The fastest retina development in normal term infants is in the 6 months after birth, and the retina becomes fully developed in the following 1–4 years. At birth, the structure and function of the peripheral retina is almost fully developed, and the changes after birth occur mainly at the posterior pole, especially at the macula. The retinal ganglion cell layer, inner nuclear layer, and rod cells in the macula move to the periphery to form a pit (foveola), whereas cone cells keep migrating from the periphery to the central macula, resulting in a contracting foveola. The fovea is immature at birth and becomes similar to that of adults only at 15–45 months, which is not associated with visual stimuli. Both inner and outer segments of cones keep growing slimmer and longer after birth, with the inner segments comparable to the adult shape at 15 months but the outer segments less than half the length of those in adults. At 45 months, the inner segments reach the adult size, while the outer segments achieve 50–70 % of the adult length. It is a crucial factor in the rapid enhancement of visual sensitivity after birth that the density of cones in the macula increases rapidly by central migration and the cones growing longer and thinner. Both the density and length of cones in the foveola will increase from birth to adulthood.

Projection areas of retinal neurons in each layer of the lateral geniculate nucleus continue to develop after birth. Immediately after birth, the neurons in the lateral geniculate nucleus are identifiable but not fully developed. There are numerous dendrites and dendritic spines that reach the maximum in 4-month-old infants. They are comparable to that in adults around the second year, showing that they are fully grown [9]. The anatomical and histological observations in primates revealed that the differentiation of six layers of neurons in the lateral geniculate nucleus has been completed before birth. The ventral layers, layer I and layer II, consist of magnocellular neurons, while layers III, IV, V, and VI are composed of parvocellular neurons. After birth, parvocellular layers develop faster than magnocellular layers, reaching the adult level at the age of 1 year or so, while the development of magnocellular layers is not comparable with that of an adult until around 2 years of age. In infancy and early childhood, the spatial resolution of the lateral geniculate nucleus receiving the foveal fibers is extremely low, which can be comparable with that of an adult until 30 weeks.

With the development of the visual function, the quantity, structure, and function of neurons in the visual cortex, as well as their synapses, may alter with circumstances, and they possess experience-dependent plasticity. There is a rapid increase of synaptic density in these neurons after birth, reaching the maximum peak at 8 months, twice as great as that at birth. With a slow decrease subsequently, the synaptic density in the visual cortex is comparable to that of an adult at 4 years. Advanced cortices develop to the adult level as late as the age of 11 [9, 10]. Increasing rapidly after birth, horizontal connections in the visual cortex form a homogeneous bundle at 7 weeks, becoming adult-type irregular projections after 8 weeks, and reach maturity by 15 months [11]. Nevertheless, a growing number of studies suggest that the function and structure in the visual cortex change through adolescence and adulthood [12–14].

Myelination of the optic nerve progresses from the distal end to the proximal end of cells. It mainly occurs in the first 3 months after birth, and it ceases when it reaches the cribriform plate at the age of 2 years [15]. Yet the myelination of some of the visual areas of extrastriate cortex and interneurons among the cortical layers requires a longer period.

The development and maturity of the visual system involves not only the structural development of the fovea, the lateral geniculate nucleus, and the visual cortex but also the refinement of axonal connections among the retina, the lateral geniculate nucleus, and the visual cortex, as well as the changes into the primary and secondary visual areas in the visual cortex. The synaptic projection from the retina onto the lateral geniculate nucleus is complete before birth, and it is further modified after birth under the regulation of visual stimulation. Ocular dominance columns, which pass through the visual cortex, will continue to develop, and orientation columns, in particular, are almost completely developed at this stage. Visual experience is essential to the completion of modification of the visual pathways from the retina to the visual cortex, and the different activities between binocular neurons induced by it also influence the formation of ocular dominance columns. In other words, visual neural circuits go through a more delicate modification in the presence of neuronal activities induced by visual experience stimulation, giving rise to topological synaptic connections at all levels of the visual pathways and finally becoming the fully developed visual system [2].

Vision refers to visual acuity. Children’s visual acuity changes dynamically. It is generally believed that it is almost fully developed at 5 years and relatively stable at 6 years and is similar to that of adults.

It is quite difficult to measure visual acuity in neonates and infants. Different examinations, such as subjective psychophysical examination and objective electrophysiological testing, show great discrepancy in results. Behavioral research observed that neonates develop slowly in grating acuity, reaching adult level as late as 3–5 years [16]. The study of visual evoked potential (VEP) discovered that, when amplitude is used as a grating frequency or contrast indicator, infants’ vision becomes comparable to that of adults between 6 months and 1 year [17]. As for preschool children, some researchers performed assessment on the visual acuities of children aged 3–6 years with the Landolt C chart and the tumbling E game, respectively. They found that, among young children aged 3–4 years, the visual acuity measured by E chart is far better than that measured by Landolt C. But for children aged 5–6 years, there is no difference in visual acuities between the two tests, and the visual acuities are indistinguishable from those of adults [18]. Therefore, children’s visual acuity test results are age specific and closely associated with testing methods as well. So particular care is required for children’s visual acuity assessment.

An external object forms a single image at the binocular corresponding retinal points and induces nerve impulses that are transmitted to the cerebral visual cortex, where two separate images can be fused into one single full image. This function is termed binocular vision or binocular single vision. Binocular visual function is divided into three levels from the basic to the advanced: simultaneous vision, sensory fusion, and stereopsis. In 1959, Hubel and Wiesel studied the interrelation of binocular vision, pointing out that the visual information converged at a very early stage [19]. In 1967, the driving cells sensitive to binocular parallax in the cat’s visual cortex were first found to be at Brodmann areas 17 and 18 [20]. Banks and colleagues stated that the sensitive period of the development of binocular vision starts several months after birth, with the peak between 1 and 3 years [21]. Leguire and colleagues investigated binocular summation of visual evoked potential among normal and strabismic children aged 1–58 months and revealed that there was a rapid increase in binocular summation response between 1.5 and 3 months, with a gradual decrease subsequently at 3–58 months [22]. They believed that it was associated with the emergence of conjugate ocular movements, sensory fusion, and stereopsis [22]. Fox and colleagues measured stereoacuity by using dynamic random-dot stereogram (RDS), demonstrating that stereoacuity emerged at 3.5–6 months. The result is consistent with the rapid development of the visual system after birth [23]. In terms of the mature stage of children’s development of binocular vision, there is a lack of systematic study. Romano and colleagues investigated the stereoscopic acuity of 1.5 to 13-year-old children with normal visual functions by means of the Titmus stereotest. They found that stereoacuity has been improving until the age of 9, and 40 s of arc in stereoacuity is the normal average level for children aged 9 and above [24]. Simons studied the stereoacuities of 3 to 5-year-old children and adults using the Frisby stereotest, Randot stereotest, random-dot stereogram, and TNO stereotest. He found that children’s binocular visual function has not yet been fully developed by the age of 5 [25]. Walraven and colleagues tested a population of 4–18 years using TNO stereotest and random-dot stereogram. They believed that stereopsis became gradually mature under the long-term stimulation of normal visual stimuli after children’s binocular vision had been established [26].