(A) Schematic of the experiments performed by Bereshpolova et al.: a single cell in the LGN of the thalamus (orange) is recorded simultaneously with a single cell (blue) in layer 4 (L4) of the primary visual cortex (V1) during a visual response. The activity across cortical layers is also measured with another electrode (grey, CSD). (B) Bereshpolova et al. found that three rules regulate the connectivity between a neuron in the LGN and an inhibitory neuron in L4: their receptive fields (shown here by an orange circle and a blue square respectively) must overlap; the termination of the LGN neuron’s axon (a location estimated by the peak in the CSD signal, grey line) must be located near the cortical neuron (small blue circle); and the delay between the firing of the thalamic and cortical neurons must be less than three milliseconds. All three conditions are met in the top left panel, so a connection is established. Only two of the three conditions are met in the other three panels, so a connection is not established in any of these cases. LGN: lateral geniculate nucleus; CSD: current source density; RF: receptive field.