Electron microscope studies of relay cells in the lateral geniculate nucleus of the CAT have shown that the retinal input of X-cells is associated with a special synaptic circuitry, termed the spine-triad complex. The retinal afferents make an asymmetrical synapse with both a dendritic appendage of the X-cell and a geniculate interneuron. The interneuron contacts in turn the same dendritic appendage with a symmetrical synaptic profile. The retinal input to geniculate Y-cells is predominately found on dendritic shafts without any triadic arrangement. We explore the integrative properties of X- and Y-cells resulting from this striking dichotomy in synaptic architecture. The basis of our analysis is the solution of the cable equation for a branched dendritic tree with a known somatic input resistance. Under the assumption that the geniculate interneuron mediates a shunting inhibition, activation of the interneuron reduces very efficiently the excitatory post-synaptic potential induced by the retinal afferent without affecting the electrical activity in the rest of the cell. Therefore, the spine-triad circuit implements the analogy of an AND-NOT gate, unique to the X-system. Functionally, this corresponds to a presynaptic, feed-forward type of inhibition of the optic tract terminal. Since Y-cells lack this structure, inhibition acts globally, reducing the general electrical activity of the cell. We propose that geniculate interneurons gate the flow of visual information into the X-system as a function of the behavioral state of the animal, enhancing the center-surround antagonism and possibly mediating reciprocal lateral inhibition, eye-movement related suppression and selective visual attention.