Why Some people Cannot View Stereograms

The basic principle behind a stereogram depends on the ability for a person to merge multiple objects into one. There are many factors that could inhibit a person's ability to see the object hidden beneath the initial surface. As an example, place your thumbs and index fingers together in the shape of a triangle, and find an object in the distance to look at through the triangle. It must be a distant object! Independently close one eye, then the other.  You will notice that you were only able to see the complete object through one eye without moving the triangle. This concept is called eye dominance. Sometimes the brain will shut off one eye, and rely on the dominant eye, if the object to be viewed doesn't come in focus. There are many people who need corrected vision that don't realize it because of the human ability to adapt.

Another possible cause is convergence. At close range (12-18 inches), your eyes converge an average of 3mm...but this is not so for everyone. Just as you have a dominant eye for distance, you will also have a dominant eye for near. After years of depending on one eye to do the majority of the work, you can depend less on convergence and adaptability. By allowing your eyes to relax, and blur, you attempt to override your brain's intent, and allow both eyes to be equally dysfunctional. Thus, images begin to overlap and the muscles that control your eyes are less likely to fix at the same point that they normally would. The same people that have problems crossing their eyes, even a little, will have the greatest frustration when attempting to view stereograms.

Other conditions such as esophoria and esotropia can cause the eye to move inwards (commonly called a lazy eye), and exophoria and exotropia (a tendency for an eye to shift toward the temple), will have an impact. A suggestion is that people could adjust their distance from the stereogram and have a greater rate of success. A distance that works for the majority doesn't always work for everyone.  Also, it would be interesting to relate the ability of a person to see stereograms to their profession, in regards to their analytical/artistic mindset.