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

85. 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