 Design and Procedures: 273 informed and consenting patients who are scheduled for elective proximal aortic reconstructive surgery ascending aorta aortic valve or root with concomitant proximal hemi- or total arch replacement via median sternotomy will be randomized to deep 20C, low 20.1C-24C, or moderate 24.1C-28C hypothermia during circulatory arrest. Cognitive testing using a standard battery will occur preoperatively baseline, at 4 weeks, and at 1 year after surgery. Neuroimaging procedures before surgery, and at 4 weeks and 1 year after surgery will consist of high-resolution anatomic, resting-state fMRI rs-fMRI and magnetic resonance spectroscopy MRS sequences. To characterize leukocyte activation, whole blood will be drawn at 5 time points: at baseline prior to surgery, before circulatory arrest, 10 minutes after reperfusion, 10 minutes after CPB, and 4 hours after CPB. 