Intracranial Pressure and Brain Function: Effects of Head Down Tilt Upon Brain Perfusion and Cognitive Performance

Inclusion Criteria: * Physically and mentally healthy male test subjects that are able and declare in writing their willingness to participate in the entire study and successfully passed the psychological and medical screening * Aged between 18-55 years old with a Body Mass Index (BMI) of 20-28 kg/m2, weight between 65-100 kg, and a height between 158-195 cm * Demonstrable medical insurance and official certificate of absence of criminal record Exclusion Criteria: * Inability to sleep on the back * Drug, medication or alcohol abuse (regular consumption of more than 20-30 g alcohol/day) * Smoking within the past 6 months prior to study commencement * Migraine or other chronic head aches * Previous psychiatric illness * Subjects suffering from weak concentration * History of psychological or central nervous disorders * Hiatus hernia * Gastro-oesophageal reflux * Diabetes mellitus * Pronounced orthostatic intolerance (\< 10 min standing) * Kidney disorder: deviations from normal values for creatinine in plasma. (Normal value: Creatinine \< 1.20 mg/dl) * Thyroid gland disorder: deviations from normal values for thyroid stimulating Hormone (TSH) in plasma. (Normal range: TSH 0.55-4.80 mUnits/l) * Anaemia: Hb under normal values (Normal values of Hb for men: 13.5-17.5 g/l) * Elevated risk of thrombosis * High likelihood of coagulopathy assessed by a clinical standard questionnaire * Chronic back complaints * History of lumbar surgery * History of lumbar spine trauma * Motor or sensory deficits as assessed by neurological examination * Contraindications against MRI * Imprisoned at the time of the study * Taking medications that may impair cognitive function, autonomic function or any of the study procedures * Ophthalmological conditions including glaucoma, retinopathy, severe cataracts, eye trauma or implants * Any other medical condition that the investigators consider a contraindication to the study procedures that would make it unsafe or confound the measurements.