What causes the visual abnormalities in schizophrenia if any?

Many: Schizophrenia results in abnormal brain function. Visual processing is managed by our brain's visual pathways leading to our occipital lobes. A fmri would reveal any abnormalities... A fun easy? Understanding is available online: http://faculty.Washington.Edu/chudler/vispath.Html.

Not sure: The visual abnormalities to what you are refering are likely called visual hallucinations and are part of the psychotic processes. There is a theory that psychosis is caused by an excess of a neurotransmitter called Dopamine in the limbic system in the brain.

Dopamine: The so-called dopamine hypothesis holds that there is an excess of the neurotransmitter dopamine in the synapses that is responsible for hallucinations of all types in schizophrenics.