Shape form preservation coupled with shape position drift appears inconsistent with proprioceptive decay because preserved movement production depends on knowledge of current limb position. Second, several researchers have shown that when shapes are drawn continuously without vision, shape position drifts but shape form is preserved ( Brown and Rosenbaum 2001 Brown et al. This finding is inconsistent with a proprioceptive decay account because participants' ability to indicate limb position did not decrease as a function of time. ( 2000) showed that with a hand position-matching task, drift distance did not grow over time when only two matching reports were made in a 20-s period. Two findings call this explanation into question, however. These results suggest that the efficacy with which proprioception conveys information about limb position is altered during prolonged periods without vision or without movement. Drift is attenuated, however, when passive or active movements are performed with the target limb between matching trials ( Paillard and Brouchon 1968) or when brief isometric contractions are performed with the target limb between reports ( Wann and Ibrahim 1993). Hand position matching experiments, in which the participant indicates the location of an occluded hand by matching its location with the other hand, show that without vision, the accuracy with which finger location is reported declines over repeated matches such that perception of limb position appears to drift ( Paillard and Brouchon 1968 Wann and Ibrahim 1993 Wolpert et al.
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Perception of limb position with respect to both the body and the external world depends on information provided by vision, proprioception, and touch ( Graziano 1999 van Beers et al. These results suggest that proprioception continues to be a reliable source of limb position information after prolonged time without vision, but that this information is used differently for maintaining limb position and for specifying movement trajectory. Inverse dynamics analysis revealed that movement preservation was accompanied by substantial modification of joint muscle torque. However, despite these dramatic changes in hand position and joint configuration, movement distance and direction remained relatively constant. This drift varied systematically with movement direction, indicating that drift is related to movement production. Over the 70 trials, the start location of each movement drifted, on average, 8 cm away from the initial start location.
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Movements were made in two directions (30° and 120°) from each of three start locations (initial shoulder angles of 30°, 40°, 50°, and initial elbow angles of 90°). Feedback was then removed, and participants were to continue on pace for the next 70 trials. Fingertip position feedback was given by a cursor during the first five trials in the series.
PRESCRIBE RIGID BODY KINEMATICS POSTVIEW SERIES
To test this hypothesis, we asked participants to perform six series of 75 repetitive movements from a visible start location to a visible target, in time with a regular, audible tone. If this account is correct, drift should degrade the accuracy of movement distance and direction over a series of movements made without visual feedback. Such drift has been attributed to a gradual reduction in the usefulness of proprioception to signal limb position.
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In the absence of visual feedback, subject reports of hand location tend to drift over time.