Reaching and Grasping

Although sensory information is used to perceive the world, ultimately perceptions must result in [Page 855]actions to have an effect. Traditionally, more research and popular interest have been devoted to sensory perception than sensorimotor actions, but that has been changing with growing awareness of the sophistication of such actions and the underlying neural mechanisms. In addition, awareness has grown regarding how body parts and potential actions influence perception. The importance of sensory information for planning and controlling actions can be noted simply by looking at the amount of brain power devoted to such processing. Sensory information from all three spatial senses—vision, audition, and touch—is integrated in the posterior parietal cortex, which constitutes about 20% of the cerebral cortex. The parietal lobes send output to motor control areas in the frontal lobes and to subcortical structures and they exchange information with brain areas in the temporal lobes involved in sensory perception. Many brain regions are involved in controlling movements of the eyes, head, body, and particularly the hands.

Given that humans are an exceptionally dexterous species, almost all of our actions upon objects are performed with our hands, and our most common actions are reaching and grasping. Reaching refers to the movement of the arm and hand to contact an object, as when pushing an elevator button or brushing a fly away. Reaching is often accompanied by grasping to acquire an object, as when picking up a coffee cup. This act of prehension, or the seizing of an object with a body part, is performed not only by human hands, but also monkey feet and tails, elephant trunks, and giraffe tongues. The focus of this entry is on reaching and prehension in humans and other primates. Moreover, the emphasis is on actions directed toward visual targets, as is usually the case, though of course the actions can also be directed toward targets based on touch or sound.