PHOTO: FSTOP123 /ISTOCK.COM
therefore enhance this aspect of cognition
specifically for individuals with relatively
lower dopamine function in the caudate
nucleus. This indicates that individual vari-
ation in dopamine function matters, both
for explaining differences in cognitive func-
tion across healthy individuals, and also for
treatment efficacy. Such reduced dopamine
function may render healthy individuals
more prone to illicitly use methylphenidate
as a study drug. Similarly, those with ADHD
who have lower caudate dopamine function
are likely to have enhanced efficacy from
this treatment (see the figure).
Unfortunately, the effects of dopamine
on cost-benefit weighting do not discrimi-
nate on the basis of the longer-term reper-
cussions of reaching the goal. For example,
methylphenidate also increases the dopa-
mine signal in response to drug-related
stimuli in those with a substance use dis-
order ( 9 ), which in the context of the find-
ings of Westbrook et al. will enhance the
willingness to expend effort to attain the
drug-related goal. Even without methyl-
phenidate’s facilitatory effects, dopamine
plays a key role in substance use disor-
ders, as all abused substances enhance
dopamine release ( 10 ). After repeated drug
use, environmental stimuli associated with
drug abuse also evoke dopamine release
in regions that include the caudate nu-
cleus ( 11 ), the same brain region shown by
Westbrook et al. to be linked with the mo-tivation to expend cognitive effort. These
drug-associated stimuli can then drive the
strong desire to acquire the drug of abuse,
contributing to relapse even when individ-
uals are trying to abstain ( 12 ). Variation in
dopamine function plays a role in this ex-
ample as well. Specifically, drug-associated
stimuli evoke more activation in reward-
related brain regions such as the striatum
and prefrontal cortex, in individuals with a
genetic variation in the gene encoding the
dopamine transporter (DAT) that results
in slower clearance of dopamine and an
amplification of the dopamine signal ( 13 ).
Determining whether a goal is “worth
the effort” is just one aspect of goal-related
decision-making. Such decisions are made
through the integration of information
processed by the striatum, including the
caudate nucleus, and other brain regions
like the frontal cortex, which allows for
learned experiences and potential future
consequences to also shape goal-related
behavior ( 3 ). The influence of dopamine
within these frontal brain regions is not
straightforward but depends on the rela-
tive balance of D1- and D2-like dopamine
receptors ( 14 ). These receptors act like a
lock into which dopamine fits, and their
activation results in the propagation of
the dopamine signal. This points out that
the “dopamine signal” is not only driven
by dopamine synthesis, as was measured
by Westbrook et al., but also involves the
activation of specific receptor subtypes.
Next steps should integrate these findings
to explain how dopamine synthesis and re-
ceptor function across brain regions work
together to affect cognition and behavior.
Such findings will create a clearer under-
standing of cognitive function and psychi-
atric diseases such as ADHD and substance
use disorders. jREFERENCES AND NOTES- M. T. Treadway et al., J. Neurosci. 32 , 6170 (2012).
- A. Westbrook et al., Science 367 , 1362 (2020).
- S. N. Haber, Dialogues Clin. Neurosci. 18 , 7 (2016).
- A. Nieoullon, Prog. Neurobiol. 67 , 53 (2002).
- E. M. Tricomi, M. R. Delgado, J. A. Fiez, Neuron 41 , 281
(2004). - M. A. Addicott et al., Pharmacol. Biochem. Behav. 183 , 14
(2019). - T. T.-J. Chong et al., Cortex 69 , 40 (2015).
- N. D. Volkow et al., Biol. Psychiatry 57 , 1410 (2005).
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ACKNOWLEDGMENTS
A.C.J. is supported by NIDA grant K02 DA042987. She pro-
vides consulting services for Axial Biotherapeutics.10.1126/science.abb0265Functional Integration of Addiction Research Lab,
Department of Psychiatry, Harvard Medical School,
McLean Hospital, Belmont, MA 02478, USA.
Email: [email protected]SCIENCEAPPLIED PHYSICSThermal light
tunnels its way
into electricity
New devices convert
low-temperature heat
into electricity
By Aaswath P. RamanI
n 1824, a 28-year-old French engineer
named Sadi Carnot published a now-
celebrated treatise that sought to un-
derstand how efficiently heat could be
converted to work ( 1 ). While Carnot
laid the foundations for our modern
understanding of thermodynamics, he also
implicitly identified a pathway for energy re-
covery and harvesting that may prove to be
an essential component of the 21st-century
response to climate change. Carnot’s epony-
mous cycle revealed that in any energy con-
version process, some heat will always be
rejected to a cold reservoir—typically the
ambient environment. In the United States
alone, 61% of the energy we consume is re-
jected in this manner as “wasted” heat ( 2 ).
However, this waste heat need not actually
be lost. It can be recovered, in principle, by
driving another energy conversion device to
generate electricity. On page 1341 of this is-
sue, Davids et al. propose and implement a
way to directly convert lower-temperature
heat into electricity ( 3 ).
Recovering heat rejected in transpor-
tation, power generation, and industrial
processes motivates a broad range of con-
temporary research in solid-state materi-
als and devices. A large fraction of waste
heat is lost radiatively, emitted as incoher-
ent, broadband electromagnetic waves at
long-wave and mid-infrared (IR) wave-
lengths. Converting this thermal radiation
to electricity has been an exciting, although
challenging, subset of this line of inquiry.
In particular, thermophotovoltaic devices
have shown considerable promise at be-
ing able to convert emitted thermal radia-
tion to electricity through a photovoltaic
mechanism. Substantial progress has been
made in thermophotovoltaics by using
photonic approaches ( 4 ) as well as near-Department of Materials Science and Engineering,
University of California, Los Angeles, CA 90049, USA.
Email: [email protected]Drugs of abuse increase caudate dopamine and
reduce the perception of effort to acquire them.20 MARCH 2020 • VOL 367 ISSUE 6484 1301