[INTRO ♪] Everyone loves a good optical illusion. There are ones that play with your perception
of color, or look like they’re swirling even though
they’re still. And how about the Müller-Lyer illusion? It’s named after the German sociologist,
Franz Carl Müller-Lyer, who came up the image in 1889. Basically, it’s a couple of arrow-like figures. How do you think the lines compare to each other? Even if you’ve seen this illusion before,
chances are the lines look like they’re slightly different lengths—but
they’re actually the same. But not everyone falls for this trick. A lot of it might actually have to do with
where you’re from and what you’re used to seeing, because optical
illusions are designed to trick your brain. The brain’s visual cortex processes everything
your eyes see, starting with two-dimensional images from
your retinas, and figuring out depth somehow. We think the brain calculates depth using
binocular disparity— the difference between what your right eye
sees and what your left eye sees— along with other cues, like the size of different
things and angles in a space. Psychologists think the Müller-Lyer illusion
hinges on those other visual cues that hint at three dimensions, and works best
on brains that are used to seeing a bunch of right angles. Many of us live and work in spaces that are
chock full of right angles, so we’re used to interpreting depth in boxy
rooms. If you’re standing on one side of a long
table, for instance, you know that the side closest to you is the
same size as the opposite. But the closer side looks bigger, and the
farther side looks smaller. That’s thanks to perspective. The Müller-Lyer illusion plays with our brain’s
sense of perspective. The arrow-like caps on the ends of each line
might trick our brains into interpreting both of them as having some
depth. So the line with inward-pointing arrow heads
is being interpreted as farther away, and the line with outward-pointing arrow heads
is being interpreted as closer. Even though our eyes might detect that the
lines are the same length, our brains might get confused about the perspective. You brain might perceive the farther line as smaller, like that far end of a table in a 3D world, and therefore think it’s really a longer line. Not all people are surrounded by so many 90-degree
angles, though. And those who aren’t seem to be less susceptible
to the Müller-Lyer illusion. In the 1960s, a group of researchers led by
psychologist Marshall H. Segall set out to test what they called the “carpentered
world” hypothesis and the “experience with two-dimensional
representations of reality” hypothesis. The researchers proposed that some peoples’
experience with “carpentered” or box-like spaces and 2D images of 3D spaces,
like photographs of rooms, helped the illusion trick their brains. They collected data from individuals in 17
groups of people around the world, showing them sets of two arrow-like lines— one with the caps pointing out, labeled a,
and one with the caps pointing in, labeled b. Then, they gathered data about how much longer
line a needs to be than line b before people perceived them as
equal. The greater the difference, the more susceptible
the group was to the illusion. The most susceptible group was from Evanston,
Illinois in the United States. And the group of Europeans living in Johannesburg,
South Africa was pretty susceptible too. On the other hand, societies of agriculturalists
and foragers, like the San from the Kalahari Desert or Bete
from the Ivory Coast, were less tricked by the illusion. And they probably didn’t spend as much time
in right-angle-filled environments. Of course, one study isn’t definitive proof
of a phenomenon. A different study from 1973 involved one group
of Americans and five groups of Zambians. And a difference was found between Zambians
living in rural and urban environments, where those living in urban environments were
more susceptible to the illusion. This supported the idea that being tricked
by the illusion could involve factors like people’s immediate
surroundings. And as psychologists have studied this illusion
more, it seems like how you perceive the lines in
the Müller-Lyer illusion is at least partially influenced by what you
see all the time, because that’s what your brain is used to
interpreting. Other researchers have tried to look into
different cases where visual perception seems to be influenced
by your surroundings and culture. A 2005 study, for instance, found that Japanese
and American undergraduates noticed changes in images differently. They were shown pictures of different scenes
like an American city, a Japanese city, or something without cultural
markers like a generic construction site. And they were given a change blindness task, where they had to pick out small changes between
very similar sets of pictures. With American scenery, all participants were
generally better at seeing changes that involved prominent objects. And with Japanese scenery, all participants
were generally better at noticing changes in the background. But with the neutral scenery, it seemed like
cultural differences showed up: Japanese students noticed more contextual
changes, and American students noticed more focal object
changes. Citing other studies that tracked participants’
eye movements, the researchers proposed that these differences
could be because different people pay attention to their surroundings
differently. Or it could mean that different environments
encourage different kinds of interpretation by our eyes
and brains. It’s important to remember, though, that
there might be other factors in play in these perception
experiments. But if these kinds of studies tell us anything,
it’s that brains are complicated, and everyone is probably seeing things from
a slightly different perspective. Thanks for using your eyes and brains to watch
this episode of SciShow Psych! If you want to learn more about weird phenomena
involving the human mind, you can go to youtube.com/scishowpsych and
subscribe. [OUTRO ♪]