The cognitive assessment test battery CERAD-Plus consistent of the Consortium to Establish a Registry for Alzheimer's Disease - Neuropsychological Assessment Battery (CERAD-NAB), Trail Making Tests A+B, and phonetic fluency (s-words), as well as the Test for Attentional Performance (TAP) developed to analyze different aspects of attention, will be performed preoperatively, at 7 days, 3 month and 12 months postoperatively. Training of all study personnel and supervision of cognitive testing will be carried out by the Memory Clinic at Basel University Hospital. Cognitive functions will be quantified using the CERAD total score, in its demographically corrected form. A correction for short-term practice effects will be carried out based on previous work.