NGF and Cognitive impairment: AAV2: Adeno-Associated Virus serotype 2, AD: Alzheimer’s Disease, ECB: Encapsulated Cell Biodelivery, MCI: Mild Cognitive Impairment, MMSE: Mini-Mental State Examination, nbM: Nucleus Basalis of Meynert, NCI: No Cognitive Impairment, NGF: Nerve Growth Factor, PET: Positron Emission Tomography, p75NTR: p75 Neurotrophin Receptor, ROS: Religious Orders Study, TrkA: Tropomyosin receptor kinase A