What is stereopsis? Why 3D movies fall flat for some people

The two eyes sit about six centimetres apart, so they see slightly different pictures of the same object. The brain merges the two and computes distance from the difference -- that ability is stereopsis, the highest level of binocular cooperation.

Stereopsis is not a simple yes-or-no; it comes in grades of fineness, measured in seconds of arc -- the smaller the number, the subtler the depth difference you can resolve. Eye clinics usually test it with random-dot patterns or a stereo test book.

Strabismus, amblyopia or a large gap between the eyes can leave the brain relying on one eye for years; fusion never forms, and stereopsis cannot develop on top of it. Finding 3D movies unimpressive, or always misjudging the pour into a cup, can be clues -- not proof, which only an examination gives.

Stereopsis matters wherever distance must be judged precisely: threading a needle, catching a ball, taking stairs, reversing a car -- and some occupational medical checks require it.

Research shows stereopsis can improve with training, reported in children and in some adults; stereopsis training usually comes after anti-suppression and fusion have laid the groundwork, step by step.

To learn your own level of stereopsis, a standard test at an eye or optometry clinic gives the answer, and the test itself takes only minutes.