Detecting monosynaptic EPSPs in vivo. A, Simultaneous intracellular recording of a V1 (L4) neuron and extracellular recording of a connected LGN neuron in response to white noise (yellow bars). Raster plot shows LGN response to additional nonrepeating stimulus blocks. B, LGN STA of V1 V_m (top), jittered STA used to remove stimulus induced correlations (middle), and difference revealing monosynaptic EPSP (bottom). “Lag” is time since LGN spike. C, Close-up of EPSP in B. D, Bootstrap significance test of EPSPs, for a sample LGN-V1 pair with an EPSP that passed (left column) and for a pair with an EPSP that failed (right column) the test. Top row, Mean and SEM of 10,000 bootstrapped estimates of the corrected STA for 10 ms before LGN spikes (pre-STA, red), representing V_m noise, and after (post-STA, blue), representing contribution of LGN. Bottom row, Difference of post- and pre-STA. Error bars indicate SEM. Ten continuous points above zero, between 1 and 4.5 ms (green dashed window), indicate a significant EPSP (see Materials and Methods).