Scientific American 201905

80 Scientific American, May 2019 Graphics by Tami Tolpa

FROM “NEURAL SUMMATION IN THE HAWKMOTH VISUAL SYSTEM EXTENDS THE LIMITS OF VISION

IN DIM LIGHT,” BY ANNA LISA STÖCKL ET AL., IN

CURRENT BIOLOGY,

VOL. 26, NO. 6; MARCH 21, 2016 (

flower images

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times much more, depending on how he processed the images.

To test this effect on the beetles themselves, Foster built a

simplified, artificial Milky Way out of single-file LED lights

on an arch over an arena. He could vary the intensity of light

on each side. The beetles could go straight if he gave them a

13 percent contrast between one end of the bright line and

the other but wavered if the contrast dropped below that.

This result indicated the animals should be able to tell the

two ends of the real Milky Way apart.

SIGNAL BOOSTERS

in addition to beetles and bees, a number of other animals

are now known to see remarkably well in dark environments:

cockroaches, lantern fish, cuttlefish, frogs and nocturnal pri-

mates such as owl monkeys. So neuroscientists are turning to

the question of how they do it. Bigger eyes collect more light,

for example, but do not gather enough photons to explain

the highly sensitive night vision that scientists have docu-

mented. Other visual processing must take place after the

rods have absorbed incoming light. In particular, animals

must be able to overcome or filter out visual “noise” created

by photoreceptor activity that does not reveal anything use-

ful about the visible world.

Noise in the visual system comes from a few different sourc-

es. One, called photon shot noise, happens when only a few pho-

tons come into photoreceptors. Because those light packets

tend to arrive sporadically, they create a variable, unreliable pic-

ture. It’s as if you shone three or four flashlights around the ceil-

ing of the Sistine Chapel at night. You would hardly be able to

appreciate Michelangelo’s complete masterpiece.

A second source of noise arises from the molecular inter-

actions in the photoreceptors themselves. A photoreceptor

senses light when an incoming photon hits a molecule called

rhodopsin. But every so often—once a minute, at most—a

rhodopsin molecule is triggered by accident, or another part

of the pathway misfires. This is called dark noise because it

can happen even in pitch-black conditions with your eyes

closed. A third source, transducer noise, results from varia-

tion in the timing and strength of the visual system’s re -

sponse to a single real photon.

Noise isn’t a big problem in broad daylight, because the tre-

mendous volume of photons hitting the eyes overwhelms

these slight variations. In the dark, however, animals need a

strategy to boost the signal to similarly noise-overwhelming

levels. They do so by summing up the signals they get from

individual photoreceptors across space and time.

Spatial summation works like this: Imagine you are at a

concert where 1,000 fans are waving their illuminated cell

phones with excitement. You can’t see the light from each indi-

vidual phone all that well. If every group of 50 concertgoers

combined the light of their phones into a single, brighter spot-

light, you’d see those 20 spotlights really well. The retina—the

sheet of tissue that contains rods and cones—does the same,

pooling the input from numerous rods into a single, bigger sig-

nal that gets sent to the brain. At the concert, you lose the pic-

ture of each individual person waving a phone, and the same

thing happens in spatial summation; the resulting image is

brighter but also coarser.

Temporal summation also increases brightness. Rods slow

Time

Slow response

Day Night

Lamina

monopolar cell

Visual

unit

Pooled signal

Pooling lamina

monopolar cell

Spatial summing effect

in low light

Temporal summing effect

in low light

Fast response

Light signals

Nonpooled signal

Low-Light Image

Improvement

During the day, there is a lot of light for high-acuity vision. But

at night, the few available photons rarely stimulate photore-

ceptor cells and do so only weakly. The result is a grainy,

obscure image. Hawkmoths solve this problem by adding up

these scarce photons across both space and time.

Spatial Summation

In dim light, sparse image signals head from photoreceptors toward

visual-processing areas of the brain. But in some insects, in an intermediate

region, lamina monopolar cells pool together signals from adjacent spots

in the eye. This creates a brighter visual unit, but it loses sharpness because

it com bines signals from different locations.

Temporal Summation

Photoreceptor cells can slow down the rate at which they pass signals back

to the brain at night. Each individual signal, at fast speeds, is too weak to stim-

ulate much of a brain response in visual areas. But by slowing their responses

down, the cells let the signals pile on top of one another, creating a stronger

stimulus. This also improves brightness, though again with less acuity.

© 2019 Scientific American