[Applause]
[Music]
thank the organizers for inviting me to
take part in today it’s been a really
inspiring day of ideas and performances
and I hope I can hold your focus for the
last little bit of time that we have
here and time is a really broad and
fascinating subject it touches on a lot
of different areas physicists and
philosophers theorized about what time
is time is a coordinate it’s a measure
of duration it’s the ever existing
entropy in the universe neuroscientists
and psychologists try to understand how
we perceive time
well more generally how the brain makes
use of it my name is Joe Patton and I’m
a neuroscientist at gym probably more
research here in Lisbon and I got
interested in time for a particular
reason and that is that what I
ultimately want to understand is how the
brain produces something we call
adaptive behavior how do we and other
animals learn through experience to
behave in ways that are good for us and
it turns out that time will role in that
ability and to give you a sense of that
I want to start with a visual analogy if
you look at this image it’s a scene
right it happens to be of Native
Americans who are hunting deer our
ability to perceive the scene and
describe it of course depends on the the
particular structure of luminance and if
this were a colored image color and
space if we were to remove that
structure by say shuffling the location
of all the pixels in this image of
course it would be impossible to tell
what this is an image of now likewise if
we were to remove the influence of time
on our experience we might have a
similarly unintelligible mess for
instance all of our experiences might be
compressed into one moment right which
is reflected in this famous quote it’s
often attributed to the physicist John
wheeler but what he actually read on the
wall of a bathroom somewhere which says
that time is Nature’s Way to keep
everything from happening at once but
actually I think about time a little bit
differently because event
have order they have a direction and
there’s some smoothness to how they
evolve right so if we were to remove
time from our experience I imagine that
events would become jumbled untethered
from the surrounding moments kind of
like shuffling the frames of a movie
that constitutes your entire life now
imagine trying to take that and using it
to learn about the consequences of your
actions in order to learn how to behave
adaptively it’s impossible we’re more
basically imagine being a young child
you know when you come into this world
you’re a blank slate a tabula rasa and
you have to learn in large part do the
relationships between events and time
how to understand really understand
anything about the world in addition to
behave adaptively we need to have a
sense of in what moment to act so time
is really fundamental in that it it
provides the structured order to our
experience that we need in order to
understand the world and make
predictions and also because we need it
to behave at the right time so when I
was setting up my research group I was I
was super excited because I felt like I
had identified this problem one of these
problems that my lab would focus on how
does the brain deal with time which was
super important for understanding how we
and other animals learn to interact with
their environment and very few people
were studying it there’s not a lot known
about it and so I thought great I have
job security security for the next few
decades but I kept running up against
this kind of uncomfortable reality and
that is that we’re pretty terrible at
timing stuff exhibit a clocks this
happens to be a clock made by a
two-year-old
and father’s day but the fact that
humanity created this machine and all of
its predecessors sundials hourglasses
water clocks astrolabes and all the
different ways that we’ve we’ve learned
we’ve invented machines to tell time is
the existence proof that our innate
sense of time isn’t very reliable if it
were we wouldn’t need clocks to tell us
what time it is
so time can you know stand still they
can fly it can disappear based on a
number of different factors for example
I’m going to show you two video clips
okay a flashing colored grids and I want
you to look straight at the center of
the video clips and I want you to tell
me which one lasted longer and don’t
cheat don’t count don’t look at your
watch or anything like that
just give me your gut impression okay
here we go
that was the first
now it’s a second okay so everyone who
thought that the first clip lasted
longer raise your hand and everyone
thought that the second thought lasted
longer raise your hand
awesome that worked perfectly okay so
the vast majority of you thought that
the first clip lasted longer but in fact
they’re identical in duration and that’s
because in a lot of studies have shown
this the rate at which sensory stimuli
change can alter your perception of time
okay and here I’m showing data from one
of those studies so what’s plotted along
the vertical axis here is the relative
estimated duration of a stimulus that
has dynamics it’s changing it’s
flickering like those stimulation before
and along the horizontal axis is the
speed of that change how fast is the
stimulus flickering and what you can see
is that the stimulus changes faster and
faster people show a tendency to
overestimate duration okay so sensory
change is one factor that can affect
your perception of time but there are
others and what I want to suggest you
today this is sort of the central idea
is that this variability in our innate
timekeeping isn’t some shortcoming it’s
not a problem that evolution hasn’t
gotten around to fixing yet but rather
it’s a feature that ultimately serves
our best interests
and I think it’s telling that when we
look into the brain we find that those
same brain systems that seem to be
critical for guiding adaptive behavior
also seem to be responsible for
variability in time perception and the
brain system I’ll spend a lot of time
focusing on today is that of dopamine
neurons in the midbrain so here I’m
showing a side or a sagittal view of the
human brain with one side cutaway and
the dopamine neurons I’ll focus on today
are located in the midbrain their cell
bodies here and this branching portion
here represents the projections or the
axons where the signals that these cells
are carrying are being projected to and
these neurons they use a
neurotransmitter they release a
neurotransmitter neuromodulator dopamine
which alters the way recipient neurons
respond to other inputs and it can do it
in the current moment or it can alter in
more permanent ways the strength of the
connections between neurons okay
something we call synaptic plasticity
and now it’s really interesting when you
consider the way these neurons can kind
of change or teach the strength of
connections between neurons when you
consider that these cells carry a really
really useful instructive signal they
seem to teach the brain to choose those
behaviors that are going to result in
reward and they act sort of like a
teacher that tells you only if you’re
surprised surprised at whether things
are better or worse than expected now
why is a teacher based on surprise
useful well if you think about it if
you’re surprised by some particular
outcome that’s an indication that you
should update your expectations you
should learn something so the next time
you encounter that situation you’re less
surprised okay these updated
expectations about the quality of the
current situation can be used to guide
your behavior towards those situations
of the highest quality okay but why am I
telling you all this about dopamine
neurons and learning and rewards I
thought this was a talk about time well
one of the reasons is that these neurons
project very very densely to another
part of the brain called the striatum
which you can see here in pink which the
work of many labs including mine is
shown to be critical for
keeping time so if we can bring down the
lights a little bit this would be a
little bit more clear but I’m showing
you is a slice now through a mouse brain
what we’ve done is we’ve labeled
dopamine neurons on one side of the
brain where the protein that fluoresce
is red and what you’re actually looking
at here are the axon terminals the
projections of these these nerve cells
and basically everything you’re seeing
here in red is this brain area the
striatum so you can just see how densely
these neurons project to this area as as
opposed to other areas where they do
project like these ones out here and now
synaptic plasticity remember those those
changes in strength in the connections
between neurons in this particular area
is thought to be important for shaping
the actions we take in a given situation
and if you were to look at the activity
of single neurons in this area during
behavior you might see something like
the following so here I’m visualizing
real data each circle here represents an
individual neuron in this brain area the
striatum and the brightness of the red
color indicates how electrically active
that cell is and if you look at the
activity of these cells you see there’s
a diversity of responding okay in this
diversity can actually be used to read
out the passage of time and this becomes
more clear if we just reorder the cells
by when they’re active if you do that
you see this kind of wave of activity
that moves over the population and just
like the hands of a clock move across
its face
so does this wave of activity across
this population of neurons okay so you
can read out time from these cells here
but what’s the evidence that the brain
is actually doing that well to get at
that you need to ask the animal this
happens to be a rat where we recorded
this activity about its sense of time if
we were working with a person we could
ask them to describe their sensations
using language but we as a society don’t
like to insert electrodes or other
devices into the human brain unless we
absolutely have to you might hit a blood
vessel or you might cause an infection
however we’re a little more tolerant of
doing this in an animal model organism
like a rat
but how do you ask a rat about its time
perception well one way to do that is to
train them to make a judgement okay was
the interval you just experienced longer
or shorter than one-and-a-half seconds
and you train them until they’re really
really good at this at this judgment and
here I’m showing behavioral data from a
rat that’s been trained to do this
we’re on what I’m plotting on the
vertical axis here is the probability
that they they categorize an interval as
long they judge it as longer than
one-and-a-half seconds against the
different intervals that we present to
them and what you can see is that for
these these intervals that are near
one-and-a-half seconds the difficult
ones their choices are variable
sometimes they’re going to judge this
interval is longer than one-and-a-half
seconds and sometimes they’re going to
judge it as shorter and we think that
most of this variability in their
judgments is actually due to variability
in their sense of time so then we can
compare the judgments that animals make
to either observations or manipulations
of neural activity that we make in the
in laboratory in order to try and
identify candidate neural signals for
timing so the first piece of evidence
that those waves of activity I showed
you are actually involved in keeping
time is that if you inject a drug into
the striatum that just perturbs activity
a little bit it doesn’t mess up the
animal’s behavior very much at all while
they’re performing this task they get
worse so here I’m showing in orange data
collected on days when we injected this
drug into the striatum okay and in green
I’m showing data on days on alternate
days when we just injected saline or
saltwater so you can see just by messing
up activity a little bit in this area
you produce a market decrease in the
sensitivity of animals judgments to time
but what about those ways of activity if
the animal is actually looking at where
that wave is and using it to tell much
time it’s past if that wave goes a
little faster they should think time is
passing faster and if that wave is going
slower they should you think that time
is passing slower and indeed if we
record from populations of neurons
during this task we see something very
similar so when that wave goes more
quickly as you can see in this red curve
animals show a system at
tendency to judge intervals as being
long particularly for those intervals
that are near the decision boundary and
the reverse is true when that wave
progresses more slowly okay so you have
this clock it’s variable sometimes it
goes faster sometimes it goes slower and
that predicts the judgments that animals
are going to make but what’s responsible
for that variability in clock speed and
this is where we come back to dopamine
so if you look through the psychological
literature and you try and find
situations where your your timekeeping
seems to be systematically altered on
the right what I’m showing our cases
where time appears to drag and this can
get confusing because this is sort of
akin to having a fast internal clock you
think that more time has passed and
actually has and these are things like
fear anxiety boredom and pain and on the
right I’m going to show cases where time
appears to fly and again this is akin to
having a slow internal clock you think
that last time has passed and actually
happens and these are things like your
motivation to approach or engage with
the situation attentional load pleasure
and novelty now interestingly most of
these cases here on the Left are
associated with decreased dopamine
signaling and most of these situations
here on the right are associated with
increased dopamine signaling in addition
conditions where dope the dopaminergic
system is damaged such as Parkinson’s
disease people with Parkinson’s disease
display kind of peculiar timing deficits
so there’s a lot of kind of indirect
evidence that dopamine neurons may be
exerting some kind of control over your
perception of time but how do we
directly test this idea so recently
bassam Matala and sofia swat is in the
lab devised and and carried out a series
of experiments to directly test whether
dopamine neurons can control your time
perception or at least the time
perception of mice so what they did was
they first trained mice to perform this
judgment was the interval longer shorter
than one-and-a-half seconds that you
just experienced
remember the data I plotted before which
is showing the probability that an
animal makes a long judgment as a
function of the different intervals
because that would be important and
that’s what they did was they measured
the activity of dopamine neurons using
an optical technique where we engineered
dopamine neurons to light up when they
were active this gave us a fast and
continuous measure of the activity of
these cells that we could then compare
to the judgments that animals were
making from childhood trial and what
they observed was striking when they
when the activity of these neurons was
higher as you can see in this blue curve
animals clocks appeared to slow down
they were more likely to underestimate
duration and the reverse was true when
these cells were less active they were
more likely to overestimate duration if
as if their clock had sped up okay so
there was this correlation between the
activity of these neurons and the
pattern the judgments about time that
animals were making but of course
correlation doesn’t imply causation so
the next they went a step further and
they applied a technique from a
relatively new field called optogenetics
the name of the name should make sense
in a moment
in order to transiently manipulate the
activity of these cells during this
behavior so what is optogenetics in our
eyes we have proteins that coupled to
light-sensitive molecules if these
molecules observe a photon they change
shape and this kicks off a series of
steps that ultimately results in a
message being sent to your brain and
that’s how we see but this process is
kind of complicated it involves a number
different proteins and genes and in the
end of the 90s in the early 2000s people
were wondering if they could kind of
harness that general idea to make
neurons controllable with light and what
these people did is they identified
another similar kind of protein that’s
expressed by algae that actually is
light-sensitive but it couples directly
to an ion channel making it easier to
kind of express potentially in neurons
and they managed to express these things
in neurons rendering them controllable
with light and this has brought about a
real revolution in neuroscience because
it allows us to go beyond
just identifying correlations between
signals we observed in the brain and
particularly havior x’ were interested
in or particular brain functions and
actually inject in signals into
particular genetically targeted cells
during behavior and so that’s what
that’s what these guys did in their
experiments they induced dopamine
neurons to express these light-sensitive
opsins and then they inserted a little
glass fiber right above the dopamine
neurons and then randomly throughout
these behavioral sessions they would
illuminate these cells to either
activate or inhibit their activity and
what they found was indeed when they
activated dopamine neurons they were
able to apparently slow animals
timekeeping consistent with the
observations they had made in the
previous experiment and conversely when
they inhibited these neurons both though
the effect was a little bit more modest
they caused the animal to apparently
speed up its timekeeping they showed a
tendency to overestimate duration so
dopamine neuron activation slows and
inhibition seems to speed timekeeping
and so I want to step back now and ask a
more general question which is which is
why why would this brain system that
seems so important for teaching us how
to act in order to get rewards also be
able to influence our sense of time and
this is the the crux of the point and
that is that a big component of your
sense of time it’s actually how
worthwhile your current course of action
is okay the quality of the current
moment is actually warping your sense of
time in order to keep you in pursuit of
those situations that are better than
expected or to cause you to disengage
and potentially move on to some some
more rewarding situation if things are
less than expected and there are a lot
of situations like this in life right
life is sequential
we’re always posed with the question
should I keep doing what I’m doing now
or should I switch and do something that
could potentially be more rewarding take
for example you’re on a first date it’s
going horribly
you’re thinking about all the other
things you’d rather be doing is each
interminable second is creeping by so
you skip dessert you leave early and
you’re off to greener pastures sooner
let’s imagine another scenario the dates
going really well
you’re completely focused on the current
moment and before you know it you’ve
been there for hours night falls and
then the Sun rises and during this
entire time you form this deep bond with
this person that just might form the
basis of a like loyal lifelong
relationship and all of its associated
benefits so the next time you notice a
discrepancy between your inner clock and
the watch that you’re wearing or the
clock on your cell phone
think about those dopamine neurons in
your midbrain and how they might be
speeding up or slowing down those waves
of neural activity in the striatum
pushing around your time perception and
all the all the work that evolution has
gone to to keep you in pursuit of things
that are good for you
thank you
you
you
[Music]
thank the organizers for inviting me to
take part in today it’s been a really
inspiring day of ideas and performances
and I hope I can hold your focus for the
last little bit of time that we have
here and time is a really broad and
fascinating subject it touches on a lot
of different areas physicists and
philosophers theorized about what time
is time is a coordinate it’s a measure
of duration it’s the ever existing
entropy in the universe neuroscientists
and psychologists try to understand how
we perceive time
well more generally how the brain makes
use of it my name is Joe Patton and I’m
a neuroscientist at gym probably more
research here in Lisbon and I got
interested in time for a particular
reason and that is that what I
ultimately want to understand is how the
brain produces something we call
adaptive behavior how do we and other
animals learn through experience to
behave in ways that are good for us and
it turns out that time will role in that
ability and to give you a sense of that
I want to start with a visual analogy if
you look at this image it’s a scene
right it happens to be of Native
Americans who are hunting deer our
ability to perceive the scene and
describe it of course depends on the the
particular structure of luminance and if
this were a colored image color and
space if we were to remove that
structure by say shuffling the location
of all the pixels in this image of
course it would be impossible to tell
what this is an image of now likewise if
we were to remove the influence of time
on our experience we might have a
similarly unintelligible mess for
instance all of our experiences might be
compressed into one moment right which
is reflected in this famous quote it’s
often attributed to the physicist John
wheeler but what he actually read on the
wall of a bathroom somewhere which says
that time is Nature’s Way to keep
everything from happening at once but
actually I think about time a little bit
differently because event
have order they have a direction and
there’s some smoothness to how they
evolve right so if we were to remove
time from our experience I imagine that
events would become jumbled untethered
from the surrounding moments kind of
like shuffling the frames of a movie
that constitutes your entire life now
imagine trying to take that and using it
to learn about the consequences of your
actions in order to learn how to behave
adaptively it’s impossible we’re more
basically imagine being a young child
you know when you come into this world
you’re a blank slate a tabula rasa and
you have to learn in large part do the
relationships between events and time
how to understand really understand
anything about the world in addition to
behave adaptively we need to have a
sense of in what moment to act so time
is really fundamental in that it it
provides the structured order to our
experience that we need in order to
understand the world and make
predictions and also because we need it
to behave at the right time so when I
was setting up my research group I was I
was super excited because I felt like I
had identified this problem one of these
problems that my lab would focus on how
does the brain deal with time which was
super important for understanding how we
and other animals learn to interact with
their environment and very few people
were studying it there’s not a lot known
about it and so I thought great I have
job security security for the next few
decades but I kept running up against
this kind of uncomfortable reality and
that is that we’re pretty terrible at
timing stuff exhibit a clocks this
happens to be a clock made by a
two-year-old
and father’s day but the fact that
humanity created this machine and all of
its predecessors sundials hourglasses
water clocks astrolabes and all the
different ways that we’ve we’ve learned
we’ve invented machines to tell time is
the existence proof that our innate
sense of time isn’t very reliable if it
were we wouldn’t need clocks to tell us
what time it is
so time can you know stand still they
can fly it can disappear based on a
number of different factors for example
I’m going to show you two video clips
okay a flashing colored grids and I want
you to look straight at the center of
the video clips and I want you to tell
me which one lasted longer and don’t
cheat don’t count don’t look at your
watch or anything like that
just give me your gut impression okay
here we go
that was the first
now it’s a second okay so everyone who
thought that the first clip lasted
longer raise your hand and everyone
thought that the second thought lasted
longer raise your hand
awesome that worked perfectly okay so
the vast majority of you thought that
the first clip lasted longer but in fact
they’re identical in duration and that’s
because in a lot of studies have shown
this the rate at which sensory stimuli
change can alter your perception of time
okay and here I’m showing data from one
of those studies so what’s plotted along
the vertical axis here is the relative
estimated duration of a stimulus that
has dynamics it’s changing it’s
flickering like those stimulation before
and along the horizontal axis is the
speed of that change how fast is the
stimulus flickering and what you can see
is that the stimulus changes faster and
faster people show a tendency to
overestimate duration okay so sensory
change is one factor that can affect
your perception of time but there are
others and what I want to suggest you
today this is sort of the central idea
is that this variability in our innate
timekeeping isn’t some shortcoming it’s
not a problem that evolution hasn’t
gotten around to fixing yet but rather
it’s a feature that ultimately serves
our best interests
and I think it’s telling that when we
look into the brain we find that those
same brain systems that seem to be
critical for guiding adaptive behavior
also seem to be responsible for
variability in time perception and the
brain system I’ll spend a lot of time
focusing on today is that of dopamine
neurons in the midbrain so here I’m
showing a side or a sagittal view of the
human brain with one side cutaway and
the dopamine neurons I’ll focus on today
are located in the midbrain their cell
bodies here and this branching portion
here represents the projections or the
axons where the signals that these cells
are carrying are being projected to and
these neurons they use a
neurotransmitter they release a
neurotransmitter neuromodulator dopamine
which alters the way recipient neurons
respond to other inputs and it can do it
in the current moment or it can alter in
more permanent ways the strength of the
connections between neurons okay
something we call synaptic plasticity
and now it’s really interesting when you
consider the way these neurons can kind
of change or teach the strength of
connections between neurons when you
consider that these cells carry a really
really useful instructive signal they
seem to teach the brain to choose those
behaviors that are going to result in
reward and they act sort of like a
teacher that tells you only if you’re
surprised surprised at whether things
are better or worse than expected now
why is a teacher based on surprise
useful well if you think about it if
you’re surprised by some particular
outcome that’s an indication that you
should update your expectations you
should learn something so the next time
you encounter that situation you’re less
surprised okay these updated
expectations about the quality of the
current situation can be used to guide
your behavior towards those situations
of the highest quality okay but why am I
telling you all this about dopamine
neurons and learning and rewards I
thought this was a talk about time well
one of the reasons is that these neurons
project very very densely to another
part of the brain called the striatum
which you can see here in pink which the
work of many labs including mine is
shown to be critical for
keeping time so if we can bring down the
lights a little bit this would be a
little bit more clear but I’m showing
you is a slice now through a mouse brain
what we’ve done is we’ve labeled
dopamine neurons on one side of the
brain where the protein that fluoresce
is red and what you’re actually looking
at here are the axon terminals the
projections of these these nerve cells
and basically everything you’re seeing
here in red is this brain area the
striatum so you can just see how densely
these neurons project to this area as as
opposed to other areas where they do
project like these ones out here and now
synaptic plasticity remember those those
changes in strength in the connections
between neurons in this particular area
is thought to be important for shaping
the actions we take in a given situation
and if you were to look at the activity
of single neurons in this area during
behavior you might see something like
the following so here I’m visualizing
real data each circle here represents an
individual neuron in this brain area the
striatum and the brightness of the red
color indicates how electrically active
that cell is and if you look at the
activity of these cells you see there’s
a diversity of responding okay in this
diversity can actually be used to read
out the passage of time and this becomes
more clear if we just reorder the cells
by when they’re active if you do that
you see this kind of wave of activity
that moves over the population and just
like the hands of a clock move across
its face
so does this wave of activity across
this population of neurons okay so you
can read out time from these cells here
but what’s the evidence that the brain
is actually doing that well to get at
that you need to ask the animal this
happens to be a rat where we recorded
this activity about its sense of time if
we were working with a person we could
ask them to describe their sensations
using language but we as a society don’t
like to insert electrodes or other
devices into the human brain unless we
absolutely have to you might hit a blood
vessel or you might cause an infection
however we’re a little more tolerant of
doing this in an animal model organism
like a rat
but how do you ask a rat about its time
perception well one way to do that is to
train them to make a judgement okay was
the interval you just experienced longer
or shorter than one-and-a-half seconds
and you train them until they’re really
really good at this at this judgment and
here I’m showing behavioral data from a
rat that’s been trained to do this
we’re on what I’m plotting on the
vertical axis here is the probability
that they they categorize an interval as
long they judge it as longer than
one-and-a-half seconds against the
different intervals that we present to
them and what you can see is that for
these these intervals that are near
one-and-a-half seconds the difficult
ones their choices are variable
sometimes they’re going to judge this
interval is longer than one-and-a-half
seconds and sometimes they’re going to
judge it as shorter and we think that
most of this variability in their
judgments is actually due to variability
in their sense of time so then we can
compare the judgments that animals make
to either observations or manipulations
of neural activity that we make in the
in laboratory in order to try and
identify candidate neural signals for
timing so the first piece of evidence
that those waves of activity I showed
you are actually involved in keeping
time is that if you inject a drug into
the striatum that just perturbs activity
a little bit it doesn’t mess up the
animal’s behavior very much at all while
they’re performing this task they get
worse so here I’m showing in orange data
collected on days when we injected this
drug into the striatum okay and in green
I’m showing data on days on alternate
days when we just injected saline or
saltwater so you can see just by messing
up activity a little bit in this area
you produce a market decrease in the
sensitivity of animals judgments to time
but what about those ways of activity if
the animal is actually looking at where
that wave is and using it to tell much
time it’s past if that wave goes a
little faster they should think time is
passing faster and if that wave is going
slower they should you think that time
is passing slower and indeed if we
record from populations of neurons
during this task we see something very
similar so when that wave goes more
quickly as you can see in this red curve
animals show a system at
tendency to judge intervals as being
long particularly for those intervals
that are near the decision boundary and
the reverse is true when that wave
progresses more slowly okay so you have
this clock it’s variable sometimes it
goes faster sometimes it goes slower and
that predicts the judgments that animals
are going to make but what’s responsible
for that variability in clock speed and
this is where we come back to dopamine
so if you look through the psychological
literature and you try and find
situations where your your timekeeping
seems to be systematically altered on
the right what I’m showing our cases
where time appears to drag and this can
get confusing because this is sort of
akin to having a fast internal clock you
think that more time has passed and
actually has and these are things like
fear anxiety boredom and pain and on the
right I’m going to show cases where time
appears to fly and again this is akin to
having a slow internal clock you think
that last time has passed and actually
happens and these are things like your
motivation to approach or engage with
the situation attentional load pleasure
and novelty now interestingly most of
these cases here on the Left are
associated with decreased dopamine
signaling and most of these situations
here on the right are associated with
increased dopamine signaling in addition
conditions where dope the dopaminergic
system is damaged such as Parkinson’s
disease people with Parkinson’s disease
display kind of peculiar timing deficits
so there’s a lot of kind of indirect
evidence that dopamine neurons may be
exerting some kind of control over your
perception of time but how do we
directly test this idea so recently
bassam Matala and sofia swat is in the
lab devised and and carried out a series
of experiments to directly test whether
dopamine neurons can control your time
perception or at least the time
perception of mice so what they did was
they first trained mice to perform this
judgment was the interval longer shorter
than one-and-a-half seconds that you
just experienced
remember the data I plotted before which
is showing the probability that an
animal makes a long judgment as a
function of the different intervals
because that would be important and
that’s what they did was they measured
the activity of dopamine neurons using
an optical technique where we engineered
dopamine neurons to light up when they
were active this gave us a fast and
continuous measure of the activity of
these cells that we could then compare
to the judgments that animals were
making from childhood trial and what
they observed was striking when they
when the activity of these neurons was
higher as you can see in this blue curve
animals clocks appeared to slow down
they were more likely to underestimate
duration and the reverse was true when
these cells were less active they were
more likely to overestimate duration if
as if their clock had sped up okay so
there was this correlation between the
activity of these neurons and the
pattern the judgments about time that
animals were making but of course
correlation doesn’t imply causation so
the next they went a step further and
they applied a technique from a
relatively new field called optogenetics
the name of the name should make sense
in a moment
in order to transiently manipulate the
activity of these cells during this
behavior so what is optogenetics in our
eyes we have proteins that coupled to
light-sensitive molecules if these
molecules observe a photon they change
shape and this kicks off a series of
steps that ultimately results in a
message being sent to your brain and
that’s how we see but this process is
kind of complicated it involves a number
different proteins and genes and in the
end of the 90s in the early 2000s people
were wondering if they could kind of
harness that general idea to make
neurons controllable with light and what
these people did is they identified
another similar kind of protein that’s
expressed by algae that actually is
light-sensitive but it couples directly
to an ion channel making it easier to
kind of express potentially in neurons
and they managed to express these things
in neurons rendering them controllable
with light and this has brought about a
real revolution in neuroscience because
it allows us to go beyond
just identifying correlations between
signals we observed in the brain and
particularly havior x’ were interested
in or particular brain functions and
actually inject in signals into
particular genetically targeted cells
during behavior and so that’s what
that’s what these guys did in their
experiments they induced dopamine
neurons to express these light-sensitive
opsins and then they inserted a little
glass fiber right above the dopamine
neurons and then randomly throughout
these behavioral sessions they would
illuminate these cells to either
activate or inhibit their activity and
what they found was indeed when they
activated dopamine neurons they were
able to apparently slow animals
timekeeping consistent with the
observations they had made in the
previous experiment and conversely when
they inhibited these neurons both though
the effect was a little bit more modest
they caused the animal to apparently
speed up its timekeeping they showed a
tendency to overestimate duration so
dopamine neuron activation slows and
inhibition seems to speed timekeeping
and so I want to step back now and ask a
more general question which is which is
why why would this brain system that
seems so important for teaching us how
to act in order to get rewards also be
able to influence our sense of time and
this is the the crux of the point and
that is that a big component of your
sense of time it’s actually how
worthwhile your current course of action
is okay the quality of the current
moment is actually warping your sense of
time in order to keep you in pursuit of
those situations that are better than
expected or to cause you to disengage
and potentially move on to some some
more rewarding situation if things are
less than expected and there are a lot
of situations like this in life right
life is sequential
we’re always posed with the question
should I keep doing what I’m doing now
or should I switch and do something that
could potentially be more rewarding take
for example you’re on a first date it’s
going horribly
you’re thinking about all the other
things you’d rather be doing is each
interminable second is creeping by so
you skip dessert you leave early and
you’re off to greener pastures sooner
let’s imagine another scenario the dates
going really well
you’re completely focused on the current
moment and before you know it you’ve
been there for hours night falls and
then the Sun rises and during this
entire time you form this deep bond with
this person that just might form the
basis of a like loyal lifelong
relationship and all of its associated
benefits so the next time you notice a
discrepancy between your inner clock and
the watch that you’re wearing or the
clock on your cell phone
think about those dopamine neurons in
your midbrain and how they might be
speeding up or slowing down those waves
of neural activity in the striatum
pushing around your time perception and
all the all the work that evolution has
gone to to keep you in pursuit of things
that are good for you
thank you
you
you
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