[Music]
this video is an illusion it might seem
like I'm moving around but I'm not I'm
just a series of completely still images
flashing quickly before your eyes your
brain is stitching those frames together
to create the perception that I'm moving
around like I would be if I were right
in front of you your brain is always
interpreting and trying to understand
the world on the fly and it doesn't
always get all the information it needs
from all your senses to figure out
what's actually happening so a lot of
the time your brain has to guess based
on what you've experienced in the past
and how humans have evolved and it
usually does a very good job of this but
once you know how to exploit the
shortcuts that the brain likes to take
it's easy to fool them and that is
exactly what optical illusions do and
they're more than just fun party tricks
so here are eight illusions that have
taught scientists a lot about how eyes
and brains process and interpret
different kinds of visual information
let's start with the illusion that
you're looking at pretty much whenever
you're on YouTube videos like this one
it works because it takes some time for
your brain to process whatever you're
looking at so if you suddenly look
somewhere else there's about a fifteenth
of a second where your brain is still
holding on to what you were just looking
at just in case it might come in handy
for processing whatever you're looking
at now this effect is called the
persistence of vision and it's why
videos fool your brain in videos the
next frame is already on the screen
before your brain lets go of the one
before it and so you naturally meld them
together into a continuous single thing
that's how a bunch of stationary images
create the illusion of motion there are
also other ways old images can stick
around stare at a bright green screen
for about a minute without blinking and
then quickly shut your eyes or look at a
white wall and you'll probably see a
magenta rectangle right where the green
used to be that magenta rectangle is an
after image and it's caused by what's
known as retinal fatigue when certain
cells in your eyes have been active for
a long enough time they can basically
run out of energy when you stare at the
green screen the green cone cells and
your eyes are constantly firing to tell
your brain that there's something green
in front of you while the red and blue
ones are just sort of sitting there then
when you look at something else those
green cones desperately need a break if
you're looking at a white wall which
should be an equal mixture of red green
and blue the greens just aren't able to
keep going so instead your eyes tell
your brain that your
looking at something that's equal parts
red and blue but not green
also known as magenta other kinds of
cells can also get overstimulated and
that's what's going on with a spiral
that sometimes called the waterfall
illusion or the motion after effects
tear at the very center of this spiral
and try not to blink keep your eyes
locked on the spot too getting sleepier
very sleepy okay but actually that's a
different thing all right now how do I
look a little bloated maybe a little
spinny the motion after effect is the
motion version of the magenta rectangle
from before your eyes have special cells
that detect movement and rotation in
different directions they are really
important to help you navigate the world
but they can get overused just like your
color cone scientists think that's what
happens when you watch the spiral the
cells that detect rotation in that
direction fire constantly while the ones
for the rotation in the other direction
aren't doing much at all the firing
cells were exhausted by the time I
showed back up on the screen so they
probably couldn't respond well enough to
match their neighbors and tell your
brain that I wasn't rotating so your
brain assumed I started rotating the
opposite way the spiral was which is why
I seemed like I was spinning in the
opposite direction of the spiral it's
called the waterfall illusion because
rocks can start to look like they're
moving up into the sky after you've been
staring at a waterfall for long enough
Sheppard's tables are named after the
cognitive scientist Roger Shepard who
published and explained the ocean back
in 1990 these two tables might seem like
they are completely different shapes one
is long and slender the other is short
and squat but they are actually the
exact same shape and size just rotated
so that one is horizontal and the other
is more vertical the trick has to do
with perspective like the kind and a
photo of train track train tracks look
like nearly vertical lines but your
brain knows that they are only like that
because they are getting farther away
from you so you don't see 2d lines you
see 3d tracks your brain tries to
interpret Shepard's vertical table in
the same way it assumes that long nearly
vertical lines mean that it's receding
into the distance which would mean that
the table on the left is much longer
than it looks this illusion actually
dates all the way back to the 1850s when
a German psychologist - named Adolf Fick
noticed that people usually think that
vertical lines are longer than
horizontal lines so the science behind
it wasn't fully explained until Shepard
came along a century late
the spinning dancer illusion was created
by Japanese web designer Nobuyuki
kiyohara back in 2003 about two-thirds
of people think the dancer is spinning
clockwise on her left foot but everybody
else sees her spinning counterclockwise
on her right foot and some people are
able to switch between the two if they
try hard enough this is called a
bi-stable illusion and it works the same
way as the famous rabbit duck illusion
and the Necker cube in a bi-stable
illusion there just isn't enough
information to tell you exactly what
you're seeing so different people's
brains make different kinds of
assumptions and settle on different
conclusions a lot of people have said
that illusions like the spinning dancer
show which side of your brain is
dominant that's not actually true just
has to do with the way you interpret the
lighting and the camera Mouse people
automatically assume that they're seeing
the dancer from above so they follow her
hand which seems to go clockwise from
that perspective others assume they're
looking at her from below and focus on
her foot which moves counterclockwise
from that angle so the dancer can move
in either direction depending on how
your brain interprets the images there
are lots of illusions where all we know
is that the brain is being tricked
because it's making some kind of
assumption brains are complicated others
we do know a lot about take this image
so boxes marked a and B might look like
different shades of grey but they are
actually the exact same color it's
called the checkered square illusion
your brain is making an assumption here
about shadows but there's a lot more to
the illusion than that it has to do with
something called lateral inhibition when
you look at a big region that's all the
same color your brain doesn't want to be
receiving all those identical signals it
would be clogging it up when it could be
focusing on something more important so
the nerves of the back of your eyes sort
of summarize what you're seeing they
send your brain signals that say things
like this entire area is gray and then
it gets brighter here along this line
and the checkered square illusion your
brain gets signals saying that a is
surrounded by lighter areas while B is
surrounded by darker areas so your brain
assumes that a must be darker and B must
be lighter and that is why a looks
darker than B there's another illusion
that happens because of the way your
brain interprets light and dark and you
might have noticed it in this very video
people pointed out all the time in the
comments every time I wear this shirt
it's called the Hermann grid illusion
where you tend to see dark
is on light lines separating darker
squares into a kind of checkerboard
pattern but you don't see the patches
when you look directly at the lines
there are two kinds of white areas in
the checkerboard grid illusion
intersections that is the areas between
four squares and stretches or the lines
between two squares like with the
checkered square illusion the illusion
on my shirt probably happens because of
the way your brain interprets different
regions based on their surroundings
between squares white is mostly
surrounded by dark so your brain assumes
these little white strips must be even
lighter as if the darker squares nearby
meant the white stretches were in shadow
but at the intersections the opposite
happens the white in the center of the
intersection is surrounded by white on
all sides so your brain assumes that
it's probably darker than it appears the
brain puts in dark spots to compensate
which is why it looks like there are
little dots all over my shirt but not
actually when you're looking directly at
them all of the illusions we've talked
about so far happened because of
neuroscience and psychology but some
have to do more with the laws of physics
mirages mirages happen because of
temperature inversions where a layer of
hot air is trapped right above the
ground by a layer of cooler air above it
light bends in air just like it does in
water but the amount that it's bent
depends on the temperature of the air if
the hot air on the ground is thick and
warm enough it can act almost like a
mirror bending incoming light back away
from the ground and into your eyes
creating a mirage
since air is pretty turbulent mirages
tend to shimmer as if light is
reflecting off of water instead of a
mirror you hear about somebody in the
desert seeing a mirage of a body of
water this is why but mirages might also
be an explanation for a famous myth the
ghost ship called the Flying Dutchman
legend has it that the Flying Dutchman
is doomed to sail the seas forever and
sailors would sometimes even see the
ghostly looking ship on the water the
ship was just a mirage it happened when
the Sun warmed the air above the ocean
but the water kept the lowest layer of
air-cool this can create a mirage that's
the opposite of what happens in the
desert instead of looking like it's
reflected off the ground light can bend
and make things look like they're
floating in mid-air a ship just over the
horizon for example might look like it's
floating above the water like some sort
of ghost ship so sometimes you might see
a non-existent
genta rectangle a bunch of dots on my
shirt were even the flying dutchman
shimmering above the horizon but they
are just optical illusions what happens
when you were brain or the laws of
physics play tricks on you thanks for
watching this episode of scishow which
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something that isn't infectious or
contagious become an epidemic and yet
the prevalence of nearsightedness in the
u.s. is pushing 40 to 50 percent among
young and that's nothing compared to
parts of East