Effects of displacement and trajectory length on the variability pattern of reaching movements

The design of the present study enabled the authors to distinguish between the possible effects of movement displacement and trajectory length on the pattern of final positions of planar reaching movements. With their eyes cl osed, 9 subjects performed series of fast and accurate movements from diffe rent initial positions to the same target. For some series, the movements w ere unconstrained and were therefore performed along an approximately strai ght vertical line. For other series, an obstacle was positioned so that tra jectory length was increased because of an increase in movement curvature. Ellipses of variability obtained by means of principal component analysis a pplied to the scatter of movement final positions enabled the authors to as sess the pattern of movement variable errors. The results showed that the o rientation of the ellipses was not affected by movement displacement or by trajectory length, whereas variable errors increased with move ment displac ement. An increase in trajectory length as a consequence of increased curva ture caused no change in variable error. From the perspective of current mo tor control theory, that finding was quite unexpected. Further studies are required so that one can distinguish among the possible effects of various kinematics, kinetics, and other variables that could affect the pattern of variable errors of reaching movements.