Double vision
When a patients looks at the point located between the two circles and behind them, the patient sees four circles: two real circles and two vertual circles at some distance from the real circles.
Binocular Vision
if the patient, while looking at the point located between the two circles and behind them, will move the real circles closer to each other, the two vertual circles will combine in one circle and the patient will see three circles: two real circles and one vertual circle behind them.
In the exercize the patient should look between the two objects and behind them to get three objects: two real and one vertual. For exercize, the patient should slowly change the distance between the circles by pressing the left or the right arrow key on the keyboard, while keep seeing these three objects.