Magno-and Parvocellular Visual Cortex Activation in Anisometropic Amblyopia, as Studied with Functional Magnetic Resonance Imaging

Purpose: We studied the functional magnetic resonance imaging (fMRI) of the visual cortex in order to activate preferentially the parvocellular (PC) or the magnocellular (MC) visual system by manipulation of the spatiotemporal characteristics of the stimuli. Then we applied this technique to a patient with amblyopia to see how these two visual systems are affected in amblyopia. Methods: We acquired the fMRI at 1.5 T in 8 normal subjects and in one patient with anisometropic amblyopia, each receiving their best refractive correction. Each subject underwent experimentation under five conditions. The MC stimulus had a low contrast, a large check size, and a high temporal frequency. The PC stimulus had a high contrast, a small check size, and a low temporal frequency. After the activation in each condition had been determined by contrasting the visual stimulation conditions with the condition at rest, the inter-eye difference was determined. Results: In normal subjects, the activation map showed different visual areas as well as some overlap between the MC and PC stimuli, and similar visual cortex activations were obtained under the right-eye and the left-eye stimulation conditions. In the patient with amblyopia, the inter-eye difference was within a 95% confidence interval for the MC stimulation, but not for the PC stimulation. Conclusions: Our fMRI results show a good localization of the activity in the MC and PC pathways, and they are consistent with the findings in the literature, suggesting that the PC visual pathway is primarily defective in amblyopia.