Complementarity of BOLD and ADC-fMRI in Mapping Brain Visual Processing in the Rat

The transition from static to dynamic vision is encoded in the superior colliculus (SC), as recently shown using blood oxygen level-dependent functional magnetic resonance imaging (BOLD-fMRI) of the rat brain. Visual stimulation at a higher frequency than the flicker fusion frequency threshold is associated with a negative BOLD response in the visual cortex, triggered by the SC. Here, we explored this mechanism in further depth using visual stimulation at low (1 Hz) and high (25 Hz) frequencies in the rat. We used both BOLD-fMRI and apparent diffusion coefficient (ADC)-fMRI to yield complementary information on brain activity during visual stimulation, from neurovascular and neuromorphological coupling perspectives. We compared responses between different brain regions (the dorsolateral geniculate nucleus, the medial and lateral parts of the SC, the corpus callosum, and the visual cortex), sexes, and field strengths (9.4 and 14 T). Results confirmed distinct BOLD responses to low- and high-frequency stimulation and highlighted for the first time that the transition from static to dynamic vision is characterized by negative BOLD in the lateral SC specifically. Whereas the BOLD response to visual stimulation depends on the vasculature properties across brain regions and sexes, we found a significant ADC-fMRI response (in the form of reduced ADC during excitatory activity) in the SC that was more consistent across visual frequencies, as well as in the corpus callosum, to which BOLD was not sensitive. Our results support an interplay between neural activity and hemodynamic response underlying the transition from static to dynamic vision, best characterized using two fMRI contrasts.