Light Stimulus Frequency Dependence of Activity in the Rat Visual System as Studied With High-Resolution BOLD fMRI

Abstract

Van Camp, Nadja, Marleen Verhoye, Chris I. De Zeeuw, and Annemie Van der Linden. Light stimulus frequency dependence of activity in the rat visual system as studied with high-resolution BOLD fMRI. J Neurophysiol 95: 3164 -3170, 2006. First published January 4, 2006 doi:10.1152/jn.00400.2005. The neurophysiology of the rodent visual system has mainly been investigated by invasive and ex-vivo techniques providing fragmented data. This area of research has been deprived of functional MRI studies based on blood oxygenation level dependent (BOLD) contrast, which allows a whole brain approach with a high spatial and temporal resolution. In the present study, we looked at the neurovascular response properties of the visual system of the pigmented rat, focusing on the visual cortex (VC), the superior colliculus (SC) and the flocculus-paraflocculus of the cerebellum (FL-PFL), using BOLD fMRI under domitor anesthesia. Visual stimulation was performed monocularly or binocularly while flashing light from a strobe unit was presented. For each structure, we assessed the flashing frequency that evoked the optimal BOLD response: Neither the VC nor the FL-PFL displayed frequency dependence during monocular visual stimulation, but were most sensitive to low frequencies (1-5 Hz) when flashing light was provided binocularly. The SC responded optimally to high flashing rates (8 -12 Hz) during both monocular and binocular stimulation. The signal intensity changes in the VC and FL-PFL were locked to the stimulation period, whereas the BOLD response in the SC showed a similar onset but a very slow recovery at offset. The VC and FL-PFL, but not the SC, showed signs of binocular competition. The observed correlation between frequency-dependent responses of different visual areas during binocular visual presentation suggests a functional relationship between the VC and FL-PFL rather than between the SC and FL-PFL. I N T R O D U C T I O NThe CNS of all vertebrates has three main visual pathways. These include the thalamofugal pathway, which is formed by the retinal projections to the primary visual cortex via the lateral geniculate nucleus of the thalamus and is involved in visual distinction of form and color as well as perception of visual motion (Hubel and Wiesel 1998); the tectofugal pathway, which is formed by a direct projection from retinal cells to the superior colliculus and is primarily involved in visual orientation and spatial attention (Wurtz and Goldberg 1972); and finally, the accessory optic system, which relays retinal slip signals for self-motion and gaze stabilization, either directly from the retina or indirectly via the visual cortex to the nuclei of the optic tract and visual tegmental relay zone, which in turn project to the inferior olive at which the climbing fiber input to the Purkinje cells in the cerebellum originate (Simpson et al. 1996). Current knowledge of the functional organization of the rodent visual system is fragmented and has been acquired exclusively through invasive or ex vivo techniques. Several s...

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