In the interest of contributing to the communal knowledge available on the Internet, here is a paper I wrote, for my class on visual optics at the University of Arizona.  I hope it may be of use to anyone doing web-based research on infant eye development.  I have included my refererences at the end.

In this paper I summarize:
    - how human vision develops in the first few years of life
    - how researches collect data
    - common abnormalities in infant vision.

Enjoy,
-Tim Miller
tmiller@optics.arizona.edu


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Infant Eye Development
OptSci 535, Visual Optics
April 30, 2002


Infants, as any parent knows, do not spring from the womb with full adult vision.  In fact, it can take years for a child to develop his visual system to near-adult levels.  Clearly as a child grows from birth, the body is still immature, and the eyes and brain have also not yet fully developed.  But it may be asked how a child’s visual system develops, and what factors limit vision as he ages.

The answer is complex, of course, and depends upon the many components in the human visual system.  In fact, even today there is debate about whether the undeveloped optics and photoreceptors of the eye, or the undeveloped neural pathways of the brain, are the primary cause of imperfect vision in the first years of life.  But it is clear that both the physiological and the neural systems are immature after birth, and take years to develop fully.  The brain’s growth and the development of intelligent perception is still a largely unknown field, but the growth of the eye is a little better understood; I will be focusing mostly on how the physical eye develops.


Testing Methods

An age-old difficulty in the field of infant vision science is of course the question of how to take useful data, given that the human subjects can’t talk, follow instructions, or even be at all cooperative.  Fortunately a variety of methods have been developed to get around this problem.

As early as six months, the experimenter can take advantage of a visual reflex of babies: when presented with an interesting high-contrast object, a baby will usually turn his attention to look at it.  Therefore the method of forced-choice preferential looking (FPL) has been developed.  A typical experiment may present a baby with two patterns: one a bar pattern with a certain contrast and spatial frequency, and the other an equivalent solid gray.  The baby will often turn to look at the more-interesting high-contrast pattern, and so using statistics it can be deduced to what extent a baby can see contrast and detail, that is, a CSF measurement.

The experimenter can also take advantage of the baby’s interest to test field of view: the baby’s attention can be drawn forward with a bright colorful object, and then another object can be slowly brought around from behind the baby, until the baby turns to look.  In this way can also be deduced what field of view the baby can see.

Another method removes entirely the reliance on a child’s visual reflex: “visually evoked cortical potentials” (VECP, or VEP).  In this method, the child’s actual brain waves are measured, by attached electrodes to the child’s scalp over the occipital cortex, the part of the brain where vision processes take place.  A reference electrode is usually placed elsewhere on the head as well.  This way, brain activity in response to visual stimuli can be directly measured.

For children older than two, there are methods to directly measure the eye’s refractive power.  However, the child is still too young to reliably follow instructions, so the accommodation of his eye must still be disabled by using eyedrops.

However, by the time a child reaches the age of four, he is old enough to follow instructions, describe what he sees, and generally interact, so that most traditional adult vision tests can be used.


Early Development of the Eye

Given these and other methods, opthamalogical researchers can study the limitations of the infant’s eye, and what changes occur in the years after birth.

At birth, the eye is far from perfect, and vision is hazy at best.  In general, a newborn can recognize motion, and can see only large or close object, that are high-contrast.  An immediate reason for this is the immaturity of the photoreceptors: the foveal cones in a baby’s eye are spaced out four times as much (in each direction) as in an adult eye.  Furthermore, the collecting area of infant foveal cones is about 25% less, as well. Therefore the infant’s fovea collects about 350 times less light than an adult.  Likewise, the rods of the eye, while fairly developed, have about one-tenth of the sensitivity of the adult set of rods, so scotopic vision is also very limited.

Color vision at birth is also pretty much nonexistent.  Newborns can only react to luminance, and can’t yet distinguish between colors and grayscale.

In the first few months after birth, the muscles that control eye pointing are still developing, so the eyes tend to wander, or even cross.

It should be noted that the visual cortex of the brain itself is also immature at this time.  The brain’s dendrites are still growing, which limits contrast sensitivity and color recognition, even if the eye were providing ideal information to the brain.  Continual visual stimuli and time will develop the neural connections, as the months progress.

After two months, the baby’s eye has improved.  The rods and foveal cones have developed, but acuity is still quite limited: roughly speaking, the baby’s eye has the equivalent of adult 20/200 vision.  However, color vision is starting to develop.  The four types of photoreceptors (the rods, and three color cones) are in place, and the red/green channel of the neural connection is working, so that the baby can distinguish some colors from grayscale.

At three months, the eye has made its most progress, and the infant can now see colors, motion, and details.  Color vision is now fully trichromatic, and the retina has developed such that the eye has the equivalent of 20/100 adult vision.  (There is evidence that the limiting factor here is not the retina, but that the neural system cannot yet encode high-frequency spatial patterns.)

Furthermore, the eye muscles have developed so that the baby’s two eyes can move together, and follow a moving object.

At this point too, the baby begins to respond to depth.  The ability to accommodate starts to develop, and the child gains experience in judging distance.

After six months, great progress has been made.  The eye has reached two-thirds of adult size.  Acuity has improved so that the eye has about 20/50 adult vision.  The eyes can coordinate for good binocular vision, and have good motion detection.  Finally, depth perception has improved, and the ability to accommodate is near to adult-quality.

At one year, the fovea is largely developed, and has a higher density of cones; the foveal cone spacing is now only 1.8 times as that found in an adult.  The neural connections have also made progress, as hand-eye coordination is being developed.  At this point, the majority of the eye’s post-birth development on the way to adulthood has been done.

At four years after birth, the eye’s visual acuity reaches adult levels.  And by six or seven years of age, contrast sensitivity and peripheral vision have reached adult levels.

There are a few long-term growth changes that occur along with those mentioned above.  The eye itself grows along with the rest of the body, and thus the length of the eye increases over the years until it reaches adult size.  The consequence of a child’s shorter eye length is that the eye’s magnification is proportionally less, and so the image projected onto the retina is smaller, and thus resolution suffers.  However, in terms of image quality, everything in the eye grows together, so decent acuity can be achieved well before the full size of the eye is reached.  Likewise, the pupil does grow over the years, but its increase in diameter roughly corresponds to the increase in the eye length, so that numerical aperture, and therefore retinal luminance, is mostly conserved.

It should be noted that premature babies may have difficulties in their visual development.  Babies born more than a month early will develop vision later, and are also at a higher risk for infant eye problems such as strabismus and amblyopia.


Infant Eye Problems:

The eyes after birth are going through major changes, and are susceptible to a particular set of problems because of their rapid development.  These include strabismus, amblyopia, and cataracts.  These abnormalities are fairly uncommon, but since early detection is difficult because of the problems mentioned above in measuring uncooperative infants, special care is taken to watch out for them.

Strabismus refers to a misalignment in the pointing of the eyes.  While wandering eyes are common right after birth, enough muscle control usually develops within the first two months that the problem goes away naturally.  However, if the eye muscles that control eye pointing are unequal in strength, or if one of the muscles is nonfunctional, then the infant will have difficulty developing binocular vision.  This will result in the child seeing two images shifted from each other, which could lead to amblyopia.  Strabismus may be treated by using an eyepatch over the good eye to force the bad eye to develop (which may take weeks or months); applying visual therapy and special eye exercises; or in the extreme, surgery to fix the eye muscles.

In amblyopia, vision in one eye is poor enough that it becomes unused by the brain.  In the extreme case, the brain ignores the input from that eye entirely.  This may be caused by any problem in one eye, such as low acuity caused by a large refractive error, or by strabismus making the use of both eyes difficult.  If this is caught early, it can be treated by fixing the bad eye, either with an eyepatch, or glasses to fix refractive error.  If amblyopia is not caught early, it is difficult to retrain the neural system, which has learned over a long time to ignore the bad eye.

Cataracts are uncommon in infants (somewhere between one in 4000 and one in 10000), but can still occur, caused by gene abnormalities.  The cataracts differ from those in adults; infant cataracts tend to be white, and localized in the crystalline lens, but still block light in the eye.  The treatment is the same as in adults: the damaged lens is removed by surgery.  To compensate for the missing lens, a synthetic intra-ocular lens may be inserted, or glasses may be prescribed.  The eye will take time to recover, but its recovery can be forced by use of an eyepatch.  Full use of the eye may be recovered in weeks or months.

These infant eye problems, if caught within the first several years of life, can often be corrected and full vision eventually recovered.  The first three to four years are critical, since this is when the brain requires constant visual stimuli.  If problems like these are not caught quickly, then the neural development may be slowed for lack of visual input, and lead to irreparable abnormalities for the rest of the child’s life.


 

References:

Martin Banks, Patrick Bennett, “Optical and photoreceptor immaturities limit the spatial and chromatic vision of human neonates.”  J. Opt. Soc. Am. A, Vol. 5, 2059-2079

http://www.yorku.ca/eye/preferlk.htm : academic tutorial site, excellent summary of infant testing methods

http://www.psy.vanderbilt.edu/courses/hon185/SpatialFrequency/SpatialFrequency.html :  academic tutorial site, explains infant eye acuity, and testing methods

http://www.pbs.org/wnet/brain/episode1/index.html :  website for PBS series, “The Secret Life of the Brain”, excellent layman’s summary of vision changes over first year of life, also cataracts.

http://www.eyes.arizona.edu/IVL/labpage.htm :  laboratory page of U. of Arizona research center, explains lab methods

http://www.allaboutvision.com/parents/infants.htm :  consumer information site, good summary of infant abnormalities, also good glossary

http://www.careforyoureyes.com/article174598-18198.html :  consumer information site, good summary of vision changes during first year

http://research.opt.indiana.edu/Labs/428/428.html : academic site, brief summary of post-birth vision