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Horizontal cells (HCs), second-order retinal neurons, form a layer of network via gap junctions and the lateral signal transmission within this HC system can be modelled by a 2-dimensional cable theory. It is well known that a dopamine-releasing feedback pathway from third-order neurons can change the receptive field size of HCs by modulating the coupling resistance of the gap junctions in a manner depending on the light/dark adaptation of the retina. In this paper, we describe our finding that the receptive field size of an HC subtype in the carp retina can be controlled by a special type of synaptic mechanism made by short-wavelength (blue and/or green) sensitive cone photoreceptors. Thus, the situation is quite different from the dopamine-mediated system, in that the receptive field size changes by modulating the presynaptic membrane resistance associated with short-wavelength cone signals, rather than by modulating the gap-junctional resistance. We also report that the synapse in question is inactivated by dark-adaptation of the retina. The present mechanism is a new plasticity scheme for modulating the efficacy of lateral signal transmission across the outer retina.