‘The Enigmatic Brain’ — Why Things That Go Bump In The Night Disappear
This article is the first in a series that explores in a straight forward way the often not so straight forward inner workings of the brain and mind.
Everyone’s experienced it. Maybe it was in a dark forest on a camping trip. Or as a kid lying in bed. It’s dark, and you catch something out of the corner of your eyes, but you can’t really make it out. You immediately shift your gaze towards it to get a better look. But when you do, it disappears. It’s no longer there, it’s gone. All you see is darkness no matter how much you squint or try to focus. As you look away though, you see it again in your periphery. And to make matters worse, everything is in sinister looking shades of gray. It’s the stuff horror movies are made of. But why? What’s going on in your eyes and brain that produces this effect?
It feels like your mind is playing tricks on you (at a really inconvenient moment). But it is actually a real phenomenon with a known physiological basis. It all has to do with how the retinas in your eyes detect visual information.
In the back of each eye is a thin sheet of cells that makes up the neural sensory retina. The retinas consist of a number of distinct layers of specialized neurons. In fact, each retina is actually an extension of the brain itself, projecting from other parts of the brain out to your eyes via the optic nerves.
The first layer of neurons in the retina are the photoreceptors. These are special light-sensitive cells responsible for converting incoming light into a neural-chemical signal that eventually gets translated into electrical impulses called action potentials that the rest of the brain can understand. The process of converting — or transducing — photons of light into a neural signal is called phototransduction. Here’s a brief description that provides an intuitive understanding of how phototransduction takes place.
There are two types of photoreceptors, rods and cones. In humans there is only one type of rod but three types of cones. To understand why scary things in the night look dark, gray, and disappear, you need three pieces of information, First, you have to know something about how rods and cones respond differently to different intensities of light. Second, you need to understand a bit about how the retina encodes color, and third, what the spatial distribution of rods versus cones is throughout the retina.
Sensitivity to Light
The brain perceives the amount of light hitting the retinas — in other words, the number of photons over some period of time — as light intensity. The greater the amount of light, the brighter and more intense it feels.
Rods and cones have different sensitivities to light intensity. Rods respond to low levels of light. As the intensity increases, the output from the rods also increases. This means that as more light hits them, they increase the volume of the neural signal they pass along to the next layer of neurons. It’s how they indicate, or encode, the intensity of the light to other parts of the brain. But eventually the rods saturate, meaning they can’t increase their output any further to continued increases in light intensity.
Luckily though, at just about the same light intensity that rods saturate, cones start to respond. At the lower light levels rods are designed to respond to, cones can’t respond at all. It is below their level of sensitivity. But when rods stop responding, the cones take over. Eventually they saturate too, but at much higher levels.
So in a dark forest or a dark room at night with the lights off, you rely on your rod photoreceptors to see. At these low light levels, there simply aren’t enough photons for the cones to detect. It’s below their sensitivity level. On the other hand, the rods are designed to see at such low light intensities.
Seeing in Color (Or Not)
The second consideration is color. What makes each type of photoreceptor unique is not how they’re built or function — they all do the same thing essentially the same way — but rather which kind of photopigment molecule they contain. All rods contain the same type of photopigment, called rhodopsin. Each of the three cone types contain a different photopigment. Each unique class of photopigment interacts with photons of different wavelengths. Your brain perceives these different photons, and mixtures of photons, as different colors.
Photons, being tiny wave-like packets of energy, have a wavelength that determines the amount of energy each packet carries. The ‘size’ of the waves that make up each photon vary, measured as the distance from peak to peak of the wave — or more technically, as the distance between any two points with the same phase. The shorter the wavelength the faster they come and the greater the ‘punch’ they carry when they hit you. In other words, the greater the energy. The longer the wavelength, the slower they come and the less energy they carry.
The photopigment is the actual molecule in photoreceptors that interacts with photons hitting the cell through the process of phototransduction. (Here’s that brief introduction to phototransduction again.) Each class of photopigment responds maximally to photons of a particular wavelength, and absorbs photons of other wavelengths progressively less as you move away from its peak response. If you plot how much a photopigment responds to different wavelengths of light the result is a bell shaped curve called its absorbance spectrum — the amount of absorbance as a function of wavelength.
The range across the four absorbance spectra for each photoreceptor type (there is overlap between them by the way), from the shortest blue light your brain can detect to the longest red, defines the human visual spectrum, the range of light (electromagnetic radiation) that we can see and perceive.
This is a tiny sampling of the much larger full span of the electromagnetic radiation spectrum, which includes everything from the much shorter very high-energy x-rays and gamma rays to the much longer wavelengths of microwaves and radio waves. The visible spectrum we see is a tiny sliver of a huge electromagnetic radiation continuum, limited by the light absorbing properties of the photopigments contained in the photoreceptors of our retinas. (Other animals have photopigments with different absorbance spectra and so can see wavelengths of light humans can’t. For example, snakes, some fish and frogs can see into the infrared. While butterflies, bees, some birds, and reindeers (yes, reindeers!) can see into the ultraviolet. Think about that next Christmas.)
The key to how the brain perceives color is to understand that it uses the overlaps in the photopigment absorbance spectra from each of the three types of cones to pinpoint the exact wavelength of light being transmitted or reflected by the object you’re looking at. This is how color is encoded.
On the other hand, because there is only one type of rod, the brain can’t differentiate wavelengths and so can’t see in color. As such, when you see with your rods, you can only see in shades of gray. In a dark forest or dark room, because you’re only able to use your rods to see, that thing that goes bump in the night looks monochromatic.
Where are Those Rod Photoreceptors?
The last piece of the puzzle is the distribution and location of rods versus cones in the human retina. They aren’t spread across the retina evenly. Cones are primarily located at a spot on your retina called the fovea, which is the point of highest acuity. As you read this article, you are fixating on the letters and words with your fovea. That also explains why you ‘see’ the sharpest under good lighting conditions compared to the dark. There are no rods in the fovea. It is cone-dominated, which depend on higher light intensities to function.
But as you move away from the fovea the cones start to drop off and the rods take over. At about 20 degrees from center the rods dominate and continue to do so all the way out to your peripheral retina. Furthermore, the rods out in the periphery ‘pool’ the information they ‘see’, so despite having a lot of rods in your peripheral retinas your visual acuity drops off compared to the fovea.
Putting it all Together
Now put the pieces all together to answer the original questions: Why is it that when it’s dark, and you catch something out of the corner of your eyes, you can’t really make it out? And if you shift your gaze towards it to get a better look it disappears.
It’s dark, so you’re using your rod photoreceptors to ‘see’ because the light levels are below the sensitivity level of the cones. Because you’re using your rods you can’t see in color, so everything looks gray. You see it in your periphery but not well, you just ‘catch’ it, because your visual acuity in the rod-dominated peripheral retina is lower than your fovea. And finally, when you do try to shift your gaze and use your fovea to see the object sharply, it disappears because your fovea is cone-dominated and, well, once again it’s below the sensitivity level of the cones.
So the next time something goes bump in the night, just remember it’s not your mind playing tricks on you, it’s neurophysiology. At least don’t feel too bad if you get caught by that monster coming at you from your closet.