Dreams

>>19223573
Neural impulses fired in our visual cortex

Now the more interesting question, is how such impulses and all that goes to support them and everything they're connected to in the brain and body can still make us "see" anything, being, as we can see, still simply material...

They are essentially, no different than your life when you're awake. I have no idea how we access them though, and why.

Pineal gland

>>19223624
Could be, the gland it's related to the light and dark perception.

>>19223611
You just said how it can make us see anything. Neural impulses in the visual cortex. That's exactly what sight is, whether it's from an external stimulus or an internal stimulus.