1 2 3 4 5 Integration of Target and Hand Position Signals in the Posterior Parietal Cortex : 6

23 24 Previous findings suggest the posterior parietal cortex (PPC) contributes to arm 25 movement planning by transforming target and limb position signals into a desired reach 26 vector. However, the neural mechanisms underlying this transformation remain unclear. 27 Here we examined the responses of 109 PPC neurons as movements were planned and 28 executed to visual targets presented over a large portion of the reaching workspace. In 29 contrast to previous studies, movements were made without concurrent visual and 30 somatic cues about the starting position of the hand. For comparison, a subset of neurons 31 was also examined with concurrent visual and somatic hand position cues. We found that 32 single cells integrated target and limb position information in a very consistent manner 33 across the reaching workspace. Approximately two-thirds of the neurons with 34 significantly tuned activity (42/61 and 30/46 for left and right workspaces respectively) 35 coded targets and initial hand positions separably, indicating no hand-centered encoding, 36 while the remaining one-third coded targets and hand positions inseparably, in a manner 37 more consistent with the influence of hand-centered coordinates. The responses of both 38 types of neurons were largely invariant with respect to the presence or absence of visual 39 hand position cues, suggesting their corresponding coordinate frames and gain effects 40 were unaffected by cue integration. The results suggest that the PPC uses a consistent 41 scheme for computing reach vectors in different parts of the workspace that is robust to 42 changes in the availability of somatic and visual cues about hand position. 43 44