To Determine the Effect of GLP1 on Cognitive Function, Brain Glucose Metabolism and Neuroplasticity.

To test the hypothesis that GLP1 can exert favourable effects on multiple aspects of brain function. To this purpose, the investigator determine whether chronic increase in GLP1 concentration as it occurs after bariatric surgery (Roux-en- Y Gastric Bypass) is associated with improvement in: * cognitive function measured by Mini Mental State Examination (MMSE) and Mental Deterioration Battery(MDB). * brain glucose metabolism measured by FDG-CT/PET * neuroplasticity measure by binocular rivalry and saccadic adaptation tests in morbid obese subjects. In order to discern the effect of GLP1 irrespective of changes in the metabolic milieu the investigator will test whether short-term GLP-1 infusion can modulate the same parameters in healthy subjects.