Visual affordances direct action: neuropsychological evidence from manual interference.

We report an experimental study of the factors that elicit manual interference in a patient with so-called "anarchic hand" behaviour in everyday life (Della Sala, Marchetti, & Spinnler, 1991, 1994) due to corticobasilar degeneration. The patient, ES, showed problems with both hands. We used tests in which ES had to respond to a left-side object with her left hand and to a right-side object with her right hand; manual interference responses occurred when she used the left hand to respond to the right-side object and the right hand to respond to left-side objects. In reaching tasks, interference responses were determined by stimulus familiarity and by the spatial relations between the hand of response and the part of the object used for action (the handle of the cup). In pointing tasks interference responses were affected by both effector and spatial uncertainty. Right hand responses were affected particularly by familiarity, and left hand responses by effector and spatial uncertainty. The results demonstrate that visual affordances (determined by object-hand compatibility) and visual familiarity can directly activate motor responses. Hand differences are discussed in terms of hemispheric specialisation for different components of motor action.