Welcome to Your Child's Brain: How the Mind Grows From Conception to College (17 page)

Though vision feels seamless, your brain actually constructs its image of the world from the neural activity in dozens of interconnected regions that specialize in particular aspects of seeing. These regions are organized into two main
pathways. The “where” pathway, which develops earlier, consists of the cortical areas that process motion and space. The “what” pathway is made up of areas that evaluate the properties of objects, including their shape, color, and patterning. Both pathways obtain information from a chain of connections that start at the retina, pass through the thalamus, and go to the primary and secondary visual areas of the cortex. From there, the two pathways diverge, involving different parts of the cortex, but with plenty of crosstalk between them.

All these cortical areas are immature at birth, making the vision of newborn babies quite poor. Babies don’t see what we see. Newborns mostly rely on sub-cortical pathways, from the retina to the
superior colliculus
, a midbrain region that controls visual-motor reflexes (like flinching away from an incoming object) and certain types of eye movements.

When the visual cortex starts to mature in the second month of life, it takes control from the subcortical pathways. This transition often does not go smoothly. At this age, many babies show
obligatory looking
, the inability to pull their gaze away from something that has caught their attention, sometimes for as long as half an hour. This difficulty is caused by the visual cortex inhibiting subcortical eye movement commands. Young babies track movement with jerky eye movements called
saccades
until two or three months of age, when cortical maturation allows them to smoothly follow a moving object with their eyes. In the first three months, babies also have difficulty focusing their eyes on faraway scenes, so they look at what is nearby (roughly seven to thirty inches away), which includes their own bodies and their parents’ faces.

The champion of the infant visual system is motion, which develops early and effectively. Babies can detect a flickering stimulus in a single location almost as well as adults as early as four weeks of age, and the flicker frequencies that they can detect become adultlike by two months. To determine motion direction, it’s necessary to associate time-based changes coming from different locations in space, a capacity that appears around seven weeks. By twenty weeks, babies can discriminate different speeds of motion. Perception of large-scale motion patterns, like raindrops seen through the windshield of a moving car, improves rapidly between three and five months and then continues to develop slowly through middle childhood. This aspect of motion processing, the most vulnerable to disruption, is impaired in some developmental disorders, including dyslexia and autism.

PRACTICAL TIP: OUTDOOR PLAY IMPROVES VISION

The stereotype of the nerdy guy with glasses has some basis in fact. Myopia (or nearsightedness) has the curious quality of being both inherited (with a heritability of about 80 percent) and strongly influenced by the environment. How this happens is a lesson on the complex ways that genes and environment can interact.

Myopia occurs when the lens of the eye focuses the visual image in front of the retina, causing faraway objects to look blurry. The incidence of myopia varies tremendously across populations, from 2–5 percent among Solomon Islanders in the 1960s to 90–95 percent among modern Chinese students in Singapore. The rate of myopia has increased considerably over the past few decades in many countries. In Israel, 20 percent of young adults were myopic in 1990, increasing to 28 percent in 2002. Similarly, in the U.S., myopia rates went up from 25 percent in the early 1970s to 42 percent in the early 2000s. These changes have been happening so fast that the explanation couldn’t possibly be strictly genetic—some external factors must be involved.

As your child’s eyes grow, the distance between the pupil and the retina needs to be matched to the focusing power of the lens to keep the image on the retina sharp. If this distance is wrong, either myopia or farsightedness results. Based on experiments with animals, we know that visual experience guides this process.

Children who spend more time outdoors are less likely to become myopic. One study compared six- and seven-year-old children of Chinese ethnicity living in Sydney, Australia, with those living in Singapore. The rate of myopia was more than eight times lower in Sydney (3.3 percent) than in Singapore (29.1 percent), despite similar rates of parental myopia (about 70 percent in at least one parent). Children in Sydney spent fourteen hours per week outside, on average, compared with three hours per week for children in Singapore.

It does not seem to matter exactly what the children do while they are outside. A U.S. study found that two hours per day of outdoor activity reduces the risk of myopia by about a factor of four compared with less than one hour per day. Playing indoor sports has no effect on visual development. Outdoor activity has a stronger protective effect for children with two myopic parents than for children with no myopic parents, suggesting that myopia-related genes may modify children’s sensitivity to environmental influences (see
chapter 4
).

Researchers are not sure why being outdoors protects children against developing myopia, but one possible explanation is that bright outdoor light is more effective than dim indoor light at driving the development of correct pupil-retina distance. Since our brains evolved under conditions in which every child spent many hours outside every day, it makes sense that our eyes may develop in a way that takes advantage of that common experience. Our current lifestyles may well lead to other unexpected consequences of this sort, as our brains are forced to adapt to a world that’s very different from the one in which their genes originated (see
Speculation: Modern life is changing our brains
).

The vision of infants is partly limited by the maturity of rods and cones, which translate light into neural signals in the retina. Cones, which provide sensitivity to color, mature rapidly. Though color vision is almost absent in newborns, four-month-old babies can see color as well as adults. Rods, which do not transmit color but detect photons in low light (which, incidentally, is why you can’t make out colors in the dark), mature by six months. Newborns can see better in their peripheral vision than in the center of their gaze, both because the cones in the peripheral retina are more mature and because cells in this part of the retina project more strongly to subcortical visual areas.

Visual acuity is easy to test because babies prefer to look at patterns. Researchers can tell whether babies can distinguish a pattern from solid gray just by whether they preferentially look at the patterned object. At three months of age, babies are still fifty times less sensitive to contrast than adults, meaning that infants find it very difficult to distinguish different shades of gray. It’s like they’re seeing the world through dense fog (see figure below). These limitations explain why the most popular toys for young infants have bold black-and-white patterns.

For depth perception, both eyes need to work together. It’s very difficult to thread a needle, for instance, with one eye closed. The two eyes don’t see exactly the same part of the visual world, and the differences between them depend on head size. The brain uses visual experience to sort this out as children grow.

Depth perception is almost nonexistent in newborns. The ability to use binocular cues develops abruptly, often in the fourth month of life.

An adult who could see as well as a newborn would be legally blind, with 20/600 vision.

From birth, babies are attracted to faces. It may be no coincidence that babies can focus best on objects about eight inches away, which is approximately the distance between the baby’s eyes and the parent’s face during feeding. Very young babies, though, are working from an approximate model of what a face looks like, as they will look at almost any round thing that has two “eyes” and a “mouth” in the right place. (This is not very surprising if you consider how poorly they see real faces.) By four or five months, their preferences are more realistic, and babies have begun to process faces differently from other objects. This change probably reflects maturation of the
fusiform face area
, a region in the temporal cortex specialized for face processing. This brain specialization enables ordinary adults to beat the world’s best computer programs in detecting subtle differences between faces. The fusiform face area appears to already be preferentially activated by faces in two-month-old infants.

Development of many visual functions requires experience during a sensitive period (see
chapter 5
). Early in cortical development, chemical cues direct axons from each visual area to innervate its appropriate target areas, where they form many more synapses than will be needed in the adult brain. Neural activity patterns then control axon retraction and synapse elimination, which fine-tune the connections so that the correct neurons talk to each other. In the primary visual cortex, for example, synapse number is greatest at eight months and then declines through middle childhood. Because different brain areas develop at different ages, the effects of visual deprivation vary with timing.

Children whose vision is impaired by cataracts provide information about the need for visual experience in human development. Babies who have cataracts from birth retain the poor acuity of newborns until their eye function is surgically restored, even as late as nine months of age. After that, with experience their acuity improves, but it does not catch up fully. Deprivation for the first three to eight months leads to acuity more than three times worse than normal at five years of age. Children who develop cataracts later on, starting between four months and ten years and lasting for two to three months on average, also end up with permanent deficits in acuity but are not as impaired as babies whose cataracts were present from birth. Global motion perception is affected by cataracts only in the first three months of life.

SPECULATION: MODERN LIFE IS CHANGING OUR BRAINS