Effects of arousal and movement on secondary somatosensory and visual thalamus

Summary All neocortical sensory areas have an associated primary and secondary thalamic nucleus. While the primary nuclei encode sensory information and relay it to cortex, the information encoded by the activity of secondary nuclei is poorly understood. We recorded juxtasomally from neurons in secondary somatosensory (POm) and visual (LP) thalamic nuclei of awake head-fixed mice with simultaneous whisker tracking and pupilometry. POm activity correlated with whether or not a mouse was whisking, but not fast precise whisking kinematics. This coarse movement modulation persisted after unilateral paralysis of the whisker pad and thus was not due to sensory reafference. POm continued to track whisking even after optogenetic silencing of primary somatosensory and motor cortex, indicating that cortical motor efference copy cannot explain the correlation between movement and POm activity. Whisking and pupil dilation were strongly correlated, raising the possibility that POm tracks arousal rather than whisker movement. LP, being part of the visual system, is not expected to encode whisker movement. However, we discovered that LP and POm track whisking equally well, suggesting a global arousal effect on both nuclei. We conclude that arousal, rather than movement per se, strongly alters activity in secondary thalamic nuclei.