Thalamus drives active dendritic computations in cortex

Perception is linked to a calcium-dependent dendritic spiking mechanism that enables the major output cells of the cerebral cortex — layer 5 pyramidal tract neurons — to combine inputs from different information streams. Which circuits activate this mechanism upon sensory input is unclear. Here we found that thalamocortical axons, which provide sensory input to cortex, target specifically the dendritic domains in pyramidal tract neurons that initiate calcium spikes. Sensory input thereby enables distal dendritic inputs preceding the stimulus to transform the first responses that leave cortex into bursts of action potentials. Thus, thalamus can drive active dendritic coupling of sensory with prestimulus information streams to modulate cortical output. Our findings indicate that thalamocortical coupling is first in a cascade of mechanisms that transform sensory input into perception.