|Wendy Suzuki, neurons
Associate professor Wendi Suzuki, Center
for Neural Science, New York University
Interview 3. juni 2003 regarding results presented in Science
(2003), 6. June, 1578-1581.
Copyright © Rasmus Kragh Jakobsen
Hello, this is Wendy Suzuki
- This i s Rasmus Kragh Jakobsen calling
about the interview.
Yes, go ahead I am free.
I would very much like to know something
about memory and the perspectives of the work presented in
Science. Am I shooting too high by asking if this study will
lead to an understanding of the neural circuits underlying
Yeah, no no that is exactly what we are
trying to get at with these studies, I mean it is a first
step towards answering those larger questions, but that is
the goal of the studies really.
- perhaps you could tell me a little
about how it is actually done.
So basically we are trying to understand
how these structures in the medial temporal lobe (MTL) allow
us to form and retain new long term memories for facts and
So memories for facts and events have been termed declarative
memories, and they are very important because our memories
for facts and event really makes us who we are. They kind
of make up our individual personalities.
So fx damage to the MTL in humans causes severe declarative
memory impairment, so cannot form new declarative memories
and you end up basically living in the present.
And also MTL as you probably know is highly vulnerable in
Which is a clinical application of this work. You need first
to understand what the NORMAL function of the MTL.
You need to have an understanding of what the normal brain
is doing and how the normal brain allows you to form and retain
Thats what our study have gone one step closer to.
- And the MTL is that in the Hippocampus
The H is part of the MTL.
That general region is the region where several structures
that we know are critical for declarative memory function.
H is one, but there is three other areas that are all critical
for declarative memory.
But this study focus om H itself.
So what we did.
It is a simple question.
We wanted to know what your braincells are doing as you learn
something new, as you form a new memory.
Why were we interested in the H?
Well we know the H is important for the ability to form these
kind of memories. If you damage, take it out in humans or
animals you no longer have that capacity to form new memories,
you fail in these tests.
So we have known for a long long time that the H is important
for memory because if you damage you longer have that ability.
But that is not the same thing as knowing what the normal
So thats what we were trying to understand. Record the activity
of individual cells as animals are performing this memory
What is it really doing is it doing something critical do
we see something interesting.
And thats what we saw :-)
We saw very dramatic plastic changes in H cells during the
learning process. So we could measure behavioural learning.
Animals are sitting there, they doing the task, they are learning
Maybe I should tell you a little bit more of the tasks they
were actually doing?
- Right, but first just clear this.
When you measure on the individual cells, how do you do that
and specifically do you know exactly what cells you are measuring?
Ok, so how we measure is that we insert
a very thin wire electrode into the bran so the tip can measure
the electrical activity of the individual cells. A standard
Recording individual neurons. Because I have an MRI I can
tell for certain that I am in the H.
- There were quite a lot of electrodes
in each monkey?
No we used single electrodes but multiple
times. Every day the animal would come and we would record
from a single electrode, and every day the animal would learn
new associations learn something new. And we would record
and monitor what the H cell that we happened to find that
day were doing.
- So the animal actually had this scraped
off area on the back of its head or something where you could
easily insert an electrode.
So let me just briefly describe what we have the animals do.
They are basically playing video games. Memory games. So they
are sitting in front of a computer monitor and the memory
game goes like this:
We show them a big complex picture, of lets say the central
park in New York. And superimposed on the picture are four
identical targets little white squares one north,
east, west and south. On the picture. So they look at this
picture and then there is a delay interval, where the picture
disappears and the targets remain. And then at the end of
the trial the monkeys are given the opportunity to make an
eyemovement towards one of the targets.
Now the rule of the game is that only one of the four targets
will give them a fruitjuice reward, which they want because
they are thirsty, but at the beginning of the trainning session
they dont know which one of the targets will give them a fruit
juice reward aasociated with that central park picture. So
they see it multiple times during the training session and
with trial and error they get a lot wrong but then they finally
get it right by chance. And then they eventually learn: Oh
it is the north target that gives me the fruit juice reward,
when ever I see the central park scene.
- What would the target be?
The target is literally a little white
square. They just move their eyes towards it in the north
That is why we call it a location scene association task.
They have to form an association between a particular rewarded
locaton, that is the target, associated with a particular
complex scene, say the picture of central park.
and so they are seeing of not only central park with the four
targets, but they see pictures of I dont know
a picture of denmark, picture of paris and all these new pictures
they have never seen before.
And with trial and error over the session they get
these pictures randomly intermixed they eventually
learn, Oh, I get it, you know central park I have to move
my eyes north, denmark south, paris west.
And so on. That is what they are learning, thats the game
they play every day.
And in terms of what does that relate to
in human memory? This form of memory is called associative
memory, it is a critical part of memory, we use it every day.
It is for example the same form of memory that we use to remember
a name of somebody new that we met. It is a name face association.
It is something we do every day, and we are not that good
at it actually. But that is also dependent on the H.
So animals now have electrodes in the H,
it is sitting there, he is playing his memory game
location scene association task game and we can show
behaviourally by monitoring correct and error trials that
he get, that he has learned these new location scene assocation.
And so he is learning them behaviourally, and we have
monitoring the individual electrical cell activity in the
And so the critical question is: Ok, does the activity of
these H cells relate in any way to the animals behaviour,
does it signal fx when learning occurs.
And the answer was YES. That is the big finding that we reported.
We see many cells in the H, that change their activity strongly
correlated with the animal behavioural learning curve.
So the learning curve goes from 25% which is chance
performance because he has four targets it is steep
all the way to 100%. Both learn them very very well.
So what we saw was parallel increases in cell activity from
a very low base line rates to very high rates at the same
time that the animals learned a particular new association.
- that wouldnt be linear I suppose?
No it is a sigmoid shape.
- In your fig 2
Yes, exactly fig 2 A
So what does that mean. Fig 2 A shows this nice correlation
between the s-shaped learning curve and the squiggly neural
activity curve that also goes up.
What we can say from this example is that the change in neural
activity seems to be signaling when learning occurs.
BUT a critical figure is 2 F, we show that for all the cells
that are changing how many of those cells are changing before
the animal actually learns the association versus after the
animals learns the association.
Now why is that important?
Well, if all the neurons change just a little bit after the
learning so the animal learned it, aha, I understand
the association I remember the association, well that means
the H cant be involed in the actual formation of the association
if the behaviour changes first and then the neuron acitivity
But we found half the H neurons changed several trials before
learning occurred. That suggests that these changes in H neurons
are signals in learning. That they are signalling when learning
is about to occur. Those neurons may be driving the actual
Those that change before are the learning neuron.
That is a critical point of this whole paper: THAT IT IMPLICATES
THE HIPPOCAMPUS IN EARLY FORMATION OF NEW ASSOCIATIONS.
- A little to actually describing memory
in terms of networks and circuits. How much can you say at
This paper focused on the contribution
of H to the early formation of new associations.
We actually have a paper that we are about to submit, looking
at how the H signals very very well learned associations.
And it is a little bit tricky because that is not waht is
in the Science paper, but I can say that we have data, that
suggest that the H does play a role in signaling long term
memory. And direction of the work is to record not only the
H but also these other MTL structures that I mentioned during
both the formation of new associations as well as the retrieval
of well-learned associations to understtand how each of these
structures are contributing to the circuit.
Thats where we are going. The Science paper in focus here
does not adress those questions.
You have identified the critical questions, and those are
the studies we are doing right now, actually.
- how about relating the cells in the
Science paper to each other, what are the circuits who is
connected to who?
Well, thats difficult to do here. What
we did identify two different subcategories of changing cells,
fig 2A is an example of changing cells.
If you compare with 2C you see the 2 different categories.
- So one goes up in a sigma curve and
the other goes down?
Exactly. But the one that goes down is
also significantly negatively correlated with the behavioural
So how we think about this is: Both these categories of cell
sustained changing cell and baseline sustained changing
cell both these categories are united in signalling
when learning occurs. One goes up and the other goes down,
but thats still a signal.
Something has changed in the brain.
2C really dont seem to be able to convey any more information
after the point learning.
They go back to baseline rate, and are not responding more.
Their big event is right at the point of learning, when they
change their activity and go back down to baseline.
But the cells we are particularly interested in are the sustained
changing cells, 2A, because those not signals when learning
occurs but that signal is maintained for as long as we are
able hold the cell. At the end of the day the electrode is
taken out and the animals goes back to its cage.
So we hypothesize that these are the cells that may be participating
in more long lasting long term trace for this particular
association. Thats a speculation. And I still havent told
you about how they connect because I dont know that. But it
is a little bit more about the circuits, the different types
of cells at least contributing to this learning process in
- Maybe I could ask you to speculate
a bit more, since they are longlasting cells having activity,
do you think that this plasticity changes until something
is learned and then we have a locked network or circuit.
Yeah, so we know something about this.
We know the H is needed at the time you are learning something,
but also during a consolidation period. Which unless I have
more practice in remembering fx your name I will forget in
a short while. Now if I have more practice in seeing your
and saying your name, say 5-6 times it is pretty hardwired.
And I need my H during all those 6-8 times early on before
it formed a long term memory.
And thats what I think these sustained changing cells
that process the consolidation proces is what I hypothesise
they are participating in.
Now what happens after it is formed in long term memory, you
dont need your H more for wellconsolidated memory.
FX if my H was damaged right now I wouldnt be able to remember
this conversation but I would remember my childhood, highschool
even graduate school memories because those are well consolidated
strong long term memory. Those memories are not dependent
on my H, but more on the neo-cortex, which is presumed to
be the storage the final kind of repository of these long
So we think in our studies of the H, that we are focusing
mainly on these areas important for initial learning and consolidation
of long term declarative memory. And that eventually these
memories become independent of the MTl and more dependent
on the reside in the neo cortex.
- Do you then have any idea on how this
No, thats we are interested in looking
at that, but the way that it is transferred is still mysterious.
As mentioned a lot this data is from lesion work, which is
fine, but because you whats gone you dont know what works
in the first place. U know what that area is important for,
but it still doesnt tell you how that area does that function.
SO THATS WHAT THIS KIND OF BEHAVIUORAL NEUROPHYSIOLOGY IS
SO USEFUL FOR BECAUSE IT ALLOWS YOU TO GO INTO THE NORMAL
BRAIN. AND ASK WHAT IS GOING ON IN IMPORTTANT TASKS LIKE ASSOCIATIVE
LEARNING OR RECALL OF ASSOCIATIONS.
- You call it hardwiring, that is strong
metaphor for what you suspect is happening, but is it really
something like hardwiring - 2, 40 or 1.000 neurons are hardwired
together and thats where your memory lies?
I wouldnt call it hardwiring, there is
certainly evidence for large and widespread networks of the
cortical neurons that are important for long term signalling,
because if you damage them you damge long term memory. but
that doesnt say they are hardwired, that just says they are
The evidence is not that it is hardwired, but that it is in
the cortex and that they are involved somehow, but no direct
evidence in supprot of hardwiring.
- But you DO believe that new wiring
is going on as these H cells are changing their activity?
Yeah, I think another important point of
this paper is that it shows that there is strong learning
related palsticity in the H. this we have expected for a very
long time but the novelty of this finding is that it is probably
one the strongest direct pieces of evidence showing Fig 2A,
learning related plasticity that one can demonstrate and analyse
in this way.
It is very different from saying, ok I am just going to remove
the H and he cant learn and therefore the H is plastic. Here
we are showing how the H is plastic, what is the timecourse
and how selective is that plasticity for particular pieces
of learned information.
- Plasticity, do you mean change in
connection or change in activity?
Change in activity, I can say nothing whether
synapses are forming, I dont think it is possible for synapses
to form this fast. No evidence for growth of synapses at this
- You need longer time for growth of
- Anybody going to look at which molecules
or proteins are needed for this, or is that a completely different
Yeah, that is a different but related field
and colleagues of mine that are molecular biologists are very
interested in identifying the genes and proteins involved
in this kind of process.
People like Eric Kandel went down in organism to Applesia
californica, a sea slug, to study the molecular mechanisms
of this exact kind of memory.
- Lower than Drosophila?
I dont know, it is bigger.
- I mentioned Drosophila because I would
have thought you would go to a strong genetic animal.
Yes, a lot of work have in fact been done
- You yourself will concentrate on long
Yes, we are in the middle of it. The next
obvious question is to use the same method on the adjacent
areas to the H.
- One electrode at a time?
We are moving to multielectrode product.
To speed up the workprocess.
- I am out of questions, but if you
would like to emphasize something that I missed or...
Yeah I guess I would say the novelty of
this finding is really that these findings provide some of
the strongest direct evidence for learning related plasticity
in the Hippocampus, I already said that, but I just want to
emphasize that, because it really is a novel finding. It is
for once direct evidence instead of inderect evidence that
you get from lession studies, that is what I think is the
novelty of this report.
- Thank you very much.
Copyright © Rasmus Kragh Jakobsen