Blake Ross, a very smart guy whose accomplishments include being the co-creator of Firefox, can't visualize things in his head simply by thinking about them—and he only just now discovered that there's anything noteworthy about that:
I have never visualized anything in my entire life. I can't "see" my father's face or a bouncing blue ball, my childhood bedroom or the run I went on ten minutes ago. I thought "counting sheep" was a metaphor. I'm 30 years old and I never knew a human could do any of this. And it is blowing my goddamned mind.
It turns out that researchers have a name for Ross's condition: aphantasia. His Facebook Note on the subject is an amazing, sometimes funny, deeply self-aware piece of writing. HM