Local and thalamic origins of ongoing and sensory evoked cortical correlations

Thalamic inputs of layer 4 (L4) cells in sensory cortices are outnumbered by local connections. Thus, it was suggested that robust sensory response in L4 emerges due to synchronized thalamic activity. In order to investigate the role of both inputs in generation of cortical synchronization, we isolated the thalamic excitatory inputs of cortical cells by optogenetically silencing cortical firing. In anesthetized mice, we measured the correlation between isolated thalamic synaptic inputs of simultaneously patched nearby L4 cells of the barrel cortex. In contrast to correlated activity of excitatory synaptic inputs in the intact cortex, isolated thalamic inputs exhibit lower variability and asynchronous spontaneous and sensory evoked inputs. These results were further supported in awake mice when we recorded the excitatory inputs of individual cortical cells simultaneously with the local field potential (LFP) in a nearby site. Our results therefore indicate that cortical synchronization emerges by intracortical coupling.