The illnesses that rob people of the ability to think, remember and create identity often begin, with pinpoint precision, in specific parts of the brain. It’s one of the biggest mysteries about brain diseases: Why do they start where they do?

Neuroscientists call this phenomenon, in which specific areas and cells in the brain are damaged by a disease while others are unaffected, “selective vulnerability.” Figuring out why it happens — and how to prevent or stop it — is a holy grail of the field.

Take Alzheimer’s, a disease that typically takes root years before memory unravels, when tangles of a protein called tau begin to accumulate and cells die off in a structure near the base of the brain called the entorhinal cortex.

In a preliminary study, scientists found that tau buildup in that origin area was linked to the progression of Alzheimer’s disease only when the region also lay close to the edge of the tentorial incisura, a notch in the brain’s outer lining.

This correlation prompts a series of…