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Visual Development & Stereopsis

An explanation of how vision develops in small children, with videos of a baby at 4 and 12 weeks showing markedly different patterns of visual behaviour.

Video Transcript:

A newborn baby will have relatively poor vision, equivalent to the top line on a visual acuity chart. The reason for this is that most newborn babies are markedly longsighted and the focusing mechanism of their eyes is not fully developed.

They are also unable to control their eye movements, so it is not at all uncommon for babies less than a month old to have a squint, or appear to have somewhat roving eye movements. However, by the sixth week of life these focusing and eye movement mechanisms are starting to mature and the connections between the eyes and the visual parts of the brain are also beginning to develop. By the 8th-10th week of life babies should be starting to smile at familiar faces and be able to “fix” their eyes on an object of interest and to “follow” this object if it moves from side to side.

As a result of continued visual stimulation the connections between the eyes and the brain, which are essential to the development of normal vision, are constantly forming between 4 weeks and 8 years of age. This is known as the “critical period” for visual development and anything that interferes with visual stimulation of one or both eyes during this period, will cause reduced vision.

Stereopsis is the ability to see objects in 3 dimensions. Each of our eyes sees a slightly different view of our surroundings and our brain joins the two images and interprets them as a 3D shape. In order to appreciate stereopsis we must have good vision in both eyes and have the ability to keep our eyes aligned together.

We can test stereopsis in the clinic in a number of different ways, either by giving each eye a slightly different view of a flat image by using red/green glasses, polaroid glasses or cylindrical grating or by presenting an actual image in depth. Children will often show a positive response to a stereo test at about 12 months of age.

Just as there is a “critical period” for the development of vision, there is also a “critical period” for the development of stereopsis, which begins at 3 months of age and continues until the age of 6-7. During this period if a child develops amblyopia and/or a squint they can very rapidly loose their stereopsis and this is why it is important to treat amblyopia and correct a squint as quickly as possible. Unfortunately if the squint has been present for some time or if the amblyopia is severe it may not be possible to restore a child’s stereopsis.

Does it matter if you do not have stereopsis?

5% of people do not have stereopsis and the vast majority of them are not affected in any way by this lack of 3D vision, as they have learnt to compensate for this relative lack of depth perception.

Children and adults who do not have stereopsis can still judge depth using clues from one eye, these include the overlapping of objects, the change of focus necessary between objects at varying distances, parallax and colour change - the further away objects are the bluer they appear. By using these strategies to assist in the judgment of depth they are able to perform equally well in situations where depth perception is used i.e. catching a ball.

Fortunately there are very few occupations which require a high degree of 3D vision, such as being a pilot or an eye surgeon.