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The neuroscience of memory - Ri Science Podcast with Charan Ranganath

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[Music]

Chon thank you so much for joining us

today here at the Royal Institution and

congratulations on your new book why we

remember um for those of you listening

or watching uh By the time this is

released the book will be out so if you

haven't read it yet this is your

official reminder to do so um your book

and your research Focus particularly on

what's going on in our brains to

facilitate our memory all the way down

to the cellular level um and how we can

use it more

effectively what inspired you to get

into this area of research and write the

book well one of the things that really

inspired me was when I was in graduate

school I was doing Clinical Psychology

and many of the people that I was

working with had U were worried that

they had brain damage or they were

referred by a doctor or sometimes even a

lawyer and uh we would do testing with

these p patients and what I would find

is is that whatever the disorder it was

whether it was something like you know

pre-alzheimer's disease to traumatic

brain injury to even things like

clinical depression uh we would see that

memory was affected it was the first

thing to go and uh at the same time I

was doing clinical work in Psychotherapy

and finding that the patients that I was

working with uh they wanted to they had

the opposite problem of too much memory

and they had these traumatic memories

that they were trying to work through

and so it really became apparent to me

how important memory is and at the same

time uh I also felt that we didn't have

a very strong scientific basis for

understanding how to treat memory

problems and so at that time functional

MRI or functional magnetic resonance

imaging was this technique that was

being just getting off the ground as a

way of looking at people's brains and

being able to see what areas of the

brain are actually doing things in

relation to memory and so I jumped on

that

opportunity so you've already mentioned

like memory underlines basically

everything we do it's our Behavior

relationships careers um and we're all

learning from the experiences that we've

had and we're retaining that information

but it's also something that a lot of us

do take for granted you've mentioned

that some people have problems with it

and it's also quite complicated could

you give us a bit of a whistle stop tour

of our current understanding of how

memory works

so it's such a big topic I'll try to

reduce it to a few key points um but one

of the uh key ideas is is that uh

memories are thought to be stored so to

speak in uh through connected networks

of neurons which are the basic cells

that are doing the Computing in the

brain and so what we think happens is

when you speak when you feel something

when you see something there are groups

of neurons that are activated that allow

you to have these conscious experiences

or even unconscious experiences things

like your heart racing when you you

something scares you and so when we have

these experiences the changes um there

changes in the connections between these

neurons and so that's what you'd call

plasticity and those changes allow us

group of cells that were active during

an event to become more easily activated

again later even when that event isn't

taking place so if I could activate some

of the neurons in this cell assembly is

what we call them uh from this

conversation presumably through these

strengthened connections the other

neurons could be brought into play and

so it only takes me a small cue to be

able to pull up the entire network of

neurons that are part of this memory so

that's like kind of the very Whistle

Stop tour I guess but then you can

certainly go deeper than that into all

the different kinds of memory and and

the way it works I mean so we've got um

I'm going to keep it very basic even

though it's very complicated but we've

got short-term memory long-term memory

um and different types of long-term

memory could you give us a bit more

about that in memory research we like to

cut things up and say this is different

and that's different but in reality when

we remember an event for instance it's

all very connected um some of the key

one of the key distinctions that I've

really been working on throughout much

of my career is the distinction between

episodic memory and semantic memory so

semantic memory would be your general

knowledge about things that you've

acquired over the years and episodic

memory would be your ability to remember

a singular event whether it's an event

that's mundane like I'm in the kitchen

and I'm trying to figure out wait a

minute why am I here and we could talk

about other kinds of episodic memory

like where did I put my keys to I'm

vividly remembering my 16th birthday or

I'm trying to remember that later on I

have to take out the trash because it's

trash day so all of these kinds of

memory are related to episodic memory

and we know that there's a brain area

called the hippocampus that's very

important for episodic memory so people

with damage to this area they can still

walk talk think but they have an

exceptional problem being able to

remember things related to a time and a

place so um there's many famous patients

including uh some that have been uh um

described at University College London

who like can learn information but they

don't have this ability to re-experience

it and tie it to a context in the way

that somebody with a hippocampus can do

and so what that tells us is is that the

hippocampus seems to be organizing and

associating our experiences so it's

getting information about who was in an

event and where the event took place and

how things transpired what was the

situation what were the feelings that we

experienced at the time and it's

associating them all and it's just

putting it together just because at this

moment in time all of these things

happen at this moment in time I was

excited to tell you about memory and at

the same time you were here and at the

same time I'm sitting on these purple

chairs or whatever this color is right

and so all of those bits are being put

together by The Hip campus and what that

allows you to do is some other time if I

walked into this room that would be

enough to cue me to be able to allow the

hippocampus to pull out all the other

bits that happened in this past event

and so that's why for instance if you

end up in the kitchen and you're trying

to figure out why am I here you actually

have to pull up information about that

past context when you decided you were

going to go into the kitchen in the

first place and so I then essentially

engage in what uh endel tulving the

psychologist who came up with this term

he called this episod U he called

episodic memory is mental time travel

and the idea is that you have to go back

in time mentally and be able to say hey

wait a minute okay I'm here in my office

and I realize that I left my phone in

the kitchen and that's why I'm there and

so otherwise what happens and this often

happens to me is I end up in the kitchen

and I forget why I'm there and then I

end up just eating something and then I

come back and then I realize why I went

to go there at the first place but being

able to visualize that context the place

and time really helps and in fact we've

uh and many others have done research

where if you scan people's brains while

they're experiencing an event and then

you scan them while they remember that

event what you can see is this this

little bit of a reset in the patterns of

activity in the hippocampus such that it

looks more like what it did in the past

so these patterns change a lot over time

but when we remember you can reverse

those effects of time so it's almost

like being in a time machine essentially

you're going backwards in time and one

of the fascinating things about this is

that as a result we can actually change

the past when we remember so we tend to

think of memory being just a replay like

you would play a movie on your phone but

in fact the act of remembering a past

event can lead to changes and updating

in the memories and this has been shown

in everything from uh rats and very

simple kinds of memories to humans and

much more complex memories so for me

personally and I know it happens to a

lot of people there are certain um like

smells and songs that bring back very

specific memories that I didn't realize

I had um why does this happen well one

of the things that I think people don't

realize this is that sometimes a memory

isn't forgotten but we just don't have

the right way to access it and so when

we hear a song for instance that comes

out of the blue and it just pops this

memory of a particular time back in our

head often it's the case that that song

was a particular soundtrack for us and

uh during a particular period in our

life and it evoked a particular emotion

and so as a result they're baked into

the context of these episodic memories

and so they provide this beautiful way

of accessing memories for a time and a

place so sometimes we had sad periods in

our life and you listen to sad music or

you have periods of time where you just

decide to get into electronic dance

music and then you go through a

classical phase and so forth and those

songs that are associated with

particular points in our life tend to be

the ones that are the best retrieval

cues to pull out these once dormant

memories and uh but it's not just of

course music smells can do this being in

a particular place can do this as long

as there's some kind of a context uh

that is unique it will help us access

those memories so lots of memory tips

and tricks out there and one that I've

heard is that if you revise for a test

in the same place you're going to take

the test um those memories should come

flooding back to you

um does that work and if so why and if

not what are the other like memory

misconceptions that are out there well I

think it can work if you're not using

good study habits in the first place so

if you're really just have this unique

memory of having studied something once

then being back in that same seat and

looking at the same scenery can help you

pull up that information that you

studied just once but ideally what you

want to do is take advantage of the fact

that memories update when we remember in

different context so if you study or

even better if you test yourself on what

you've learned let's say in this room

and then you go home and you do it then

you go to a your favorite Cafe and you

do it what happens is every time you

recall this uh information that you

studied now what happens is is it

becomes updated so it's not tied to this

one place it can now generalize to

different places and at some point what

happens is is that the memory for the

information you need is accessible

anywhere and so ideally what you want is

you don't want to have to be sitting in

one place to remember everything that

you're trying to remember you want to be

able to get it from anywhere else and so

I think one you mentioned a memory

misconception I think one misconception

is is that if you study read the same

thing over and over over again and then

you take the test in the same spot or

whatever that you're going to retain

this information and it's just not true

actually you retain much more

information if you force your brain to

struggle and so by that I mean that um

you try to test yourself and sometimes

you can even test yourself before you

study and that can actually enhance

memory too and so this is a a method

that I talk about in the book called

error driven learning and the idea is is

that when you actually force yourself to

try to pull up information from memory

and challenging

circumstances what happens is when you

do get the right answer essentially

there's changes We Believe anyway uh

based on our computer models of the

brain that essentially there's parts of

a cell assembly that gives rise to a

memory that are not necessarily very

helpful and adaptive and there's some

that are very very effective right so

what happens is is that uh the memory

can get reconfigured and tuned up so

that the bad connections go and the good

connections are

strengthen and that only happens though

if we allow ourselves to be stress

tested in the first place so even though

there is a long way to go to

understanding Neuroscience um we have

come a remarkable way but there is a lot

of progress that we need to make um we

are also sat in the Royal Institution uh

in the theater where the Christmas

lectures are filmed so I thought I'd go

over a couple of Christmas lectures that

we've had in the past are based on

neuroscience and memory um so we've had

Richard Gregory who gave his lectures

the intelligent ey back in 1967 which

was the first Christmas lectures ever

broadcasting color and only the second

ever broadcast on bbc2 we've had Susan

Greenfield delivered uh the Christmas

lectures in 1994 titled the Journey to

the Center of the brain so that's quite

fun and then the most recent one is

Bruce Hood who in 20

delivered the series called meet your

brain how have you seen the

understanding of memory and Neuroscience

develop over your career well one of the

big developments that I've seen over my

career has been the ability to move away

from thinking that memory is just one

brain area to really thinking about

networks in the brain and how they

interact with each other and uh so

because before brain Imaging was really

big people would focus on specific areas

of the brain like the hippocampus but

now one of the things we appreciate is

that memory is more than just the

hippocampus it's just that some areas of

the brain it's harder to find disorders

that affect it so for instance there's a

whole network of brain areas called the

default mode Network and people used to

think this is just a network that comes

on when you're not doing anything uh and

now we realize it's actually

exceptionally important for memory and

if you look at the pathology in the

brain that happens over the course of

Alzheimer's disease the hippocampus is

part of the you know Ground Zero but

then soon soon after you start to see

pathology throughout the whole default

mode

Network um you mentioned Alzheimer's

disease um so obviously we've got

neurodegenerative diseases that are

marked by memory loss um what goes wrong

to cause these like memory

dysfunction well it's still under debate

but one of the ideas that's out there is

is that

brain areas aren't it's not like you can

go to any part of the brain and find

connections to every other part of the

brain right it's just like if I go to a

tube station there's certain lines that

you can get onto and there's other lines

which aren't really connected to each

other and then there's some lines that

are like uh there's some stations that

are hubs where you can transfer from one

station to another and you can think of

the brain in a similar kind of way that

there are these networks of Highly

interconnected neurons where if you're

on one part of the line it's very easy

to get to another part of the line but

then there are some networks where the

between network connections are very

weak and so what that means is it allows

the brain to both be specialized but at

the same time it gives some flexibility

to allow areas of the brain to talk to

each other now one of the consequences

of this is if there's pathology uh like

the buildup of uh toxic proteins like uh

towel for instance that what it can do

is you can get this uh spread from one

neuron to another to another and so that

can cause an entire network to basically

collapse when you have the have the

spread of a neurogenerative disease like

that and your team at UC Davis are

working on ways to catch signs of

Alzheimer's earlier um and try and

prevent damage before it starts how does

that work well so there's many different

approaches that are being tried out

right now to catch uh the pathology very

very early because one of the big

discoveries in Alzheimer's research uh

that was that you can actually start to

see buildup of heavy buildup of this

protein called amalo which is thought to

play a part in the whole Cascade of

events that causes the brain damage and

Alzheimer's uh and if people were trying

to do drug Discovery what they would do

is they would try out these drugs and

people after they've been diagnosed with

a clinical memory problem but now we

know that by the time someone has memory

problem that's easy to detect what they

find is is that there's already pretty

significant loss of neurons and you

can't really reverse that so if you want

to come up with a intervention you want

to be able to intervene before that

brain damage really kicks in and so now

we realize there's this long preclinical

period And so one of the uh one approach

that's being done is to look at blood

plasma uh tests and that that looks very

promising uh in our lab we're hoping to

develop behavioral tests that you know

any neurologist could give so that

there's different ways of identifying

people in these early stages and so one

of uh our lines of research is focused

on this interaction between the hip

campus and the default Network and we've

got good data now from a very large

group it was a um 346 people scanned

actually in the University of Cambridge

and we analyzed that data and found that

there are these certain points when

people people are say watching a movie

for instance where the hippocampus seems

to come online and that's related to

memory and uh and the default Network

too and these uh activations reduce as

people get older but what's interesting

is that independent of age people with

poorer memory performance show less of

this variability in the hippocampus and

so what that tells us is that we can use

these as biomarkers for memory problems

or bi ological markers so to speak so

keeping on the theme of looking into the

future and future research um we get

this question a lot on well artificial

intelligence is a big topic we've just

had the 2023 Christmas lectures on it um

and it's being integrated into society

uh more and more so how will artificial

intelligence

um impact our memory or is there any

concern around that well there's there's

many different kinds of concerns uh what

I would say is is uh certainly one issue

that I I worry about is uh what you can

see with a lot of the issues that come

up with technology in terms of our

thinking and our memory functions are

not really related to the technology per

se but the way we interact with it and

so one of the things that I think about

quite a bit is uh now for instance with

uh um this actually happened a few years

ago when I would start using email

programs they would use AI to finish my

sentences and about onethird of the time

I would say you know uh no this isn't

what I want and just type it myself

onethird of the time I'd say this is

exactly what I want just fill it in

onethird of the time I'll say that's

good enough let's just go with that

because I didn't want to type the rest

of the sentence and it's not what I

would have wanted to type but it goes

anyway but the brain as I said is very

plastic and it's constantly reorganizing

to be more and more efficient and so as

a result what happens is is that often

people can start to just learn to adapt

exceptionally well to whatever is being

suggested to them and the downside of

that is is that people can actually

become less creative and you might end

up just sort of seeing this like funnel

towards Mass mediocrity essentially if

everyone it's not that chat GPT would

write what you want it's more like

everyone would start writing like at GPT

I hope that doesn't

happen I hope so to I hope not do um so

you mentioned the potential lack of

creativity how does memory help us be

creative so memory is incredible in

humans because we do things that are

very different than typical generative

AI systems so generative AI they just

absorb massive amounts of data and then

try to find patterns in it humans

actually take in very weird kinds of

data like we don't even see the entire

world in front of us we just get little

Snippets of the World by moving our eyes

around and so as a result we actually

put together much more Rich kinds of

inferences for much less

information so you wander around the

world you meet people maybe in your when

you're in uni you lived in a dormatory

with uh friends who you wouldn't have

met if it just didn't happen that you

were living in the same place and these

kinds of unique experiences somebody

gives you a book for your birthday and

you read it and it's something

completely outside of your normal

experience you can start to build

connections between ideas and if you

look at some of the great artists some

of the great musicians poets there are

actually people who took a diverse range

of influences from their lived

experiences and just found unusual

connections between them and this is the

kind of thing that we can do because of

episodic memory episodic memory allows

us to take these unique moments in our

lived experiences and then recombine

them and put them together into new

things and synthesize them and this

would be something that is not present

in at least current AI systems because

they're designed to just take the common

elements of whatever they're trained

with as opposed to the distinctive

unique weird experiences that we have

and also in your book you mentioned to

me in the same thread uh imagination

um how does memory help us with that

well so one of the early memory

researchers uh that's been just a huge

influence in the field was Frederick

Bartlett um who was actually at the

University of Cambridge and Bartlett

proposed a really radical idea remains

radical to this day that essentially we

don't remember the past by replaying a

video in our heads even though it feels

like that we actually only get little

glimpses of those past experien es so we

don't activate the entire set of neurons

that we're part of an event we actually

just get little bits and pieces but then

we're able to use our knowledge and use

our goals and use our everything that we

have to build a story and imagine how

the past could have been and that's the

act of remembering and so this is why

there's this just amazing link between

memory and

Imagination uh so for instance a number

of researchers including Demis hbus who

started uh Google Deep Mind and uh

Elanor Maguire uh discovered that many

patients who had memory problems also

had a very impoverished imagination and

when people started to scan brains while

healthy people were just imagining or

healthy people were remembering

experiences from their own lives what

they found was is that basically the

same networks of brain areas in the

hippocampus and the default mode Network

come alive and so there seems to be this

element where imagination is the core of

memory but also memory is the core of

imagination that is if you have a memory

problem your ability to think about to

imagine how things could be in the

future or even Imagining the present and

I know that might sound a little weird

that we can imagine the present but you

know one thing that we're doing in this

conversation is we've got this common

ground because I have some understanding

of of what you are asking me that goes

beyond the simple words that you put out

right and memory is very important for

being able to connect those dots and

that kind of uh ability to use memory

flexibly to generate in things that

aren't in front of us is exceptionally

important for imagination and for

creativity but it also creates some huge

problems because we have a very hard

time telling the difference between

things that we imagine versus things

that actually happen so I don't know

about you but I often have cases where I

get an email and uh I think oh yeah I've

responded to that and then a week later

it comes back and it says urgent please

reply and I realized that I didn't

respond to it I just thought about it

and then I said oh this is how I should

reply and then I got interrupted and

distracted and I never did it and so we

have this real challenge of being able

to say is this something that I thought

about or is this something that actually

happened and sometimes people can

actually get so far out in this that

they can remember events that never

actually took place if they imagine them

too vividly so I'm sure that everyone uh

listening to this and watching this uh

podcast episode has one thing that they

can never quite remember um it's at this

point that I'm meant to say one thing

that I can't remember but ironically I

can't actually remember what I can't

remember so we're going to move on um

and one of the themes in your book is

how we can use our memory uh to our

advantage and more efficiently uh for

all our sakes especially mine uh what's

the

secret I think the secret is you have to

first accept the fact that memory is not

supposed to be an effortless process

where you can record every part of your

experience and replay it exactly as it

happened I think a lot of people start

with this assumption that it should be

effortless it should be complete and it

should be 100% accurate and when you

start with those assumptions you're

going to make mistakes and that's in

some sense normal but on in another

sense it won't help you remember the

things that you want to remember it well

um so we know that the information that

tends to get forgotten uh is suffering

because memories compete with one

another and this is a way that one of

the many ways in which the human brain

is different from machine intelligence

where we really

store Many Memories uh with

overlapping Assemblies of neurons and so

what happens is is that activating one

memory can actually compete with or

inhibit another memory so if you take

the kinds of things that happen every

day like you know I put down my keys and

then later I'm saying where did I put my

keys it's a very difficult problem

because you have memories of where I put

my keys the an hour ago versus 3 hours

ago versus one day ago versus two days

ago and so it's not a matter of just

remembering where I put my keys but

finding the right memory of where I put

my keys that's the issue so how do you

beat that interference one way you can

beat that interference is by first of

all capturing and paying attention to

what are the important features in your

environment and that sounds very easy

but it's not especially in the world we

live in you know people are always being

bothered by uh text alerts and so forth

we're often doing one thing but engaging

in all sorts of mental simulation and

planning for other things that we're

supposed to be doing and actually

engaging in a lot of memory processing

you know thinking about the future

thinking about the past and so that

really distracts us from the present

moment and makes it that we're not even

there in the first place when you put

down your keys um and we know that the

kinds of memories that really benefit um

and are able to come up at a moment's

notice are the ones where we have a lot

of sensory detail for instance um just

Vivid information about the sights and

the sounds and the smells and the

emotions that make a memory distinct

from other memories that they could be

competing with and so anything that you

can do to focus yourself on those moment

ments that make an event unique will

help you remember what you need to

remember and I think part of what we

need to do is actually go ahead and

think about well what do we want to reme

remember in the first place because if

we're not going to remember everything

then the question is what do you need to

remember what do you want to remember

and how do you want to remember it so

we've covered how we can access memories

with unique events and different senses

um but there are obviously some lify

lifestyle choices which lead to memory

loss um for one the consumption of

alcohol um why does this happen well

alcohol is one substance and there are

other drugs like this too like for

instance benzodiazapines like valum and

Xanax that actually can have an amnestic

effect because essentially there are

certain uh chemicals that are released

in the brain during particularly

interesting or new experiences for

instance or emotional experiences and

alcohol can block some of those uh

changes that happen when these chemicals

are released in the brain and so what

happens is is that it actually can block

the formation of a new memory as a

result so sometimes you can even see

after someone's had alcohol that they

have better memory for the stuff that

happened before they started drinking

but worse memory for the things that

happen while they were drinking because

what happens is that now you've lost all

this memory for while you were drunk and

so the stuff that happened before is now

not suffering from competition as a

result um alcohol has another

interesting effect which is that when

you are under the influence of alcohol

you're in a particular mental context so

I think many people after like three or

four drinks are in a different mental

state than they are in their daily life

hopefully anyway and so as a result you

can't access whatever memories you did

form because you'd have to get back into

that mental state again and there's some

evidence suggests that in fact if you

did form memories while you're under the

influence of alcohol you can get better

access to it if you're remembering under

the influence of alcohol which I don't

recommend but nonetheless that yeah and

there's other effects of alcohol so for

instance uh it can give you these

rebound things where it messes with your

sleep so even though alcohol can help

you get to sleep sleep quality is

terrible you wake up in the middle of

the night and all those factors can lead

you to have the next day um from all the

sleep deprivation among other things uh

your brain isn't going to function as

well in particular the prefrontal cortex

is an area of the brain that's important

for allowing us to focus on what we need

to learn and it allows us to be able to

reconstruct the context of past events

so that we can actually pull up the

right memories that we're looking for

and the prefrontal cortex is really shut

down when we're in a state of sleep

deprivation so there's many of factors

uh lifestyle factors and of course

alcohol is not the only drug that

affects memory there are many others uh

um THC for instance and marijuana can

affect memory um psychedelic drugs

actually have interesting effects on

memory we're still trying to unpack all

of them uh they do seem to change

plasticity in a way that can affect

memory for while you're under the

influence but I think the effects are

actually quite complex and interesting

uh but the effects on memory are

probably the reason why some uh

psychedelic drugs like MDMA seem to have

uh an effect on traumatic memories

because often what happens is people

retrieve these old memories but you're

also producing these changes in the

ability to form new memories while

you're retrieving these old memories and

so what can sometimes happen is the

memory can become updated in interesting

ways so I wish we could continue this

conversation all day but in true RI

fashion I'm going to put you on the spot

to finish the episode not that I haven't

been throughout this whole thing um so

if there is one thing you want people to

remember from this episode or the book

again everyone should read it uh what

would it be I would say that the one

thing I would want people to take away

is that memory is not about the past

it's about the present and the future

and that we're constantly using memory

to be able to tell ourselves everything

from where am I at the given moment what

time is it to things that are the big

choices that we want to make in life I

mean I know when I chose my career path

part of my key decisions were always

based on what are the memories that I

have of Past Times where I've been happy

what are the kinds of contexts in which

I function well in right and so uh I

really want people to think about memory

as this resource that they can draw upon

uh for making better decisions and for

being more present and for choosing the

kinds of Life uh paths that you want to

take because the thing is it's going to

influence your behavior and if you're

not aware of it then memor is in the

driver's seat but you really just want

it to be the

co-pilot excellent Sharon thank you so

much for joining us today um at thank

you for such an incredibly insightful

episode excellent thank you

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