Human Visual Orientation in Weightlessness

There is still a great deal that we do not understand about human visual orientation, both on Earth and in weightlessness. Our current models are useful in parsing and understanding the different types of 0-G illusions, but the models cannot yet be used to make quantitiative predictions for individual subjects, since they are largely heuristic and incomplete. For example, we need to better understand the effects of fluid shift and otolith unweighting on the gravireceptor bias terms in our models, and have reliable ways of predicting or measuring their magnitude and time course in 0-G. The orientation model presented in this paper is a simple one, and does not include the effects of surface contact forces, which can have a major effect when present. We also know that visual and vestibular angular velocity cues influence the SV, and in certain situations can cause static illusions such as “aviator’s leans,” but these effects are omitted from the current model. Why does susceptibility to “levitation” illusion gradually increase with age on Earth? The stability of the Aubert illusion in individuals suggests idiotropic bias is relatively constant in 1-G, but does it change after months of living in 0-G, in an environment where a “floor” is no longer consistently beneath us? Can we develop models for the way humans represent 3-D spatial frameworks, and validate them? After living in space for many months, will humans develop a more robust ability to establish 3-D spatial frameworks, and turn them over in our minds? My hope is that continued scientific research in weightlessness aboard the space station and its successors will ultimately help provide answers to these questions.