Proprioceptive guidance and motor planning of reaching movements to unseen targets

The ability to make accurate reaching movements toward proprioceptively defined target locations was studied in seven normal subjects who were trained to reach to five different targets in a horizontal plane, with no vision of hand or target. The task consisted of moving a handle from a fixed origin to each target location, fast and accurately. Target locations were learned in training sessions that utilized acoustic cuing. Most movements were rapid, with a bell-shaped velocity profile. The error in target reproduction, which constituted the difference between the position consciously identified as the correct target location and the real target location, was calculated in each trial. This was compared with the error in preprogrammed reaching, which constituted the difference between the point in space where the initial fast movement toward the target ended and the target location. The absence of significant differences between these two error types indicated that the transformation from an internal representation of target location into a motor program for reaching to it did not introduce an additional reaching error. Learning of target locations was done only with the right hand, yet, reaching of both hands was tested. This allowed a comparison between the subjects' ability to utilize a transformed spatial code (reaching with the untrained hand) and their ability to use a direct sensory-motor code (reaching with the trained hand). While transformation of the spatial code was found to reduce it's accuracy, utilization of this code in motor programming again did not appear to introduce an additional error.