Why you ‘see’ things in the dark, according to an ophthalmologist

In 1999, Daniel Myrick and Eduardo Sánchez made one of the definitive horror films of the era on a budget of around $60,000. The Blair Witch Project is a study in omission, in the glaring absence of the visual effects characteristic of the genre. Instead of baroque prosthetic gore and over-the-top CGI effects, the film relies on silence and darkness for much of its 81 minutes.

Watch it on a small screen and much of the magic evaporates, but watch it in a dark cinema as entranced audiences across the United States once did, and the effect is terrifying. This is effective in large part because of the way our brains fill negative space. In the absence of light, we imagine the worst.

Off-screen, there are good reasons why so many children (and some adults) are afraid of the dark: our eyes really play tricks on us. Turn off the lights and you’ll still see faint colors and flickers of moving shapes. In low light conditions, you might suddenly find yourself more aware of movement at the edge of your peripheral vision. Add to that a vivid imagination and it’s easy to imagine witches, monsters under the bed, or the fear of your choice hiding in the dark.

Popular science spoke with Dr. Scott E. Brodie, professor of clinical ophthalmology at Columbia University Medical Center, about the science behind why we see things in the dark.

Don’t believe everything you see

It is tempting to view the flow of information that passes through our retinas as the reality of the world around us. “We generally think of our vision as being very accurate: what we see is what’s actually there,” Brodie explains. But this is not always the case. “There’s neurology, there’s biochemistry in there – and [our vision] can be deceived.

There is a simple way to see the limits of our visual systems. “There are what are called optical illusions, which help illustrate that there are real physical and chemical processes that underlie vision,” Brodie explains. German vision specialist Michael Bach offers a whole series of optical illusions on his website. Scroll through and you’ll see an illusory color appear out of nowhere in Benham’s Top, or chromatic afterimages dot the screen in Hinton’s Lilac Chaser.

Vision depends as much on your brain as it does on your eyes. Video: The Visual System: How Your Eyes Work, National Eye Institute, NIH

For a DIY demonstration, close your eyes, then very gently press your index finger against the upper edge of the bony socket of one eye. Move the pressure toward your eyeball, then move your finger from side to side. You should see a black circle with shiny edges moving in the opposite position as your finger.

“It’s an example of your eye seeing something visual, something you would interpret as a light phenomenon when there’s no light at all,” Brodie explains. “It’s simply a mechanical distortion of the retina, which causes nerve cells in the retina to do things that your brain interprets as a circular pattern.”

This particular visual phenomenon of seeing light in the absence of a light source is called phosphenes. The term phosphenes is derived from the Greek words meaning “light” and “show”. Phosphenes are usually a reaction to mechanical pressure or some kind of specific external stimulation. They also appear in response to physical trauma. If you’ve ever hit your head and found yourself “seeing stars, that’s what’s really happening”

Your eyes try to see in the dark, with mixed results

Even when the lights go out, your retina never stops working. In dim lighting, your rod cells, which are photoreceptors that are very sensitive to light, become more active. Because they are clustered more toward the edges of your retina, you may be more aware of your peripheral vision.

“In the dark, the retina is just as active, more or less, than in the light,” explains Brodie. “It’s just that more of the activity is dominated by off cells than by on cells. And very slight variations or quantum variations in activation will stimulate the circuits, even if there is no light around.”

In short, there is a lot going on on the chemical and neurological level, which means that we can “see” things that don’t really exist. Maybe you even “see” things (or at least think you do) in the dark. These seemingly random bits of color and light are actually closed-eye hallucinations, also known as closed-eye visualizations (CEVs).

Unlike phosphenes, you don’t need to bang your head to see them: you don’t have to do anything at all.

Not quite black

Contrary to what one might think, when one is immersed in total darkness or when one closes one’s eyes, what one sees is not actually darkness, but rather a very specific, uniform dark gray called eigenggrau. The word, which comes from the German for “intrinsic gray”, was coined by physicist Gustav Fechner, who explored the phenomenon in his Method of Limits experiment in the 19th century. In it, he tried to reduce variable stimuli (light, in this case) and see how that impacted human perception.

The reason we don’t see total darkness is again because of this visual noise, which comes from signals from our optic nerve that our brain interprets as flickers of light. Think of it as a kind of static coming from your own optical system rather than the world around you.

“Ultimately, there is noise in the visual system, which becomes a predominant sensation in the absence of light,” says Brodie. This does not mean visual noise only exists in the dark; it’s just that when our retinas absorb so much information about the world around us, we can’t really see the superimposed noise. However, turn everything else off and suddenly it’s hard to ignore.

It should be noted that there are other factors beyond our visual system that can make it appear as if there is something in the dark. When our retinas aren’t absorbing a lot of information, our other senses kick into high gear, meaning we suddenly hear every little bump in the night.

You may also notice that you feel more aware of your body itself. You can feel where your hands are even if you can’t see them exactly. This is thanks to proprioception, which allows your brain to keep tabs on movement and spatial location throughout the body.

So the next time you feel nervous in the dark, pay close attention to your other senses, try to stay calm, and always remember that your eyes can, in fact, deceive you.

This story is part of the Popular Science book Ask Us Anything Serieswhere we answer your craziest and most burning questions, from the ordinary to the unusual. Do you have something you always wanted to know? Ask us.

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