That sounds really similar to what I often do. I often start picturing a tree, then I force one transition and feel like I let the brain switch images without my involvement.

It's really hard to describe it but here is a simplified example.

Tree -> zoom into the leaves which are actually a field of grass -> there is a small pond -> pond is now growing -> I see a sky now -> random colors and shapes are showing up... And it keeps on going and I don't even notice when I fall asleep.