Frames of reference and control parameters in visuomanual pointing.

Three hypotheses concerning the control variables in visuomanual pointing were tested. Participants pointed to a visual target presented briefly in total darkness on the horizontal plane. The starting position of the hand alternated randomly among 4 points arranged as a diamond. Results show that during the experiment, movement drifted from hypometric to hypermetric. Final positions depended on the starting position. Their average pattern reproduced the diamond of the starting points, either in same orientation (hypometric trials), or with a double inversion (hypermetric trials). The distribution of variable errors was elliptical, with the major axis aligned with the direction of the movement. Statistical analysis and Monte Carlo simulations showed that the results are incompatible with the final point control hypothesis (A. Polit & E. Bizzi, 1979). Better, but not fully satisfactory, agreement was found with the view that pointing involves comparing initial and desired postures (J. F. Soechting & M. Flanders, 1989a). The hypothesis that accounted best for the results is that final hand position is coded as a vector represented in an extrinsic frame of reference centered on the hand.