Fig. 4.

Photostimulation of layer 5 pyramidal neurons lead to intercolumnal spread of postsynaptic activity. (a) Diagram depicting arrangement for photostimulation of layer 5 pyramidal neurons in column C. (b) VSD image taken at 13.2 ms after the onset of a 3 ms 460 nm light flash to stimulate layer 5 pyramidal neurons (indicated by the dotted box at location 2), showing the spread of excitatory responses up to layer $2/3$ and along layer 5. Barrels are indicated in white and lettered accordingly, showing the location of A to E columns. (c) Different peak latencies in layers $2/3$ and 5 responses show that responses initiated in layer 5 spread toward layer $2/3$. (d) VSD image taken at 13.2 ms after a $50\mu \mathrm{A}$ electrical pulse was delivered to layer 5. Strong responses tended to spread upwards along the column. (e) Electrically stimulated slices show a similar pattern of peak latencies in layers $2/3$ and 5, indicating that responses initiated in layer 5 spread toward layer $2/3$.