Why and how we perceive tilt perception in space—an experimental plan during long-term space lifein the international space station—

We are planning an experiment to determine why and how we perceive tilt in space. Tilt perception was thought to disappear in space, where gravity does not exist as a reference for tilt. Nevertheless, astronauts reported perceived head roll-tilt and ocular counter-rolling (OCR) in a centrifuge experiment in space. To explain these unexpected findings, during long-term space life we will measure the subjective visual body axis (SVBA) and OCR at various body roll-tilt angles relative to the inner-cabin vertical, with and without head roll-tilt relative to the body, with and without visual information. We are also currently performing ground-based experiments to collect control data. In this article, we introduce these space and terrestrial experiments.