I underwent French Desert Warfare training recently. The instructors were French Marines, but the unit undergoing the training was the French Foreign Legion (a number of different French legions cycle through the course and it just happened to be the Legion's turn). For them, the physical aspects of what we were doing were very easy. They march overland something like 45 miles per week on average--the longest march on this course was a "mere" 21 miles.
But we didn't just go on four long (for me) marches, we went on marches in the pitch black of moonless nights. Twice there were clouds. I could barely see the man in front of me, yet I marched along over rough terrain, often off any trails, in a very rocky part of the country of Djibouti.
As my eyes struggled to find an sign of a rock that would trip me and see the man in front of me (far too often, my sight globes failed at each task), the sensory deprivation of the dark created a strange effect. Odd shapes began to swim into my field of view--the outline of amoebas once, once a grid of little cabbages. Other things appeared too and I wondered briefly if the effect might be a function of some misfiring of the rods and cones in my eyes. As soon as I thought that, a grid of blue rods and red cones interspaced with one another appeared in my view.
So then I realized in was my imagination supplying the images. Sure, there was some linkage to the eyes, but the basic instrument making pictures was my mind.
I understood, as I stumbled over rocks in the dark, that this is why I'd been so terrified of darkness as a child. My imagination filled the void of night with its own ideas of what would inhabit such a sinister space as to hide itself from my view. Monsters crawled everywhere in my boyhood imagination.
And then, having thought this, what did I see on the march? Monsters, of course, crawling everywhere. I was more annoyed by the phenomenon than frightened.
But then a story idea popped into my mind. What if a neuroscientist researcher found out that the nerves in the eyes that overlay the rods and cones (the light receptors--and yes, the receptors are underneath the nerve fibers they send signals to) actually had the ability to directly receive input from a source that showed signs of being coordinated? An input other than light, something noticeable only when the eyes were deprived of light?
Intrigued, the scientist seeks a means to enhance the effect. He finds a chemical that boosts the eye nerves' ability to see this direct input that does not come from light. He tries the chemical on himself.
And suddenly, he finds himself surrounded by dark, twisted, malevolent shapes, which had been with him the entire time, but he only just then discovered the means to see them. Demons...