Being primates we have forward directed eyes and so large parts of the visual field of one eye overlap with the other. If we keep our head still and compare what we see when we open first one eye and then switch to the other, we can compare what each eye sees. If you repeat that comparison and then attend to objects that near and far you will notice that the nearer objects appear to move more: the view seen by each eye is increasingly different the nearer that objects are.
That disparity in the views is the basis of our ability to judge the distance to objects, giving us our depth perception. When we are born our brains have not developed the neural machinery to make use of disparity cues from our two eyes to give us depth perception. There is a critical period between birth and about two years when the depth perception circuits are mainly wired up in our brains. Some refining of that can occur later, but less and less the older we are. Amblyopia is when the wiring process goes wrong, leaving people with poor to no depth perception. This occurs when vision in one or both eyes is poor compared to normal during the critical period. Amblyopia can also cause poor pattern recognition, poor visual acuity, and low sensitivity to contrast and motion. Between 2 and 5% of the population of Western countries have Amblyopia.
For normal depth perception the foveas of our two eyes always need to be aligned to whatever object we are looking at. If a child has an eye-alignment error amblyopia can occur. This condition is easy to recognise because one eye of the child appears to be misaligned, some or all of the time. This is incorrectly called a lazy-eye. A more insidious issue is where the eyes are aligned but the vision in one eye is poor relative to the other. This can occur by mis-development of the clear focusing parts of one eye (Figure 1) or something like a cataract making vision of one eye foggy. Children tend not to notice that one eye is working poorly: their good eye covers for the other. So unless a professional checks a child’s eyes by age 2 to 3 years, amblyopia can occur.
Traditional treatment involves correcting the issue, mis-matched gaze direction or poor visual clarity, and then covering the good eye with a patch for several hours per day. This makes the brain attached to the formerly affected eye work harder. This wakeup-call seems to help build depth perception when the patch is off. With the advent of low-cost headsets for 3D virtual and augmented reality the opportunities for controlled doses of disparity and other cues to train the depth perception system are becoming more practical. Vision ACTion and its partners are working in this field to correct amblyopia, and Public Health programs to prevent it.