(a) The spread of fluorescence signal after 3-pulse or 10-pulse injections of Congo red (to mimic MK801) into the layer 1 of the motor cortex. Green square represents the region for imaging dendritic Ca2+ spikes. (b) The fluorescence signals of Congo red spread to a region 426 ± 11 μm after 3 pulses and 612 ± 39 μm after 10 pulses in layer 1 (n = 4 mice for each group). (c) The fluorescence signals of the imaged region after 3 and 10-pulse injection of Congo red. (d) The fold change in fluorescence after 10-pulse injection of Congo red relative to 3-pulse injection. The large difference in fluorescence intensity between 3-pulse and 10-pulse injection was partially due to that the concentration of Congo red was lower at the electrode tip (~20 μm) than at the shaft. As the result, the amount of Congo red released from the first 1-2 pulse injection was less than that from the subsequent pulse injections (n = 4 mice). Data are presented as mean ± s.e.m. *P < 0.05, Wilcoxon–Mann–Whitney test.