Corticothalamic modulation of somatosensory thalamic tactile processing.

The brain's processing of sensory information involves intricate interactions between feedforward and feedback pathways, including corticothalamic feedback. Although feedback from cortical Layer 6 to the sensory thalamus is known to regulate sensory signalling, its precise function remains elusive. This study delves into the impact of Layer 6 feedback on sensory transmission in the ventral posteromedial nucleus using in vivo electrophysiology recordings in lightly anesthetized rats. By local administration of drugs to the barrel cortex during thalamic recordings, we investigate how corticothalamic neurons influence the transformation of tactile stimuli into neuronal discharge characteristics. Our findings reveal that increasing cortical dynamics enhances thalamic response magnitude at low stimulus intensities but decreases it at high intensities, whereas reducing cortical dynamics produces the opposite effect. Moreover, we observe bidirectional cortical influence on thalamic neurons extending to stimulus magnitude-dependent sensory adaptation and burst propensity modulation by Layer 6 dynamics. Specifically, increased cortical dynamics reduce thalamic sensory adaptation and increase burst propensity at low stimulus intensities, with no observed change at high intensities, whereas decreased cortical dynamics elicit opposite effects. We show that thalamic neurons can discriminate between stimuli, with cortical influence varying by stimulus intensity. Increased cortical dynamics enhances discrimination at low intensities, whereas reduced dynamics has the opposite effect. Our findings suggest that cortical control of ventral posteromedial nucleus tactile transformation is not a binary switch but a dynamic modulator, adjusting thalamic transformations in real time based on cortical dynamics. This mechanism finely tunes sensory processing to meet environmental and behavioural demands. KEY POINTS: The study investigates touch processing in the brain by examining interactions between brain regions. Specifically, we study how cortical Layer 6 influences sensory signal processing in the thalamus. We manipulated Layer 6 activity with drugs and observed resulting changes in thalamic touch responses. Increased cortical activity enhanced weak touch signals but dampened strong ones in the thalamus; lower activity had the opposite effect. Increased cortical dynamics reduced thalamic sensory adaptation and increased burst propensity at low stimulus intensities, with no change at high intensities. The study shows that the brain's control over how it processes sensory information is not just an on/off switch but a dynamic system that adjusts in real time to different situations.