Movement-related activity surpasses touch responses in secondary somatosensory thalamus

Each primary sensory cortex gets input from corresponding primary and secondary thalamic nuclei. While primary thalamic nuclei are characterized by their sensory responses, the degree to which secondary thalamus encodes sensory and non-sensory signals is unknown. In the whisker system, the primary nucleus is the ventral posterior nucleus (VPM) and the secondary nucleus is the posterior medial nucleus (POm). While VPM sends precise whisker touch signals to cortex, POm responses are not well understood. Unlike VPM, POm is interconnected with many cortical areas, including motor cortex and association areas. POm, as a recipient of both bottom-up whisker signals and top-down cortical signals, might integrate touch with contextual signals such as reward or movement. Using two-photon microscopy through a gradient index (GRIN) lens, I have assessed the POm response to touch with multi-whisker passive deflections of different velocities, to reward with water droplets, and to self-movement by measuring whisking and licking. POm activity had weak touch responses and was dominated by self-generated movements. My results suggest that POm is driven by self-movement or the internal state signals that accompany it, such as arousal. Next, I investigated whether these representations change when mice learn sensory-reward associations. I demonstrate that POm activity continues to be dominated by whisking and licking and does not acquire selectivity for reward-associated sensory stimuli. We propose a model in which the representation of movements within POm may facilitate learning sensory features in cortex by creating a window for plasticity around relevant stimuli.