Spread of PV interneuron responses and determination of conduction velocity. Gradient contrast image (A) and fluorescence image (B) of a BC slice (both taken with the high-resolution Kiralux camera) with stimulating electrode in L5 (black star at the tip). Layer boundaries and barrels are visible in L4 in A. In B, layer boundaries are indicated with dashed lines, and column boundaries with dotted lines (in C as well). (C) Heatmap of response SNR from the slice shown in A and B. Stimulation site indicated by red star (position differs slightly from A and B due to different cameras). Red outlines define ROIs ⁓20 μm thick spanning the stimulated column. Black arrow indicates direction of propagation. (D) Top: Trace of fluorescence versus time for ROI indicated by asterisk in C and E. Gray shading indicates the 20 ms measurement window expanded below. Bottom: Latency (purple line) is the time from stimulation (dashed line) to half-maximal change in fluorescence of the rising phase. (E) Traces of fluorescence versus time for 21 ROIs in sequence from bottom up (increasing distance from stimulation electrode). ROI 1 was closest to the electrode, and ROI 21 was farthest from the electrode. ROIs 1, 13 (star), and 21 are labeled in C. Dashed line indicates stimulation time, and asterisk indicates trace corresponding to the ROI marked with asterisk in C (thirteenth from bottom). Traces show PV interneuron responses with advancing latency. Higher noise in darker regions closer to the edge of the slice reduces SNR (e.g. ROI 21). (F) Latency plotted versus distance from the site of stimulation. The relationship between latency and distance was significant (R = 0.94, P = 3 × 10⁻⁸). The inverse of the slope gave an interlaminar conduction velocity of 231 μm/ms.