Laurinemily via Wikimedia Commons // CC BY-SA 2.5
Laurinemily via Wikimedia Commons // CC BY-SA 2.5

How Our Eyes See Everything Upside Down

Laurinemily via Wikimedia Commons // CC BY-SA 2.5
Laurinemily via Wikimedia Commons // CC BY-SA 2.5

by Katie Oliver

Beliefs about the way visual perception works have undergone some fairly radical changes throughout history. In ancient Greece, for example, it was thought that beams of light emanate from our eyes and illuminate the objects we look at. This "emission theory" ["a href="https://web.archive.org/web/20111008073354/http://conference.nie.edu.sg/paper/Converted%20Pdf/ab00368.pdf" target="_blank">PDF] of vision was endorsed by most of the great thinkers of the age including Plato, Euclid, and Ptolemy. It gained so much credence that it dominated Western thought for the next thousand years. Of course, now we know better. (Or at least some of us do: There’s evidence that a worryingly large proportion of American college students think we do actually shoot beams of light from our eyes, possibly as a side effect of reading too many Superman comics.)

The model of vision as we now know it first appeared in the 16th century, when Felix Platter proposed that the eye functions as an optic and the retina as a receptor. Light from an external source enters through the cornea and is refracted by the lens, forming an image on the retina—the light-sensitive membrane located in the back of the eye. The retina detects photons of light and responds by firing neural impulses along the optic nerve to the brain.

There’s an unlikely sounding quirk to this set-up, which is that mechanically speaking, our eyes see everything upside down. That’s because the process of refraction through a convex lens causes the image to be flipped, so when the image hits your retina, it’s completely inverted. Réné Descartes proved this in the 17th century by setting a screen in place of the retina in a bull’s excised eyeball. The image that appeared on the screen was a smaller, inverted copy of the scene in front of the bull’s eye.

So why doesn’t the world look upside down to us? The answer lies in the power of the brain to adapt the sensory information it receives and make it fit with what it already knows. Essentially, your brain takes the raw, inverted data and turns it into a coherent, right-side-up image. If you’re in any doubt as to the truth of this, try gently pressing the bottom right side of your eyeball through your bottom eyelid—you should see a black spot appear at the top left side of your vision, proving the image has been flipped.

In the 1890s, psychologist George Stratton carried out a series of experiments [PDF] to test the mind’s ability to normalize sensory data. In one experiment he wore a set of reversing glasses that flipped his vision upside down for eight days. For the first four days of the experiment, his vision remained inverted, but by day five, it had spontaneously turned right side up, as his perception had adapted to the new information.

That’s not the only clever trick your brain has up its sleeve. The image that hits each of your retinas is a flat, 2D projection. Your brain has to overlay these two images to form one seamless 3D image in your mind—giving you depth perception that’s accurate enough to catch a ball, shoot baskets, or hit a distant target.

Your brain is also tasked with filling in the blanks where visual data is missing. The optic disc, or blind spot, is an area on the retina where the blood vessels and optic nerve are attached, so it has no visual receptor cells. But unless you use tricks to locate this blank hole in your vision, you’d never even notice it was there, simply because your brain is so good at joining the dots.

Another example is color perception; most of the 6 to 7 million cone photoreceptor cells in the eye that detect color are crowded within the fovea centralis at the center of the retina. At the periphery of your vision, you pretty much only see in black and white. Yet we perceive a continuous, full-color image from edge to edge because the brain is able to extrapolate from the information it already has.

This power of the mind to piece together incomplete data using assumptions based on previous experience has been labeled "unconscious inference" by scientists. As it draws on our past experiences, it’s not a skill we are born with; we have to learn it. It’s believed that for the first few days of life babies see the world upside down, as their brains just haven’t learned to flip the raw visual data yet. So don’t be alarmed if a newborn looks confused when you smile—they’re probably just trying to work out which way up your head is.

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10 Science-Backed Tips for Getting a Cat to Like You
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Like so many other humans, you might find cats to be mysterious creatures. But believe it or not, it’s not that hard to make friends with a feline, if you know what to do. Here are some tips on how to effectively buddy up with a kitty, drawn from scientific studies and my own experience as a researcher and cat behavioral consultant.

1. LET THE CAT CALL THE SHOTS.

When we see cats, we really want to pet them—but according to two Swiss studies, the best approach is to let kitty make the first move.

Research done in 51 Swiss homes with cats has shown that when humans sit back and wait—and focus on something else, like a good book—a cat is more likely to approach, and less likely to withdraw when people respond. (This preference explains why so many kitties are attracted to people with allergies—because allergic people are usually trying to not pet them.) Another study found that interactions last longer and are more positive when the kitty both initiates the activity and decides when it ends. Play a little hard to get, and you might find that they can’t get enough of you.

2. APPROACH A CAT THE WAY THEY GREET EACH OTHER (SORT OF).

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Felines who are friendly with each other greet each other nose to nose. You can mimic that behavior by offering a non-threatening finger tip at their nose level, a few inches away. Don’t hover, just bend down and gently extend your hand. Many cats will walk up and sniff your finger, and may even rub into it. Now that's a successful greeting.

3. PET CATS WHERE THEY LIKE IT MOST …

They're very sensitive to touch, and generally, they tend to like being petted in some places more than others. A small 2002 study demonstrated that cats showed more positive responses—like purring, blinking, and kneading their paws—to petting on the forehead area and the cheeks. They were more likely to react negatively—by hissing, swatting, or swishing their tails—when petted in the tail area. A more recent study validated these findings with a larger sample size—and many owners can testify to these preferences.

Of course, every animal is an individual, but these studies give us a good starting point, especially if you're meeting a cat for the first time.

4. … AND IF YOU GET NEGATIVE FEEDBACK, GIVE THE CAT SOME SPACE.

There are plenty of signs that a cat doesn't like your actions. These can range from the overt—such as hissing and biting—to the more subtle: flattening their ears, looking at your hand, or twitching their tails. When you get one of those signals, it’s time to back off.

Many of the owners I work with to correct behavioral issues don't retreat when they should, partially because they enjoy the experience of petting their cat so much that they fail to recognize that kitty isn’t enjoying it too. You can’t force a feline to like being handled (this is especially true of feral cats), but when they learn that you’ll respect their terms, the more likely they will be to trust you—and come back for more attention when they're ready.

5. DON’T OVERFEED YOUR CAT.

Many think that food equals love, and that withholding food might make your kitty hate you, but a recent study of obese felines from Cornell University showed the opposite is true—at least for a period of time. About a month after 58 overweight kitties were placed on a diet, three-quarters of their owners reported that their dieting felines were more affectionate, purred more often, and were more likely to sit in their owner's lap. This adorable behavior came with some not-so-cute side effects—the cats also begged and meowed more—but by week eight, both the good and bad behavior had abated for about half the animals.

Regardless of whether a diet makes your pet cuddlier, keeping your pet on the slender side is a great way to help them stay healthy and ward off problems like diabetes, joint pain, and uncleanliness. (Overweight animals have difficulty grooming themselves—and do you really want them sitting on your lap if they can’t keep their butt clean?)

6. PLAY WITH THEM—A LOT.

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Most of the behavior problems that I've witnessed stem from boredom and a lack of routine playtime. No one thinks twice about walking their dog every day, but many people fail to recognize that felines are stealth predators who need a regular outlet for that energy. A recent study suggested that cats prefer human interaction over food, but a closer look at the data demonstrated that what really attracted them to humans was the presence of an interactive toy. One of their top choices is a wand-style toy with feathers, strings, or other prey-like attachments that evoke predatory behavior. Daily interactive play is a great way to bond with them when they’re not in the mood to cuddle—and to keep them fit. Try the Go-Cat Da Bird or any of Neko Flies interchangeable cat toys.  

7. KEEP YOUR CAT INDOORS.

A study conducted in Italy showed that felines who stayed mostly indoors (they had one hour of supervised access to a small garden each day) were more “in sync” with their owners than felines who were allowed free access to the outdoors. The indoor kitties were more active during the day, when their owners were likely to be active, and less active at night, when humans like to sleep. (Many people believe cats are nocturnal, but they are naturally crepuscular—active at dawn and dusk.)

8. SOCIALIZE CATS WHEN THEY'RE YOUNG.

Multiple studies have shown that just a few minutes a day of positive handling by humans helps kittens grow up to be friendlier and more trusting of humans. The ideal age to socialize kittens is when they're between 2 and 9 weeks old. One 2008 study found that shelter kittens that had been given a lot of "enhanced socialization"—additional attention, affection, and play—were, a year later, more affectionate with their owners and less fearful than other kittens adopted from the same shelters.

You can help socialize kittens by volunteering as a foster caretaker. Fostering ensures they get plenty of interaction with people, which will help them will be comfortable around potential adopters. You'll also be doing your local shelter a huge favor by alleviating overcrowding.

9. TAKE THE CAT'S PERSONALITY—AND YOUR OWN—INTO CONSIDERATION WHEN ADOPTING.

If you want to adopt an older animal, take some time at the shelter to get to know them first, since adopters of adult cats report that personality played a big role in their decision to take an animal home permanently and had an impact on their satisfaction with their new companion. Better yet, foster one first. Shelters can be stressful, so you'll get a better sense of what an animal is really like when they're in your home. Not all cats are socialized well when they're young, so a cat may have their own unique rules about what kinds of interactions they're okay with.

It's also key to remember that a cat's appearance isn't indicative of their personality—and it's not just black cats who get a bad rap. In 2012, I published a study with 189 participants that showed that people were likely to assign personality traits to felines based solely on their fur color. Among other things, they tended to think orange cats would be the nicest and white cats the most aloof. (Needless to say, these are inaccurate assumptions.) And it's not just the kitty's personality that matters—yours is important too. Another study I conducted in 2014 of nearly 1100 pet owners suggested that self-identified “cat people” tend to be more introverted and anxious when compared to dog people. (We’re also more prone to being open-minded and creative, so it’s not all bad.) If you’re outgoing and active, a more playful feline could be for you. If you prefer nights spent snuggling on the couch, a mellow, shy-but-sweet lovebug could be your perfect pet.

10. BE A KEEN OBSERVER OF THEIR BEHAVIOR.

Overall, use your common sense. Be a diligent and objective observer of how they respond to your actions. Feline body language can be subtle—something as small as an eye-blink can indicate contentment, while ear twitches might signal irritation—but as you learn their cues, you'll find yourself much more in tune with how they're feeling. And if you adjust your behaviors accordingly, you'll find soon enough that you've earned a cat's trust.

Mikel Delgado received her Ph.D. at UC-Berkeley in psychology studying animal behavior and human-pet relationships. She's a researcher at the UC Davis School of Veterinary Medicine and co-founder of the cat behavior consulting company Feline Minds.

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11 Facts About Fingernails
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Whether there's dirt beneath them or polish atop them, your fingernails serve more than just decorative purposes: They help keep your fingertips safe and have a multitude of special functions that even your doctor might not be aware of. “The nails occupy a unique space within dermatology and medicine in general, particularly because they are such a niche area about which few people have expertise,” Evan Rieder, assistant professor in the Ronald O. Perelman Department of Dermatology at NYU Langone Health, tells Mental Floss.

1. FINGERNAILS HAVE FOUR MAIN PARTS.

Along with skin and hair, nails are part of the body's integumentary system, whose main function is to protect your body from damage and infection. Fingernails have four basic structures: the matrix, the nail plate, the nail bed, and the skin around the nail (including the cuticle).

Fingernail cells grow continuously from a little pocket at the root of the nail bed called the matrix. The pale, crescent-shaped lunula—derived from Latin for "little moon"—on the nail itself is the visible portion of the matrix. If the lunula is injured, the  nail won't grow normally (a scarred lunula can result in a split nail), and changes in the lunula's appearance can also be signs of a systemic disease.

Fingernail cells are made of a protein called keratin (same as your hair). As the keratin cells push out of the matrix, they become hard, flat and compact, eventually forming the hard surface of the nail known as the nail plate. Beneath that is the nail bed, which almost never sees the light of day except when there's an injury or disease.

Surrounding the matrix is the cuticle, the semi-circle of skin that has a tendency to peel away from the nail. The skin just underneath the distal end of the fingernail is called the hyponychium, and if you've ever trimmed your nails too short, you know this skin can be slightly more sensitive than the rest of the fingertip.

2. THEY GROW AT A RATE OF 0.1 MILLIMETERS A DAY ...

That's about 3 to 4 millimeters per month. But they don't always grow at the same speed: Fingernails grow more quickly during the day and in summer (this may be related to exposure to sunlight, which produces more nail-nourishing vitamin D). Nails on your bigger fingers also grow faster, and men's grow faster than women's. The pinky fingernail grows the slowest of all the fingernails. According to the American Academy of Dermatology, if you lose a fingernail due to injury, it can take up to six months to grow back (while a toenail could take as much as a year and a half).

3. ... BUT NOT AFTER YOU'RE DEAD.

You've probably heard that your fingernails keep growing after death. The truth is, they don't, according to the medical journal BMJ. What's actually happening is that the skin around the base of the fingernails retracts because the body is no longer pumping fluids into the tissues, and that creates a kind of optical illusion that makes the nails appear longer.

4. ITS ESTIMATED THAT 20 TO 30 PERCENT OF PEOPLE BITE THEIR NAILS.

Scientists say it's still unclear why, but they suspect nail-biters do it because they're bored, frustrated, concentrating, or because it just feels comforting (and anxiety doesn't seem to play a big role). Perfectionists who don't like to be idle are very likely to have the habit. Biters expose themselves to the dangerous crud that collects underneath the nail: The hyponychium attracts bacteria, including E. coli, and ingesting that through nail-biting can lead to gastrointestinal problems down the line. Biting can also damage teeth and jaws.

5. HUMAN FINGERNAILS ARE BASICALLY FLAT CLAWS.

Our primate ancestors had claws—which, like nails, are made of keratin. As human ancestors began using tools some 2.5 million years ago (or even earlier), evolutionary researchers believe that curved claws became a nuisance. To clutch and strike stone tools, our fingertips may have broadened, causing the claws to evolve into fingernails.

6. THE NAIL ACTUALLY MAKES YOUR FINGERTIP MORE SENSITIVE.

While the fingernail may be tough enough to protect tender flesh, it also has the paradoxical effect of increasing the sensitivity of the finger. It acts as a counterforce when the fingertip touches an object. "The finger is a particularly sensitive area because of very high density of nerve fibers," Rieder says.

7. FINGERNAILS CAN REVEAL LUNG, HEART, AND LIVER DISEASES.

"One of the most interesting facts about fingernails is that they are often a marker for disease within the body," Rieder says. Nail clubbing—an overcurvature of the nail plate and thickening of the skin around the nails—is a particularly significant sign of underlying illness, such as lung or heart disease, liver disease, or inflammatory bowel disease. Two-toned nails—whitish from the cuticle to the nail's midpoint and pink, brown, or reddish in the distal half—can be a sign of kidney and liver disease. Nails that are two-thirds whitish to one-third normal can also be a sign of liver disease. However, little white marks on your nails, known as milk spots (or punctate leukonychia) are just the remnants of any kind of trauma to the nail, from slamming it in a door to chewing on it too fervently.

8. YOU CAN GET A COMMON SKIN DISEASE ON YOUR NAILS.

Psoriasis is "typically thought of as a skin disease, but is actually a skin, joint, and nail disease, and when severe, a marker of cardiovascular risk," Rieder says. Psoriatic fingernails may have orange patches called oil spots, red lines known as splinter hemorrhages, lifting of the edges of the nails, and pits, "which look like a thumb tack was repeatedly and haphazardly pushed into the nails," he says.

Doctors often prescribe topical or injected corticosteroids to treat psoriatic nails, but using lasers is an emerging and potentially more cost-effective technique. Rieder relies on a pulsed dye laser, which uses an organic dye mixed with a solvent as the medium to treat nail psoriasis, "which can be both medically and aesthetically bothersome," he says. This laser is able to penetrate through the hard nail plate with minimal discomfort and "to treat targets of interest, in the case of psoriasis, blood vessels, and hyperactive skin," Rieder says.

9. ANCIENT CULTURES DISPLAYED SOCIAL STATUS WITH NAIL ART.

Painting and other forms of decorating nails have a history of offering social and aesthetic cues through variations in nail color, shape, and length, Rieder says. In fact, he adds, in some cultures ornate and well-decorated fingernails "serve as a proxy for social status."

Five thousand years ago in China, men and women of the Ming Dynasty aristocracy grew their nails long and covered them with golden nail guards or bright home-made polishes. The long nails allegedly announced to the world their social rank and their freedom from performing menial labor.

10. A FORMER BEAUTICIAN HELD THE WORLD RECORD FOR THE LONGEST NAILS.

Lee Redmond of Utah started growing her nails in 1979 and kept at it until she held the world record for "longest fingernails on a pair of hands ever (female)" in 2008. Her right thumbnail was 2 feet, 11 inches and the collective length of all her nails was 28 feet, 4 inches. She also applied nail hardener daily and painted them a reflective gold. Unfortunately, she broke her nails in a 2009 car accident and has no plans to regrow them.

More recently, the man who holds the Guinness record for the "longest fingernails on a single hand—ever" chose to chop them off at Ripley's Believe It Or Not! in New York City in July 2018. Shridhar Chillal of Pune, India started growing the nails of his left hand in 1952, when he was 14 years old. At last count, the total length measured 29 feet, 10.1 inches.

11. THE FIRST NAIL CLIPPERS WERE PATENTED IN 1875.

Today, biters don't have to use their teeth to trim their nails. While the earliest tools for cutting nails were most likely sharp rocks, sand, and knives, the purpose-built nail clipper—though it might be more accurately called a circular nail file—was designed by a Boston, Massachusetts inventor named Valentine Fogerty and patented in 1875. The nail clippers we know today were the design of inventors Eugene Heim and Oelestin Matz, who were granted their patent for a clamp-style fingernail clipper in 1881.

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