The aim of the study is to understand the relationship between intracranial pressure regulation, cerebral tissue oxygenation and cognitive functioning. More specifically, the study tests the hypothesis that head down tilt will increase intracranial pressure (not measured in this study, but demonstrated in previous studies), will induce venous congestion and facial swelling, decrease intracranial tissue oxygenation and hamper brain functioning. The objectives of the study therefore are to assess young healthy people during head-down tilt (HDT), and to assess cognitive brain functioning, cerebral tissue oxygenation (non-invasively), frontal skin thickness, cerebral perfusion and neuronal functioning via event-related potentials.

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.