The fluctuation-based regime of thalamocortical circuitry.

Neocortical neurons in primary visual cortex (V1) integrate excitatory inputs from both thalamic and cortical sources to generate selectivity for orientation, direction and disparity. Feedforward models have posited that thalamocortical input can drive these selectivities, but the relative sparsity of thalamic synapses raises questions about the sufficiency of this input. To determine whether thalamic drive is sufficient to evoke cortical responses, we quantified the current threshold required to evoke action potentials in vivo using whole-cell recordings in mouse V1. We then isolated thalamic input via optogenetic cortical silencing and compared its strength to the measured spiking threshold. We find that although average thalamic input is below the threshold for action potential generation, trial-by-trial fluctuations in thalamic drive are sufficient to evoke visually driven action potentials. These results demonstrate that thalamic excitation alone can drive cortical spiking, but its effectiveness depends on fluctuations in synaptic input.