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1 Neuroscience, 1New York College of Osteopathic Medicine, Old Westbury, New York, United States
2 Ashton Graybiel Spatial Orientation Lab, Brandeis University, MS033, Waltham, Massachusetts, United States
3 Ashton Graybiel Spatial Orientation Lab, Brandeis University, Waltham, Massachusetts, United States
* To whom correspondence should be addressed. E-mail: erabin{at}nyit.edu.
Lightly touching a stable surface with one fingertip strongly stabilizes standing posture. The three main features of this phenomenon are: fingertip contact forces maintained at levels too low to provide mechanical support; attenuation of postural sway relative to conditions without fingertip touch; and center of pressure (CP) lags changes in fingertip shear forces by ~250ms. In the experiments presented here, we tested whether accurate arm proprioception and also whether the precision fingertip contact afforded by the many degrees of freedom of the arm are necessary for postural stabilization by finger contact. In our first experiment, we perturbed arm proprioception and control with biceps brachii vibration (120 Hz, 2mm amplitude). This degraded postural control, resulting in greater postural sway amplitudes. In a second study, we immobilized the touching arm with a splint. This prevented precision fingertip contact but had no effect on postural sway amplitude. In both experiments, the correlation and latency of fingertip contact forces to postural sway were unaffected. We conclude that postural control is executed based on information about arm orientation as well as tactile feedback from light touch, although precision fingertip contact is not essential. The consistent correlation and timing of center of pressure movement and fingertip forces across conditions in which postural sway amplitude and fingertip contact are differentially disrupted suggests posture and the fingertip are controlled in parallel with feedback from the fingertip in this task.
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