What causes the visual abnormalities in schizophrenia if any?

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.
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.