The goal of this observational study is to determine the effects of cervical hyperextension on cerebral oximetry and cognitive functions in healthy individuals without other additional factors such as anesthesia and surgical stress. The main questions it aims to answer are:

• Can cervical hyperextension cause changes in cerebral oxygenation in non-anesthetized healthy individuals?
• Can cervical hyperextension cause changes in cervical and cerebral blood flow in non-anesthetized healthy individuals?
• Can cervical hyperextension cause changes in cognitive functions in non-anesthetized healthy individuals?
• Can cervical hyperextension cause changes in optic nerve sheath diameter in non-anesthetized healthy individuals? Participants will be monitored with cerebral oximeter device and asked to answer Montreal cognitive function test before and during cervical hyperextension position. The researchers will evaluate the changes in cervical and cerebral blood flow ultrasonographically before and during cervical hyperextension position.

Some operations are performed in the cervical hyperextension position. During the cervical hyperextension position, changes in neck blood flow as a result of stretching of neck vessels can occur. These changes can be observed as an increase or decrease in cerebral blood flow. Changes in cerebral blood flow can lead to changes in brain oxygenation and affect cognitive functions.

Aim: This study aims to show the effects of cervical hyperextension on cerebral oximetry and cognitive functions in healthy individuals without other additional factors such as anesthesia and surgical stress.

Method: Healthy volunteers who meet the specified criteria and sign informed consent will be included in the study. All volunteers will be asked to answer validated Montreal cognitive assessment test in Turkish. Cerebral oximetry will be monitored by 2 probes placed on the frontal bone while all volunteers are lying in a supine position with the neck in a neutral position. Basal cerebral oximetry values will be recorded. In the same position cervical blood flow values and optic nerve sheath diameter will be measured by the same and experienced researcher non-invasively with a Doppler ultrasonography device and recorded. Optimum cervical hyperextension position will be obtained by placing the standard pad under the neck of the volunteers. In the meantime cerebral oximetry values will be recorded. Cervical blood flow will be re-measured in this position non-invasively with a Doppler ultrasonography device and recorded. Cerebral oximeter values will be recorded every 5 minutes for 30 minutes. In the end of 30 minutes period of hyperextension position the cervical blood flow and optic nerve sheath diameter will be measured again non-invasively with a Doppler ultrasonography device and recorded. Immediately afterwards, the Montreal cognitive assessment test will be repeated without changing the position. After all measurements are completed, the procedure will be terminated by resting the volunteer in neutral position for 15 minutes.

Inclusion Criteria:

• 18-65 years, healthy volunteers

Exclusion Criteria:

• diabetes mellitus,
• chronic obstructive pulmonary disease,
• Liver failure,
• Kidney failure,
• Past coronary arteria by-pass graft history,
• Hemoglobinopathies,
• Anemia,
• hypertensive patients,
• Alzheimer's disease,
• neurodegenerative diseases,
• mental retardation,
• Vertebrobasilar system anomalies detected in magnetic resonance-angiography

55 healthy volunteers between age of 18-65 years who meet the specified criteria, agreed to participate and signed informed consent will be included in the study.