ATTENTION IN ACTION OR OBSTRUCTION OF MOVEMENT - A KINEMATIC ANALYSISOF AVOIDANCE-BEHAVIOR IN PREHENSION

Obstacle avoidance strategies are of two basic but interrelated types: moving around an obstacle to that body parts do not come too close, a nd slowing down. Ln reaching-to-grasp, avoidance may involve the trans port component, the grasp formation component, or both. There has been little research that has directly examined obstacle avoidance strateg ies during reaches-to-grasp. Several recent reports describe experimen ts in which reaches-to-grasp were made when nontarget objects were pre sent in the workspace; The effects of these nontargets were interprete d as being due to their distracting effects rather than their obstruct ing effects. The results of these studies are reinterpreted as being d ue to the non-target's obstructing effects. The obstacle interpretatio n is more parsimonious and better predicts the pattern of results than the distracter interpretation. Predictions of the obstacle interpreta tion were examined in an experiment in which participants were require d to reach to grasp a target in the presence of another object in vari ous locations. The results were exactly Inline with the interpretation of the object as an obstacle and the data show how grasp and transpor t movements are subtly adjusted so as to avoid potential obstacles. It is proposed that people move so as not to bring body parts within a m inimum preferred distance from nontarget objects within the workspace. What constitutes-the preferred distance in a particular context appea rs to depend upon the speed of movement and a variety of psychological factors related to the cost that a person attaches to a collision.