Corticothalamic feedback locally modulates network state.

As the cortex receives most of its input from the thalamus, cortical layer 6 (L6) in turn sends a massive glutamatergic projection back to the corresponding thalamic areas. L6 feedback is morphologically and physiologically distinct from driver excitatory inputs, being, according to Sherman and Guillery, a modulatory pathway, akin to classical modulators. Here we tested this hypothesis by examining the effect of L6 corticothalamic activation on thalamocortical oscillations and network state, using extracellular recordings and optogenetic stimulation. We found that different patterns of L6 activity can promote the transition to both light- and deep sleep-like states, as well as to desynchronized activity. Low frequency L6 activation entrained sleep spindles in a temporal manner, while tonic L6 activity abolished spindling and promoted delta-rich sleep. Stronger L6 activation desynchronized the network, inducing gamma oscillations. These changes were confined to the activated thalamocortical circuit, suggesting that corticothalamic feedback acts as a local modulatory subsystem.