Functional Magnetic Resonance Imaging of Neural Activity in Rat CNS in Response to Chromatic Stimuli

Abstract

In rodents, dichromatic color vision is based on comparisons of M-cones and UV-cones. However, little is known about the location and distribution of cortical regions carrying color information in the rodent. Therefore, we used functional magnetic resonance imaging (fMRI) to measure the blood-oxygenation level dependent (BOLD) signal generated by a heterochromatically modulated stimulus. Two stimulus conditions were designed to favor either luminance or chromatic pathways individually. M-cone quantal catches of an ultraviolet light (DW= 465 nm) were adjusted to match those of a green light (DW= 545 nm). The lights were modulated in counter-phase at 1 Hz to isolate chromatic signals produced by S-cones. To acquire a response to luminance increments and decrements, 545 nm light was modulated at 1 Hz without the 465 nm light. BOLD responses were measured for each stimulus condition. Diffuse activation in V1 with lesser activation in the superior colliculus and the dorsal lateral geniculate (DLG) nucleus was observed in response to the chromatic condition. The luminance condition produced a more robust activation of the superior colliculus and DLG. This approach will allow us to evaluate the functional consequences of newly introduced opsin molecules targeted to either M- or S- photoreceptors in rodents.