The initial stages of the process of vision are made up of:
1-transmission and refraction of light by the eye’s optics
2-transformation of light energy into electrical signals by photoreceptors
3-coding of signals by retinal neural circuits.
The last two mentioned steps take place in the retina. The retina is the innermost layer tissue that forms the eye. Although its location is peripheral, it is part of the central nervous system because it was originated from an evagination of the diencephalon. This layer consists of five types of neurons: photoreceptors, bipolar cells, ganglion cells, horizontal cells and amacrine cells. These neurons are arranged in alternating layers allowing the formation of a direct chain to the flow of information from the photoreceptors to the optic nerve. The axons of the ganglion cells make up the optic nerve, this nerve continues on its way to the thalamus (dorsolateral geniculate nucleus) to finish in the primary visual cortex or striate cortex. In addition, ganglion cells direct their endings to the superior colliculus (orientation movements of the head and eyes), the pretectal region (reflex control of the pupil and lens) and hypothalamus (regulation of circadian rhythms).
There is a differentiation between the nasal and the temporal retina that decides if the information acquired by the photoreceptors is sent to one hemisphere or to the other. When the information that is captured by the nasal retina it reaches the optic chiasm, it crosses hemisphere and therefore, the information is analyzed in the contralateral hemisphere. However, when the information of the temporal retina reaches the optic chiasm, it continues in the same hemisphere where the analysis is performed.