In the 1820s, a dinner party brought together two brilliant minds: Charles Babbage, known as the father of computing, and William Herschel, the astronomer who discovered Uranus. During this gathering, Herschel posed an intriguing question to Babbage: “How can you see two sides of a coin at the same time?” Babbage’s clever response was to use a mirror, but Herschel demonstrated an even more ingenious solution by spinning the coin. As it spun, both sides appeared to blend together, creating the illusion of simultaneous visibility.
This clever trick inspired an Irish doctor to invent a device called the thaumatrope. It featured two images on either side of a disc, and when spun, the images merged into one. This simple toy became wildly popular in the 19th century and laid the foundation for modern moving images, including movies, cartoons, and video games. The thaumatrope demonstrated how a sequence of still images could trick our brains into perceiving motion.
Spinning discs like the thaumatrope were part of a broader category of “philosophical toys” that captivated people worldwide. These toys eventually led to the development of the film projector. In a projector, a sequence of still images is projected onto a screen, creating the illusion of movement. The magic happens as the projector rapidly flashes these images, with a shutter blocking the light between frames, so our brains perceive a continuous motion.
The phenomenon that allows us to perceive motion from still images is known as “persistence of vision.” This concept dates back to ancient Greece and Egypt, where philosophers observed that lightning streaks seemed to linger after the flash. Over time, scientists like Leonardo da Vinci and Isaac Newton explored these mysteries. They discovered that our brains hold onto images slightly longer than our eyes do, creating the illusion of continuous motion.
When we watch movies or play video games, our brains are tricked into seeing motion through a process called “apparent motion.” This occurs because our brains interpret a series of still images as a single moving object. For example, in video games, characters appear to move smoothly, even though they are just a sequence of static frames. Our brains fill in the gaps, creating a coherent narrative.
This ability to perceive motion is crucial for survival. Our ancestors needed to detect movement to avoid predators. Interestingly, our brains make these interpretations retrospectively, a phenomenon known as “postdiction.” Our brains gather information from different frames and then construct a story of what happened, even if it didn’t occur in real-time.
Our understanding of motion perception has evolved significantly over time. While early theories likened the eye to a camera, capturing slices of reality, we now know that the brain plays a crucial role in interpreting visual information. The brain processes data from the retina and constructs a narrative that makes sense of the world around us.
Visual illusions, like those created by early toys and modern media, continue to intrigue scientists. They offer insights into how our brains work and how we perceive reality. These illusions challenge our understanding of vision and highlight the brain’s remarkable ability to create coherent stories from limited information.
In conclusion, the world of visual illusions and motion perception is a fascinating field that reveals the complexities of our brains. From the simple thaumatrope to modern digital screens, our ability to perceive motion has shaped our understanding of reality. As technology advances, we continue to explore the mysteries of perception, uncovering new insights into how our brains construct the world we see.
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Design and construct a thaumatrope using simple materials like paper, string, and markers. Draw two related images on either side of a disc, such as a bird and a cage. Spin the disc rapidly to observe how the images merge into one. Reflect on how this activity demonstrates the concept of persistence of vision.
Watch short clips from classic films or animations. Identify scenes where apparent motion is particularly effective. Discuss with peers how the filmmakers used sequences of still images to create the illusion of continuous movement. Consider the role of frame rate and editing techniques in enhancing motion perception.
Play a video game that features smooth character animations. Pause the game at various points to examine individual frames. Analyze how your brain fills in the gaps between frames to create a seamless experience. Share your observations on how postdiction influences your perception of motion in the game.
Research and select a visual illusion related to motion perception, such as the rotating snakes illusion. Present the illusion to classmates and gather their reactions. Discuss the scientific principles behind the illusion and how it challenges our understanding of visual perception.
Participate in a debate on the evolution of theories about visual perception. Divide into groups, with each group representing a different historical perspective, such as ancient philosophers, Renaissance scientists, or modern neuroscientists. Argue your perspective’s contributions to our current understanding of motion perception and visual illusions.
Thank you to Wren for supporting PBS. Hey smart people, Joe here. In the 1820s, there was a dinner party. Mathematician Charles Babbage was there with astronomer William Herschel. You might know Babbage as the father of computing and Herschel as the astronomer who discovered Uranus. It sounds like a fun dinner party. After dinner, Herschel asked Babbage a question: “How can you see two sides of a coin at the same time?” Babbage’s answer, “Just look at its reflection in the mirror,” was a creative solution, but Herschel had an even better answer. He spun the coin on the table, and like magic, both sides of the coin seemed to blend together as if they were seen at the same time.
This story got around until an Irish doctor heard it and was inspired to create a device with two pictures on either side of a disc. When spun, the two images became one. This invention became known as the thaumatrope, one of the most popular toys of the 19th century. Although it was eventually mostly forgotten, it laid the groundwork for modern moving images, including movies, cartoons, and video games. Every moving image we view on screens can trace its origin to this simple illusion, which takes still images flashed in sequence and tricks our brain into perceiving motion.
Spinning discs are one of the simplest ways to create illusions that blend images into one. In the 19th century, they inspired a range of so-called philosophical toys, including a spinning disc with slits cut around it. When viewed in a mirror, a series of still images on one side appear as one moving image, creating the illusion of apparent motion. These toys captivated people around the world and eventually led to the invention of the film projector.
To create the illusion of movement, a sequence of still images is fed between a light source and a set of lenses, projecting a series of pictures on a screen. However, simply projecting a scrolling stream of still images is not enough to create the illusion of motion; we would only see an indecipherable blur. The magic happens in the projector, where one still frame is projected on the screen, then the screen goes black while the film advances one full step, showing the next still image. The film doesn’t move continuously but is advanced frame by frame many times every second. Inside the projector is a carefully timed shutter that blocks the light, so whenever the film is moving, you only see black. When these images are flashed on the screen at a fast enough rate, we perceive a moving picture.
Before around 2010, most movies you saw in theaters were projected this way. Most of the time, the movie wasn’t actually moving, and you spent half your time in the theater completely in the dark. Today, films and other moving pictures are projected digitally or displayed directly on digital screens. Modern screens no longer flicker to black between still images; instead, millions of individual pixels are refreshed dozens of times per second. However, the effect is still the same: what is beamed into your eyes is a series of still images, whether on big or small screens. Thanks to this technology, billions of minds have been tricked into seeing moving images that aren’t really there.
These early forms of moving picture entertainment have roots that may even trace back to prehistoric cultures, which is fascinating. It’s been said that our understanding of vision was changed as much by these toys as the field of biology was changed by the invention of the microscope. These toys inspired fundamental questions about how our brains work, how we perceive the world, and how we construct reality itself. Scientists today are still using these illusions to tackle those questions.
I made something that I have to show you right off the bat. It’s my Eagleman trope. Yes, that works surprisingly well even over video. Why does this work? It’s because when the brain sees something, even if it’s very rapid, it can’t turn on and off that quickly. Your brain sees things for longer than your eye does. This phenomenon is called persistence of vision.
The idea of visual persistence goes back long before Babbage’s dinner party, even to ancient Greece and Egypt. Early philosophers noticed that streaks from lightning seem to linger for a moment after the flash or that the sun stays in their vision after they look away. In the 11th century, an Arab mathematician and philosopher noted that a flickering flame seemed to appear where it was a moment before. Da Vinci and Newton even devoted time to these mysteries. Early explanations centered on the idea that light temporarily burns a scene into our eyes, which is then wiped clean for the next scene.
In 1765, an Irish mathematician conducted an experiment with glowing embers, calculating the velocity needed to spin to see a streak of light make a complete circle. He estimated the refresh rate of the eye to be around 130 milliseconds. You can try this experiment for yourself: if you rapidly move a point of light, it appears to leave trails behind. These illusions are not simply artifacts of a camera or shutter speed; you can see them in person.
The theory that the eye acts like a camera capturing slices of the world in single frames has persisted for a long time, but it’s incorrect. It has to do with what’s happening in the brain. Many processes occur in the retina, and then the information travels to the visual thalamus and the visual cortex, where various areas interact.
The effects in these toys and movies today create an illusion neuroscientists call apparent motion. When you see a series of still images appear to move, it’s your brain that’s been fooled, not your eye. Look at this arrangement of dots; they are just dots, right? But when played rapidly in succession, they can appear to form something else. Depending on how the dots are arranged relative to each other, our brains can interpret them in very different ways, even inventing different characters.
Now, consider two dots. Does it look like one is chasing the other? Or perhaps they’ve switched? In reality, one dot is simply moving erratically around the other, but when you add a moving background, you perceive something else. This may feel like a trick, but there’s no reason to think dots chase each other; they’re just dots. The position of one of the dots never changes; it’s a story invented by your brain.
Did it move, or did they just blink on and off? Why does our brain invent stories that don’t exist? These little white lies our brains tell us are important for understanding visual information in our world. If I see a bird flying and it goes behind a tree, then a moment later I see the bird emerge on the other side, my motion detectors interpret that as smooth motion. The bird didn’t disappear; our brains want to maintain the idea of object permanence.
I played a lot of Mario Brothers growing up on the old Nintendo. When you look at what’s happening frame by frame, you see a character standing still, then suddenly a character with his fist up a few pixels away, with nothing in between. But when you play the game, you feel as if the character smoothly jumped into the air. That’s what apparent motion is about. Your brain does all kinds of computations and concludes that the character must have moved.
Your brain makes an unconscious decision that images seen at different places and times represent the same object, a concept known as correspondence. The snake in the classic game isn’t moving; it’s just a series of blocky shapes. However, our brain’s object permanence interprets the shape as the same snake between frames, assembling those shapes into motion based on our understanding of how snakes usually move in the real world.
Even in modern video games, while the picture quality has improved and the technology has increased the number of images played each second, what we see as continuous motion is still just a series of still images. Interestingly, the brain can only do this in retrospect. It collects data from frame one, then from frame two, and retrospectively concludes that it must have moved smoothly between those points.
This is illustrated by a visual illusion called the colorify effect. If I show a red dot and then a blue dot, you might think it moved from one position to the other, but you may also have the impression that it changed color halfway through. This perception only occurs after the second dot appears, even though it feels like you saw the color change in real-time. Your brain is going back and writing a story that never happened in between those two points.
I coined the term postdiction for this phenomenon. It’s the opposite of prediction, meaning your brain collects information about a scene before it retrospectively determines what it thinks it saw. You’re living about half a second in the past. When you think a moment has occurred, it’s already happened a while ago. Your brain collects information from your senses, which come in at different speeds, and stitches it together to create a coherent narrative.
This raises interesting questions about how our brains evolved. Our species has only been exposed to illusions like these for a couple of centuries, but our brains are much older. We didn’t evolve with cartoons, TV, or video games, but the ability to sense motion, even the illusion of motion, has been crucial for our survival. Our ancestors who accurately detected motion were more likely to survive encounters with predators.
This illustrates that our perceptual world is built from cells trying to do their best. We’ve found a loophole, a trick we can play on these cells, but we certainly didn’t evolve to see movies. I’m glad we did, though; otherwise, we wouldn’t be having this conversation, and no one would watch YouTube. I appreciate this reality.
I’ve always been fascinated by visual illusions because we open our eyes and perceive the world as reality. It’s intriguing to see how the brain constructs this reality. It’s like being a fish in water and being asked to describe water; you wouldn’t be able to do it. Visual illusions are like bubbles that come up past you, prompting you to wonder, “What is that?” To understand how the brain works, you almost have to break it or find its shortcomings. It’s doing the best it can with limited information.
We wouldn’t know what we’re missing, so we think it’s doing a good job. However, maybe in a hundred years, when we all wear glasses to detect infrared and ultraviolet light, we’ll realize it has been doing a poor job all along. I look forward to experiencing that.
So why do videos, movies, or games work? The old idea that our eyes function like a camera, with motion burned into our eyes like light hits film, turns out to be incomplete. Our eyes are excellent at sensing the universe and send information to our brain, but our brains know that isn’t how the universe works. They fill in the gaps and blend what we see with something that isn’t really there.
Imagine living in a dark, silent room, cut off from the outside world, with only periodic notes passed under the door. Your image of reality would be a story you write based on that limited information. That’s how it is for your brain. In something as normal as watching TV or a movie, it’s not the actors creating the story; it’s your brain.
So what does this all mean? It means that this video is an illusion, but it’s one you can trust. Stay curious. Thank you to Wren for supporting PBS. Wren is a website where you can calculate your carbon footprint and offset it by funding projects that plant trees, protect rainforests, and remove carbon from the atmosphere. By answering a few questions about your lifestyle, you can find out your carbon footprint and how to reduce it. While no one can reduce their carbon footprint to zero, you can offset what remains after reducing. Subscribers receive monthly updates on the projects they support, including tree planting and rainforest protection. You can learn more about Wren’s climate projects at wren.co.
And as always, a big thank you to everyone who supports the show on Patreon. We could not make these videos without you. If you’d like to learn more about how you can support the show, just click the link in the description or watch this rapidly spinning disc. You’re getting very sleepy. You want to click the button and support the show on Patreon.
Motion – The change in position of an object with respect to time and its reference point. – In physics, the study of motion involves understanding concepts like velocity, acceleration, and the forces that cause these changes.
Perception – The process by which sensory information is interpreted by the brain to form a coherent understanding of the environment. – In physics, perception can influence how we interpret the motion of objects, such as the apparent speed of a car when viewed from different angles.
Images – Visual representations of objects formed by lenses or mirrors, often studied in optics. – The formation of images by concave mirrors is a fundamental concept in the study of optics within physics.
Brain – The organ that processes sensory information and coordinates responses, crucial for interpreting physical phenomena. – The brain’s ability to process visual information is essential for understanding complex physics concepts like wave interference patterns.
Illusion – A misleading or distorted perception of reality, often used to demonstrate principles in physics and psychology. – Optical illusions can be used to illustrate the principles of light refraction and reflection in physics.
Vision – The faculty of sight, which involves the detection and interpretation of light by the eyes and brain. – In physics, understanding the mechanics of vision is crucial for studying how lenses and optical instruments work.
Physics – The natural science that studies matter, energy, and the fundamental forces of nature. – Physics provides the foundational principles that explain the behavior of the universe, from subatomic particles to galaxies.
Toys – Objects designed for play, which can also be used to demonstrate physical principles in an educational context. – Simple toys like spinning tops can be used to demonstrate principles of angular momentum and rotational motion in physics.
Narrative – A structured account of events or concepts, often used to explain complex scientific ideas. – A well-crafted narrative can help students understand the historical development of key physics theories, such as relativity.
Evolution – The gradual development of systems or theories, often used to describe changes in scientific understanding over time. – The evolution of quantum mechanics has significantly altered our understanding of atomic and subatomic processes in physics.
