Study of the: Effect of Head Rotation in Prone Position on Regional Cerebral Oxygen Saturation.

Inclusion Criteria: * Lower back surgery * Age \[18-80\] * ASA group I-III * free and painless movement of the neck (including flexion, extension and rotation) Exclusion Criteria: * Pain during neck movement as mentioned under inclusion criteria. * known history of cervical spine pathology or pathology of central nervous system including cerebrovascular disease * any stenosis of the carotid vessels or known symptoms associated with such * BMI \> 35