Thalamus drives coupling of information streams in cortical dendrites

Perception is linked to a calcium-dependent spiking mechanism that is built into the distal dendrites of layer 5 pyramidal tract neurons (L5PTs) – the major output cell type of the cerebral cortex. It is yet unclear which circuits activate this cellular mechanism upon sensory stimulation. Here we found that the same thalamocortical axons that relay sensory signals to layer 4 also densely target the dendritic domain by which pyramidal tract neurons initiate calcium spikes. Distal dendritic inputs, which normally appear greatly attenuated at the cell body, thereby generate bursts of action potentials in cortical output upon stimulus onset. Our findings indicate that thalamus drives an active dendritic mechanism to couple sensory signals with top-down information streams into cortical output. Thus, in addition to being the central hub for sensory signals, thalamus is also likely to ensure that the signals it relays to cortex are perceived by the animal.