1. Light at Night: Cognitive Disrupter
Light at Night: Cognitive Disruptor
in Drosophila melanogaster
Justin Nelson
Dublin Coffman High School
2. Introduction
By the 1930’s most houses in the US had electricity.
For the first time in history, humans were being
exposed to artificial light at night.
The recently published “World Atlas of Artificial Night
Sky Brightness” documents that greater than 80% of
world’s people live under light polluted skies (Falchi et
al., 2016).
There has not been a comparable study of house-hold
light levels at night, but technological advances have
increased the number of light sources that are now
standard in our homes.
3. Thus, with the evolution of lighting over the past 100 years…
……there has been an increase in night-time exposure to light
4. Other Modern Night-Time Sources of Light Exposure
Until recently, it was assumed by most people that light at night was innocuous.
However, chronic exposure to light at night is now hypothesized to increase risk
for several serious health conditions
5. Rationale for Current Study: Chronic exposure to light at night
over years has been linked to cancer, heart disease and
obesity in older adults. However, short-term effects on
younger individuals have not been studied. Thus, the goal of
the study became to determine whether a week of light at
night exposure could affect brain function; cognitive function
was chosen because of its critical function in daily life and the
existence of well-described cognitive tests in many species.
Comparative Question: Does light exposure at night affect
cognitive function in Drosophila melanogaster (fruit flies)?
7. Rationale: Chronic exposure to light at night has been linked
to cancer, heart disease and obesity in older adults. However,
short-term effects on younger individuals have not been
studied.
Comparative Question: Does light exposure at night affect
cognitive function in Drosophila melanogaster (fruit flies)?
Hypothesis: If light at night impairs cognitive function, then
flies exposed to light at night will take longer to learn
suppression of a positive phototaxic response in a T-maze
than flies maintained in the dark nights.
8. What Is Positive Phototaxis and How Does a T-Maze Work?
The T-maze exploits a fly’s normal positive phototaxic
response. It gives the fly a choice between spending time
in a lighted arm and a darker arm. Most flies prefer the
lighted arm, so they will cross over to it quickly and stay
there. However, if a sufficiently aversive stimulus is added
to the light side (for example, very bitter quinine), then
flies learn to suppress the phototaxic response and will
not cross over into the lighted arm. There are eight 2-
minute trials.
Positive phototaxis means that flies will
walk toward light. This is a normal,
natural behavior.
9. Experimental Design
Flies purchased from
Carolina Biological
emerge from pupa and
are sexed and
separated into tubes
with same sex flies.
Day 0 ~ week 1
Culture tubes are
randomly assigned
to dark nights (DN;
0 lux for 12 h) or
light at night (LAN;
5 lux for 12h).
~ week 2
Flies tested in T-
maze for positive
phototaxic response
and learned
suppression of
phototaxic response
7 nights
of DN or LAN
10. Four Experimental Groups:
Dark Night Males (DN; n=8)
Dark Night Females (DN ; n=8)
Light at Night Males (LAN; n=8)
Light at Night Females (LAN; n=8)
Experimental Design
Screening for Positive Phototaxis
(movement toward light)
Trials 1-8 in T-maze
(light arm paired with diet quinine water, dark arm
paired with water. The quinine and water are added
to Whatman paper placed in the arms of the maze)
Retention Trial
4 h after Trial 8, no pairing
with quinine or water
11. Experimental Design
Independent Variables: Night-time light exposure and sex
Controlled Variables: Age, culture tube preparation, same “blinded”
experimenter, time of day for testing (3:30-6:30 pm)
Dependent Variables: latency to cross into the lighted tube (sec) and total time
spent in the lighted tube during each 2 min trial (sec).
12. Positive phototaxis is movement toward light and is a normal behavior for flies.
There were no significant group differences in latency to cross into the lighted
arm of the T-maze on the first trial (P>0.05; n=8/group; mean + SEM). These
data suggest that phototaxis is comparable among the experimental groups.
The data were analyzed using analysis of variance (ANOVA).
13. Across training trials in the T-maze, there was a significant increase in latency
to cross into the lighted arm which had bitter quinine on the Whatman paper
(P<0.01; n=8/group; mean + SEM) among dark night (DN) flies relative to light
at night (LAN) flies. These data suggest that the DN flies learned to suppress
phototaxis better than the LAN flies. There was no sex differences in behavior.
The data were analyzed by two way repeated measures ANOVA.
Improvingtaskperformance
Increase in
latency across
trials suggests
the DN flies
learned to avoid
lighted arm
No increase in
latency across
trials suggests
the LAN flies did
not learn to
avoid lighted
arm
14. Across training trials in the T-maze, there was a significant decrease in total
time spent in the lighted arm (P<0.01) among dark night (DN) flies relative to
light at night (LAN) flies. Together, these data suggest that the DN flies learned
to suppress phototaxis better than the LAN flies. There was no sex difference in
behavior. The data were analyzed by two way repeated measures ANOVA.
Improvingtaskperformance
The decrease in
latency across
trials suggests
the DN flies
learned to avoid
lighted arm
No decrease in
duration across
trials suggests
the LAN flies did
not learn to
avoid lighted
arm
15. Retention of the Task (4 hours later). During the 60 second retention task, the
latency to cross into the lighted arm was significantly shorter for the LAN flies
than the DN flies (P<0.01; mean + SEM). There was no sex difference in
performance of this task. The data were analyzed by ANOVA.
16. Experimental Design
Negative Geotaxis: Movement away from Earth’s gravitational pull.
Goal: To determine whether general locomotor is altered in response to
exposure to light at night. It is important to rule out potential locomotor
differences among experimental groups in a cognitive task that requires
locomotor behavior.
Procedure: Ten flies are placed in a culture tube that is lightly tapped until
all are on the floor of the tube, then the percent of flies that cross a line 6
cm up within 10 sec is recorded.
0 sec ~2 sec ~5 sec
17. There were no significant group differences in negative geotaxis (P>0.05;
n=30/group; mean + SEM). More than 90% of the flies in each group crossed the
line within 10 sec. These data suggest that locomotor activity is comparable
among the groups. The data were analyzed using Chi Square statistical test.
18. Conclusions:
Flies kept in dark nights (DN) learned to suppress positive phototaxic
behavior when the light was paired with bitter diet quinine water.
Flies exposed to light at night (LAN) displayed cognitive deficits in both
learning and retaining the suppression of photopositive behavior
relative to DN flies.
There were no sex differences in the learning task.
LAN did not significantly affect positive phototaxic behavior or negative
geotaxis.
Implication: Light at night is not necessarily an innocuous
stimulus and even a relatively short exposure can affect
behavior. The effects of light at night on a wide range of
human behaviors and health measures should be studied.
19. Future Directions:
To determine if some wavelengths of light are less detrimental than others. For
example, red light (~650 nanometers) at night does not alter circadian rhythms.
To expand the study to test other types of cognition, as well as assess other
behaviors, such as aggression, depression, and circadian patterns of activity.
Also, in a preliminary study I weighted some of the flies. There was no difference
between the mean body masses of DN and LAN males, which were
approximately 75% of the mean body mass of DN females (this sex difference is
expected), but only 50% of the body weight of LAN females. In other words, LAN
females had higher average body mass than all other groups. One of the
negative health consequences of LAN in humans is obesity; surprisingly obesity
can also be studied in flies. I would like to determine why there was a sex
difference in LAN effects on body mass and how LAN is changing growth curves.
20. Thank You!
Dr. Courtney DeVries for sponsoring my project.
Dr. Zach Weil for providing suggestions and feedback.
Dr. John Chaston for lending me the maze after I
wrote to ask him for dimensions so I could 3-D print
one (most other people I emailed about dimensions
didn’t respond)
My parents for not getting mad when some flies
escaped into our house….
21. References:
1) Aceves-Pina et al., 1983. Learning and Memory in Drosophila, Studied with Mutants. Cold Spring Harbor Symposium on
Quantitative Biology, 48: 831-840.
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and Behavior, 30(4):330-41.
3) Carolina Drosophila Manual 2005.
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and Behavior, 30:330-41.
8) McFadden E, Jones ME, Schoemaker MJ, Ashworth A, Swerdlow AJ. 2014. The relationship between obesity and exposure to
light at night: cross-sectional analyses of over 100,000 women in the Breakthrough Generations Study. American Journal of
Epidemiology, 180(3):245-50.
9) Quinn WG, Harris WA, Benzer S. 1974. Conditioned behavior in Drosophila melanogaster. Proceedings National Academy
Sciences U S A. 71(3):708-12.
10) Rybnikova NA, Haim A, Portnov BA. 2016. Does artificial light-at-night exposure contribute to the worldwide obesity
pandemic? International Journal of Obesity, 40(5):815-23.
11) Smith WW, Thomas J, Liu J, Li T, Moran TH. 2014. From fat fruit fly to human obesity. Physiology Behavior. 136:15-21.
12) Stevens RG, Zhu Y 2015. Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem? Philos
Trans R Soc Lond B Biol Sci. 370(1667).
Photo credits:
1) Alamy Stock Photos: Earth picture, and associated fly picture. Two pictures
merged by Justin Nelson using Affinity Photo Software.
2) Shutterstock Photos: Fly on leaf and light bulbs.
3) Justin Nelson: all other photos and videos.
