The Journal of Neuroscience, February 15, 1998, 18(4):1528-1545
Pointing to Kinesthetic Targets in Space
Gabriel
Baud-Bovy1 and
Paolo
Viviani1, 2
1 Department of Psychobiology, Faculty of Psychology
and Educational Sciences, University of Geneva, Carouge, Switzerland,
and 2 Laboratory of Perception, Action and Cognition,
LAPCO, Vita-Salute University HSR, Milan, Italy
An experiment investigated in human adults the sensorimotor
transformation involved in pointing to a spatial target identified previously by kinesthetic cues. In the "locating phase," a
computer-controlled mechanical arm guided the left [condition LR
(left-right)] or right [condition RR (right-right)] finger of the
blindfolded participant to one of 27 target positions. In the
subsequent "pointing phase," the participant tried to reach the
same position with the right finger. The final finger position and the
posture of the arm were measured in both conditions. Constant errors
were large but consistent and remarkably similar across conditions,
suggesting that, whatever the locating hand, target position is coded
in an extrinsic frame of reference (target position hypothesis). The
main difference between the same-hand (RR) and different-hand (LR)
conditions was a symmetric shift of the pattern of endpoints with
respect to the midsagittal plane. This effect was modeled accurately by assuming a systematic bias in the perception of the postural angles of
the locating arm. The analysis of the variable errors indicated that
target position is represented internally in a spherical coordinate
system centered on the shoulder of the pointing arm and that the main
source of variability is within the planning stage of the pointing
movement. Locating and pointing postures depended systematically on
target position. We tested qualitatively the hypothesis that the
selection of both postures (inverse kinematic problem) is constrained
by a minimum-distance principle. In condition RR, pointing posture
depended also on the locating posture, implying the presence of a
memory trace of the previous movement. A scheme is suggested to
accommodate the results within an extended version of the target
position hypothesis.
Key words:
kinesthetic pointing; frames of reference; arm movements; arm posture; sensorimotor transformations; inverse kinematics; position
sense
Copyright © 1998 Society for Neuroscience 0270-6474/98/1841528-18$05.00/0
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