A 3D stereogram is an image hidden in another image. Those who passed the exam for the driving licence, can have an idea about this, because they have tests examining accuracy and visual attention.
Still, 3D stereograms are quite different. They exam the ability of a person to emerge multiple objects into one. It's not the attention but the technique of viewing, that makes a 2D image to transform suddenly in a 3D one...like a miracle I would say.
Even if I discovered the stereograms just a cupple