THE PREMOTOR CORTEX AND NONSTANDARD SENSORIMOTOR MAPPING

We often gaze at and attend to an object while preparing to reach towa rd and grasp it, and continue doing so when the plan is executed. Elab orate machinery, much of it in the brainstem and spinal cord, provides control systems for the spatially congruent guidance of the eyes, lim bs, and body toward targets in visual space. We will use the term stan dard mapping for the sensorimotor transformations that underlie such b ehavior. Despite the commonsense character of standard mapping, the ta rgets of gaze, attention, and reaching can be dissociated from each ot her. We can attend to stimuli in locations that differ from the target of action. We can gaze in one direction while reaching in another. An d we can guide spatial action with nonspatial stimuli, such as when, i n conditional motor tasks, the color of an object instructs a movement elsewhere in space. All of these situations, and many others, call fo r a process that we term nonstandard mapping, wherein the central nerv ous system must reject the commonplace correspondences among visuospat ial stimuli, gaze, attention, and reaching movements. We focus in this article on the possibility that premotor cortex underlies nonstandard mapping and, therefore, the behavioral flexibility that such a proces s allows.