Our big human brain visualizes and dreams in the same area of the occipital lobe. This area is collectively known as the secondary visual cortex. When we visualize, knowingly or not, we are priming the pump for sleep and dreams. It is not unusual, however, to be visualizing about one thing and then find yourself dreaming about something completely different. This is because are brains are going through a kind of housekeeping process as we sleep... keeping important data whilst discarding useless memories we had during the day. This is what dictates our dream subject matter. However, a trick I have learned is to visualize the last dream you can recall... especially is a dream you just awoke from. This has opened the door to many lucid dream experiences for me over the year.