While visual input is normally treated as a unimodal stimulus, it is able to influence other sensory cortices and modulate their processing through multisensory interaction^{1, 2}. Since the visual system responds differently to stimuli of different intensities and frequencies, the corresponding responses may alter crossmodal influences. Yet, the effect of different parameters of visual input to crossmodal influences remains largely unexplored. In this study, light stimulation of different intensities and frequencies were presented to normal rodents and their BOLD responses were compared to characterize the crossmodal responses evoked by visual stimuli of different intensities and frequencies.Visual stimulation: Adult male rats (n=12, 300-350g, Sprague Dawley strain) were anesthetized with mixed air and 1.3% isoflurane during fMRI scanning. Binocular visual stimulation was delivered to the rats through an optical fiber placed 1.5cm in front and along the midline of the eyes. Blue light at 1Hz and 10Hz (10% duty, peak power=2mW (n=8) and 0.1mW (n=4)) was presented in a block-design paradigm (20s on and 40s off). fMRI acquisition and analysis: fMRI data was acquired at 7T using GE-EPI (FOV=32×32mm², matrix=64×64, α=56°, TE/TR=20/1000ms, 12 contiguous slices with 1mm thickness). Data were preprocessed before the standard GLM analysis was applied to identify significant BOLD responses (p<0.001).Figure 1 shows the BOLD activation maps upon 1Hz and 10Hz light stimulation at peak power of 2mW and 0.1mW. The visual pathway (e.g. visual cortex (VC), superior colliculus (SC) and lateral geniculate nucleus (LGN)) and other brain regions (e.g. auditory cortex (AC), secondary somatosensory cortex (S2), cingulate cortex (Cg) and caudate putamen (CPu)) were activated during 1Hz 2mW light stimulation, while only the VC, SC and LGN were activated during 1Hz 0.1mW light stimulation. Under 10Hz light stimulation, only SC and LGN were activated at both light power levels. Figure 2 shows 1Hz 2mW light evoked similar BOLD increase among different cortical regions (VC, AC, S2 and Cg) and caudate putamen. Figure 3 compares the BOLD responses in the visual pathway evoked by different visual stimuli. It shows the BOLD increase in the SC is smaller during 1Hz light stimulation at 2mW compared to 0.1mW (p<0.05, unpaired t-test on averaged BOLD signal change). Previous studies showed the functional and structural connections among the visual cortex, sensory cortices and subcortical regions in multisensory brain interaction or processing^1-5. Strong low frequency light stimulation evoked responses in the visual pathway, auditory cortex, secondary somatosensory cortex, cingulate cortex and caudate putamen. Responses in AC and S2 likely reflect the interaction among these areas and the visual system, while the response in CPu may suggest sensory integration through CPu when strong low frequency light were delivered. Earlier studies described the involvement of the cingulate cortex in attentional modulation of the sensory processing⁶. The activation in the Cg may provide attentional control to multisensory interaction among VC, AC and S2. Upon strong low frequency light stimulation, BOLD response in the SC is smaller and more confined. This possibly resembles the converging cortical influences to SC during multisensory integration^{7, 8}. Since a low population of neuron in the visual cortex responds to high frequency stimuli⁹, high frequency stimuli, compared to low frequency stimuli, induce less activation in the visual cortex, which may account for the insignificance or the absence of the visual cortex activation upon 10Hz light stimulation. Note that 2mW 10Hz light stimulation did not evoke responses in those sensory and attention circuits. This may suggest that large-scale brain activations propagated from the visual cortex. Since visual cortex does not response to 1Hz stimulus only, the large-scale brain activation would be expected upon strong light at other low frequencies. Whether such large-scale brain activation possesses frequency dependence at or below 1Hz remains to be elucidated. This is the first visual fMRI study that demonstrated large-scale brain activation using strong low frequency light stimulation. Such stimulus evoked responses in the visual system and brain regions that play a role in multisensory interaction and attentional modulation of sensory processing. The activations in those brain regions likely propagated from the visual cortex and influenced subcortical responses. Our current study provides a functional understanding to cortical crossmodal activity and its influences on the subcortex upon visual stimuli of different intensities and frequencies.