Vision is the faculty of being able to see. The human eye allows us to have a sense of sight, enabling us to learn more about the world than we do with any other senses, hence why it is so crucial to maintain our vision by getting routine checkups. I will discuss many properties of vision development including Visual acuity, contrast sensitivity, the retina, color vision and depth perception. An infant’s vision changes drastically in the first year of their lives (American Academy of Ophthalmology, 2020). Babies are born with very near-sighted vision and can’t focus on items further than 25 cm (10 inches) from their face. Over the next 12 months, a baby's vision will gradually strengthen, improving to near normal (Aboutkidshealth.ca, 2020). Newborns can detect changes in brightness, and distinguish between stationary and mobile objects. Although most visual elements aren’t fully developed, with increases in: distances between the cornea and retina, pupil size, and stronger rods and cones, visual ability improves drastically (En.wikipedia.org, 2020). The axial length of an eye is 17mm from birth which grows to 21mm by age 2. At birth babies are adjusting to light and beginning to focus and see up to 12 inches in front of them(Complete Eye Care, 2020), up to 6 months they’re reaching and tracking for objects, recognizing people and focusing and up to 12 months they’re able to judge distances pretty well and seeing color clearly. (American Academy of Ophthalmology, 2020)
Visual acuity is the ability of the eye to see and distinguish fine details. It can be influenced by a variety of factors such as: color, brightness, and contrast. (wiseGEEK, 2020) Visual acuity uses the muscles of the eye (ciliary muscles and muscles of orbit) as well as the retina and fovea to focus on objects. A newborn’s acuity is 6/133 (measured from a 6 meters distance using teller cards) and develops to 6/6 at 6 months. At 2 months, babies have control over eye muscles movements which become stronger, but they still see unclear images as the fovea and retina are still developing, and acuity improves to 6/45. Due to the small distance between the cornea and retina of 17mm, infants have smaller retinal images. At 4 months acuity improved to 6/18 vision and at 6 months to 6/6 vision.
The diagram shows the relative acuity of the human eye on the horizontal meridian.
In human foveal cone receptors, spacing declines after birth, the increased density if cones contribute to the development in increasing visual acuity. The density of cones at birth is 19 cones/100mm2 to 42 cones/100mm2 in adulthood (Barnard.S, Edgar.D)
A recent study showed that two weeks before birth, cone density distribution was 20–30 x 103/mm2 across the retina. Density increased on a postnatal day 1 around the foveal center and reached a 45–55 x 103/mm2 by day 10. Between days 10 and 33, cone density at the foveal center increased dramatically, reaching 283 x 103/mm2 by 3.5 months and 600 x 103/mm2 by 5.4 months. Peak foveal density then lowered to 440 x 103/mm2 at 6 months and onwards. (Ncbi.nlm.nih.gov, 2020) This further proved that a high cone density (photopic vision) results in high visual acuity.
Another reason why visual acuity improves in infants is due to the development in the ciliary muscles. These develop from mesenchyme and allow infants to accommodate their vision by contraction/relaxation. To be able to measure visual acuity is an important factor in understanding pediatric vision. One way to measure an infant’s vision is to test sensitivity to visual details by using a set of black stripes. Studies show that a child of one week can distinguish a grey field from a fine black-striped field, one foot away, showing that they will look at a simulated pattern rather than a plain background. As vision develops, infants can differentiate strips in lines that are closer together, so by changing up distances between the lines, we are able to measure VA (En.wikipedia.org. (2020)
Contrast sensitivity is the difference in luminance that allows us to distinguish an object, it is a very essential component of functioning vision. It measures the amount of contrast needed to detect the presence of a grating of different spatial frequencies (units’ are cycles/degrees). Sensitivity at its peak is said to be 4 cycles/degree which shifts to higher frequencies with age.
Contrast sensitivity functions for individual subjects with static grating patterns (Ncbi.nlm.nih.gov. (2020).
In a study, contrast sensitivity development was measured using the sweep VEP method from ages 2 to 40 weeks old (48 infants) and 10 adults. The sweep VEP estimate of grating acuity showed a slow increase in spatial frequency with age, in the first month; 5 c/deg and reaching 16.3 c/deg at 8 months. Results indicated that increases in peak sensitivity and spatial resolution lead to increased contrast sensitivity development. At lower spatial frequencies (5 cycles/deg) CS develops at a rapid rate up to 10 weeks after birth. Development of higher spatial frequency and grating acuity sensitivity continues to develop until 40 weeks after birth. (gratings over 15 cycles/deg). Development after 9 weeks is influenced by changes in the retina(fovea) and cortex (Norcia, Tyler and Hamer, 2020) Wilson (1988) suggested that changes in the spatial scale/curve are caused by the movement of foveal cones. This allows the progression of the shift toward increased spatial frequencies, and the growth of foveal cones causes a rise in sensitivity (Barnard.S, Edgar.D)
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The retina is a thin layer of light-sensitive tissue made of photoreceptor cells that is responsible for detecting the color and intensity of light. The purpose of the retina is to receive and convert light into nerve impulses, and send them to the brain via the optic nerve (Healthline, 2020)
From a study on cadaver neonate retinas, researched and performed by Yuodelis and Hendrickson, showed that the diameter of the fovea decreases from 1000 micrometers at birth to 650-700 at 45 months. The main reason why the diameter decreases is due to cones migrating towards the central fovea, increasing its density from 18 cones/100 mm2 in newborns to 42 cones/100 mm2 in adults. Postnatal cone development demonstrates maturation, elongation and increase packing density. Electro-oculography results have shown electrical activity in the cones of newborn babies proving that the cones are functioning despite the immaturity of vision development. The more peripheral regions develop faster than the foveal region; amacrine, bipolar and ganglion cells also migrate as the fovea starts to develop during the first 4 months, as a result, light entering the eye will be detected by a changing array of receptor cells. Myelination of the optic nerve begins and completes for the ganglion cell region by the 6th month after birth (Barnard.S, Edgar.D)
Colour vision is the ability to differentiate among a range of wavelengths of light waves and to see different colors. Colour perception is coordinated by a process between neurons that stimulates different types of photoreceptors by light entering the eye. Those photoreceptors then output impulses via neurons to the brain. Colour perception starts with cells called cones, there are three different types of cones, each of which contains different types of pigments; longer wavelengths, middle wavelengths, and lastly the shorter wavelengths, resulting in trichromatic color vision (Encyclopedia Britannica, 2020) (En.wikipedia.org, 2020)
Colour vision improves at a steady pace over the first 12 months of life, this is due to the strengthening of cones in the fovea. Red is the first color a newborn sees once color vision develops. At one month, babies become sensitive to the brightness of colors and will spend most of their time looking at bold colors and contrasting patterns compared to lighter colors. Vision is limited as nerve cells in the retina that control vision development aren’t developed yet. Around 2 months, a baby is able to tell the difference between two shades of gray that differ by 0.5 percent in brightness, at 4 months babies are able to distinguish an entire range of colors and by 6 months babies are able to see 6/6 vision. (theAsianparent - Your Guide to Pregnancy, Baby & Raising Kids, 2020)
Depth perception is the ability to see the world in three dimensions and to judge the distance of objects. In order to be able to have depth perception, binocular vision is needed. If binocular vision isn't possible(blind in one eye, etc.), then other factors can aid for some sense of depth perception such as: Motion parallax - which occurs when we move our head back and forth, objects move at different speeds depending on their distance, Interposition - When objects overlap allowing those with monocular vision to judge distances of objects and lastly aerial perspective - this is when color and contrast together tell us the distance of an object: when light travels from a distance, it's scattered, this light blurs the outlines of things and our brain interprets this as being farther away.(Verywell Health, 2020)
Newborns aren't able to tell distinguish or see two images. A 1 month can focus on objects 8 to 10 inches from their face however after this vision starts to quickly improve. By 2 months, they can focus on faces, at 3 months babies can follow moving objects and reach for things in front of their face. From months 5 to 9, eye-body coordination improves. From 10 to 12 months, improvements occur in hand-eye coordination and judging distances. The development of depth perception occurs around the time infants start crawling (Hello Motherhood, 2020)
From research, it has been proved that the strength of eye muscle control has a strong positive correlation with depth perception.
Researchers Gibson and Walk developed a study in which an apparatus called the Visual Cliff to test depth perception in infants. Infants were placed on a surface that contained a steep drop and shallow illusion on either half of the surface. In fact, both sides were covered in glass making it a plane surface allowing infants to crawl or move safely. From their experiment, it was found that most infants from 6–14 months would not cross from the shallow side to the deep side due to fear of heights. Gibson and Walk concluded that by six months an infant has developed a sense of depth.
To show that depth perception occurs before infants start to crawl, 1.5-month-old infants were placed on the deep end of the visual cliff, the 1.5-month-old infants' heart rate decreased, and interest was shown by the infants rather than fear. However, when repeated with six-month-old infants, their heart rates increased rapidly and showed signs of strong discomfort. Therefore, it could be concluded that sometime around 4–5 months, depth perception is developed fully (En.wikipedia.org, 2020).
In conclusion, at birth, pupils can’t fully dilate; lens curvature is practically spherical; the retina is undeveloped, and the infant suffers from astigmatism and mild farsightedness. The baby has poor fixation ability and is unable to discriminate color. By 3 months, control of head movements has improved, and is attraction occurs for colored targets as well as acknowledging smaller targets. They are also able to associate objects with an event (e.g., the bottle and feeding). From 5-6 months, infants have the ability to see an object in their hands, the infant is aware of their surroundings and can look around easily; the infant can observe objects from close distances and can accommodate the eyes to do so. Between 6-9 months acuity improves to almost mature levels. Between 9 months to 1 year, the infant can spot small monuments nearby and watches faces trying to imitate expressions. Infant is very alert to new people around them. They can also recognize familiar and unfamiliar people. (Encyclopedia on Early Childhood Development, 2020)
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