How optical illusions trick your brain Nathan S. Jacobs
Check this out:
Here’s a grid, nothing special,
just a basic grid, very grid-y.
But look closer,
into this white spot at the center
where the two central vertical
and horizontal lines intersect.
Look very closely.
Notice anything funny about this spot?
Yeah, nothing.
But keep looking.
Get weird and stare at it.
Now, keeping your gaze
fixed on this white spot,
check what’s happening
in your peripheral vision.
The other spots, are they still white?
Or do they show weird flashes of grey?
Now look at this pan for baking muffins.
Oh, sorry, one of the cups is inverted.
It pops up instead of dipping down.
Wait, no spin the pan.
The other five are domed now?
Whichever it is, this pan’s defective.
Here’s a photo of Abraham Lincoln,
and here’s one upside down.
Nothing weird going on here.
Wait, turn that upside down one right side up.
What have they done to Abe?
Those are just three optical illusions,
images that seem to trick us.
How do they work?
Are magical things happening
in the images themselves?
While we could certainly be
sneaking flashes of grey
into the peripheral white spots
of our animated grid,
first off, we promise we aren’t.
You’ll see the same effect with a grid
printed on a plain old piece of paper.
In reality, this grid really is just a grid.
But not to your brain’s visual system.
Here’s how it interprets the light
information you call this grid.
The white intersections are surrounded by
relatively more white on all four sides
than any white point along a line segment.
Your retinal ganglion cells notice that
there is more white around the intersections
because they are organized to
increase contrast with lateral inhibition.
Better contrast means it’s easier
to see the edge of something.
And things are what your eyes
and brain have evolved to see.
Your retinal ganglion cells don’t
respond as much at the crossings
because there is more lateral inhibition
for more white spots nearby
compared to the lines,
which are surrounded by black.
This isn’t just a defect in your eyes;
if you can see, then optical illusions
can trick you with your glasses on
or with this paper or
computer screen right up in your face.
What optical illusions show us
is the way your photo receptors and brain
assemble visual information
into the three-dimensional world
you see around you,
where edges should get extra attention
because things with edges
can help you or kill you.
Look at that muffin pan again.
You know what causes confusion here?
Your brain’s visual cortex operates on
assumptions about the lighting of this image.
It expects light to come from
a single source, shining down from above.
And so these shading patterns could only
have been caused by light shining down
on the sloping sides of a dome,
or the bottom of a hole.
If we carefully recreate these clues
by drawing shading patterns,
even on a flat piece of paper,
our brain reflexively creates
the 3D concave or convex shape.
Now for that creepy Lincoln
upside down face.
Faces trigger activity
in areas of the brain
that have specifically evolved
to help us recognize faces.
Like the fusiform face area and others
in the occipital and temporal lobes.
It makes sense, too,
we’re very social animals
with highly complex ways of
interacting with each other.
When we see faces,
we have to recognize they are faces
and figure out what
they’re expressing very quickly.
And what we focus on most
are the eyes and mouth.
That’s how we figure out if someone
is mad at us or wants to be our friend.
In the upside down Lincoln face,
the eyes and mouth were
actually right side up,
so you didn’t notice anything was off.
But when we flipped the whole image over,
the most important parts of the face,
the eyes and mouth, were now upside down,
and you realized something fishy was up.
You realized your brain had taken
a short cut and missed something.
But your brain wasn’t really being lazy,
it’s just very busy.
So it spends cognitive energy
as efficiently as possible,
using assumptions about visual information
to create a tailored, edited vision of the world.
Imagine your brain
calling out these edits on the fly:
“Okay, those squares could be objects.
Let’s enhance that black-white contrast
on the sides with lateral inhibition.
Darken those corners!
Dark grey fading into light grey?
Assume overhead sunlight
falling on a sloping curve. Next!
Those eyes look like most eyes I’ve seen before,
nothing weird going on here.”
See? Our visual tricks have
revealed your brain’s job
as a busy director of 3D animation
in a studio inside your skull,
allocating cognitive energy and
constructing a world on the fly
with tried and mostly – but not always – true
tricks of its own.