The long-term goal of this research is to advance the investigators knowledge of how the brain combines the information of multiple sensory systems coding for spatial oriention and how adaptation to vestibular imbalance influences spatial orientation. In healthy human subjects verticality perception is accurate while upright. After prolonged roll-tilt, humans show a systematic bias in perceived direction towards the previous roll-tilted position (so-called "post-tilt bias"). This effect could either be related to adaptation to the roll-tilted position (shifting the "null" position) or it could be related to changes in torsional eye position due to prolonged static roll-tilt (ocular counter-roll). While in the first case a post-tilt bias is predicted independently of the experimental paradigm used, the second hypothesis predicts a post-tilt bias only if retinal cues are integrated. In order to evaluate these hypotheses, the investigators will assess perceived vertical using both vision-dependent and vision-independent paradigms.

Inclusion Criteria:

• ages 18-65 years
• informed consent
• absence of exclusion criteria

Exclusion Criteria:

• peripheral-vestibular deficit
• disturbed consciousness
• history of sensory deficits
• visual field deficits
• other neurological or systemic disorder which can cause dementia or cognitive dysfunction
• intake of antidepressants, sedatives, or neuroleptics
• pregnancy, unless excluded by a negative pregnancy test
• known neck pain or status post neck trauma