The superior parietal lobule of macaque monkey: relative influence of gaze and static arm position during reaching.

The superior parietal lobule (SPL) integrates somatosensory, motor, and visual signals to dynamically control arm movements. During reaching, visual and gaze signals are used to guide the hand to the desired target location, while proprioceptive signals allow to correct arm trajectory, and keep the limb in the final position at the end of the movement. Three SPL areas are particularly involved in this process: V6A, PEc, PE. Here, we evaluated the influence of eye and arm position upon single neuron activity of these areas during the holding period at the end of arm reaching movements, when the arm is motionless and gaze and hand positions are aligned. Two male macaques (Macaca fascicularis) performed a foveal reaching task while single unit activity was recorded from areas V6A, PEc, and PE. We found that at the end of reaching movements the neurons of all these areas were modulated by both eye position and static position of the arm. V6A and PEc showed a prevalent combination of gaze and proprioceptive input, while PE seemed to encode these signals more independently. Our results demonstrate that all these SPL areas combine gaze and proprioceptive input to provide an accurate monitoring of arm movements.SIGNIFICANCE STATEMENTThis study shows that the SPL areas V6A, PEc and PE combine eye and static arm positions signals to build an estimate of the limb posture at the end of a reaching movement. The degree of integration of gaze and proprioceptive information changes from a joint processing of these signals in the caudal-most areas V6A and PEc (Brodmann area 7), to a more independent encoding in PE (Brodmann area 5). Our results support the existence of a functional trend in the SPL, with the anterior part dealing mainly with limb representation based on proprioception and the posterior one linking gaze and arm position signals for encoding reaching.