Functional magnetic resonance imaging (fMRI) investigation of the circuitry for blue-yellow color vision

Abstract

The neural circuit underlying blue-yellow color vision in the rat was investigated using high resolution functional magnetic resonance imaging (fMRI) under a variety of stimulus conditions. Rodents and other mammals share evolutionarily conserved neural circuits for processing the output of two classes of spectrally distinct cone photoreceptor. Unlike other mammals, their availability combined with their brain and body size make them ideal subjects for experiments using a high field strength 9.4T MRI system coupled with pharmacological intervention. Herein, the well established agonist of metabotropic glutamate receptors, 2-amino-4-phosphonobutyric acid (APB), was used intravitreously to block the ON-pathways in the rat retina. A computer controlled binocular visual stimulator was designed to operate in the MRI scanner. Light from arrays of colored LEDs was delivered via fiber optic bundles to produce binocular stimulation. fMRI responses were compared under stimulus conditions that isolated either S or M cones. Regions analyzed in the functional images included the dorsal lateral geniculate nucleus, the lateral posterior nucleus, the superior colliculus, the primary visual cortex, and higher visual areas. Standard theory attributes the perception of blueness to comparisons between S-cones and M-cones made by small bistratified ganglion cells. However, when the ON-pathways (i.e. S-cone input) were inhibited the fMRI showed cortical activation, indicating that the small bistratified cells normally provide inhibitory rather than excitatory input to cortex. This suggests that the sensation of blueness is mediated by OFF responses. Based on known circuitry the best candidate for generating these OFF responses is horizontal cell feedback from S-cones on neighboring M-cones.