Frequency-dependent thalamocortical transfer limits sustained cortical expression of sensory drive

Frequency-dependent sensory drive must pass through thalamocortical pathways before cortical expression, yet how high-frequency input is transformed into sustained cortical activity remains unclear. We combined 11.7-T fMRI, pathway-specific optogenetic stimulation, tuned-burst thalamic drive, and dynamic causal modeling (DCM) in dexmedetomidine-sedated rats to examine the ventral posterolateral thalamus (VPL)-to-primary somatosensory forelimb cortex (S1FL) pathway. Peripheral forepaw stimulation revealed intermediate-frequency cortical tuning, whereas thalamic nuclei remained responsive at high frequencies. Direct S1FL activation evoked sustained high-frequency blood-oxygen-level-dependent responses, indicating that cortex can express high-frequency drive when upstream relay is bypassed. In contrast, direct VPL activation produced strong thalamic responses but weak, onset-dominated high-frequency cortical responses, and tuned-burst stimulation showed temporal compression before cortical expression. DCM indicated preferential enhancement of VPL-to-S1FL effective connectivity at intermediate frequencies. These findings identify a frequency-dependent bottleneck in VPL-to-S1FL transfer, rather than local cortical responsiveness, as a constraint on sustained cortical expression of temporally dense sensory input.