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 guy who found uranus sounds like a fun dinner party well after  dinner herschel asked babbage a question how can you see two sides  of a coin at the same time well babbage's answer just look at its  reflection in the mirror pretty creative solution but herschel had an even better answer he spun  the coin on the table like magic both sides of the coin seemed to blend together as if they were  seeing them at the same time this story got around until an irish doctor heard it and it gave him an  idea he put two pictures on either side of a disc when he spun them between string the two images  became one this became known as the thaumatrope this was one of the most popular toys of the 19th  century but it was eventually mostly forgotten except that it wasn't this and the toys that it  inspired are the reason you can watch this video it's why movies exist and cartoons video games  every moving image we view on screens big or small can trace its origin to this simple illusion one  that can take still images flashed in sequence and fool our brain into perceiving  motion so what makes this all work spinning discs are one of the simplest ways  to create illusions that blend images into one and in the 19th century they inspired a  whole range of so-called philosophical toys including this one a spinning disc with several  slits cut around it now when viewed in a mirror a series of still images on one side appear as one  moving image the illusion of apparent motion these inspired a series of victorian cinematic toys they  were commercialized throughout the 19th century selling by the thousands and captivating  people around the world these cinematic toys became more and more complex eventually  giving birth to the era of the moving picture despite the fact that no one quite knew how  they worked ultimately those toys directly led to this invention 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 but simply projecting a scrolling stream of still  images alone is not enough to create the illusion of emotion we merely see an indecipherable  blur this illusion is about what you don't see what actually happens in a projector is one  still frame is projected on screen then the screen goes black while the film is advanced  one full step and the next still image is shown the film doesn't move continuously but instead is  advanced frame by frame many times every second inside the projector is a carefully timed shutter  that blocks the light so that 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 it's likely that any movie that you saw in a theater was projected like this most of the  time the movie wasn't actually moving and you spent half your time in the theater completely  in the dark today of course films and other moving pictures are projected digitally  or displayed directly on digital screens as you can see in these incredible clips from our  friends the slo-mo guys modern screens no longer flicker to black between still images instead  millions of individual pixels are refreshed dozens of times per second but the effect is still  the same what's getting beamed into your eyes is a series of still images and that's true on screen's  big or small thanks to this technology in the last century and a half or so billions of minds  have been tricked into seeing moving images that aren't really there many of us spend  hours every day staring at these illusions and never even think twice about it and it all  traces back to these these were the first forms of moving picture entertainment though there  are some hints from caves and artifacts that prehistoric cultures may have made versions of  their own long before as well which is pretty cool 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 literal toys inspired fundamental questions about how our brains work  how we perceive the world and how we construct reality itself and scientists today are still  using these illusions to tackle those questions i made something that i have to show you right off  the bat i hope this works it's my eagleman trope yes that works surprisingly well even over video  i like it why does this work i mean this is like a toy that people played with in the in the  1830s or something like what is happening so why does that work it's because when the  brain sees something even if it's very rapid your brain's unable to turn it on and off that  quickly so your brain sees things for longer than your eye does this is what's called persistence of  vision the idea of some kind of visual persistence goes back long before babbage's dinner party even  to ancient greece and egypt early philosophers noticed that streaks from lightning seemed to  stick around for a split second after the flash or that the sun stayed in their vision after  they looked away don't stare at the sun ever okay the guy who invented this did an experiment  where he stared at the sun and he went blind just don't do it people it's bad for you 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 all of those early explanations centered on the idea that the light that enters our  peepers temporarily burns a scene into our eyes then it's basically wiped clean our eyes  refresh and then the next scene is burned in and so on in 1765 an irish mathematician  did this experiment with glowing embers he calculated the velocity you'd have to spin to  see the streak of light make a complete circle and estimated this supposed refresh  rate of the eye to be 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 my camera or the shutter speed you can see them if you try this in person this  theory that the eye acts like a camera with some refresh rate capturing slices of the world in  single frames stuck around for a long time but it's wrong that's right it has to do with what's  going on in the brain so there are many things that happen in the retina and that's just the  first stage and then you go back into the brain to an area of the visual thalamus and then the visual  cortex and then all these areas the visual cortex all of these things are interplaying the effects  in these toys and movies and screens 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 and how it really works is pretty freaky look at this arrangement of dots and here's a slightly  different arrangement and another these dots are just dots right but played rapidly in succession  at some point these dots become something else and depending on just how those dots are arranged  relative to each other as they move our brains can write very different stories about what  we're seeing even invent different characters now look at these two dots does it look like  one is chasing the other or perhaps they've switched well none of those are true one dot  is simply moving erratically around the other but when you add a moving background suddenly you  perceive something else that 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 even changes you made  it do that it's a story invented by your brain how about this did it move or do they just blink  on and off so why are our brains inventing stories that don't exist well these little white lies  our brains tell us are an important part of understanding the visual information in our world  if i see a bird flying and it goes behind the tree then a moment later i see the bird emerge on  the other side of the trunk and for my motion detectors that's perfectly fine i say oh great  there was there was motion there smooth motion the bird didn't disappear birds just don't do  that in the real world exactly exactly and this and this is this notion of object permanence your  brain is is wanting to say i i want to still hang on to that bird wherever it went i played a lot  of mario brothers growing up you know like on the old nintendo and actually when you look at  what's happening frame by frame like you've got a little plumber man standing there and then  all of a sudden you have like a plumber guy with his fist up a few pixels away and there's  nothing in between that but when you play the game you have this sensation that this character  smoothly jumped up in the air that's exactly right and that's what apparent motion is about your  brain does all kinds of computations and says oh wait here's mario and there was a mario over  here it must be the same mario and he has moved your brain makes an unconscious decision that  images seen at different places and at different times represent the same object this is called  correspondence the snake in the classic game isn't moving it's simply a series of blocky shapes but  our brain's object permanence interprets the shape as the same snake between frames and based on our  understanding of how snakes usually move in the real world we assemble those shapes into motion  even in modern video games while the picture's quality has definitely improved the technology has  increased the number of images played each second what we see is continuous motion is just a series  of still images what's really interesting is that the brain can only do this in retrospect so in  other words it collects up the data from frame one then from frame two and then it retrospectively  says i it must have moved smoothly between those things and part of the way we know this is  from a visual illusion called the color phi effect which is if i show let's say a red dot  and then i show a blue dot you will think that it moved from one position to the other but you will  also have the impression that it changed color halfway between and that's only possible  after the second dot has appeared even though it feels like oh i'm seeing  this thing and it changes color to blue you can only know the position and the color after  it's over so your brain is going back and writing a story that never happened in between those two  things that's exactly right and actually i coined the term for this uh some years ago called  post-diction it's the opposite of prediction which is to say you know your brain collects  up all the information about a scene before it retrospectively says what it thinks  it saw you know you're living about probably about half a second in the past so  when you think the moment now occurs it's already happened a long time ago your brain is  collecting up all this information including what's coming through your eyes and your ears  and your fingertips and your toes and everything which comes in at different speeds your brain has  to collect all that up stitch it together and then say okay here's what i think happened i'm really  starting to question reality more and more as we talk here here's another example of this there are  cells in your eyes even in the visual processing centers of your brain that respond fast enough to  detect rapidly flickering light but what's really interesting is there's a threshold where your  brain flips it decides to ignore that flickering and you suddenly see a constant light source when  the rate of flicker is higher than 35 flashes per second or so your brain says i'll just smooth this  out to look like real life even though your eyes and parts of your brain are sensing flickering  light movie cameras record 24 images per second but if you played that back at 24 flickers  per second it would look like a strobe light film projector shutters flicker two or three times  for each still image digital displays refresh two or three times before the image changes it's over  that threshold so your brain ignores the flicker when you think about this that your brain  is taking in all of this information from your senses it's comparing it with your past  experience about how things in the universe should behave then your brain goes back and  writes the story of what it thinks you saw it brings up a lot of interesting questions like  how did our brains end up this way i mean our species has only been staring at illusions  like these for a couple of centuries but our brains are way older than that we didn't  evolve with cartoons and tv and video games around or even spinning picture discs but being  able to sense motion even the illusion of motion has been a big part of our species  survival you know maybe 99 times out of 100 that wasn't a tiger in the grass it was just  an illusion of motion but our only ancestors who survived are the ones whose motion detectors  got it right the hundredth time too to me what i think it illustrates is that you know your whole  brain your whole perceptual world is built out of cells that are just trying to do the best job  they can essentially we found a little loophole a trick that we can play on these cells but we  certainly didn't need to evolve to see movies i'm glad we did though otherwise we wouldn't be  able to talk here and no one would watch youtube i like this reality i've always been attracted  to visual illusions because um you know we we open our eyes and there's the world and we take  it to be reality and so it's very interesting to see how the brain constructs this reality you  know it's like if you were a fish in water and you were asked to describe water you wouldn't be  able to do it but if you see a bubble that comes up past you might think what is that thing well  that's exactly what visual illusions are to us you almost have to break the brain or find  its shortcomings to figure out how it actually works it's doing the best that it can i guess with  with limited information well we wouldn't know what we're missing right so we think it's doing a  good job but maybe in a hundred years when we all you know wear glasses for detecting  infrared and ultraviolet and the rest of the electromagnetic spectrum maybe we'll say wow it's  actually been doing a terrible job this whole time i look forward to living like  a bee that sounds amazing so why do videos work or movies or games well  the old idea that our eyes work like a camera that motion is burned into our eyes the way light  hits film that turns out to be wrong or at least incomplete your eyes are really good at  sensing the universe and they do sense the individual still images you are watching and  send that information to your brain but our brains know that isn't how the universe really  works so it fills in the gaps and blends this into something else something that isn't really  there imagine if you lived in a black silent room cut off from the outside world the only  descriptions of the outside world that you get are periodic notes passed under the door your image  of reality will 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 isn't the actors that are creating the  story it's your brain so what does this all mean