Linear transformation of thalamocortical input by intracortical excitation.

Neurons in thalamorecipient layers of sensory cortices integrate thalamocortical and intracortical inputs. Although we know that their functional properties can arise from the convergence of thalamic inputs, intracortical circuits could also be involved in thalamocortical transformations of sensory information. We silenced intracortical excitatory circuits with optogenetic activation of parvalbumin-positive inhibitory neurons in mouse primary visual cortex and compared visually evoked thalamocortical input with total excitation in the same layer 4 pyramidal neurons. We found that intracortical excitatory circuits preserved the orientation and direction tuning of thalamocortical excitation, with a linear amplification of thalamocortical signals of about threefold. The spatial receptive field of thalamocortical input was slightly elongated and was expanded by intracortical excitation in an approximately proportional manner. Thus, intracortical excitatory circuits faithfully reinforce the representation of thalamocortical information and may influence the size of the receptive field by recruiting additional inputs.