I saw this on CNN’s Web site today and thought it was interesting. I knew babies didn’t see as well as the rest of us, but I never knew exactly how vision development worked or realized that even toddlers don’t have the same visual acuity as older children or adults. Although the article discusses how cataracts in infancy can impact vision development, it would be interesting to learn how other vision impairments and the approximate time they develop impact vision. (If the impairment develops at 3 or 6 months, for example, how does that impact future vision as compared to impairment from birth as discussed in the article?)
The original post is available at http://thechart.blogs.cnn.com/2011/09/19/learning-to-see-how-vision-sharpens/?hpt=hp_bn6
Editor’s Note: Sandra Aamodt and Sam Wang are the authors of Welcome to Your Child’s Brain, a guide to what’s really going on in the mind from conception to college.
Babies are born nearly blind. You may think that your newborn is gazing into your eyes, but what she actually sees is a vaguely face-shaped blur, associated with loving sounds and possibly milk. How she develops mature vision is mostly automatic, requiring involvement from you only at a few key points.
Though vision feels seamless, the brain constructs its image of the world from neural activity in dozens of interconnected regions that specialize in particular aspects of seeing. All these cortical areas are immature at birth, so babies’ acuity starts out forty times worse than adults’ and doesn’t become equal until four to six years of age. Indeed, an adult who could see as well as a newborn would have 20/600 vision.
The champion of the infant visual system is motion, which develops early and is relatively difficult to disrupt. At four weeks of age, babies can detect a flickering stimulus in a single location almost as well as adults. Babies can discriminate motion direction around seven weeks and speed by twenty weeks. Perception of global 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, autism, fragile-X syndrome, and Williams syndrome.
From birth, babies are attracted to faces. 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 is preferentially activated by faces already in two-month-old infants.
Visual experience influences the development of face-recognition expertise starting in infancy. Six-month-olds are as good at distinguishing individual monkeys as individual people, but by nine months, babies become better at identifying people and lose the ability to recognize monkey faces. During this period, babies also become better at distinguishing faces within their own racial group than within other racial groups, probably because most babies have more visual experience with their own group than with others. This process probably involves the sculpting of synaptic connections by experience to tune perception to the characteristics of the local environment.
Children whose vision is impaired by cataracts demonstrate the need for visual experience in the development of seeing. 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. With experience, their acuity improves, but deprivation for the first three to eight months leads to acuity more than three times worse than normal at five years of age. Global motion perception is affected by cataracts only in the first three months of life.
So under most conditions, development of vision goes fine—though parents do need to watch out for exceptional cases needing further attention. Routine well-baby exams should catch most issues of this sort, but early treatment of any sensory deficit, if possible, will help to minimize later difficulties. Luckily, problems in visual development are the exception. In most cases, parents get a free ride and can simply sit back and watch their child’s new abilities grow.