Ordinarily – or a few years ago, at least – I would have walked into a conference room with thirty colleagues around the table and been prepared either to present or to consult on a case. This day was different. Today, I was the case.

The brain-imaging study that brought me to San Francisco in September 2015, more than two years after my retirement, would establish a baseline for measuring my cognitive loss going forward. Dr. Gil Rabinovici had invited me to take part in a long-term brain-imaging study for subjects at risk for Alzheimer’s disease. This work-up involved two PET scans, a high-resolution MRI, and two days of cognitive testing. One of the PET scans could detect the abnormal amyloid protein found in plaques, which are seen up to twenty years before onset of cognitive impairment. The other PET scan could detect the abnormal tau protein found in the neurofibrillary tangles of Alzheimer’s disease.