Fig. 3.

Photostimulation of layer 5 neurons elicits postsynaptic responses in layer 2/3. (a) All-optical stimulation of neurons with 460 nm light-emitting diode (LED) light and recording of excitation by voltage-sensitive dye (VSD) fluorescence. The mosaic micromirror array was used to photostimulate specific regions of the slice. (b) Diagram depicting photostimulation of layer 5 pyramidal neurons in columns A to E is shown in the top-left panel. Other panels are VSD images showing the time-dependent spread of circuit activity from layers 5 to $2/3$ in the somatosensory cortex. A 460 nm light flash (5 ms) was used to photostimulate layer 5 pyramidal neurons and images taken at respective durations after starting photostimulation are shown. Barrels are indicated in white to indicate the location of columns A to E. (c) VSD image taken at 17.6 ms after the onset of a 5 ms 460 nm light flash to stimulate layer 5 pyramidal neurons (indicated by the dotted area at location 2). (d) Postsynaptic responses measured in layer $2/3$ were blocked with bath application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) ($10\mu \mathrm{M}$). In these and all other traces, horizontal bars indicate the timing of photostimulation. Transient fluorescence increases caused by excitation of the VSD during photostimulation have been blanked out.