1. Subjects were presented with a cylinder, whose orientation with respect to the vertical and location in space varied from trial to trial. They grasped a similar cylinder in their hands and were instructed to align the grasped cylinder with the target cylinder. In some experiments the task was performed from memory, and subjects attempted to reproduce both location and orientation of the cylinder. In others, they attempted to reproduce only its orientation, either from memory or while the cylinder was in view. 2. Multivariate linear regression analysis was used to determine persistent and variable errors in performance. This analysis related the subjects' performance (reproduced orientation) to target parameters (location and orientation). 3. We have interpreted the experimental results starting from the assumption that there are two parallel neural processes underlying reaching and grasping: one relating proximal arm motion to target location and the other relating distal hand motion to target orientation. 4. Variable errors did not vary with task conditions, even when subjects were asked to dissociate target orientation from target location by matching target orientation with the arm at the side, irrespective of the location of the target. This finding suggests that the neural transformations involving target location and target orientation are performed in parallel. 5. Persistent errors did vary with task condition. The subjects made the largest errors in matching target orientation when the target was in view, but they were asked to match its orientation at a location that differed from that of the target. These errors depended mostly on the elevation of the target and on its slant (inclination relative to the vertical). They were related to the posture of the arm in a manner that suggested that matching orientation is influenced by both extrinsic (spatial) and intrinsic (arm posture) parameters. 6. The fact that persistent errors depend on target location and on arm posture as well as on target orientation implies that the neural transformation from target orientation to hand orientation is not independent of the transformation dealing with target location.
Attention Humans