Effects of arousal and movement on secondary somatosensory and visual thalamus

Abstract

All neocortical sensory areas have an associated primary and secondary thalamic nucleus. While the primary nuclei encode sensory information for transmission to cortex, the nature of information encoded in 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 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 during optogenetic silencing of primary somatosensory and motor cortex and after lesion of superior colliculus, indicating that motor efference copy cannot explain the correlation between movement and POm activity. Whisking and pupil dilation were strongly correlated, raising the possibility that POm may track arousal rather than movement. LP, being part of the visual system, is not expected to encode whisker movement. We discovered, however, that LP and POm track whisking equally well, suggesting a global effect of arousal on both nuclei. We conclude that secondary thalamus is a monitor of behavioral state, rather than movement, and may exist to alter cortical activity accordingly.