Differential effect of task conditions on errors of direction and extent of reaching movements.

Invariant patterns in the distribution of the endpoints of reaching movements have been used to suggest that two important movement parameters of reaching movements, direction and extent, are planned by two independent processing channels. This study examined this hypothesis by testing the effect of task conditions on variable errors of direction and extent of reaching movements. Subjects made reaching movements to 25 target locations in a horizontal workspace, in two main task conditions. 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 same horizontal workspace, but target location was displayed on a vertical screen in front of the subjects. For both tasks, variable errors of movement extent (on-axis error) were greater than for movement direction (off-axis error). As a result, the spatial distributions of endpoints about a given target usually formed an ellipse, with the principal 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 movement amplitude in the two task conditions. This suggests that variable errors of direction and extent can be modified independently by changing the nature of the sensorimotor transformations required to plan the movements. This finding is further evidence that the direction and extent of reaching movements appear to be controlled independently by the motor system.