But other of Wenger's exercises get you to visualize specific things like a door that you then go through... They are not image streaming though...
The image streaming he taught was to relax and recall a past vivid experience to another person or a tape recorder. Recall everything about the experience, access all the memory of all of your senses, relive the experience as much as you can, and SAY IT ALL out loud. This makes the brain hemisphere link work harder, which increases memory ability and is supposed to increase IQ by .8 points per 20 minutes of practice. He quoted some study on this, but I do not recall it. I am sure he would answer the question if you asked.
I believe this technique this is covered in his book on photoreading. You can buy it at the web site listed above. The key is getting the right brain total database of past experiences practicaly linked out through your verbal logical left brain into words.
I would believe there is a point of diminishing return. Most of us are not use our brains efficiently, but who knows. How do you rate the IQ of people who can do very complicated math in their heads, etc etc?
Lacking a really good definition of what intelligence even is, knowing the limits is hard. Some of the things people have done with Photoreading, things I have done in my work, defy logical explanation. Any time you can really tap the right brain, the left brain can not really grasp it. IQ specs is a very left brain concept. True intelligence is so much more than what those tests can quantify.
I assure you that image streaming really works, in that I really see extremely engaging things when I image stream. It is closer to having a very controlled lucid dream than anything else I have experienced.
As for all of the IQ benefits posited by Win's book, I don't know. I do know that it has started to become an invaluable source of creative inspiration from which to draw. It is also EXTREMELY entertaining.
I have some adaptations to the basic methods discussed in the book. I don't use a tape recorder, nor do I speak to another person. I speak aloud to myself, sometimes I whisper to myself when I think I can be overheard. I also find it good to cover my eyes with my hands sometimes to block out ambient light.
If all you can see is splotches of light when you start, then describe them in as much detail as you can. Eventually, you may realize that you are having difficulty concentrating on that description because another image is trying to force it's way into your conscious mind. This is what you are looking for... this new image is the one coming from your subconscious... just describe it and you are on your way to image streaming.
Hope this helps.