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We can see the world in colour due to receptors, known as rods and cones, which are found in our retina. They contain different pigments, which absorb certain wavelengths of light better than others. Rods do not mediate colour vision and are responsible for our ability to see in dim light as they have high photosensitivity. There are three types: blue-sensitive, green-sensitive and red-sensitive cones (short, medium and long wavelength cones with peak absorptions at 430, 530, 560nm.) When they are stimulated in different proportions we are able to see different colours. Although humans are born with all 3 types of cone cells, we are not able to see in colour at birth as parts of the brain receiving these nerve signals need to mature with age, along with the cone cells themselves.
Colour categorisation is a skill that infants develop early in life. Infants can distinguish between red and green at 2 months, and with time, between blues and yellows. An expert in colour vision, Franklin, devised a test that involved presenting infants with a coloured screen on which a dot appears. Both the dot and coloured screen may have been from the same colour category or a different colour category. It was observed how quickly the infant looked at the coloured dot. The speed at which they looked at the coloured dot helped to determine how different it appeared from the colour of the screen. Franklin and her team carried out another experiment involving 170 babies, being shown two squares of the same colour and two of different colours. This experiment helped to derive how many colour categories infants possess – they found that these categories were likely to be red, purple, yellow-brown, green and blue whilst other colour categories with pink and orange develop with language.
You can see that infants look more quickly towards a coloured dot when it comes from a different colour category. In other words, babies can tell that different colours of blue are all, well, “blue”. Colour categories, it seems, are not just down to language, they are in some way “hardwired”.
Contrast sensitivity measures the amount of contrast needed to detect the presence of a grating of different spatial frequencies, ranging from coarse to fine. Spatial frequency is measured in cycles per degree, and peak sensitivity is reached at 4 cycles/deg – this grating is resolved with only 1% contrast, whereas higher and lower spatial frequencies require higher contrast.
An experiment in which CSF’S were obtained longitudinally from infant monkeys (5-6 weeks postnatally) showed that with time, sensitivity increased at all spatial frequencies. (Boothe et al.,1980) However, sensitivity to low spatial frequencies become adult-like earlier than that for the high-frequency range. E.g, sensitivities for 1-5 cycles/deg reach adult levels by 20 weeks, compared to gratings greater than 15 cycles/deg which still improve at 40 weeks. These changes shift the CSF curve upwards (increasing sensitivity) and peak response shifts to the right (higher spatial frequencies. A 4:1 rule is devised because visual functions develop four times faster in a monkey than an infant, therefore we can relate these results to human visual development.
Changes in contrast sensitivity occur due to changes in the retina and the development of the visual cortex. Wilson (1988) suggested that migration of foveal cones produces a change in spatial scale and therefore a progressive shift of mechanisms turned towards higher spatial frequencies. The growth of the foveal cone outer segments causes an increase in mechanism sensitivity (Bernard, Edgar, 1995, p.54), as well as tighter packing of cones.
In the image above we can see the improvements in contrast sensitivity from 3 to 9 months, as the infant is able to see more low contrast elements and fine detail, and how similar this is to what the adult is able to see. Retinal factors take 4+ years to reach full adult maturity. Vision rapidly improves after birth because of the size, shape and density of cones found in the fovea. The cone length is 16x shorter in newborns. Visual acuity is further limited because the cone packing density is also 4x less in newborns which reduces spatial sampling.
Cone waveguide is not developed in the infant, meaning the cones do not have a funnel-like appearance, which is the waveguide property. The waveguide property allows light to enter only at one aperture and guides it into the interior space of the receptor.
The image shows the difference in a cone without the waveguide property on the left. When light enters this cone, it is not guided towards the interior space of the aperture and continues in a straight line. The cone with waveguide properties continually guides the light ray. Banks and Bennet (1988 ) estimated that the adult central foveal cone lattice absorbs 350 times more quanta than the newborn central foveal lattice, i.e for every quantum of light absorbed in newborn cones, roughly 350 quanta are effectively absorbed in adult foveal cones.
Post retinal factors also affect visual development. The speed of conduction depends on synapses and the structural properties of axons. When light focuses onto the retina, the optic nerve transports nerve impulses from receptors to the brain. A critical component is the thickness of the myelin sheath which acts as an electrical insulator. It causes the nerve impulses to ‘jump’ from one uninsulated node to the next, resulting in rapid saltatory conduction. However, myelination is incomplete at birth therefore vision is limited and improves during growth.
Axon diameter is limited at birth and continues to grow throughout the first few years. A larger axon diameter means there is less resistance to ion flow, increasing the speed of nervous transmission. Maturation of these regions of the brain undergo conspicuous growth in infancy but may not completely mature until adulthood (suggested by postmortem studies).
Synaptogenesis is also important in neural development. They begin to form prior to week 27 of pregnancy and reach peak density after birth. In late pregnancy as well as early postnatal life, synaptic density increases within the primary visual cortex. Density doubles from 2 to 4 months of age, and declines after the age of one. Along with synaptogenesis, neurons increase their dendritic arborizations, extend their axons and myelination occurs. This is followed by a period called synapse elimination during which the nervous system fine-tunes neural connectivity; which involves eliminating interconnections between redundant or non-functional neurones. This continues for over a decade.
To summarise, vision over the first 12 months experiences a rapid improvement. The most important changes occur in the rapid infantile phase because of emmetropization, retinal factors, as well as post retinal factors involving the visual cortex, myelination and synaptogenesis. These factors combined to give rise to colour vision, improved contrast sensitivity and a higher VA in the first 12 months.
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