![]() However, what POm activity encodes remains a mystery. POm is thus positioned to strongly influence sensory computations in S1 and do so in ways that are highly distinct from VPM. POm is a stronger driver of layer 2/3 cells than cortico-cortical synapses and can enhance sensory responses in pyramidal neurons of layers 2/3 and 5 ( Mease et al., 2016 Zhang and Bruno, 2019). Whereas VPM innervates cortical layer 4 and the border of layers 5B and 6, POm projects to the apical dendrites of layer 1 as well as layer 5A ( Wimmer et al., 2010). It receives input from S1, motor cortex, posterior parietal cortex, the zona incerta, and many other subcortical regions in addition to brainstem afferents ( Chiaia et al., 1991 Olsen and Witter, 2016 Trageser and Keller, 2004). Compared to the primary nucleus VPM, the secondary nucleus POm has broader receptive fields, longer-latency sensory responses, and poorly encodes fine aspects of whisker touch such as contact timing and stimulus frequency ( Diamond et al., 1992 Masri et al., 2008 Moore, 2004 Moore et al., 2015). In rodents, the facial whisker representation of primary somatosensory cortex (S1) is tightly integrated with two thalamic nuclei: the ventral posteromedial nucleus (VPM) and the posterior medial nucleus (POm) ( Deschênes et al., 2016 Petersen, 2007). Unlike primary nuclei, the secondary nuclei are interconnected with many cortical and subcortical regions, and their role in sensation and cognition is poorly understood. The primary nuclei are the main source of sensory input to the cortex and respond robustly to sensory stimulation with low latency ( Chiaia et al., 1991 Constantinople and Bruno, 2013 Sherman and Guillery, 2002 Wimmer et al., 2010). ![]() These nuclei can be subdivided into primary and secondary (often termed ‘higher-order’) nuclei ( Guillery and Sherman, 2002 Herkenham, 1980 Phillips et al., 2019). Somatosensory, visual, auditory, and gustatory cortex are each reciprocally connected with a specific subset of thalamic nuclei. We conclude that secondary thalamus monitors behavioral state, rather than movement, and may exist to alter cortical activity accordingly. The semblance of movement-related activity is likely instead a global effect of arousal on both nuclei. ![]() Indeed LP, which is not part of the whisker system, tracked whisking equally well, further indicating that POm activity does not encode whisker movement per se. Whisking and pupil dilation were strongly correlated, possibly reflecting arousal. This phenomenon also continued during optogenetic silencing of somatosensory and motor cortex and after lesion of superior colliculus, ruling out a motor efference copy mechanism. This coarse movement modulation persisted after facial paralysis and thus was not due to sensory reafference. POm activity correlated with whisking, but not precise whisker kinematics. We recorded juxtasomally from secondary somatosensory (POm) and visual (LP) nuclei of awake mice while tracking whisking and pupil size. While primary nuclei transmit sensory information to cortex, secondary nuclei remain poorly understood. Neocortical sensory areas have associated primary and secondary thalamic nuclei.
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