DIFFERENTIAL EFFECT OF TASK CONDITIONS ON ERRORS OF DIRECTION AND EXTENT OF REACHING MOVEMENTS

Invariant patterns in the distribution of the endpoints of reaching mo vements have been used to suggest that two important movement paramete rs of reaching movements, direction and extent, are planned by two ind ependent processing channels. This study examined this hypothesis by t esting the effect of task conditions on variable errors of direction a nd extent of reaching movements. Subjects made reaching movements to 2 5 target locations in a horizontal workspace, in two main task conditi ons. In task 1, subjects looked directly at the target location on the horizontal workspace before closing their eyes and pointing to it. In task 2, arm movements were made to the same target locations in the s ame horizontal workspace, but target location was displayed on a verti cal screen in front of the subjects. For both tasks, variable errors o f movement extent (on-axis error) were greater than for movement direc tion (off-axis error). As a result, the spatial distributions of endpo ints about a given target usually formed an ellipse, with the principa l axis oriented in the mean movement direction. Also, both on- and off -axis errors increased with movement amplitude. However, the magnitude of errors, especially on-axis errors, scaled differently with movemen t amplitude in the two task conditions. This suggests that variable er rors of direction and extent can be modified independently by changing the nature of the sensorimotor transformations required to plan the m ovements. This finding is further evidence that the direction and exte nt of reaching movements appear to be controlled independently by the motor system.