Figure 2. Synaptic input mapping in AFC slices. A, Bright-field image showing recording arrangement, including orientation of the 256-site stimulus grid (yellow box), a single stimulus location (*), and the soma of the recorded neuron (triangle). B, Example of EPSC evoked by glutamate uncaging at one stimulus location. C, Array of traces recorded during sequential photostimulation at the 256 sites in the stimulus grid. Responses that were contaminated by direct stimulation of the postsynaptic neuron's dendrites are blank. D, Synaptic input map. Pixel values represent the mean current over a 50 ms poststimulus time window. Black pixels represent traces contaminated by dendritic responses. E, Traces from region of interest (ROI) (gray brackets in D) showing strong layer 2/3 inputs for this neuron. F, Excitation profile. This example shows an AP evoked by perisomatic stimulation of a cKO layer 2/3 pyramidal neuron, recorded in cell-attached mode. The location of the stimulus grid is indicated by the yellow box. G, APs occurred at several locations immediately surrounding the soma. H, Mean distance of AP-generating sites from the soma, an estimator of the spatial resolution of photostimulation (Vexc), did not differ in WT and cKO. Circles, Data points from individual neurons; line and box, mean ± SEM. I, Total number of APs per excitation profile, an estimator of the intensity of photostimulation (NAP), did not differ in WT and cKO. J, Example of a Nissl-stained section of AFC (in a WT mouse). Cells were counted within a 0.1 × 0.2 mm ROI in layer 2/3 (box and inset) to assess relative cell density. K, Number of layer 2/3 neurons per ROI, expressed as cells per 10,000 μm2 in a 50-μm-thick slice (ρcell), did not differ in WT and cKO.