A Period Of Slumber Helps The Brain Distinguish
Core Emotions From Background Details
August 14, 2008
As poets, songwriters and authors have described, our memories range from misty
water-colored recollections to vividly detailed images of the times of our
lives.
Now, a study led by researchers at Beth Israel Deaconess Medical Center (BIDMC)
and Boston College offers new insights into the specific components of emotional
memories, suggesting that sleep plays a key role in determining what we remember
- and what we forget.
Reported in the August 2008 issue of the journal Psychological Science,
the findings show that a period of slumber helps the brain to selectively
preserve and enhance those aspects of a memory that are of greatest emotional
resonance, while at the same time diminishing the memory's neutral background
details.
"This tells us that sleep's role in emotional memory preservation is more
than just mechanistic," says the study's first author Jessica Payne, PhD, a
Harvard University research fellow in the Division of Psychiatry at BIDMC.
"In order to preserve what it deems most important, the brain makes a
tradeoff, strengthening the memory's emotional core and obscuring its neutral
background."
Previous studies have established the key role that sleep plays in procedural
memory, demonstrating that the consolidation of procedural skills (such as
typing or playing the piano) is greatly enhanced following a period of sleep.
But sleep's importance in the development of episodic memories - in particular,
those with emotional resonance- has been less clear.
"Emotional memories usually contain highly charged elements - for example,
the car that sideswiped us on the ride home - along with other elements that are
only tangentially related to the emotion, such as the name of the street we were
traveling on or what store we'd just passed," explains study author
Elizabeth Kensinger, PhD, an Assistant Professor in the College of Arts and
Sciences at Boston College. "We were interested in examining whether sleep
would affect memory for all of these elements equally, or whether sleep might
allow some of the event features to decay at a faster rate than others."
The authors tested 88 college students. Study participants were shown scenes
that depicted either neutral subjects on a neutral background (a car parked on a
street in front of shops) or negatively arousing subjects on a neutral
background (a badly crashed car parked on a similar street). The participants
were then tested separately on their memories of both the central objects in the
pictures and the backgrounds in the scenes. In this way, memory could be
compared for the emotional aspects of a scene (the crashed car) versus the
non-emotional aspects of the scene (the street on which the car had crashed.)
Subjects were divided into three groups. The first group underwent memory
testing after 12 hours spent awake during the daytime; the second group was
tested after 12 nighttime hours, including their normal period of nighttime
sleep; and the third baseline group was tested 30 minutes after viewing the
images, in either the morning or evening.
"Our results revealed that the study subjects who stayed awake all day
largely forgot the entire negative scene [they had seen], with their memories of
both the central objects and the backgrounds decaying at similar rates,"
says Payne. But, she adds, among the individuals who were tested after a period
of sleep, memory recall for the central negative objects (i.e. the smashed car)
was preserved in detail.
"After an evening of sleep, the subjects remembered the emotional items
[smashed car] as accurately as the subjects whose memories had been tested only
30 minutes after looking at the scenes," explains Kensinger. "By
contrast, sleep did little to preserve memory for the backgrounds [i.e. street
scenes] and so memory for those elements reached a comparably low level after a
night of sleep as it did after a day spent awake."
"This is consistent with the possibility that the individual components of
emotional memory become 'unbound' during sleep," adds Payne, explaining
that "unbinding" enables the sleeping brain to selectively preserve
only that information which it calculates to be most salient and worthy of
remembering. A real-world example of this tradeoff, she adds, is the
"weapon focus effect" in which crime victims vividly remember an
assailant's weapon, but have little memory for other important aspects of the
crime scene. Traumatic memories, such as the flashbacks experienced among
individuals with post-traumatic stress disorder, can demonstrate similar
disparities, with some aspects of an experience seemingly engraved in memory
while other details are erased.
"Sleep is a smart, sophisticated process," adds Payne. "You might
say that sleep is actually working at night to decide what memories to hold on
to and what to let go of."
This study was supported, in part, by grants from the National Science
Foundation and the National Institute of Mental Health. Coauthors include
Elizabeth Kensinger, PhD, of Boston College, Robert Stickgold, PhD, of Beth
Israel Deaconess Medical Center; and Kelley Swanberg of Harvard University.
Beth Israel Deaconess Medical Center is a patient care,
teaching and research affiliate of Harvard Medical School, and consistently
ranks among the top four in National Institutes of Health funding among
independent hospitals nationwide. BIDMC is clinically affiliated with the Joslin
Diabetes Center and is a research partner of Dana-Farber/Harvard Cancer Center.
BIDMC is the official hospital of the Boston Red Sox.
Beth
Israel Deaconess Medical Center
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