PubMed · 14341055
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
B POPOV. 1965. THE BIO-ELECTRICALLY CONTROLLED PROSTHESIS.. https://pubmed.ncbi.nlm.nih.gov/14341055/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Humans are able to stabilize their movements in environments with unstable dynamics by selectively modifying arm impedance independently of force and torque. We further investigated adaptation to unstable dynamics to determine whether the CNS maintains a constant overall level of stability as the instability of the environmental dynamics is varied. Subjects performed reaching movements in unstable force fields of varying strength, generated by a robotic manipulator. Although the force fields disrupted the initial movements, subjects were able to adapt to the novel dynamics and learned to produce straight trajectories. After adaptation, the endpoint stiffness of the arm was measured at the midpoint of the movement. The stiffness had been selectively modified in the direction of the instability. The stiffness in the stable direction was relatively unchanged from that measured during movements in a null force field prior to exposure to the unstable force field. This impedance modification was achieved without changes in force and torque. The overall stiffness of the arm and environment in the direction of instability was adapted to the force field strength such that it remained equivalent to that of the null force field. This suggests that the CNS attempts both to maintain a minimum level of stability and minimize energy expenditure.
Watt et al. (Exp Brain Res, 2000, 135:411-416) suggested that a reduced field of view causes objects to appear closer than their physical distance. This suggestion is based on the observation that individuals terminated open-loop prehension prematurely when pretending to grasp a paper rectangle initially viewed through a reduced field of view. We tested Watt et al.'s suggestion in an open-loop pointing task. In experiment 1, 21 participants pointed at targets in three locations (20, 30 and 40 cm relative to the starting position) in three viewing conditions (full, 16 degrees and 4 degrees field of view). No difference in accuracy was found between conditions but the reduced field of view led to an increase in end-point variability across trials. We interpret these results as indicating that a reduced field of view decreases precision but does not necessarily affect object localisation. In experiment 2, we asked participants to reach-and-grasp a real object under the same three open-loop viewing conditions but without vision following movement onset. The experimental design ensured that haptic feedback was available, which could be used to calibrate reaching movements. We found that the reduced field of view caused no changes in grasp but we observed changes in the transport kinematics consistent with increased variability in the perceptual estimate of target location. Notably there were no changes in the spatial path (expected from movements to a closer location). In experiment 3, we repeated the Watt et al. design but removed vision and forced participants to rely on memory. In this condition we found the same undershoots as described by Watt et al. We conclude that a reduced field of view is neither necessary nor sufficient for underestimation and suggest that a reduced field of view decreases precision. This can cause participants to undershoot and/or alter the movement kinematics but we argue that such findings cannot be ascribed unambiguously to perceptual underestimation as they may reflect strategic alterations in behaviour.
Explore the source record for details and available documents.