May 2019, ScientificAmerican.com 81ALAMY (1 ); CHRIS COLLINGRIDGE (2 ); MALCOLM SCHUYLAlamy(^3 ); ANUP SHAHGetty Images( macaque)their activity down, summing up the input from incoming pho-
tons over, say, 100 milliseconds. We experience this effect when
we see a shooting star in the night sky. The star is a point at any
given moment, but the brain interprets the sight as a line because
it is summing those points over a period of time. Again, there’s a
trade-off. This type of summation makes it easier to detect
objects, but it blurs them when they move.
In some insects, spatial and temporal summation happen in
parallel, and it occurs in cells farther back toward the brain,
according to biologist Anna Stöckl, now at the University of
Würzburg in Germany. Stöckl, when she was a graduate student
under Warrant, positioned hawkmoths in front of a computer
screen showing a pattern of scrolling black-and-white stripes.
Then she cut a tiny hole in the back of each moth’s head and
poked electrodes into its cells. Her goal was to stimulate the
photoreceptors with each alternating stripe and compare their
activity with that of other nerve cells deeper in the brain, in the
optic lobe. This area gets the signal after any processing or sum-
mation has occurred, so differences between the unprocessed
“input” at the photoreceptor and the “output” in the optic lobe
would indicate that the brain altered the visual signal.
Comparing these input and output values, Stöckl calculatedthat when she transferred moths from light to dark, the size of a
“pixel” in their optic lobe quadrupled, showing that they used spa-
tial summation. She also found that moths used temporal summa-
tion, slowing their vision in the dark so they added up input over
220 milliseconds. The combination allowed the hawkmoths to see
well at light levels 100 times dimmer than when summation was
not in use, Stöckl reported in a 2016 paper.
“This hasn’t been shown in any other animal apart from
hawkmoths, but the principle is so basic that it would be hard
to believe it isn’t widespread,” Warrant says.
Another approach that animals use is to filter out noise, say
scientists who have investigated visual noise-canceling methods
used by mice and monkeys. While not on a par with hawkmoths,
these mammals do reasonably well at night. Researchers have
found there are at least two threshold points on a path between
their photoreceptors and the brain that allow only strong signals
through and reject those likely to be noise. Midway along this
path are gatekeepers called rod bipolar cells. These cells, it turns
out, are tuned to send the “photon detected” signal onward only
if they receive significant input from rods. Several incoming pho-
tons at once are strong enough. But single photons, and much of
the noise in the system, might not be. A second cellular gate lies
deeper in the optic system on this same path. This gate blocks
errant signals that are missed by the first one or that arise after
that point. The result is nearly noiseless vision, says Petri Ala-
Laurila of the University of Helsinki, one of the scientists who
identified the process.FORWARD-LOOKING
despite all this research, Warrant says, scientists are just
beginning to understand animals’ ability to see in the dark and
how they manage to do so. Studies of the genes and light-sensi-
tive molecules that nocturnal animals possess can offer new
clues. For example, some night-active lemurs have genes and
pigments that indicate their eyes might be sensitive to blue or
green, which could help them distinguish blue seeds and green
leaves in twilight. And some bats—which, contrary to popular
wisdom, are not blind—also possess genes tied to color vision.
Still, having the genes and molecules to detect color does not
prove an animal’s brain uses that information after twilight. For
example, some light-sensitive molecules are involved in maintain-
ing bodily rhythms that have nothing to do with vision. Therefore,
scientists still need to perform behavioral experiments, such as
those carried out on the hawkmoths and frogs, to show those mol-
ecules play a role in night sight. That work may indicate that the
molecules are not used in the dark—or it could reveal sight-
enhancing tricks researchers have not yet envisioned.IN DIM LIGHT, sweat bees ( 1 )
detect detailed patterns, dung
beetles ( 2 ) navigate by star-
light, hawkmoths ( 3 ) blend
visual signals to brighten imag-
es, and southern pig-tailed
macaques ( 4 ) filter interfer-
ence from what they see.123 4MORE TO EXPLORE
Nocturnal Colour Vision—Not as Rare as We Might Think. Almut Kelber and Lina S. V.
Roth in Journal of Experimental Biology, Vol. 209, No. 5, pages 781–788; March 1, 2006.
Vision in Dim Light. Eric Warrant in Visual Ecology. Thomas W. Cronin, Sönke Johnsen,
N. Justin Marshall and Eric J. Warrant. Princeton University Press, 2014.
Vision in Dim Light. Compiled and edited by David O’Carroll and Eric Warrant. Theme
issue of Philosophical Transactions of the Royal Society B, Vol. 372; April 5, 2017.
FROM OUR ARCHIVES
Household Pest Sees the Light. Rachel Nuwer; Advances, March 2015.
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