A dream’s vivid visuals made while your brain manages memories
This morning my first grader described a dream wherein our family was evacuating her elementary school on bicycle because an asteroid was due to hit it soon. After commenting on how small an asteroid it would have to be that we could even flee the blast radius, she mentioned just how real it all seemed. Which makes sense, because for a lot of her brain at the time, it sort of was, even if her eyes and ears didn’t have much to do with putting it all together.
Visuals without views
Just as you view the photos saved and interpreted on your camera’s memory, not the light coming through the lens, we perceive the world that our brain assembles, not necessarily what our eyes see. This is more obvious when you consider a visual memory, as neurologically it’s nearly the same as seeing something, just without the stimulus of a new image coming through your eyes first. The hippocampus is thought to help manage our memories, and coordinates with our brain’s visual centers, like the parietal and temporal lobes. So as the hippocampus brings up the script of the memory, our visual centers activate as if we were seeing something in front of our face, rather than behind it.
Midnight memory management
A similar scenario is central to your dreams, but with a bit more confusion thrown into the mix. We dream during the Rapid Eye Movement (REM) phase of sleep. The pons deactivates some communication with the rest of the body through the spinal cord, allowing your body to continue resting instead of physically reacting to the mental activity taking place (your eyes, on the other hand, continue to move through this phase. Quite rapidly.) The thalamus and various learning centers are then activated, with the hippocampus acting like a sort of staging area for memories. Some may be encoded in the neocortex, as a long-term memory center, while others are discarded. This process of reworking a day’s memories appears to be critical to learning, as people who miss REM sleep have trouble retaining learned information. On the other end of the spectrum, infants engage in more REM sleep than adults, befitting the immense amount of novelty their brains are trying to record and organize into functional memories.
In all this sorting and sifting of memories, it’s thought that dreams are our frontal cortex’s attempt to make sense of the jumble of data flying through our sleeping heads. The vividness of each dream is thanks to our retrieved memories activating perception centers, and any possible narrative is our brain trying to rationalize what would likely be an unfamiliar set of experiences when taken as a whole. Unless you really did find yourself late for class while naked in high school on a regular basis.
Source: Brain Basics: Understanding Sleep: Dreaming and REM Sleep, National Institute of Neurological Disorders and Stroke