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Laurinemily via Wikimedia Commons // CC BY-SA 2.5

How Our Eyes See Everything Upside Down

Original image
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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The Body
11 Interesting Facts About Lymph Nodes

The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

The lymphatic system is a crucial part of your body's ability to fight off infection and viruses. It's a key player in the immune system that functions by circulating lymphatic fluid through a series of lymph vessels all throughout your body. This fluid gathers up anything foreign, such as viruses and bacteria from your body tissues and flushes them to your lymph nodes, where immune cells attack whatever isn't helping your body. 

Mental Floss spoke to Adriana Medina, an internal medicine doctor with a specialty in hematology and oncology at the Alvin and Lois Lapidus Cancer Institute at Sinai Hospital in Baltimore, Maryland, about these important tissues. 

1. THERE ARE HUNDREDS OF NODES.

They're about size and shape of a pea, and hundreds of them are scattered all throughout the body. In order to fight many little pathogens and clear out unhelpful debris, your body needs a lot of nodes to rally to these causes, according to Medina. 

2. LYMPH NODES ARE HOME TO IMPORTANT IMMUNE CELLS.

"The lymph nodes are in charge of harboring lymphocytes," says Medina. Your body makes two main types of these immune cells, B-lymphocytes and T-lymphocytes (or B- and T-cells), which are crucial to your body's ability to fight off infections of all kinds. There are many sub-classes of the T-cells because "they are very important to attack infection," says Medina.

3. LYMPHOCYTES ESCORT FOREIGN INVADERS OUT.

When your lymph nodes receive some sort of foreign debris they recognize isn't ours, Medina says, "the B-lymphocytes are in charge of making antibodies." These antibodies "leave with the toxic substance," and signal other immune cells to come in and attack the cells.

4. WHERE DO ALL THE TOXINS GO?

Once the lymphatic fluid has grabbed up its targets, most of it returns to your blood stream, Medina explains, which is why it's so important for lymph cells to do their job: kill what aims to harm you before it gets flushed back into your system.

5. THERE ARE MANY CAUSES OF SWOLLEN LYMPH NODES.

When your immune system senses a foreign invader, be it a virus, bacteria, vaccine, or even some medications, it preps the lymph nodes to make antibodies and lymphocytes to fight off the offender. This also increases the amount of lymphatic fluid in the node, which can make it swollen and tender. Most of the time swollen lymph nodes are not a big cause for concern.

6. A HARD, RUBBERY LYMPH NODE IS A PROBLEM.

A lymph node that is harder rather than soft and persists for several weeks is worth a doctor visit. While lymph nodes can be tender or swollen and mobile when infected, "when there is a [cancerous] malignance…they're hard, rubbery, they don't move, and they don't go away. The lymph nodes are always telling us something."

7. YOU ARE THE PUMP FOR YOUR LYMPHATIC SYSTEM.

Unlike your blood, which has the heart to pump it through your body, your lymphatic fluid doesn't have a pump. Instead, it relies upon gravity and pressure, which you create when you move around, as well as light massage.

8. WHERE YOU FIND VEINS, YOU FIND LYMPHATIC VESSELS.

The lymphatic system and the circulatory system are separate systems, but connected, running in tandem like underground networks of streams. "Lymphatic vessels are distributed along the body wherever we have arteries [or] veins," says Medina.

9. YOUR LYMPH NODES AND YOUR SPLEEN WORK TOGETHER.

"The spleen is like one big lymph node," Medina says of the organ that lives between your stomach and diaphragm. "The spleen is able to produce additional blood cells in case we need it to." Additionally, she explains, many toxic substances are filtrated through the spleen. However, if something happens to your spleen and it needs to be removed, you can live without it; you just may become more prone to infection and require more vaccinations to protect you against aggressive viruses.

10. STAGES OF CANCER ARE DETERMINED BY THE NUMBER OF AFFECTED LYMPH NODES.

The easiest cancers to treat are those that remain in the tissue where they first occur. However, in metastatic cancers, cancer cells migrate to the lymph nodes, which can cause cancer to spread. "When the cancer is detected in lymph nodes, we have to try to find out how many lymph nodes are involved," Medina says. "Lymph node involvements [determines] the prognosis of the cancer." When lymph node involvement occurs, "the treatment has to be more aggressive," she says, often adding radiation to a regime of chemotherapy and other drugs.

11. RESEARCHERS ARE TURNING THE BODY'S OWN LYMPHOCYTES INTO CANCER FIGHTING TREATMENTS.

Breakthroughs in immunotherapy known as Car T-cell therapy turn the body's own immune system into a weapon against cancer by engineering patients' own immune cells to recognize and attack their tumors, according to the National Cancer Institute. "What's happening—it's just beautiful—is that [researchers] are using B-lymphocytes to fight not only breast cancer, but leukemia and lymphomas," Medina explains. "The results are so good and encouraging, changing chances of survival."

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The Body
13 Intriguing Facts About the Sciatic Nerve

The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

If you say someone's getting on your nerves, you could just cut to the chase and say they're getting on your sciatic nerve—this nerve is plenty big enough for both minor and major irritations. It's the largest nerve in the body, running a lengthy route from each side of your lower spine, deep into your buttock, wrapping around to the back of the thigh and into the foot. Mental Floss spoke to Loren Fishman, medical director of Manhattan Physical Medicine and Rehabilitation in NYC and associate clinical professor at Columbia Medical School. Here are 13 things we learned about this important part of the nervous system.

1. AT ITS LARGEST POINT, IT'S ABOUT AS BIG AROUND AS A MAN'S THUMB.

No wonder this nerve hurts when it gets irritated—at its biggest point, it's one heck of a large nerve, says Fishman. 

2. THE SCIATIC NERVE IS ACTUALLY MADE UP OF FIVE NERVES.

The sciatic nerve is more accurately five nerves that come together on the right and left sides of the lower spine. Technically, the fourth and fifth lumbar nerves and the first three nerves in the sacral spine come together and merge into the unified sciatic.

3. WITHOUT SCIATIC NERVES, YOUR LEGS WOULD BE WEAK NOODLES.

"The sciatic nerve gives feeling and strength to the muscles and skin of the calf and foot, supplies sensation from the joints, bones, and just about everything else below the knee," says Fishman.

4. THE SPINAL CORD'S CONNECTED TO THE THIGH BONE.

The nerve connects the spinal cord with the outside of the thigh, the hamstring muscles in the back of the thigh, and the muscles in your lower leg and feet. This is why sciatic nerve impingement often results in muscle weakness, numbness and/or tingling in the leg, ankle, foot, and toes.

5. INJURIES TO THE SCIATIC NERVE OFTEN AFFECT THE CONNECTION TO THE BRAIN RATHER THAN THE NERVE ITSELF.

After severe spinal cord injury, the nerve itself is often just fine, but the connection between it and the brain has been severed, Fishman says. Until now, there's been no way to fix such injuries, but "recent work with stem cells has begun to restore the connection in dogs and other animals."

6. BACK INJURIES ARE THE MOST COMMON CAUSE OF SCIATIC PAIN.

A variety of lower back problems can lead to pain that radiates along the sciatic nerve. Most commonly, sciatica pain is caused when a herniated disc at the L5 (lower lumbar back) irritates the S1 (sacrum) nerve root in the lower spine. The exiting nerve roots are highly sensitive, and the bits of the disc that herniate contain inflammatory proteins such as interleukin and tumor necrosis factor that can also aggravate the nerve.

7. SCIATIC PAIN CAN BE A MEDICAL EMERGENCY.

In a small number of people, a condition called cauda equina syndrome (so named because the nerve bundle at the base of the spinal cord resembles a horse's tail) can masquerade as sciatica—but it also usually causes weakness that extends to bowel or bladder incontinence and sometimes weakness or loss of sensation in the legs that gets progressively worse. In this case, immediate medical attention should be sought, and recovery may not be as quick as with common sciatica.

8. ANCIENT GREEKS AND ROMANS COULDN'T DISTINGUISH BETWEEN JOINT AND NERVE PAIN …

When the ancient Greek and Roman physicians were treating the pain we now commonly know as sciatica, they believed it stemmed from "diseases of the hip joint," according to a 2007 study in Spinal Cord. It wasn't until 1764, write the authors, "that leg pain of 'nervous' origin was distinguished from pain of 'arthritic' origin."

9. …AND HIPPOCRATES TREATED IT WITH THE BOILED MILK OF A FEMALE ASS.

Among the many treatments Hippocrates and his ilk came up with for this painful condition were: "Fumigations, fasting, and subsequently, laxatives, and ingestion of boiled milk of the female ass." In his Treatise of the Predictions, Hippocrates noted that elderly patients with "cramps and colds at the loin and the legs" would experience their pain for up to a year, whereas young people could be free of pain in about 40 days.

10. SCIATICA DERIVES ITS NAME FROM THE 15TH CENTURY.

The modern name for the disease, according to Fishman, comes from 15th-century Florence. "They called sciatica ischiatica, since they thought it came from tuberculosis that worked its way down to the ischial tuberosity (the sit-bones)," Fishman says. These medieval doctors had the cause wrong, but the name stuck.

11. SOMEWHERE BETWEEN 1900 AND 1925 PHYSICIANS CONNECTED HERNIATED DISCS TO SCIATIC PAIN.

Different researchers in different countries began to make sciatic breakthroughs when doing autopsies on corpses with fractured or herniated discs, where they noticed compression on the sciatic nerve.

12. WEIGHT HAS LITTLE INFLUENCE ON SCIATIC PAIN, BUT HEIGHT DOES.

A 1991 cross sectional study of 2946 women and 2727 men published in Spine found that neither gender nor body mass made any difference in the likelihood of developing sciatica. Body height did, however, in males between the ages of 50 and 64, with taller men being more likely to have the condition. Other studies have found a similar link [PDF]. Over 5'8"? Your risk is higher. 

13. SUFFERING FROM SCIATICA? YOU'RE NOT ALONE.

Sciatica has a surprisingly common negative impact on daily life. "Low back pain and sciatica are the second biggest reason for lost days of work—just behind the common cold," says Fishman. The condition is most commonly found in people over 50 and rarely seen in anyone under 20 years old—and then it most often has a genetic cause.

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