Infant visual development

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A seven-week-old human baby following a kinetic object. Infant vision.jpg
A seven-week-old human baby following a kinetic object.

Infant vision concerns the development of visual ability in human infants from birth through the first years of life. The aspects of human vision which develop following birth include visual acuity, tracking, color perception, depth perception, and object recognition.

Contents

Unlike many other sensory systems, the human visual system – components from the eye to neural circuits – develops largely after birth, especially in the first few years of life. At birth, visual structures are fully present yet immature in their potentials. From the first moment of life, there are a few innate components of an infant's visual system. Newborns can detect changes in brightness, distinguish between stationary and kinetic objects, as well as follow kinetic objects in their visual fields. However, many of these areas are very poorly developed. With physical improvements such as increased distances between the cornea and retina, increased pupil dimensions, and strengthened cones and rods, an infant's visual ability improves drastically. The neuro-pathway and physical changes that underlie these improvements in vision remain a strong focus in research. Because of an infant's inability to verbally express their visual field, growing research in this field relies heavily on nonverbal cues including an infant's perceived ability to detect patterns and visual changes. The major components of the visual system can be broken up into visual acuity, depth perception, color sensitivity, and light sensitivity.

By providing a better understanding of the visual system, future medical treatments for infant and pediatric ophthalmology can be established. By additionally creating a timeline on visual perception development in "normal" newborns and infants, research can shed some light on abnormalities that often arise and interfere with ideal sensory growth and change.

Development

Acuity

Infants' eyes develop significantly after birth. The muscles of the eye such as ciliary muscles - become stronger after two months of age, allowing infants to focus on particular objects through contraction and relaxation. Their retinal images are also smaller compared to adults due to shorter distances from the retina to the cornea of the infants' eye. A newborn's pupil grows from approximately 2.2 mm to an adult length of 3.3 mm. Eyesection.svg
Infants' eyes develop significantly after birth. The muscles of the eye such as ciliary muscles – become stronger after two months of age, allowing infants to focus on particular objects through contraction and relaxation. Their retinal images are also smaller compared to adults due to shorter distances from the retina to the cornea of the infants' eye. A newborn's pupil grows from approximately 2.2 mm to an adult length of 3.3 mm.

Visual acuity, the sharpness of the eye to fine detail, is a major component of a human's visual system. It requires not only the muscles of the eye – the muscles of orbit and the ciliary muscles – to be able to focus on a particular object through contraction and relaxation, but other parts of the retina such as the fovea to project a clear image on the retina. The muscles that initiate movement start to strengthen from birth to 2 months, at which point infants have control of their eye. However, images still appear unclear at two months due to other components of the visual system like the fovea and retina and the brain circuitry that are still in their developmental stages. This means that even though an infant is able to focus on a clear image on the retina, the fovea and other visual parts of the brain are too immature to transmit a clear image. Visual acuity in newborns is very limited as well compared to adults – being 12 to 25 times worse than that of a normal adult. [3] It is important to note that the distance from the cornea at the front of the infant's eye to the retina which is at the back of eye is 16–17 mm at birth, 20 to 21 mm at one year, and 23–25 mm in adolescence and adulthood. [1] This results in smaller retinal images for infants. The vision of infants under one month of age ranges from 6/240 to 6/60 (20/800 to 20/200). [4] By two months, visual acuity improves to 6/45 (20/150). By four months, acuity improves by a factor of 2 – calculated to be 6/18 (20/60) vision. As the infant grows, the acuity reaches the healthy adult standard of 6/6 (20/20) at six months. [5]

One major method used to measure visual acuity during infancy is by testing an infant's sensitivity to visual details such as a set of black strip lines in a pictorial image. Studies have shown that most one-week-old infants can discriminate a gray field from a fine black stripped field at a distance of one foot away. [6] This means that most infants will look longer at patterned visual stimuli instead of a plain, pattern-less stimuli. [7] Gradually, infants develop the ability to distinguish strips of lines that are closer together. Therefore, by measuring the width of the strips and their distance from an infant's eye, visual acuity can be estimated, with detection of finer strips indicating better acuity. When examining an infants preferred visual stimuli, it was found that one-month-old infants often gazed mostly at prominent, sharp features of an object – whether it is a strong defined curve or an edge. [8] Beginning at two months old, infants begin to direct their saccades to the interior of the object, but still focusing on strong features. [9] [10] Additionally, infants starting from one month of age have been found to prefer visual stimuli that are in motion rather than stationary. [11]

Faces

Newborns are exceptionally capable of facial discrimination and recognition shortly after birth. [12] [13] Therefore, it is not surprising that infants develop strong facial recognition of their mother. Studies have shown that newborns have a preference for their mothers' faces two weeks after birth. At this stage, infants would focus their visual attention on pictures of their own mother for a longer period than a picture of complete strangers. [14] Studies have shown that infants even as early as four days old look longer at their mothers' face than at those of strangers only when the mother is not wearing a head scarf. This may suggest that hairline and outer perimeter of the face play an integral part in the newborn's face recognition. [15] According to Maurer and Salapateck, a one-month-old baby scans the outer contour of the face, with strong focus on the eyes, while a two-month-old scans more broadly and focuses on the features of the face, including the eyes and mouth. [10]

When comparing facial features across species, it was found that infants of six months were better at distinguishing facial information of both humans and monkeys than older infants and adults. They found that both nine-month-olds and adults could discriminate between pictures of human faces; however, neither infants nor adults had the same capabilities when it came to pictures of monkeys. On the other hand, six-month-old infants were able to discriminate both facial features on human faces and on monkey faces. This suggests that there is a narrowing in face processing, as a result of neural network changes in early cognition. Another explanation is that infants likely have no experience with monkey faces and relatively little experience with human faces. This may result in a more broadly tuned face recognition system and, in turn, an advantage in recognizing facial identity in general (i.e., regardless of species). In contrast, healthy adults due to their interaction with people on a frequent basis have fine tuned their sensitivity to facial information of humans – which has led to cortical specialization. [16]

Depth perception

To perceive depth, infants as well as adults rely on several signals such as distances and kinetics. For instance, the fact that objects closer to the observer fill more space in our visual field than farther objects provides some cues into depth perception for infants. Evidence has shown that newborns' eyes do not work in the same fashion as older children or adults – mainly due to poor coordination of the eyes. Newborn's eyes move in the same direction only about half of the time. [17] The strength of eye muscle control is positively correlated to achieve depth perception. Human eyes are formed in such a way that each eye reflects a stimulus at a slightly different angle thereby producing two images that are processed in the brain. These images provide the essential visual information regarding 3D features of the external world. Therefore, an infant's ability to control his eye movement and converge on one object is critical for developing depth perception.

One of the important discoveries of infant depth perception is thanks to researchers Eleanor J. Gibson and R.D. Walk. [18] Gibson and Walk developed an apparatus called the visual cliff that could be used to investigate visual depth perception in infants. In short, infants were placed on a centerboard to one side which contained an illusory steep drop (“deep side”) and another which contained a platform of the centerboard (“shallow side”). In reality, both sides, covered in glass, was safe for infants to trek. From their experiment, Gibson and Walk found that a majority of infants ranging from 6 to 14 months-old would not cross from the shallow side to the deep side due to their innate sense of fear to heights. From this experiment, Gibson and Walk concluded that by six months an infant has developed a sense of depth. However, this experiment was limited to infants that could independently crawl or walk. [18] To overcome the limitations of testing non-locomotive infants, Campos and his colleges devised an experiment that was dependent on heart rate reactions of infants when placed in environments that reflected different depth scenarios. Campos and his colleagues placed six week-old infants on the “deep end” of the visual cliff, the six week-old infants' heart rate decreased and a sense of fascination was seen in the infants. However, when seven month-old infants were lowered down on the same “deep end” illusion, their heart rates accelerated rapidly and they started to whimper. Gibson and Walk concluded that infants had developed a sense of visual depth prior to beginning locomotion. Therefore, it could be concluded that sometime at the spark of crawling around 4–5 months, depth perception begins to strongly present itself. [19]

Cues

From an infant's standpoint, depth perception can be inferred using three means: binocular, static, and kinetic cues. As mentioned previous, humans are binocular and each eye views the external world with a different angle – providing essential information into depth. The convergence of each eye on a particular object and the stereopsis, also known as the retinal disparity among two objects, provides some information for infants older than ten weeks. With binocular vision development, infants between four and five months also develop a sense of size and shape constancy objects, regardless of the objects location and orientation in space. [20] From static cues based upon monocular vision, infants older of five month of age have the ability to predict depth perception from pictorial position of objects. [21] In other words, edges of closer objects overlap objects in the distance. [22] Lastly, kinetic cues are another factor in depth perception for humans, especially young infants. Infants ranging from three to five months are able to move when an object approaches them in the intent to hit them – implying that infants have depth perception. [20]

Color vision

Infants are often attracted to shiny bright objects with strong contrast and bold colors. Infant looking at shiny object.jpg
Infants are often attracted to shiny bright objects with strong contrast and bold colors.

Color vision improves steadily over the first year of life for humans due to strengthening of the cones of the eyes.[ citation needed ] Like adults, infant color vision derives from three cone cell types with long-, mid- and short-wavelength opsins that are sensitive to different parts of the visible range. The signals from these cones recombine in the precortical visual opponent process to form a luminance channel and two chromatic channels (red-green and blue-yellow) that comprise an individual's trichromatic color gamut. The number of colors an infant can see is proportional to the size of their gamut, which is proportional to the dynamic range of each of the three channels. These dynamic ranges increase with age, leading to the development of color vision.

It is generally accepted across all current research that infants prefer high contrast and bold colors at their earlier stages of infancy, rather than saturated colors. [23] One study found that newborn infants looked longer at checkered patterns of white and colored stimuli (including red, green, yellow) than they did at a uniform white color. However, infants failed to discriminate blue from white checkered patterns. [24] Another study – recording the fixation time of infants to blue, green, yellow, red, and gray at two difference luminance levels – found that infants and adults differed in their color preference. Newborns and one-month-old infants did not show any preference among the colored stimuli, while three-month-old infants preferred the longer wavelength (red and yellow) stimuli to the short-wavelength (blue and green) stimuli, and adults had the opposite. However, both adults and infants preferred colored stimuli over non-colored stimuli. This study suggested that infants had a general preference for colored stimuli over non-colored stimuli at birth, though infants were not able to distinguish the different colored stimuli prior to the age of three months. [25]

Research into the development of color vision using infant female Japanese macaques indicates that color experience is critical for normal vision development. Infant monkeys were placed in a room with monochromatic lighting limiting their access to a normal spectrum of colors for a one-month period. After a one-year period, the monkey's ability to distinguish colors was poorer than that of normal monkey exposed to a full spectrum of colors. Although this result directly pertains to infant monkeys and not humans, they strongly suggest that visual experience with color is critical for proper, healthy vision development in humans as well. [26]

Light sensitivity

The threshold for light sensitivity is much higher in infants compared to adults. From birth, the pupils of an infant remain constricted to limit the amount of entering light. In regards to pupil dimensions, newborns' pupils grow from approximately 2.2 mm to an adult length of 3.3 mm. [2] A one-month-old infant can detect a light threshold only when it is approximately 50 times greater than that of an adult. By two months, the threshold decreases measurably to about ten times greater than that of an adult. The increase in sensitivity is the result of lengthening of the photoreceptors and further development of the retina. Therefore, postnatal maturation of the retinal structures has led to strong light adaptations for infants. [27]

Vision abnormalities in infants

Vision problems in infants are both common and easily treatable if addressed early by an ophthalmologist.

Critical warning signs

Vision problems

See also

Related Research Articles

<span class="mw-page-title-main">Visual cortex</span> Region of the brain that processes visual information

The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and then reaches the visual cortex. The area of the visual cortex that receives the sensory input from the lateral geniculate nucleus is the primary visual cortex, also known as visual area 1 (V1), Brodmann area 17, or the striate cortex. The extrastriate areas consist of visual areas 2, 3, 4, and 5.

Blindsight is the ability of people who are cortically blind to respond to visual stimuli that they do not consciously see due to lesions in the primary visual cortex, also known as the striate cortex or Brodmann Area 17. The term was coined by Lawrence Weiskrantz and his colleagues in a paper published in a 1974 issue of Brain. A previous paper studying the discriminatory capacity of a cortically blind patient was published in Nature in 1973. The assumed existence of blindsight is controversial, with some arguing that it is merely degraded conscious vision.

<span class="mw-page-title-main">Color constancy</span> How humans perceive color

Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects.

<span class="mw-page-title-main">Phi phenomenon</span> Optical illusion of apparent motion

The term phi phenomenon is used in a narrow sense for an apparent motion that is observed if two nearby optical stimuli are presented in alternation with a relatively high frequency. In contrast to beta movement, seen at lower frequencies, the stimuli themselves do not appear to move. Instead, a diffuse, amorphous shadowlike something seems to jump in front of the stimuli and occlude them temporarily. This shadow seems to have nearly the color of the background. Max Wertheimer first described this form of apparent movement in his habilitation thesis, published 1912, marking the birth of Gestalt psychology.

<span class="mw-page-title-main">Peripheral vision</span> Area of ones field of vision outside of the point of fixation

Peripheral vision, or indirect vision, is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in the "corner of one's eye". The vast majority of the area in the visual field is included in the notion of peripheral vision. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision refers to medium eccentricities, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.

<span class="mw-page-title-main">Visual system</span> Body parts responsible for vision

The visual system is the physiological basis of visual perception. The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system and the neural system.

Dichromacy is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two primary colors to be able to represent their visible gamut. By comparison, trichromats need three primary colors, and tetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked with monochromatic light. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light.

<span class="mw-page-title-main">Visual acuity</span> Clarity of vision

Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sharpness of an image on its retina. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.

<span class="mw-page-title-main">Fovea centralis</span> Small pit in the retina of the eye responsible for all central vision

The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.

Stereopsis is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image because they are in slightly different positions in one's head. These positional differences are referred to as "horizontal disparities" or, more generally, "binocular disparities". Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real three-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".

<span class="mw-page-title-main">Photopic vision</span> Visual perception under well-lit conditions

Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.

In cognitive neuroscience, visual modularity is an organizational concept concerning how vision works. The way in which the primate visual system operates is currently under intense scientific scrutiny. One dominant thesis is that different properties of the visual world require different computational solutions which are implemented in anatomically/functionally distinct regions that operate independently – that is, in a modular fashion.

<span class="mw-page-title-main">Bird vision</span> Senses for birds

Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

<span class="mw-page-title-main">Chromostereopsis</span> Visual illusion whereby the impression of depth is conveyed in two-dimensional color images

Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red–blue or red–green colors, but can also be perceived with red–grey or blue–grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.

Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.

Perceptual learning is learning better perception skills such as differentiating two musical tones from one another or categorizations of spatial and temporal patterns relevant to real-world expertise. Examples of this may include reading, seeing relations among chess pieces, and knowing whether or not an X-ray image shows a tumor.

<span class="mw-page-title-main">Parasol cell</span>

A parasol cell, sometimes called an M cell or M ganglion cell, is one type of retinal ganglion cell (RGC) located in the ganglion cell layer of the retina. These cells project to magnocellular cells in the lateral geniculate nucleus (LGN) as part of the magnocellular pathway in the visual system. They have large cell bodies as well as extensive branching dendrite networks and as such have large receptive fields. Relative to other RGCs, they have fast conduction velocities. While they do show clear center-surround antagonism, they receive no information about color. Parasol ganglion cells contribute information about the motion and depth of objects to the visual system.

Form perception is the recognition of visual elements of objects, specifically those to do with shapes, patterns and previously identified important characteristics. An object is perceived by the retina as a two-dimensional image, but the image can vary for the same object in terms of the context with which it is viewed, the apparent size of the object, the angle from which it is viewed, how illuminated it is, as well as where it resides in the field of vision. Despite the fact that each instance of observing an object leads to a unique retinal response pattern, the visual processing in the brain is capable of recognizing these experiences as analogous, allowing invariant object recognition. Visual processing occurs in a hierarchy with the lowest levels recognizing lines and contours, and slightly higher levels performing tasks such as completing boundaries and recognizing contour combinations. The highest levels integrate the perceived information to recognize an entire object. Essentially object recognition is the ability to assign labels to objects in order to categorize and identify them, thus distinguishing one object from another. During visual processing information is not created, but rather reformatted in a way that draws out the most detailed information of the stimulus.

<span class="mw-page-title-main">Vernier acuity</span>

Vernier acuity is a type of visual acuity – more precisely of hyperacuity – that measures the ability to discern a disalignment among two line segments or gratings. A subject's vernier acuity is the smallest visible offset between the stimuli that can be detected. Because the disalignments are often much smaller than the diameter and spacing of retinal receptors, vernier acuity requires neural processing and "pooling" to detect it. Because vernier acuity exceeds acuity by far, the phenomenon has been termed hyperacuity. Vernier acuity develops rapidly during infancy and continues to slowly develop throughout childhood. At approximately three to twelve months old, it surpasses grating acuity in foveal vision in humans. However, vernier acuity decreases more quickly than grating acuity in peripheral vision. Vernier acuity was first explained by Ewald Hering in 1899, based on earlier data by Alfred Volkmann in 1863 and results by Ernst Anton Wülfing in 1892.

<span class="mw-page-title-main">Davida Teller</span> American psychologist

Davida Young Teller was a professor in the Departments of Psychology and Physiology/Biophysics at the University of Washington, Seattle, Washington. She was a leader in the scientific study of infant visual development.

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