When Robin Williams died by his own hand four years ago, the world sucked in for air, shocked that a man with Williams’s gifts and seemingly inexhaustible energy could perform an act of such desperation.

In the months that followed, news trickled out that Williams had been suffering from Parkinson’s disease and a pernicious form of dementia. His body had been affected—and he felt, as his widow put it, that he was “losing his mind.”