00:00Don't you think Harry Styles looks a bit strange?
00:04Of course, he's upside down.
00:06But that's not what we're talking about.
00:08There's something even stranger here.
00:10And I'll give you a second to try to guess.
00:13Look closely at every detail of this photo.
00:16So, have you noticed it?
00:18Look at his mouth and his eyes.
00:20Although everything else is inverted,
00:22these two elements seem normal.
00:24In other words, they are correctly oriented.
00:27However, I bet you only noticed it after I told you, didn't you?
00:32This phenomenon is called the Thatcher effect.
00:35Our brains are designed to recognize faces in their natural position
00:39by analyzing them globally and identifying the details in an instant.
00:43But when a face is turned over,
00:45it becomes much more difficult to detect anomalies.
00:48So, at first glance,
00:50you might think that the mouth and eyes are also upside down,
00:53just like the rest of the face.
00:55Be careful.
00:56This is not a simple monotonous image of black and white stripes.
01:00I assure you that there is something,
01:02or rather someone, hidden behind these lines.
01:05To solve this mystery,
01:07you just have to shake your head quickly.
01:09If it seems uncomfortable or inappropriate to you,
01:12especially in the presence of your colleagues,
01:14there is an alternative.
01:16If you use a laptop,
01:18tilt the screen slightly,
01:20back and observe the image from an oblique angle.
01:23Do you see Taylor Swift appearing?
01:25Yes, here she is.
01:27Masks have something disturbing
01:29that always makes me uncomfortable.
01:31And here, things are going to get even more disturbing.
01:34For now, it's just a mask turning very slowly.
01:38Well, go on.
01:39Oh, wait.
01:40Do we see the front of the mask again?
01:42This is not really the case.
01:44The mask is still rotating.
01:46This phenomenon is called the illusion of the hollow face,
01:49or illusion of the hollow mask.
01:51What happens is that our brain
01:53lets itself be fooled by perceiving the back of the mask,
01:56the concave part,
01:58as if it were the front,
01:59that is, the convex part.
02:01In other words, instead of seeing the back as hollow,
02:04most of us imagine it as a normal face
02:06coming out towards us.
02:08This illusion works because our brain
02:10relies on what it expects to see,
02:12a relief face,
02:14rather than on what is really there.
02:16In a way, it refuses reality by saying,
02:18No, I'm sure it's a classic face.
02:21Here is another fascinating illusion
02:23based on rotation.
02:25Give us your opinion in the comments.
02:27Do you see a young woman turning to the right or to the left?
02:30This illusion,
02:32called the rotating dancer,
02:34or the illusion silhouette,
02:36has intrigued netizens since 2003.
02:39Some say they see the rotation
02:41in the direction of the needles of a watch,
02:43while others are convinced
02:45that it is done in the opposite direction.
02:47So, who is right?
02:49The short answer? Both.
02:51The detailed answer?
02:53It is an optical illusion
02:55that plays with our perception of depth.
02:57At certain moments of rotation,
02:59it becomes difficult to determine
03:01if the dancer's arms move
03:03in front or behind her body.
03:05In response, our brain guesses,
03:07creating the illusion that it can turn
03:09in one direction or the other.
03:11What makes this illusion even more interesting
03:13is that by adding a few landmarks,
03:15the dancer stops moving.
03:17He then becomes able to discern
03:19the direction of the rotation,
03:21although the initial illusion
03:23works in both directions.
03:25Can you spot an anomaly on these photos?
03:27Strangely, you may not have noticed
03:29that the upper half of the two clichés
03:31represented exactly the same image
03:33of Ryan Gosling.
03:35But why do we feel
03:37that these are two different photos?
03:39This phenomenon is known as
03:41composite face illusion.
03:43With the lower half of another,
03:45our brain interprets it
03:47as a whole new face.
03:49Each half disturbs the perception of the other,
03:51making them indissociable.
03:53However, this illusion works mainly
03:55when the face is in its normal position.
03:57If we turn it or unalign
03:59the two parts,
04:01our brain manages to better dissociate them,
04:03which allows us to recognize
04:05the original faces more easily.
04:07This then reveals that the two upper parts
04:09are perfectly identical.
04:11Why do we see Adele's head floating
04:13alone in the middle of nowhere?
04:15Let's add a background to give a little context.
04:17Oh, wow!
04:19This is not exactly what I expected.
04:21But let's go.
04:23Now, look around Adele.
04:25Do you think the background is moving or not?
04:27Do you have the answer?
04:29In reality, the background gives the impression of moving,
04:31but it is totally still and flat.
04:33If you have any doubts,
04:35pause the video.
04:37You will see that the purple and yellow waves
04:39seem to continue to move
04:41in a frenetic way.
04:43This phenomenon is due to a pattern
04:45specially designed to deceive your brain,
04:47using contrasting colors,
04:49shapes and positions
04:51that create this illusion of movement.
04:53Our brain interprets
04:55the variations of light as a movement.
04:57Hence this impression that the lines move.
05:01But there is even more fascinating.
05:03Fix your eyes only on Adele
05:05and try not to blink.
05:07Stay focused on her.
05:09So, did you notice that the illusionary movement
05:11seemed to slow down?
05:13Amazing, isn't it?
05:15Many optical illusions
05:17found on the internet rely on the effect of contrast.
05:19They play on the way
05:21our brain interprets things
05:23according to comparisons.
05:25Our perceptions can be amplified or reduced
05:27depending on what they are confronted with.
05:29Do you want an example?
05:31Let's take a picture of Dua Lipa.
05:33Let's duplicate it and apply a gray filter.
05:35The two images
05:37are exactly the same color and shade.
05:39However,
05:41when we add a light gray background to one
05:43and a black background to the other,
05:45our perception changes.
05:47The gray tones of Dua Lipa's pictures
05:49seem different, don't they?
05:51If you don't perceive the effect with this image,
05:53don't panic.
05:55Here is a simpler demonstration.
05:57Look at these two gray rectangles in the center.
05:59They are strictly the same color.
06:01However, you probably see
06:03the one at the bottom as darker.
06:05Why?
06:07Because it is surrounded by a lighter shade,
06:09unlike the one at the top.
06:11Everything depends on the way
06:13our brain interprets the elements
06:15by taking into account their environment.
06:17Ready for the adventure?
06:19Look at this ruler
06:21while we prepare to put the sails on.
06:23So, in which direction does it turn?
06:25Right or left?
06:27Well, it depends,
06:29because you have the possibility to control it.
06:31Now, tilt your head to the left
06:33and you will see the ruler turn in one direction.
06:35Now, tilt it gently to the right,
06:37without rushing.
06:39Do you notice what happens?
06:41It looks like the ship is changing course.
06:43A similar phenomenon occurs here.
06:45This time, the image of the circle
06:47remains completely still,
06:49but you can give it the appearance of movement.
06:51Focus on the black dot in the center
06:53and swing your head back and forth.
06:55You probably see the circle turn.
06:57This illusion is explained by the way
06:59our brain decodes the direction of the movements
07:01based on the ends of the lines
07:03and their angles.
07:05These contradictory signals
07:07create an impression of rotation.
07:09You probably have the impression
07:11that this image is tilted,
07:13as if the columns, especially those in the center,
07:15seemed to be curving.
07:17But once again, your brain plays tricks on you.
07:19Look carefully.
07:21If we add white lines forming a rectangle,
07:23you will notice that the vertical and horizontal lines
07:25are actually perfectly straight.
07:27Let's finish with a little fun exercise.
07:29The next time you pass in front of a retro barber,
07:31stop to observe the usual pole.
07:33Hypnotizing, isn't it?
07:35The red and blue stripes
07:37give the impression of going up
07:39or down endlessly.
07:41But in reality,
07:43they do not move at all in this direction.
07:45They simply turn around the pole,
07:47horizontally.
07:49What creates this illusion
07:51is the diagonal orientation of the stripes.
07:53When the pole turns,
07:55instead of perceiving a simple circular movement,
07:57we have the impression that the stripes
07:59go up or down along the pole.
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