Subjective postural orientation and visual vertical during slow pitch tilt for the seated human subject

To assess subjective postural orientation and the visual vertical during slow pitch tilt, subjects were seated, restrained and in darkness in a simulator. They estimated when they were tilted 0 degrees, 45 degrees, and 90 degrees forwards and backwards during cycles of pitching tilted at 1 degree x s-1 and at the same time set a 5-cm luminous cube, cockpit-mounted at 60 cm from the nasium, to be Earth vertical. Experimental protocols assessed the following: 1) the effect on postural estimates of seeing the cube versus darkness; 2) comparison of visual vertical settings at actual 45 degrees and 90 degrees tilts; 3) the adaptive effect of repeating estimates and settings; 4) the effect of a prolonged tilt on subsequent postural and visual vertical estimates; and 5) the comparative effect of tilting the body in alignment.

Seated, subjects' estimates of postural tilt were consistently greater than machine tilt, and similar in darkness to when seeing the cube. Overestimations increased with preceding tilts in the opposite direction, particularly when tilting from forwards whence subjects estimated they were backwards when the machine was tilted forwards. Postural estimates became more accurate with repetition but visual verticals deteriorated. Duration of conditioning tilt did not effect estimates. Visual vertical settings were largely accurate throughout. Labyrinthine defective subjects performed similarly to normals. Subjects frequently reported they felt disoriented during tilting.

Seated subjects' postural estimates were 'accurate' if one assumes that the reference shifted from the head to a trunk-leg axis on backwards tilt, however, subjects were unaware of this. Postural and visual vertical estimates could have been based solely on proprioception. Shifts in reference for verticality could be a factor in misperception of attitude in air and ground vehicles when accelerating or climbing.