At the age of three years

TNO-test is one of many stereo tests. It often works well as an introduction to the test situation if a child is shy like this girl. The child first shows where she sees two balls without the special red-green lenses, then whether she sees more balls through the glasses. To test further requires telling where a piece is missing in a circle. This is easier if the child places a piece of cardboard with the same form on the test picture.