Despite their awareness of being in microgravity, astronauts often reach for a falling object too soon, as though they expected the normal pull of the earth's gravity. The source of this expectation may be hardwired. Italian researchers showed volunteers two groups of animations, one in which a ball jumped up and bounced down under normal gravity, g, and the other in which the ball accelerated upward in each bounce. Subjects were better at predicting the ball's return when its behavior matched that of normal gravity.

Brain scans showed that the subjects were accessing the vestibular cortex, which controls our sense of balance and awareness of body location. The researchers propose that this brain region models the effect of gravity by comparing the head's motion in different directions and sending its calculations to the visual and motor systems. Catch the April 15 Science for the weighty details.