Before making it onto a canvas or a plinth, an artwork is often conceived in the “mind’s eye.” Or is it?

When artist Isabel Nolan realized, only recently, that she has a rare neurological condition that prevents her from visualizing her thoughts, the diagnosis cleared up a lifetime of confusion. But it also raised new questions, about the nature of creativity, of selfhood, and the strange idiosyncrasies in how we perceive the world.

Aphantasia—the inability to conjure mental images—was only formally defined and named in 2015, by the British neurologist Adam Zeman.…