Neuroimaging During Pure Oxygen Breathing

The investigators will conduct a non-randomized clinical trial to examine the effect of pure oxygen breathing on the brain. The study will compare cerebral blood flow, cortical electrical activity, and cognitive performance in 32 persons during room air (21% oxygen) breathing and pure oxygen (100% oxygen) breathing. Subjects will be used as their own controls. The investigators aim to:

1. Determine whether breathing 100% oxygen changes blood flow through the brain. The investigators will learn whether brain blood flow is increased, decreased or stays the same.
2. Determine if changes that might occur in brain blood flow are also accompanied by changes in the brain's electrical activity (EEG).
3. Learn whether changes in the speed at which the brain processes information (cognitive function) accompany changes in brain blood flow and electrical activity that may be seen.

The investigators will conduct a crossover design clinical trial to compare the effect of room air breathing (21% inspired oxygen) with pure oxygen breathing (100% inspired oxygen) on brain blood flow and cortical electrical activity. The study involves a one-time data collection taking place at University Hospitals Cleveland Medical Center on the Case Western Reserve University campus in Cleveland, Ohio. The investigators will perform neuroimaging (MRI) with electroencephalographic (EEG) cortical network mapping and cognitive assessments in all participants during room air breathing and again while breathing 100% pure oxygen. Oxygen will be delivered through a non-rebreather mask. Arterial blood partial pressure of oxygen (PaO2) will be measured twice, from arterial blood samples drawn during breathing room air prior to the MRI scan and again after 30 minutes of breathing 100% oxygen immediately following neuroimaging. Thus, the investigators will be able to determine if breathing pure oxygen may temporarily change brain blood flow and breathing, leading to changes in cognitive status such as euphoria or slowed reflexes. Information gained in this study may have direct operational relevance by helping us to understand one potential cause of "Unexplained Physiologic Events" that are reported in some aircraft pilots when flying at high altitude. Information gained could lead to development of new gas mixtures for use by personnel working in low oxygen high altitude environments.

Eligibility Criteria:
Inclusion Criteria:

1. Persons recruited from currently approved Human Subject Panel for high altitude studies at Wright Patterson Air Force Base, Dayton, OH. Volunteers will have documentation of past exposure to hypobaric conditions, either from past high-altitude flight, as hypobaric chamber personnel, or as participants in previous/current high altitude studies. Persons without past exposure to hypobaric conditions were also eligible to participate.
2. Height 152.5-195.5 cm, weight 40-135 kg.

Exclusion Criteria:

1. Persons who have contraindications to MRI such as cardiac pacemakers, intracranial aneurysm clips, metallic implants or external clips within 10 mm of the head; implanted metallic devices such as pumps or previously implanted neurostimulation devices; cochlear implants, defibrillators, pacing wires, body piercings that cannot be removed, metal filings such as shrapnel, tattoos on the head and neck, or medical conditions contraindicated for MRI safety.
2. History of claustrophobia