Organization of the posterior parietal cortex in galagos: I. Functional zones identified by microstimulation.

We used half-second trains of intracortical microstimulation to study the functional organization of the posterior parietal cortex (PPC) in prosimian galagos. These trains of current pulses evoked meaningful behaviors from the anterior, but not posterior, half of PPC. Stimulation of dorsal PPC caused contralateral forelimb movements, including defensive, hand-to-mouth, and reaching movements. Defensive and hand-to-mouth movement territories overlapped, although hand-to-mouth movements were usually evoked from more rostrolateral sites than defensive movements. Reaching movement sites were typically more caudal than defensive or hand-to-mouth movement sites. Stimulation of the most medial PPC sites evoked complex movements of forelimbs and hindlimbs. Ventral PPC commonly represented defensive face movements. Similar defensive movements, with the addition of widely opening the mouth to expose the teeth, were elicited from a small area in front of the PPC defensive face zone. Sometimes defensive face movements occurred with forelimb movements. Thus, subregions of PPC relate to different ethologically relevant categories of behavior. Most movements were initiated within 33-100 msec after stimulus onset. Face, eye blink, and ear movements were generally less delayed than forelimb movements. The present results in galagos, together with those obtained from macaque monkeys by Graziano and coworkers (Graziano et al. [2002a] Neuron 34:841-851; Cooke et al., [2003] Proc. Natl. Acad. Sci. U.S.A. 100:6163-6168), suggest that the functional involvement of the PPC in specific types of sensorimotor behavior evolved early in the course of primate evolution and that networks for complex movements involving motor and posterior parietal areas are characteristic of all primate brains.