What Triggers Image Impulses?

I'm not sure what to call them, but I sometimes have random and vivid visual memories of particular locations, most of which appear to be completely unrelated to my current experience. I could be standing in the kitchen, riding in the car, or taking a shower. Actually, it does seem that these image impulses occur most frequently when I'm in some sort of half-resting state. Other than that, they seem to be completely unpredictable in terms of timing and content. They typically don't include any significant set of events, and last for somewhere between 5 to 10 seconds. Their most memorable feature is the location/landscape itself.