The Paris Review tells the story of a project begun in 1963 by Bruce McAllister, then a precocious high school student:
…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?
The responses are artifacts from a time before correspondence became instantaneous. Read the whole article to see the responses McAllister received from John Updike, Ray Bradbury, Ralph Ellison, and more.