Optical imaging with high spatial and temporal resolution of neural activity in rat cortical slices was used to investigate the dynamics of signal transmission through neural connections in the visual cortex. When inhibition due to gamma-aminobutyric acid was slightly suppressed, horizontal propagation of excitation in both the supra- and infragranular layers became prominent. This propagation was not affected by vertical cuts in either the supra- or infragranular layer, which suggests that excitation is at least partially conveyed horizontally by reciprocal vertical connections between neurons in these layers.