Translator: Virginie vW Reviewer: Jehanne Almerigogna
Good evening!
A skydiving friend has asked me to solve a problem tonight.
The problem is this:
In the skydiving field, they say:
"Free-falling for one minute is really short,
but one minute with your fingers stuck in the car door, is extremely long!"
That's tonight's problem!
So, what does that tell us?
Well, at least two things:
First, that we can experience time, feel time,
and in fact, as human beings,
we can even project ourselves into the future or the past,
thus defying the laws of physics!
The second thing it tells us
is that we're not really defying the laws of physics,
as the feeling of time is not always consistent
with the measurements we can obtain in physics.
In other words, perceptual time is not always accurate.
What I wanted to say now is,
on the one hand, we have a definition of time in the physical world,
represented with the top arrow,
and temporal distortions, or psychological time, here at the bottom.
How do you go from physical time to psychological time,
what is there between the two?
There is, of course, a brain
which is a complex organ made of billions of neurons
which communicate with each other using electric signals.
These neurons will talk to each other either locally or over larger distances.
And we can record these electric signals in different ways.
In humans, we use a non-invasive technique
called magneto-electroencephalography,
commonly referred to as MEG,
which is a tool that allows us to record magnetic fields
that are emitted by neurons communicating with each other.
And when there is electrical current, there is also a magnetic field,
and this is what a MEG records.
I will now show you a video of a MEG
which we've matched with an anatomical MRI.
We had a person sitting in the MEG,
fixing their eyes on distant point
and trying not to think about anything.
And we observed the activity you will see here on your left.
Here, you have half a brain, the left hemisphere,
which you see here from the outside and then, you'll see the middle of it.
What I'd like you to focus on is the rhythm of the activity.
The red color shows you the activity of the neurons.
In reality, each colored area corresponds to thousands of neurons
being active at the same time.
And you should observe a certain rhythm
that we call "brain oscillations"
which are named here, as they each have a name.
These frequency bands are really natural oscillations,
biological rhythms that are intrinsic to the brain,
that we can observe in a resting state.
And if you can't see it well in the video,
I'll play some music we created,
which reflects the neural activity recorded from the temporal lobe
again, in a patient who was in a resting state, but awake.
This music was composed by Alain Destexhe.
(Music)
I think you can hear it now.
(Music)
Well, I'll stop here.
OK, so we have a biological rhythm,
but how is it related to time perception?
Or perception in general?
The fact that we can perceive the world?
Let's see a drawing by Wittgenstein
which represents either a duck or a rabbit.
In fact, it's a bistable perception,
meaning that every 2 or 3 seconds, you perception should change,
now you see a rabbit, now you see a duck.
What you have to realize here,
is that there is absolutely nothing changing in the drawing.
The switches you perceive are created by the rhythmicity of your brain.
Now, this reflects the rhythmicity of the brain,
but not your perception of time just yet.
Here, you are perceiving a duck or a rabbit.
If I were to ask you to pay attention to the duration of your perception,
then it would be time perception, but only if I did ask you that.
Now, I will talk about cognitive neuroscience
by making you understand the difficulty
of saying that we have physical time on one side,
brain dynamics that we can now measure with modern tools,
and then time perception itself.
If I show you this color here, which is a shade of blue,
and if I wanted to understand how your brain would encode this blue,
I could put you in a machine and see which of your neurons would turn blue.
Of course, in reality, no neurons will turn blue.
But considering this simple reasoning, why would there be time in the brain?
If the brain uses a neural code to encode information from the physical world,
why wouldn't time be encoded in the same way
that light, color, sound, etc.
This is the first problem we are confronted with,
when we try to understand how time is perceived,
As it is not just linear time in the brain.
The second problem is that the brain uses time,
temporal information from the physical world, for other things.
For example, as you all know, bats produce sounds,
they wait for the return of the echo,
and based on that, they form a representation of a spatial world,
that we, humans, cannot necessarily have access to.
But for bats, this is a spatial representation of the world.
Also, a lot of other mammals can use this system.
They use it to measure distances, but in terms of temporal delays.
Here, you can see the representation of a head,
with the source of a sound located on its right.
And of course, this sound and the acoustic waves
will reach the right ear before the left ear,
and in the auditory system of some mammals,
you have groups of neurons
which are capable of creating a spatial representation of the auditory source
from differences on the order of microseconds.
Hence, time at the level of microseconds, for the brain,
may represent space, and not time.
Another problem is what you are experiencing now:
you see me, I'm moving, I'm talking, you can also hear me.
You auditory system will respond to the sound you hear
about ten milliseconds after I produced that sound.
Your visual cortex will respond
roughly 50 milliseconds after I started moving my lips.
And yet your perception of me is coherent.
It's not like you hear me before you see me move, everything is coherent.
We should also add the differences in the speeds of light and sound.
So basically, your brain must establish coherent representations in time
in spite of all these delays,
whether external or intrinsic to its own connectivity.
So, how does the brain do it?
I will present to you three working hypotheses
currently being tested in neurosciences,
and I'll try to show how the brain can create time,
that is, create psychological time,
what we feel and experience as time.
The first thing the brain can do, through neural oscillations
and this brain dynamic I showed you earlier,
is to split time.
If you represent time as an arrow, with a past, a present and a future,
each of these moments can have a specific temporal granularity.
An oscillation will occur which can integrate time,
in a way that past, present and future, for the brain, become one segment.
This illustrates the "thick present" which Husserl talked about.
Hence the brain, depending on its dynamic,
will have a different temporal granularity of a representation of time,
on different scales and frequency ranges.
A second possibility is the predictive power of brain dynamics.
It is a bit like when you listen to music:
there is a rhythm from which you can predict the next musical note.
Brain dynamics can similarly help us predict something.
Here, you probably felt time, right?
The idea was to make you pay attention to time.
I spoke in a certain rhythm,
and you were expecting me to continue, then suddenly I stopped.
So then, the brain started paying attention to time,
and suddenly time expanded.
When you pay attention to time, the present expands.
The psychological present, of course, not the physical one.
The third working hypothesis is one that we tackle in my group.
We are interested in the phase of neural oscillations.
If your brain is resting without stimuli,
it has its oscillations at different frequencies,
so that when an event occurs,
it can do so at any moment during an ongoing oscillation.
This is what the blue and green lines show here:
you have the same oscillation, the same frequency,
but events take place at different phases of this oscillation.
And this can have dramatic consequences on how the brain will deal with time.
For example, we recently found that the auditory cortex
could slightly shift its oscillatory phases
with respect to those seen in the visual cortex.
We hypothesize that the brain tries to integrate all these delays
in order to make the perceptual world more coherent.
Now, other time models exist.
The ones I have mentioned so far
deal mostly with short time-scales, with milliseconds.
You also have the internal clock, which is also based on oscillations.
and is more focused.
Those I mentioned before were distributed systems all around the brain.
The internal clock is more concerned with how we encode time
on the scales of seconds and minutes.
Its a sub-cortical system based in the basal ganglia,
constantly in communication with the cortex through oscillations.
It works a little bit like filling up a container.
Time here becomes a substance that spills into a container,
which we call the accumulator,
and this stops when the duration is over.
This flow can have a speed,
so that we can adjust our internal clock in terms of speed.
Once the accumulation is over,
what is left in the accumulator can be compared
with durations that we have in memory.
What is a minute? What are two minutes? Etc.
And then, we will be able to assess the duration we felt.
I told you there was a problem to solve.
There is a time paradox, and you are all experts on it.
When, in the present, you experience an event of short duration,
like free-fall,
or a very long one, like your fingers stuck in the car door.
In reality, when you remember these events in the future,
for example, in ten years' time, you will think about your free fall,
and at that moment, this free fall in your memory will last an eternity,
but your fingers stuck in the car door, you will have nearly forgotten.
It's the same thing with the time spent waiting in a doctor's waiting room,
and the duration of a first date.
So, a time that is long in the present had been a short time in the past,
and a time that is short in the present was most likely long in the past.
And so, if there is a moral to this story?
I'll suggest one:
Do not think about the present, just live it!
Thanks!
(Applause)