Previous research has demonstrated the ability of diffuse red light to suppress activity in the magnocellular (M) visual pathway. An earlier psychophysical study found that a subset of nonpsychotic relatives of persons with schizophrenia showed the opposite effect when compared to healthy adults (Bedwell et al., 2003), suggesting a novel biobehavioral marker for the disorder. The present study attempted to replicate and explore the mechanism for this effect using fMRI. Results provide physiological evidence that the M pathway response to red light is in the opposite direction than expected in a subset of nonpsychotic relatives of persons with schizophrenia.