The origin of contraversive pushing - Evidence for a second graviceptive system in humans

Background: Stroke patients may exhibit the peculiar behavior of actively pushing away from the nonhemiparetic side, leading to lateral postural imbalance and a tendency to fall toward the paralyzed side. This phenomenon has been called the "pusher syndrome." Objective: The current study analyzes the mechanism leading to contraversive pushing. Methods: The subjective postural vertical (SPV) and subjective visual vertical (SW) were determined in five consecutively admitted patients with severe contraversive pushing and in controls. Whereas adjustment of the SPV reflects the perceived upright orientation of the body, the SW provides a sensitive and direction-specific measurement of peripheral and central vestibular dysfunction. Results: The deficit leading to contraversive pushing is an altered perception of the body's orientation in relation to gravity. Pusher patients experience their body as oriented "upright" when it is tilted 18 degrees to the nonhemiparetic, ipsilesional side. In contrast, perception of the SW was undisturbed. Conclusions: A separate pathway seems to be present in humans for sensing the orientation of gravity apart from the one for orientation perception of the visual world. This second graviceptive system decisively contributes to humans' control of upright body posture. Contraversive pushing occurring after stroke lesions may represent the behavioral correlate of a disturbed neural representation of this system.