> > Wolf "Maybe the Martians are transmitting neural images of red rocks directly into our brains, but we don't have the sense to tell" spirit
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> This reminds me of a book I used to have called "Lost Race of Mars" about a family who makes an extended visit to Mars because the dad was a scientist and was studying Martian life. Tortoises with eyes that look up as well as the standard forward-facers, cats, etc... well, the kids find a cave where the Old Martians live. The reason they're so hard to find is that they're psychic and can project illusions into humans' minds, so that the opening doesn't appear.
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> Nyper"Sorry, I don't remember the time frame of the story, the copyright date, or the author's name"old
>

I have that book somewhere. It's by Robert Silverberg. A thin, old paperback with a bright red cover, and line drawings of children wearing canary yellow spacesuits while meeting "Martians" in a cave. I don't remember anything at all about the plot, though. Even your description of people mentally projecting fantasies into the mind doesn't remind me what the story was about. :-(

Larry Niven has several stories which deal with illusions (both physically real and imagined ones) and how they get projected or presented to the consciousness. Like, for example, how demons would appear to people.
Dave tells me C.S. Friedman has a series called the Cold Fire trilogy, where a mysterious element called "The Fae" causes the minds of all the settlers on the planet to create all the stuff of their dreams and nightmares. There are probably many Star Trek episodes dealing with similar ideas (developed to an insane degree purely by holodeck technology.)