Memories in the Dreaming Brain
[Music]
i’d like to ask you to pause for a
moment
right now and bring to mind the last
dream you can remember whether this was
last
night last week or last month
now my bet is the dream you’re thinking
of probably
incorporates some people places
and experiences from your real-life
memories
maybe you dreamed of a conversation you
had with your mother
uh yesterday maybe you dreamed of an
old friend you haven’t seen in many
years
maybe you dreamed of the trip you’ll be
going on
next month well the question i’ll be
talking about today
is why did you dream that
why did those particular experiences
from your life
show up in your dream and indeed why do
any of us
dream the strange bizarre and seemingly
random things that we do
well today i’ll be offering one possible
answer to that question which is that
dreaming reflects kind of a
post-processing of memory in the
sleeping brain
a process that we psychologists call
memory consolidation
now i know in your life you’ve heard
many different explanations for
why we dream and why we dream what we do
and most of those have focused on
dreaming as
originating from some mysterious or
unknown provenance hundreds of years ago
for example
it was most commonly thought that dreams
were hidden messages from
gods or spirits and then of course
around the turn of the last century
freud popularized the notion that dreams
are instead
hidden messages from a hidden part of
our own minds
the unconscious mind well as interesting
as those ideas are
there’s actually little or maybe no
scientific evidence that dreams arise
from either of
those mysterious sources outside the
self
instead neuroscientists like me view
dreaming as the natural result
of a normal brain process that unfolds
during sleep
this consolidation of
memory and there’s a few reasons why
i think dreaming does arise from our
memory
first of all in the last several decades
neuroscientists have discovered
that specific patterns of brain activity
representing our recent memories are
replayed back again when we fall asleep
now that’s a pretty big claim so let me
step back
and explain for a moment what i mean by
that
this replay of memory was first
discovered
in rodents in a rat’s brain
in a area called the hippocampus deep
inside the medial temporal lobe
of the brain there is a special type of
cell
called a place cell and place cells are
called place cells
because they only become active or fire
when i’m standing
in a particular place so
in my hippocampus there is a place cell
that fires
only when i am standing in this
particular spot
let’s call it cell 85 and then there’s
another place cell that fires only when
i’m standing in this
particular spot say cell 27
and another that fires only when i’m
standing here
say cell 18. so what this
means is that as i move across this
stage
we could define my movement through
space
as just a sequence of brain cell
firings 85 27
18 or 18 27
- and the discovery is
when a rat or a human falls asleep
these sequences that represent our real
life
experiences play back again
when we fall asleep just as if the real
experience were unfolding again
87 25 15 over and over
and over again and the
accuracy of these sequences is good
enough so that
just by analyzing a rat’s brain activity
we can see where it is walking
in the maze that it experienced in
wakefulness we can see
from its sleeping brain that now the rat
is dreaming
that it’s running ahead turning right
heading toward
the cheese
and this replay of memory in the
sleeping brain
might explain why sleep is so good for
human memory
in fact across the last decades we have
discovered that if you sleep
after you learn something you’ll
remember that information
better later on in humans and animals
and this holds true for a wide different
variety of types of learning so this is
whether you’re talking about memorizing
words
or maybe learning how to type or
solving a problem and the benefit of
sleep holds true
whether you’re talking about a very
short daytime nap
or maybe a full night of sleep
in all of these instances sleeping after
learning
boosts memory for whatever it is
that you learned just prior
but it’s not just sleep that boosts
memory
another piece of evidence that leads me
to my views is that
when humans sleep and then
dream about something that they just
learned
their memory is especially
improved for example in one series of
studies in our own laboratory
we had human participants run through a
virtual
maze a 3d style virtual maze on the
computer
and using a game controller they had to
escape the maze
over and over and over again
while participants who were randomly
assigned to sleep
after this learning task boosted their
memory
not surprising based on prior research
but also those who slept and reported
that they
dreamed about the maze improved their
ability to find the
exit ten times more than participants
without
the maze-related dreams so there’s
another piece of evidence
that leads me to my view that dreaming
is a result
of activating memory in the brain
but at this point you might be thinking
to yourself
well that neuroscience that’s all well
and good
but my dreams are not a simple replay
of memory my dreams are strange
weird bizarre things that could never
happen in real life
and you may be right maybe you didn’t
dream of
a conversation you had yesterday with
your mother
maybe in your dream you were speaking to
your mother
and then she put on two skis made of
banana peels and slid down a mountain of
ice cream
okay that didn’t happen in real life
that is not
a replay of any one real experience that
you had
but what we know is that when memories
from your life
do appear in dreams they rarely
appear in their entirety instead
the dream will take bits and pieces of
several
different memories from the previous
days weeks or even years
and combine those together into a novel
bizarre but sometimes useful
unique scenario that does not
mimic exactly any one real life event
but combines together
several of those so
perhaps you did dream of your mother
because you spoke to her recently
but maybe you also ran into your
friend’s mother at baskin-robbins eating
ice cream and ice cream reminds you of
the banana splits that you used to get
at the ice cream store when you went
there as a child
so in fact the dream is not so similar
to any one experience but these
seemingly random and unrelated elements
the mother the ice cream the bananas
are each drawn from separate memories
and pulled together into a
semi-coherent whole
so to recap what i’ve told you about
is some evidence from neuroscience that
one
in humans and animals recent experiences
are replayed by your brain when you fall
asleep
and what’s more this activity during
sleep boosts your memory
improves your ability to remember
information
later on but that’s especially the case
when those replayed memories make it
into the conscious
experience of our dreams
so to me this suggests that dreaming
might not be
quite so mysterious as it’s often
discussed as being
instead dreaming might be the natural
logical result of this process of
memory reactivation and consolidation
that happens each night when we fall
asleep
and functions to strengthen and
stabilize memories over time
converting them into a more permanent
form
of long-term storage in fact
this is thought now to be one of the
fundamental functions of sleep
that this consolidation process is
required to take
a freshly minted new memory and turn it
into
this stable long-term memory
so i would tell you that dreaming is
not just a curiosity it’s not a silly
waste of time
and it’s not a hidden message either
from the gods or from your unconscious
mind
instead dreaming may really be a side
effect
of the hard work that your brain is
doing
processing memories while you sleep
you