ADVANCES
16 Scientific American, October 2019 Illustration by Brown Bird Design
Faster processingActual pathPerceived pathPerceived pathSlower processing Faster processing Slower processingNEUROSCIENCE
Depth-Defying
Illusion
A common visual correction may
distort 3-D motion perception
The lenses in human eyes lose some abili-
ty to focus as they age. Monovision—a
popular fix for this issue—involves pre-
scription contacts (or glasses) that focus
one eye for near-vision tasks such as read-
ing and the other for far-vision tasks such
as driving. About 10 million people in the
U.S. currently use this form of correction,
but a new study finds it may cause a
potentially dangerous optical illusion.
Nearly a century ago German physicist
Carl Pulfrich described a visual phenome-
non now known as the Pulfrich effect:
When one eye sees either a darker or a
lower-contrast image than the other, an
object moving side to side (such as a pen-
dulum) appears to travel in a three-dimen-
sional arc. This is because the brain pro-
cesses the darker or lower-contrast image
more slowly than the lighter or higher-
contrast one, creating a lag the brain
perceives as 3-D motion.
Johannes Burge, a psychologist at the
University of Pennsylvania, and his col-
leagues recently found that monovision
can cause a reverse Pulfrich effect. They
had participants look through a device
showing a different image to each eye—
one blurry and one in focus—of an object
moving side to side. The researchers
found that viewers processed the blurrier
image a couple of milliseconds faster than
the sharper one, making the object seem
to arc in front of the display screen. It
appeared closer to the viewer as it moved
to the right (if the left eye saw the blurry
image) or to the left (if the right eye did).
“That does not sound like a very big deal,”
Burge says, but it is enough for a driver at
an intersection to misjudge the location
of a moving cyclist by about the width of
a narrow street lane ( graphic ).
Burge and his colleagues had expected
the opposite: that the brain would process
the blurry image more slowly because of
its lower contrast, similar to the traditionalPulfrich effect. They resolved this paradox
by showing that blur reduces the contrast
of fine details more than that of coarse
ones. Because the brain takes more time
to process fine details, the blurry image
is processed faster. The researchers
published their study in August in Cur-
rent Biology.
Douglas Lanska, a retired University
of Wisconsin neurologist who has studied
the Pulfrich effect and was not involved
in the study, calls the findings “intriguing”
but says, “My guess is that the modeling
overestimated the real-world impact
some.” The reverse Pulfrich effect should
be tested outside the laboratory,
Lanska adds.
Burge and his team found they could
correct the effect by tinting the blurrier
lens, creating a classic Pulfrich effect that
cancels out the reverse one. The brain
may also compensate for the limitations
of monovision—but further study is need-
ed, Burge says. These misperceptions are
rare, he notes, suggesting that “under nor-
mal circumstances, our visual systems are
exquisitely well calibrated.” — Tanya LewisThe Reverse Pulfrich Effect
Monovision, a common visual
correction in which a lens in one
eye is focused for near vision and
the other for far vision, results in
one eye forming a blurry image at
a given distance. In a phenomenon
dubbed the reverse Pulfrich effect,
the brain processes the blurry
image more quickly than the sharp
one, creating an illusion in which a
moving object or person (such as
a cyclist) appears either farther
( left ) or closer ( right ) than he or
she actually is. To err on the side
of caution, the far lens could be
placed in the right eye in countries
where people drive on the right
side of the road, and in the left eye
where people drive on the left,
researchers suggest.