a, Sample and applied light stimulus S, inducing bidirectional propagation. b–d, Schematic representation (b) of the applied light protocol in space and time (x–t) with pre-conditioning stimuli p1 and blocking stimuli bL and bR to set tissue refractoriness before stimulus S, resulting in a right-side (c) or a left-side (d) unidirectional block. Here p1 is 350 ms, bR and bL are 50 ms and S is 10 ms. Irradiance levels are high (1,200 W m−2) and medium (700 W m−2). Activation maps show isochrones at 100 ms spacing.