The cross exercise

I have an interest about knowing the source from where the author of TFoC got this exercise.

In this exercise only an image and your eyes are needed. It is said it could be helpful to us.

I think crossed sight keeps cerebral hemispheres busy, and inducts us to see and focus what our normal sight can't notice.

Also, It is said that it helps to improve the communication between cerebral hemispheres. If you want to know what I am talking about, you can go here.