Retinal neurons are interconnected vertically as well as horizontally. Light signals received by photoreceptors are partially processed at the first synapse in the retina, namely the synapse between photoreceptors and bipolar cells. Processing is achieved through spatial distribution of photoreceptors into receptive fields consisting of a center and an antagonistic surround. Central photoreceptors make contacts with bipolar cells directly whereas peripheral photoreceptors are wired to bipolar cells indirectly via horizontal cells. Because horizontal cells secrete inhibitory neurotransmitter gamma-aminobutyric acid, the illumination of the surround photoreceptors causes the bipolar cell to respond oppositely, as if the central photoreceptors were illuminated. Regarding the type of postsynaptic receptors and the effects of the neurotransmitter glutamate secreted by photoreceptors on the bipolar cells, the latter can be divided into two groups. On-center bipolar cells respond to photoreceptor stimulation with depolarization, whereas off-center bipolar cells respond with hyperpolarization. All bipolar cells secrete glutamate into their synapses with ganglion cells. The complexity of the retina is further increased by different types of amacrine cells which laterally interconnect bipolar and ganglion cells in the inner plexiform layer of the retina. Due to differences in arrangement, density and convergence of photoreceptors on different parts of the retina as well as communication between various types of bipolar, amacrine and ganglion cells, the retina enables a certain degree of integration and processing of the visual information before it is conveyed to the brain.