I also experience the same thing. At first I thought that I must have a filter I can't control that blocks imagery and it finally lifts as I'm falling asleep. But then I learned that the part of the brain that controls dreams is not the same area as the part that controls visual imagery. So I think that it's the dreaming area of the brain that is becoming active and that's what we are "seeing" as we fall asleep.

I also experience the same thing. At first I thought that I must have a filter I can't control that blocks imagery and it finally lifts as I'm falling asleep. But then I learned that the part of the brain that controls dreams is not the same area as the part that controls visual imagery. So I think that it's the dreaming area of the brain that is becoming active and that's what we are "seeing" as we fall asleep.