Aim 5. Baseline NCFHEB PET imaging will be conducted followed by administration of physostigmine. Preferably, this will be done on the same day. However, at times when there is not enough radiotracer or when a subject is not able to tolerate a longer scan day. Therefore, some subjects may complete the study over two separate days (preferably within 1 month apart based on the availability of PET scanning times and subject's schedule).