Figure 4. Laser illumination increases spiking in Arch-expressing PV+ neurons in L2/3.

a, Possible sources of increased inhibition to PNs. PV+ suppression disinhibits PN1 and increases excitatory input to downstream neurons PN2 and IN2. This unidentified inhibitory neuron IN2 could represent a distinct PV-negative inhibitory cell type, or potentially PV+ neurons themselves. PV+ neurons could provide increased inhibition to PNs if the increase in excitatory drive was strong enough to overcome the suppressive effect of Arch. b, To test this idea, we targeted extracellular recordings to PV+ cells in L2/3 and L4 using 2-photon microscopy. Left: PV+ neurons expressed membrane-bound Arch-GFP; right: PV- controls did not. Cells filled gradually with red Alexa dye over the course of the recording, or with current pulses at termination. c, Extracellular spike width and spontaneous firing rate for n=21 Arch-GFP-positive PV+ cells (red) and n=11 Arch-GFP-negative PN controls (black) collected in the same penetrations. Inset shows peak-aligned waveforms computed from 100 spikes. d, Example L2/3 PV+ neurons showing increased spike count on Arch•PV+ trials. Stimuli, WN at 0–70 dB; values are mean±SEM of 20 repetitions. See Figure S1 for example cells in L4. e, Spike times for example cell 4dii, pooled across all repetitions of the stimulus. Arrows, median spike time at 70 dB. f, Percent change in spike count plotted against recording depth for PV+ neurons in L2/3 and L4. Note the transition from suppression to enhancement near the L3–L4 boundary. Open circles, 2-photon targeted recordings (n=21 PV+ neurons); black squares, blind-patch recordings from Figure S1 (n=5 PV+ neurons). Black arrowheads correspond to example cells in 4d. Values are median, IQR across best stimuli (75th percentile). g, Integrate-and-fire model of a circuit with 2 layers of feedforward inhibition, representing cortical layers 2/3 and 4. When a hyperpolarizing current is applied to both PV+ neurons (I_Arch = −0.5 nA), PV+ activity is suppressed in the first layer (gi) but enhanced in the second layer (giii). E-I balance is disrupted in the first PN (gii), but the increase in PV+ activity “rebalances” synaptic input to the second PN (giv). Blue: excitatory input to the first layer of the model.