No-one really knows what causes schizophrenia. One theory is that too much activity involving the neurotransmitter dopamine in a part of the brain called the striatum causes the delusions associated with psychosis but it is unclear when the excess dopamine activity starts to occur in the course of the illness. A brain-imaging study of 43 people by researchers at the Institute of Psychiatry in London compared people with schizophrenia, people with prodromal symptoms of schizophrenia (who show certain symptoms and have a high risk of going on to develop full-blown schizophrenia) and a healthy control group. The researchers found that people with schizophrenia had the highest levels of dopamine activity but that those people with prodromal symptoms also had a raised level of dopamine activity compared to the control group. The higher levels of dopamine activity were found in a region of the striatum called the associative striatum.
Howes, Oliver D. ... [et al] - Elevated striatal dopamine function linked to prodromal signs of schizophrenia Archives of General Psychiatry January 2009, 66(1), 13-20