Take the sur­prise some have expressed in recent years upon find­ing out that the expres­sion to "pic­ture" some­thing in one's head isn't just a fig­ure of speech. You mean that peo­ple "pic­tur­ing an apple," say, haven't been just think­ing about an apple, but actu­al­ly see­ing one in their heads? The inabil­i­ty to do that has a name: aphan­ta­sia, from the Greek word phan­ta­sia, "image," and prefix - a, "with­out."
The word we focused on was scotoma, a term from ophthalmology that means a patch of blindness in the field of sight. Every human eye has one. It is the point where the optic nerve exits the retina and no image can form. We do not see the hole because the brain fills it in. Ralph Levinson, a retired ophthalmologist and our guest that week, called this "a profound statement on life and cognition."