Functional magnetic resonance imaging of the magnocellular visual pathway in nonpsychotic relatives of persons with schizophrenia
Bedwell, Jeffrey Scott
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Physiological research with monkeys and psychophysical research with humans has demonstrated the ability of diffuse red light to suppress activity in the magnocellular (M) visual pathway. In contrast, a previous psychophysical study found that a subset of nonpsychotic relatives of persons with schizophrenia showed the opposite effect, suggesting a novel biobehavioral marker for the disorder. Other research suggests that persons with schizophrenia have a dysfunctional M pathway under neutral (non-red) light conditions. However, it does not appear that these findings have been explored with physiological methods in nonpsychotic relatives, which would provide support for genetic contributions. The current study used physiological methodology to explore whether, as a group: 1) healthy adults show suppression of the M pathway in response to diffuse red light; 2) nonpsychotic relatives of persons with schizophrenia have a dysfunctional M pathway under neutral light conditions; and 3) nonpsychotic relatives of persons with schizophrenia have a differential M pathway response to red light. Functional magnetic resonance imaging (fMRI) was used to investigate a group of 13 nonpsychotic relatives of persons with schizophrenia and 11 controls. Moving concentric rings were presented on both red and green backgrounds to stimulate the M pathway. The fMRI signal strength in bilateral cortical region V5 (MT) was measured as a marker of M pathway functioning. Statistically significant results were limited to measures of fMRI signal in right hemisphere V5 relative to signal from bilateral V5, and suggested that: 1) the control group had reduced M pathway activity in response to diffuse red light; 2) the relative group had a hypoactive M pathway; and 3) a subset of the relatives had the opposite M pathway response to diffuse red light. The differential M pathway response to red light in the relatives remained a statistical trend after controlling for M pathway signal from the neutral (green) background condition. Results provide preliminary evidence that genetic risk for schizophrenia is related to a hypoactive M pathway and an independent differential response (increase in activity) of the M pathway to red light. These features may be more evident in the right hemisphere when examining cortical region V5.