It's quite simple really... your brain has an easier time seeing and hearing things that don't actually exist when there are fewer cues in the environment that suggest the hallucination isn't real. Reduce those cues (by removing light, taking off glasses, closing your eyes, etc.) and you will get a more "intense" image.
Speaker imaging
Considering my speakers definitely image better when I am not wearing my glasses leads me to conclude imaging ain't really real but relies on the power of suggestion. Sort of like some folks who prefer to listen with the lights off. Any comments?