Frontal cortical areas of the monkey brain engaged in reaching behavior: A 14C-deoxyglucose imaging study

The ( 14C)-deoxyglucose method was employed to study whether different areas of the primate frontal lobe are involved in different aspects of reaching behavior. To this end, we mapped the functional activity of the frontal motor cortical areas in three monkeys performing reaching movements with one forelimb. The first monkey had to capture a peripheral visual target with a saccade and a forelimb-reach together, the second monkey had to reach a peripheral visual target with one forelimb while fixating a central target, and the third one had to reach a peripheral memorized target with one forelimb in complete darkness while the eyes maintained a straight ahead direction. The extent and intensity of activations were compared to those of three respective control monkeys: a saccade-control, a fixation-control, and a dark-control. The primary somatosensory (S1) and motor (F1) forelimb representation, the S1- and F1-trunk representation, the F2-dimple region, areas F3-forelimb, F4, F5-bank of arcuate sulcus, F7-ridge, the dorsal bank of cingulate sulcus, and 24c were activated in all reaching monkeys regardless of accompanying visual stimulation and oculomotor behavior. Interestingly, the S1-forelimb activation in the monkey reaching to memorized targets in complete darkness was more pronounced than that in the monkeys reaching to visual targets in the light, indicating that increased somatosensory processing compensates for the absence of visual feedback. On the other hand, areas F2-periarcuate, F5-convexity, F6, and 23 were preferentially activated by reaching to visual targets and remained unaffected during reaching to memorized targets when no visual feedback was available.