Shift of subjective reference and visual orientation during slow pitch tilt for the seated human subject.

We examined the ability to assess subjective orientation and orientation of an external visual object during pitch tilt. Subjects were seated, restrained, and in darkness in a simulator and estimated when they were 0 degree, 45 degrees, and 90 degrees forwards and backwards from upright during pitching at 1 degree/s. They temporarily stopped in these positions and set a 5 cm luminous cube, cockpit mounted at 60 cm from the nasium, to earth vertical. Estimates of subjective tilt were consistently greater than actual tilt. Overestimations were increased by preceding tilts in the opposite direction, particularly when tilting from forwards, where subjects sometimes estimated they were tilted backwards when the machine was tilted forwards. Subjects were surprised with their estimates, and reported disorientation. Regardless, settings of the visual vertical made "intuitively" were largely accurate. Subjective estimates could be construed as "accurate" if one assumes that the rostro-caudal axis of the head was referenced for estimates of upright and forwards and a trunk-leg axis for backwards. Because labyrinthine defective patients behaved as normal subjects, task performance must have been based on proprioception. The overestimation of tilt is exploited in fairground illusions and may account for the common experience when driving, that hills seem much steeper than they are.