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Excerpt
Weightless conditions were used by McIntyre et al. (2001) to investigate whether an implicit awareness of acceleration due to gravity contributes to the mechanism through which the nervous system synchronises movement to catch a falling ball. Their experiment suggests that theories which view visual sensory information alone as acting to estimate time-to-contact (TTC) wrongfully neglect the role that the internal modelling of Earth's gravity plays in initiating catching movements.
The human visual system is good at estimating velocity but poor at establishing the acceleration of a movement. Theoretically, an increased accuracy of TTC estimates can be facilitated through the inclusion of an internal model of gravity. McIntyre et al. (2001) examined the role of such an internal model using a catching task; subjects caught a 400 gram ball projected downwards at 0.7, 1.7 or 2.7 m/s from a starting point 1.6 m above their outstretched hand under different gravitational conditions. On Earth catching responses were well synchronised with the ball's arrival, with anticipatory forearm rotation occurring approximately 200 ms before contact. In 0 g the peak anticipatory biceps EMG occurred earlier relative to impact compared to 1 g. This suggests that that the nervous system is indeed modelling for an anticipated acceleration due to gravity. Adaptation to the altered gravitational conditions occurred slowly; some adaptation in forearm rotation developed over time in space, with later trials showing diminished amplitude of the premature erroneous movement and a later upward movement just before impact. McIntyre et al. (2001) propose that this slow adaptation is due to the set-up of the Spacelab that encourages astronauts to continue to use the Earth-valid model in 0 g, despite vestibular, pressure and visual cues, which indicate weightless conditions. Spacelab has identifiable floors, ceilings and overhead lighting and astronauts most commonly adopt an "upright" posture, providing a strong sense of up and down.
