DMT, particularly at the lower end of the intensity spectrum, plays off of those phosphene light afterimages in your vision. When you rub your eyes, you are stimulating these phosphenes. I don't think it has as much to do with DMT being processed by the retinas as much as it is a modulation to the experience you can create by doing so. There are many ways to modulate the experience, from breathing to concentration to changes in posture, lighting, sound and so on.