Since Jackson's time, physiologists, and more recently, experts in functional imaging, have repeatedly analyzed these three levels of motor organization and found them to be valid but to have remarkably complex relationships. Motor and sensory systems, although separated for practical clinical purposes, are not independent entities but are closely integrated. Without sensory feedback, motor control is ineffective. And at the higher cortical levels of motor control, motivation, planning, and other frontal lobe activities that subserve volitional movement are preceded and modulated by activity in the parietal sensory cortex.