One of the most common measurements of “how well a person sees” is visual acuity. For older children and adults, optometrists typically use a letter chart to determine the smallest letters the patient is able to recognize. Just about everyone is familiar with this concept of 20/20 visual acuity. But how can you measure visual acuity in an infant? Obviously the baby is not going to be reading the letters on a chart 20 feet away.
Another way to measure visual acuity uses black and white stripes. As the stripes become skinnier and more closely packed together, it becomes more difficult to resolve the stripes. They begin to blend together and look like a uniform grey space. Vision scientists studied this phenomenon in infants and developed a method called “Forced Preferential Looking” that is now used to measure visual acuity in babies.
Babies love to look at interesting things; human faces, toys, and complex patterns. If you show a baby a card with black and white stripes on one side and grey space on the other, they usually prefer to look at the stripes because it is more interesting than the empty grey space. This assumes that the baby can tell the difference between the stripes and the grey space. If the stripes are too close together and the baby cannot resolve the stripes, both sides of the card will look the same and the baby will no longer have a preference to look at the stripes.
This is the science behind the use of Teller Acuity Cards. The optometrist will show the baby a card with big fat stripes on one side and grey space on the other and watch to see if the baby “prefers” to look at the stripes. Here is a great photo of these cards in action. If the baby does prefer the stripes, then the baby is shown another card with slightly skinnier stripes. This procedure is continued until the baby no longer shows a preference for looking at stripes versus the grey field. The stripe width of the last card where the baby seemed to have a preference is converted to a visual acuity measurement.
Although the stripe width is typically converted to an acuity measurement such as 20/20 or 20/100, these measurements are not equivalent.
There has been a considerable amount of research using this technique to help us interpret the results. This method seems to work best for babies from 6 to 12 months of age. We know what visual acuity measurements to expect for babies at different ages and this will allow the doctor consider whether your baby sees well or may have a visual problem. Of course, the earlier a problem is detected, the earlier treatment can be initiated. So be sure to take your baby to see an optometrist during the first year of life!