RESUMO
Twelve human subjects performed a reciprocal precision aiming task of varying difficulty (index of difficulty=4, 5, or 6) while vision of the ongoing movement was available either continuously or intermittently. In the intermittent conditions, vision of the moving end-effector was available at regular intervals (equal to 100%, 125%, or 150% of the movement durations measured under continuous visibility conditions) for varying amounts of time (75%, 50%, or 25% of the duration of the interval). Movement time (MT) increased with both increasing task difficulty and decreasing availability of visual information. Increases in MT were brought about by the same systematic changes in the kinematic characteristics of movement, whether task difficulty increased or availability of visual information decreased. At higher levels of task difficulty, subjects organized their movements so as to make visual information available at particular instances (at the start and at the end of the aiming movement).