We describe the photochemical behavior of aqueous solutions containing the Co(CN)(6)(3-) complex ion and the trans-chalcone form Ct of the 4'-methoxyflavylium ion (AH(+)). It is shown that, under the experimental conditions used, the photochemical reaction leading from Ct to AH(+) shows an off-on-off behavior upon excitation with a continuous light source or with successive light flashes. The described systems perform as threshold devices and as XOR (eXclusive OR) logic gates. They behave as rudimental artificial neuron-like systems in the sense that their outputs (formation of the AH(+) species, with a consequent change in the spectroscopic properties) are the result of an elaboration of two distinct (light) inputs by chemical reactions in solution.