New research indicates that decreased cravings for pleasure may be
at the root of a core symptom of major depressive disorder. The
research is in contrast to the long-held notion that those suffering
from depression lack the ability to enjoy rewards, rather than the
desire to seek them.
The research, led by Vanderbilt psychologists Michael Treadway and David Zald, was published Aug. 12 by the online journal PLoS One.
"This
initial study shows that decreased reward processing, which is a core
symptom of depression, is specifically related to a reduced willingness
to work for a reward," Treadway, a graduate student in psychology,
said.
Decreased motivation to seek and experience
pleasurable experiences, known as anhedonia, is a primary symptom of
major depressive disorder. Anhedonia is less responsive to many
antidepressants and often persists after other symptoms of depression
subside. However, understanding the different components of anhedonia -
the desire to obtain something pleasurable versus experiencing pleasure
- has been difficult for researchers to determine in humans.
"In
the last decade and a half, animal models have found that the
neurotransmitter dopamine, long known to be involved in reward
processing, is involved in craving or motivation, but not necessarily
enjoyment," Treadway said. "To date, research into reward processing in
individuals with anhedonia has focused on enjoyment of rewards, rather
than assessing the drive to work for them. We think this task is one of
the first to do that."
Treadway and his colleagues devised the
Effort-Expenditure for Rewards Task, or EEfRT, to explore the role of
reduced desire and motivation in individuals reporting symptoms of
anhedonia. EEfRT involved having individuals play a simple video game
that gave them a chance to choose between two different tasks, one
hard, one difficult, to obtain monetary rewards. Participants were
eligible but not guaranteed to receive money each time they completed a
task successfully.
The "hard" task required pressing a
button 100 times within 21 seconds using one's non-dominant little
finger and carried a potentially higher reward than the easy task,
which required pressing a button 30 times in seven seconds using one's
dominant index finger. The subjects were told at the beginning of each
trial whether they had a high, medium or low probability of winning a
prize if they successfully completed the trial. The participants could
choose which trials they completed and were given 20 minutes to perform
as many tasks as possible.
The researchers found that subjects
who reported symptoms consistent with anhedonia where less willing to
make choices requiring greater effort in exchange for greater reward,
particularly when the rewards were uncertain.
"Consistent
with our hypotheses, we found that individuals with self-reported
anhedonia made fewer hard-task choices," the authors wrote. "These
findings are consistent with theoretical models linking anhedonia to
decreased (dopamine levels)."
"By addressing the
motivational dimension of anhedonia, our findings suggest a plausible
theoretical connection between dopamine deficiency and reward
processing in depression, which may eventually help us better
understand how anhedonia responds to treatment," Treadway said.
###
Zald
is an associate professor of psychology. Treadway and Zald's co-authors
were Joshua W. Buckholtz, a Vanderbilt doctoral student in
neuroscience, Ashley Schwartzmann, research analyst, and Warren
Lambert, a research associate in the Department of Special Education
and at the Vanderbilt Kennedy Center for Research on Human Development.
Funding from Vanderbilt University and the National Institute on Drug Abuse supported the research.
For more Vanderbilt news visit VUCast, www.vanderbilt.edu/news.
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