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Life’s a blur — but we don’t see it that way

This story was originally published by Knowable Magazine.

An image of a painting by French artist Georges Seurat, “A Sunday Afternoon on the Island of La Grande Jatte.” Lines scribbled over the scene come from an experiment that tracked how the human eye jerks around as it takes in the details.

The lines scribbled over this famous Georges Seurat painting
come from an experiment that tracked how the human eye jerks around
as it takes in the details of the scene.

CREDIT: R. WURTZ / DAEDALUS 2015 /
PUBLIC DOMAIN

The image above, “A Sunday Afternoon on the Island of La Grande
Jatte,” was painted in 1884 by French artist Georges Seurat. The
black lines crisscrossing it are not the work of a toddler wreaking
havoc with a permanent marker, but that of neuroscientist Robert
Wurtz of the National Eye Institute in the US. Ten years ago, he
asked a colleague to look at the painting while wearing a contact
lens–like contraption that recorded the colleague’s eye movements.
These were then translated into the graffiti you see here.

Art lovers may cringe, yet it is likely that Seurat would have
been intrigued by this augmentation of his work. The movement
Seurat kick-started with this painting — Neo-Impressionism — drew
inspiration from the scientific study of how our vision works.
Particularly influential was the pioneering research of Hermann von
Helmholtz, a German physician, physicist and philosopher and author
of a seminal 1867 book, Handbook of Physiological
Optics
, on the way we perceive depth, color and
motion.

One of the questions that occupied Helmholtz, and quite possibly
Seurat, is why we don’t perceive the constant eye movements we make
when we are scanning our surroundings (or a painted representation
of them). Consider that the lines above were drawn in just three
minutes. If we saw all those movements as we made them, our view of
the world would be a blur of constant motion. As Wurtz and his
Italian colleagues Paola Binda and Maria Concetta Morrone explain
in two articles in the Annual Review of Vision Science,
there’s a lot we know about why that doesn’t happen — and more yet
to learn.

via GIPHY

A short movie of an eye making saccades, shown in slow
motion.

CREDIT: WEEKEND WAY VIA GIPHY

Beginning with the basics: The only things we can ever hope to
see are those that send or reflect light toward our eyes, where it
might end up hitting the retina, a layer of nervous tissue that
covers the back two-thirds of the inner eyeball. There, the complex
image of whatever we are looking at is first translated into
activity of individual light-sensitive photoreceptor cells. This
pattern is then transmitted to a variety of neurons in the retina
that specifically respond to certain colors, shapes, orientations,
movements or contrasts. The signals they produce are sent up to the
brain through the optic nerve, where they are interpreted and put
back together in a progression of specialized areas in the visual
cortex.

Yet to transmit all the information that reaches our retina at
the resolution we are used to would require an optic nerve with
roughly the diameter of an elephant’s trunk. Since that would be
rather unwieldy, only one tiny area of the retina — called the
fovea — provides this kind of resolution. So in order to grant all
the interesting features of our environment their moment in the
foveal spotlight, we move our eyes around — a lot — in darts that scientists call saccades. (French
for “jerks,” the word was coined in 1879 by French ophthalmologist
Émile Javal.) Saccades are guided by what we are paying attention
to, even though we are often blissfully unaware of them.

An illustration lays out the basic structure of the eye, including the fovea, where images are rendered in high resolution. Eye jerks known as saccades allow different parts of a person’s field of view to move into line of sight of the fovea.

This illustration laying out the basic structure of the eye
shows where the fovea — where images are rendered in high
resolution — is situated. Eye jerks known as saccades allow
different parts of a scene to come into the line of sight of the
fovea.

There are a number of reasons why these movements don’t
transform our view of the world into a blur of motion. One is that
the most distinct things in our field of view may render us blind
to other stimuli that are fleeting and faint: Objects that are in
clear sight when our eyes don’t move are likely to make a more
vivid impression than the blur in between. Scientists refer to this
phenomenon as visual masking, and it is thought to be very common
in real-life situations where a lot is going on at the same
time.

If scientists set up experiments in a way that avoids this
visual masking, it reveals that our brains can perceive
the less noticeable things. This can be done, Morrone explains, by
showing people nothing but very faint and short-lived visual
stimuli on an otherwise empty background. Under these conditions,
surprising things may happen. When researchers create a motion very
similar to what we should normally perceive when we make a saccade,
by rapidly moving a mirror around in front of people’s eyes, those
people do report seeing movement — and they often find it rather
disturbing. Since we do not notice our constant saccades, this
suggests that the brain specifically suppresses the signals that
reach our retina while a saccadic eye movement is in process. And
indeed, experiments have shown that if something appears during a
saccade, we may miss it entirely.

But suppression does not adequately explain why the image in our
mind’s eye is so stable. If we were to see our surroundings from
one angle, then see nothing, and then suddenly see it from another
angle, that would still be unsettling. Instead, as Wurtz and others
have shown, a kind of remapping happens even before we move our
eyes. In experiments with macaques that were trained to make
predictable saccades, brain cells that receive signals from one
particular spot in the retina switched from responding to things
currently in view there to things that would show up only after the
saccade. And that happened before the monkeys moved their eyes. In
this way, Wurtz thinks, the current image is gradually replaced by
the future one.

So how do these brain cells know in advance that a saccade is on
the way? Scientists theorized for many years that this would
require them to receive an additional signal, from the brain area
that gives the command for eye movement. And they have shown that
such signals do occur, arriving at areas of the brain involved in
coordinating what we see and where we will look next. Wurtz and
others believe that this kind of signal nudges brain cells to start
responding to things that their part of the retina will see only
after the saccade.

Photograph of Georges Seurat, amended to show his eyes making saccades. Like other artists of his time, Seurat was interested in the workings of human visual perception.

Georges Seurat, along with other artists of his time, was
interested in the workings of human visual perception.

CREDIT: WIKIMEDIA COMMONS / PUBLIC DOMAIN / GIF
BY KNOWABLE

All of this is very likely to work almost exactly the same way
in humans as it does in monkeys. But if you ask people what they
see right before a saccade, as Morrone and Binda have done, they don’t
report a gradual replacement of one image by another before their
eyes move. Instead, anything they’re shown during a 100-millisecond
period right before the saccade becomes visible only after the
saccade ends. The result of this delay is that stimuli appearing at
different times within that short period before the saccade may all
be perceived at the same time — 50 milliseconds after it ends.

And if these stimuli are sufficiently similar, they might be
perceived as fused together into one thing, even when they were
shown at slightly different times or places before the eye
movements. Binda and Morrone call this time window right before the
saccade the confusion period. The things we see may literally be
con-fused — fused together — by our vision, and then more
conventionally confused — mistaken for each other — in our
minds.

In real life, this fusion of similar elements across space and
time during saccades might actually help to prevent
confusion, because the continuity helps us to grasp that things we
saw before and after a saccade are the same, even if they have
moved or if the light has shifted. So though the mechanism may seem
very sloppy, Binda and Morrone believe this sloppiness usually
works to our advantage.

A similar sort of desirable imprecision might be what allows us
to enjoy Seurat’s painting in the first place. Instead of a perhaps
more accurate perception of colorful collections of distinct dots,
a beautiful Sunday afternoon emerges. Hats off to that — or, as the
French would say: “Chapeau!”

This story was originally published by Knowable Magazine. Knowable Magazine is an independent journalistic endeavor from Annual Reviews.

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