Background: Visual motion processing is compromised in schizophrenia, as shown in deficient velocity discrimination. Processing of motion signals comprises progressive stages along the geniculate-striate-extrastriate-cortex pathway. Based on neurophysiologic and brain lesion studies, a velocity discrimination deficit can implicate early-stage motion processing if it is contrast-dependent or late-stage motion processing if it is contrast-independent.
Methods: To determine which stage underlies the deficient velocity discrimination in schizophrenia, we examined the effects of visual contrast on velocity discrimination. We measured velocity discrimination thresholds in schizophrenia patients (n = 34) and normal control subjects (n = 17) at both low and high contrasts, using each subject's contrast detection threshold to equate contrast levels.
Results: Schizophrenia patients showed poor velocity discrimination that improved little with high contrast, whereas normal control subjects showed enhanced velocity discrimination with increased contrast.
Conclusions: The finding that the velocity discrimination deficit in schizophrenia is independent of contrast modulation implicates the later, rather than the earlier, stages of motion processing, which is mediated in the extrastriate cortex.