Fluorescence of 3 in brain slices containing the dorsal cochlear nucleus (DCN) after photoactivation. (A) 4× image of the DCN slice (left), with overlaid 8 × 8 photostimulation grid (40 μm spacing). 20× image of the DCN (right) with the overlaid grid. The grid was photostimulated 5 times with 1 ms pulses (355 nm, ~5.5 mW). (B) Heat map of normalized fluorescent signals. ΔF/F is the fluorescence change after photostimulation divided by the initial fluorescence. (C) Comparison of the average photoactivated fluorescence of sensor 3 in the molecular layer versus the deep layer (regions outlined by yellow boxes in B, n = 6, p < 0.0001, unpaired t test).