All of those seem to make some sense. I like the one about imagery forming on imagery with strengthening synapses. I frequently experience a strange sensation just upon waking where I recall not only my dream from that night but dozens and dozens of dreams and imagery going back as far as several years. Also, dreams I had many years back ofetn play a role in current dreams, even though none of the dream events can really be linked to anything that has actually occurred in my life. Its interesting stuff!