(A) Schematic of the technical approach. L2/3 neurons of urethane-anesthetized Six3-cre/Ai32 mice were recorded in whole-cell mode and ChR2-expressing L4 afferent fibers were activated using a fiber-coupled LED.

(B) Pattern of expression of ChR2 in the barrel cortex (scale bars: left, 500 μm; right, 200 μm).

(Ci–Civ) (Ci) Example trace of highly active L2/3 neuron (top) and sparsely spiking L2/3 neuron (bottom). (Cii) Bimodal distribution of membrane potential (MP). (Ciii) Thresholds for stimulation during Up states and Down states were 0.5 mV negative to the mean Up state MP (red dotted line) and 0.5 mV positive to the mean Down state MP (blue dotted line), respectively. (Civ) Mean MP at Up states and Down states for all cells recorded (n = 92 neurons in N = 81 mice). Box-and-whisker plots represent maximum, upper quartile, mean (cross), median, lower quartile, and minimum values.

(D) Example trace of light-evoked EPSP during Down state. A closed-loop was used to elicit EPSPs only at MP negative to Down state threshold. Spikes are truncated for clarity. See also Figures S1 and S2

(E) Ten overlaid traces of light-evoked EPSPs during Down states (gray) and their mean (black).

(F) Diagram of the experimental design. EPSPs were monitored for 10 min using a 2-ms light pulse only during Down states (<0.1 Hz). Subsequently, one of eight protocols was applied (100 repetitions at <0.2 Hz): light stimulation at Down states only (control); light-stimulation at Down states followed (1) or preceded (2) by postsynaptic spike; light pulse during Up states only (3); light pulse during Down states paired with postsynaptic depolarization (4); light pulse during Up states paired with postsynaptic hyperpolarization (5); and presynaptic light stimulation during Up states followed (6) or preceded (7) by a postsynaptic spike. Following the plasticity protocol, EPSP was monitored by light stimulation during Down states (<0.1 Hz) for 20 to 30 min.