what is thought to cause hallucinations

from The Brain Series by Charlie Rose, Episode 5, The Developing Brain:

Stephen Warren of Emory University says at the end of the program: "I think that if we could figure out how genes influence, say, schizophrenia... How do you generate a hallucination because you have a mutation in some genes? I mean, that's a profound progress from a simple gene and a protein and a synapse to a circuit that's somehow recreating experience that occurred earlier."

This quote reveals that there is currently no awareness as to how a gene can influence schizophrenia. It also reveals that the current idea is that a "mutation in some genes" causes hallucinations, which are considered the recreation of previous experience.