that there was a process which produced rapid online tracking of stimulus rotation and this had to be overcome by a slower process which dictated grasping in accordance with knowledge of tool use.