ACCURACY OF PLANAR REACHING MOVEMENTS .1. INDEPENDENCE OF DIRECTION AND EXTENT VARIABILITY

This study examined the variability in movement end points in a task i n which human subjects reached to targets in different locations on a horizontal surface. The primary purpose was to determine whether patte rns in the variable errors would reveal the nature and origin of the c oordinate system in which the movements were planned. Six subjects mov ed a hand-held cursor on a digitizing tablet. Target and cursor positi ons were displayed on a computer screen, and vision of the hand and ar m was blocked. The screen cursor was blanked during movement to preven t visual corrections. The paths of the movements were straight and thu s directions were largely specified at the onset of movement. The velo city profiles were bell-shaped, and peak velocities and accelerations were scaled to target distance, implying that movement extent was also programmed in advance of the movement. The spatial distributions of m ovement end points were elliptical in shape. The major axes of these e llipses were systematically oriented in the direction of hand movement with respect to its initial position. This was true for both fast and slow movements, as well as for pointing movements involving rotations of the wrist joint. Using principal components analysis to compute th e axes of these ellipses, we found that the eccentricity of the ellipt ical dispersions was uniformly greater for small than for large moveme nts: variability along the axis of movement, representing extent varia bility, increased markedly but nonlinearly with distance. Variability perpendicular to the direction of movement, which results from directi onal errors, was generally smaller than extent variability, but it inc reased in proportion to the extent of the movement. Therefore, directi onal variability, in angular terms, was constant and independent of di stance. Because the patterns of variability were similar for both slow and fast movements, as well as for movements involving different join ts, we conclude that they result largely from errors in the planning p rocess. We also argue that they cannot be simply explained as conseque nces of the inertial properties of the limb. Rather they provide evide nce for an organizing mechanism that moves the limb along a straight p ath. We further conclude that reaching movements are planned in a hand -centered coordinate system, with direction and extent of hand movemen t as the planned parameters. Since the factors which influence directi onal variability are independent of those that influence extent errors , we propose that these two variables can be separately specified by t he brain.