The Scientist November 2018

11.2018 | THE SCIENTIST 53

DIEGO FERNANDEZ, NIMH; ©

ISTOCK.COM, JANULLA

PROJECTION: Intrinsically photosensitive retinal ganglion cells help LIT UP: Suppressing activity in the amygdala (red) reduces fear.

transmit light signals from the retina to mood-regulating neural circuits.

NEUROSCIENCE

Overcoming Fear

THE PAPER

L.D. de Voogd et al., “Eye-movement intervention enhances extinction

via amygdala deactivation,” J Neurosci, 38:8694–706, 2018.

EYEING THE PROBLEMS

Some psychotherapists coach patients to recall traumatic memories

as they make back-and-forth eye movements, tracking the therapist’s

hand. The procedure, eye movement desensitization and reprocessing

(EMDR), helps lessen the power of those memories, but how it works

“has been kind of unknown,” says psychologist Joseph Dunsmoor of

the University of Te x a s at Austin.

ENHANCING EXTINCTION

Lycia de Voogd of Radboud University in the Netherlands and her

colleagues sought to integrate EMDR and a form of conditioning

known as fear extinction, a way of lessening fear through repeated

exposure to a stimulus. They gave 24 healthy subjects electric shocks

to their fingers as the participants looked at blocks of color on a screen.

The next day, the participants simply looked at the blocks, with or

without tracking a moving dot with their eyes for 10 seconds. On the

third day, the researchers reapplied the shock to subjects as they

looked at the color blocks again in order to reinstate the fear response.

SETTLE DOWN

EMDR in tandem with fear extinction dampened skin conductance, a

measure of fear, more than extinction alone. Additionally, fMRI scans of

participants revealed that reduced fear recovery corresponded with less

activation in the fear-processing amygdala. Both a working memory task,

which involved keeping track of a number sequence, and guided eye

movements independently tamped down activity in the amygdala while

activating brain pathways involved in controlling emotion.

LOOKING FURTHER

Dunsmoor, who was not involved in the study, notes that knowing the

mechanism underlying EMDR could help identify other techniques to

help patients deal with trauma.

—Sukanya Charuchandra

NEUROSCIENCE

Mood Lighting

THE PAPER

D.C. Fernandez et al., “Light affects mood and learning through distinct

retina-brain pathways,” Cell, 175:71–84.e18, 2018.

SEASONAL BLUES

Depressive feelings associated with fewer hours of daylight in winter

were once considered an indirect consequence of circadian rhythm

disruption. But in 2012, chronobiologist Samer Hattar, then of Johns

Hopkins University, and colleagues showed that light can boost mood

scores—along with learning ability—in mice, even when sleep and

circadian rhythms are unperturbed.

THE THIRD CELL

To understand these effects, the researchers looked at recently

discovered photoreceptors known as intrinsically photosensitive retinal

ganglion cells (ipRGCs), which unlike rods and cones play no role in

image formation. “Anatomical data suggested that [the] cells can

directly influence several brain areas involved in mood and learning

functions,” study coauthor Diego Fernandez of the National Institute

of Mental Health, where Hattar now works, writes in an email.

FORK IN THE ROAD

Unexpectedly, transgenic mice with different populations of ipRGCs ablated

revealed two independent pathways mediating mood and learning. One

set of ipRGCs projected to the suprachiasmatic nucleus, a brain region

associated with circadian function—although rhythms were unaffected in

the animals. That pathway mediated light’s effects on learning, while cells

projecting to the perihabenular nucleus in the thalmus regulated mood.

“We were stunned that they are completely dissociable,” Hattar says.

OUT OF THE DARK

The results further support the circadian-clock independence of

some of light’s effects, and could illuminate the mechanisms behind

neuropsychiatric disorders associated with certain light conditions, says

Lily Yan, a neuropsychologist at Michigan State University. Hattar’s team

is now keen to understand more about light’s effects, he says. “Why

should light enhance your mood?” —Catherine Offord