Whisker pad stimulation with different frequencies reveals non-uniform modulation of functional MRI signal across sensory systems in awake rats

Primary sensory systems are traditionally considered separate units, but emerging evidence highlights notable interactions between them. Using a quiet and motion-tolerant zero-echo time functional magnetic resonance imaging technique, we examined brain-wide cross-sensory responses to whisker pad stimulation in awake and anesthetized rats. Our results indicate that whisker pad stimulation activated not only the whisker-mediated tactile system, but also auditory, visual, high-order, and cerebellar regions, demonstrating brain-wide cross-sensory and associative activity. Based on response characteristics, non-core regions responded to stimulation in a markedly different way compared to the primary sensory system, likely reflecting distinct encoding modes among primary sensory, cross-sensory, and integrative processing. Lastly, while low-order sensory activity was detectable under anesthesia, high-order processing and the complex differences between primary, cross-sensory, and associative systems were evident only in the awake state. This study reveals novel aspects of the cross-sensory interplay of the whisker-mediated tactile system and underscores the challenges of observing these phenomena in anesthetized rats.