In his theory, Julian Jaynes describes the role hallucinations played in an earlier mentality, prior to the development of subjective consciousness. He predicted that hallucinations were more common in the normal population than was known at the time, and this has been confirmed in literally hundreds of studies over the past three decades. Below are books supporting this aspect of Jaynes's theory:
The Origin and Mechanisms of Hallucinations
Wolfram Keup (Ed.), New York: Plenum Press, 1970.