1963: man, would've loved to see PKD's answer; arguably at the height of his (speed-fueled) powers round then.
"In 1963, a sixteen-year-old San Diego high school student named Bruce McAllister sent a four-question mimeographed survey to 150 well-known authors of literary, commercial, and science fiction. Did they consciously plant symbols in their work? he asked. Who noticed symbols appearing from their subconscious, and who saw them arrive in their text, unbidden, created in the minds of their readers? When this happened, did the authors mind?"
Ayn Rand's answer is instructive.