PubMed · 7373788
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J Elliott. 1980-06-13. Better artificial arms under development.. https://pubmed.ncbi.nlm.nih.gov/7373788/
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A novel obstacle avoidance paradigm was used to investigate the planning of human reaching movements. We explored whether the CNS plans arm movements based entirely on the visual space kinematics of the movements, or whether the planning process incorporates specific details of the biomechanical plant to optimize the trajectory plan. Participants reached around an obstacle, the tip of which remained fixed in space throughout the experiment. When the obstacle and the start and target locations were rotated about the tip of the obstacle, the visually specified task constraints retained a rotational symmetry. If movements are planned in visual space, as indicated from a variety of studies on planar point-to-point movements, the resulting trajectories should also be rotationally symmetric across trials. However, systematic variations in movement path were observed as the orientation of the obstacle was changed. These path asymmetries can be accounted for by a class of models in which the planner reduces the likelihood of collision with the obstacle by taking into account the anisotropic sensitivity of the arm to external perturbations or uncertainty in joint level control or proprioception. The model that best matches the experimental results uses planning criteria based on the inertial properties of the arm.
We explored the degree to which vision may alter kinaesthetic perception by asking participants to view their hand through a prism, introducing different horizontal deviations, while trying to align their fingers above and below a thin table. When the visual image of one hand was displaced this overwhelmed kinaesthetic judgements and participants reliably reported that they felt their limbs were aligned, even when they were laterally mis-aligned by as much as 10 cm. This effect, however, was mediated by 'visual capture' and when the task was attempted in a darkened room with limb position indicated by an LED taped to the finger, kinaesthesis dominated and participants reported that the LED seemed to become detached from their finger tip. In both light and dark conditions the finger was clearly visible and only the background detail was extinguished. Hence, in perceiving limb position, it appears that we believe in what we see, rather than in what we feel, when the visual background is rich, and in what we feel when the visual background is sparse.
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