Here’s [David] Eagleman and [Don] Vaughn’s theory in nutshell: The role of dreams is to ensure that the brain’s visual cortex is stimulated during sleep. Otherwise, if the visual system were deprived of input all night long, the visual cortex’s function might degrade.
We know that the visual cortex, in the brain’s occipital lobe, can start to respond to non-visual signals if it is deprived of visual input. In blind people, for instance, the occipital lobe strongly responds to touch. This rewiring or repurposing of under-utilized brain areas is a form of neuroplasticity.
If we are in a dark place, or it’s night, we get little or no visual input. So — in theory — our visual cortex would be vulnerable to ‘takeover’ by other senses, every single night. Dreams, on this view, are our brain’s way of defending the integrity of our visual system by keeping it active.
As I said, I love the ingenuity of this theory, but I don’t really buy it.
For one thing, Eagleman and Vaughn’s theory only makes sense if neuroplastic repurposing of the cortex happens very quickly. For the visual cortex to need defending, harmful neuroplasticity would need to occur in the space of a few hours. The authors do discuss evidence that rapid neuroplasticity can occur, but they don’t show any evidence that these rapid changes are strong enough to be harmful.