Reaching to objects of interest in immediate extrapersonal space is an important motor activity of primates in everyday life. Recent studies of the activity of single cells in various brain regions of behaving primates have provided new insights into the brain mechanisms underlying reaching. These studies are discussed below with emphasis on parametric studies of the relations of the neuronal activity to the direction of reaching.
Several brain areas are involved in the initiation and control of reaching. The study of the role of the various areas in this function was made possible by the advent of a technique that allowed the recording of the activity of single cells in the brain of behaving animals during reaching. The technique (Lemon, 1984) is indispensable for the study of the neural mechanisms underlying motor aspects of behavior. Typically, monkeys are trained to perform various motor tasks and then microelectrodes are inserted through the dura into the brain area of interest to record extracellularly the electrically isolated action potentials of single cells. This combined behavioral-neurophysiological experiment provides a direct tool and one with fine-grain by which the brain mechanisms underlying performance can be studied.