Vision

Testing Methods

Two main methods have been used to test babies’ vision: preferential looking and habituation. Preferential looking relies on the fact that even newborns will look longer at a pattern than at plain gray—if they can see the pattern. They will also look longer at one pattern than another if they can tell the difference between them and naturally prefer one to the other. When infants show no natural preferences, another technique, habituation, is used to determine whether two patterns can be discriminated. For the habituation technique, babies are shown one pattern over and over again until they get bored with it. Then, they are shown a new pattern and the old pattern. If babies can tell the difference between the two, their looking time will increase to the new pattern but not to the old one.

As soon as children are old enough to give reliable verbal responses, typically at about 5 years of age, their vision can be tested using psychophysical techniques adapted from those used for testing adults, namely, asking the participant to name or choose the correct alternative.

Vision at Birth

Newborn infants can see, but their acuity is 40 times less that of adults. They have very limited peripheral vision and no binocular vision—each eye seems to work independently, seeing a bit off to the same side but nothing past the nose. Newborns’ sensitivity to contrast is about 50 times less than that of adults, but they do have some color vision: They can discriminate gray from red, yellow, green, orange, and turquoise, providing the stimuli are sufficiently large and highly saturated. They show no evidence of discriminating gray from blue or purple or chartreuse, colors that depend on the less mature receptors feeding into the blue/yellow pathway than on the more mature receptors feeding into the red/green pathway.

Newborns also can discriminate a 90° difference in orientation, providing evidence that the visual cortex is functional at birth because that is the first area in the visual pathway capable of orientation discrimination. They can differentiate some geometric shapes, such as a circle from a cross or a triangle, but not if the shapes have an identical frame around them. Thus, the visual cortex is functional when it comes to discriminating orientation but not when it comes to discriminating shapes inside identical frames. Newborns can also recognize their mother based on visual cues alone, but this discrimination is based solely on differences in the hairline and not on differences in the facial features. Finally, newborns show no evidence of discriminating even large differences in the direction of motion. However, they already can perceive biological motion, a type of motion depicted by point lights on the key joints of a moving animate organism, and one that requires not only the integration of the moving dots but also deciphering form from them.

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