These are very unique times for brain research. The aperitif for the course will thus highlight the present “brain-excitements” worldwide. You will then become intimately acquainted with the operational principles of neuronal “life-ware” (synapses, neurons and the networks that they form) and consequently, on how neurons behave as computational microchips and how they plastically and constantly change - a process that underlies learning and memory. Recent heroic attempts to realistically simulate large cortical networks in the computer will be highlighted (e.g., “the Blue Brain Project”) and processes related to perception, cognition and emotions in the brain will be discussed. For dessert we will deliberate on the future of brain research, including the questions of “brain and art”, consciousness and free will. For more information see the course promo below and read “About the course.”
How Sensory Information Guides Motion
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Do curso por Hebrew University of Jerusalem
Sinapses, Neurônios e Cérebros
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Perception, Action, Cognition and Emotions
During this module we will have a special lecture which will be given by Prof. Israel Nelken from the Hebrew University in Jerusalem, who will discuss "Perception, action, cognition and emotions". Until today we have focused mainly on the function of single cells and small networks. Module #8 dwells into higher level computations and especially "the story of sound". The auditory system converts sound into electrical signals. Hair cells, basilar membrane, the cochlea and others are all tools which the brain uses in order to translate the outer world into neuronal activity. The next step is perception – processing the sensory information into useful representation. One beautiful example is the case of using binaural cues for the localization of sound. Perception leads the organism to actions – the brain predicts the world and computes the action that will yield a maximal reward and avoid punishment. What happens in the case of surprises? Here, higher level computation is needed. We will end the lecture with emotions – What are they, how are they represented in the brain and how they affect our actions.
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Idan SegevProfessor
Computational Neuroscience
In this part of the lecture, I am going to discuss the connections between the
auditory system and motor systems and in particular, the way that sounds guide
head turns. The model animal in which such issues
have been studied most and I'll describe the best is not a mammal actually, it's a
bird, it's the barn owl. The barn owl is a fascinating animal.
It hunts it can hunt in complete darkness.
Although it stands out, the barn owls they prefer to, to hunt at dusk, not in
complete darkness. They can hunt also in complete darkness.
barn owls turn out to be wonderful models for auditory system.
But also it turns out that they are good models for stereo vision.
They have the two eyes pointing forward and they have very good stereo vision
that is based on very similar principles to stereo vision in, in mammals.
But we are going to discuss the audition in barn owls.
When they hunt, they can hear the rustling of the mouse and the information
that reaches the ear, tells them both about the direction that the azimuth of
the mouse and how far it is below them. Em, and the main cue for detecting the
azimuth of the sound source for a barn owl's ear is like in mammals, the
integral time difference. So in this illustration, you have the
recording of two microphones that were in the two ears of the of the barn owl.
There was a sound, a short sound presented somewhere in the in the space.
And you can see that in this case, the sound was closer to the left ear, so it
reaches first the left ear and then the right ear.
And in the case of the barn owls, their two ears are asymmetric.
So the left ear hears better sounds that come from below and the right ear hears
better sound that comes from above and in consequence, if the sound source is below
the barn owl, it will be louder in the left ear and softer in the right ear.
So, in the case of the barn owls because of the asymmetric ears, inter aural level
differences have to do not with azimuths like in mammals who have symmetric ears
but with elevation. Now, barn owls are very good at detecting
sound source. If there is a barn owl, if you, if
there's a sitting barn owl and you present a click somewhere in the space,
the barn owl will turn its head pretty automatically.
to the sound source. Similarly, if you flash light.
So barn owls turn their heads very very well both to visual targets and to
auditory targets. And this plot shows a bunch of eh, eh,
head turns. The head turns were all to the, were to
different directions, but in this plot they're all normalized as if the
direction of the turn was at zero. And what you see are the shifts relative
to the target location, and the the orange color represents head turns that
were evoked by an auditory target and the purple circles represents head turns that
were evoked by a visual target. So, these head turns both to auditory
targets and to visual targets are essentially as precise the one as the
other. Now here's the manipulation that's done
on the barn owls in order to study the head turn behavior.
When they are young, this is the young barn owl.
It looks more like a chick than a barn owl but it still doesn't have its
feathers that create the, the, the typical shape of the head.
The experimental [INAUDIBLE] put prisms on its eyes.
So these goggles are prisms that shift the visual field by 20 degrees or so to
the side. So now when the when you, you present a
sound, when you present a light from the middle it will be, it will follow the
lighting of the barn owl as if it was shifted to this side.
[BLANK_AUDIO]. When you do this to the to the chick.
Then initially when you start testing it, the visual head turns are shifted, like
the prisms. But the auditory head shifts to auditory
targets from the same locations stay are still to the right direction.
So, initially the prisms do what you would expect, they shift the visual
field. So, their heads turns, their visual head
turns are now wrong in a way. The barn owl shifts its head to the
direction it thinks the target is and this direction is shifted because of the
because of the prisms by a by some angle. This why the visual, visual head turns
are now wrong. But the auditory head turns are
unaffected because auditory information is independent of the visual information.
However, after a few weeks. The auditory head turns shift and much
again the visual head turns. So, the fact that the barn owl sees the
world shifted, shifted now causes him also to hear the world shifted.
this is a kind of funny response. You would like the, effect to be the
other way, the visual, a visual, a head turns the getting back to the correct
direction instead of the auditory head shifts ahead heads turns to move toward,
to the wrong direction but it has some kind of a ecological validity to it.
As a rule, if you don't use prisms which are presumably not very common in the
natural world. Vision would be more precise than
audition special localization so it makes sense that if something happens and the,
the world shifts around. It's the visual system that will tell the
auditory system how to interpret the direction of the sound source.
And finally, if you take out the, the, prisms, then the visual head turns
immediately shift back to their correct location, but the auditory head turns
wrong immediately after you remove the prisms.
And it takes some time for the auditory head turns with enough time and under the
correct conditions, it will shift back and will become normal again.
So what is the new [INAUDIBLE] circuit that underlie these effects.
First of all, the way the fact that visual and auditory stimuli cause head
turns. And secondly, the facts that vision
teaches the auditory system where things are in the world.
This is as schematic view of the brain of a barn owl.
So this is a spinal cord, this is the cerebellum, and this is the forebrain,
what would be the cortex and all of these structures in humans.
And as in mammals, the important part of the auditory system of the barn owl for
this task is in the brain stem. So, we have structures very similar to
the MSO in mammals, that calculate the interval time differences.
And we have information about the interaural level differences that's
calculated in the brain stem and about other features of sounds.
And all of these different [INAUDIBLE] that calculate the different features of
sounds integrate in a structure called the inferior collicullus.
From the inferior colliculus, in a few steps in a, in a number of steps that I
will describe in a moment, the auditory information gets to a place called the
optic tectum and the optic tectum is both a sensory structure and a motor
structure. The optic tectum gets a visual, gets
input from many different senses. It has visual and auditory inputs, as
well as a somato sensory inputs and other typed as a, as a sensory inputs and it
among other things it project down to the brain stem and to a, a spinal cord to
areas which are a, a, which a, guide a, orientation movements.
The barn owl, for the barn owl, which almost doesn't move its eyes in their
sockets, the main orientation movement is the head turn.
For mammals that do move their eyes in their socket you get both eye movements
and, if the target is far in the periphery, also head head movements.
So the optic tectum is equivalent to [UNKNOWN] structure in mammals which is
called the superior collicullus, guides, is the center, central guiding post for
orientation movements, both in birds and in mammals.
So, this is a circuit and I want to to concentrate now on the way that the
auditory and visual information are both together are integrated.
So then, I'm going to concentrate again on the calculation of the interaural time
differences, on the presentation of the interaural time differences and the
reason for that is because this is the best studied part of the circuit.
It's not the only part but the best studied part of the circuit.
So we have the inferior colliculus and in inferior colliculus some representation
is still separate from different frequencies.
So remember that in the cochlear we already separated sounds into its
frequency component and this frequency components are processed separately as
you go up. Along the stations of the auditory mid
brain so that for example, neurons that are sensitive to eight kilohertz lie in a
different place than neurons that are sensitive to six kilohertz and neurons
that are sensitive to four kilohertz. For each one of these frequencies, you
have a bunch of neuron which is sensitive to a difference interaural time
difference. So if you go to the eighth kilohertz
place, you will find neurons that are sensitive to fire most when the
interaural time difference is zero. So they are sensitive to some sounds
right in the middle, and you will have neurons that are sensitive most to
interaural time differences of a few tens of microseconds to to one side or to the
other. And so you have a range of neuron with,
range of interaural time difference activity.
Same thing as six kilohertz, you have a bunch of neurons, all of them are
sensitive to six kilohertz but if you present the sound with different ITD's,
you will see that some of the neurons fire when the ITD is zero, other, when
the ITD is 20 or 40 or 50 or 100 micro seconds.
The same thing for the four kilohertz neurons.
Now, when you represent space, the frequency is immaterial as sounds can
come from the right and be at eight kilohertz or four kilohertz or six
kilohertz, all three of them together. So, if you want to represent space, you
want to somehow integrate over, get rid of the frequency dependence.
And this is exactly what happens in the next type of processing in the barn owl.
The neurons in the inferior colliculus project to a to the next station on the
pathway, which is called ICX. So, that's an external nucleolus of the
inferior collicullus, and this projection has a very special structures.
All neurons that are sensitive for ITD of zero microseconds, from four kilohertz at
six kilohertz to eight kilohertz all converge together to the same location in
the ICX. All neurons which are sensitive to ITD of
50 micro seconds, independent of the frequency for six or eight kilohertz, all
converge to a different place in the ICX and so on and so forth.
So, in the ICX, you get an axis of interaural time difference.
Which is now independent of frequency, independent of which sound you've
presented, what are the properties of the sound that you've presented.
The neurons in the ICX will be sensitive only to the interaural time difference of
these sounds. So, the ICX is the first place where you
get a. Real, truthful roughly stimulus
independent space map, auditory space map for neurons are sensitive to space and
not to other features of the sound. So this is the ICX.
Next station is the optic tectum. Okay, as I told you before the optic
tectum has now a, a lot of different inputs including visual inputs and the
visual inputs are ordered in the optic tectum, so visual inputs come from
straight ahead would go to one point those that go slightly sideways to a
different point and those that goes to far sideways.
Will go to a third point in the optic tectum.
And then what is right now magic but we will see in a moment how it comes about
is the fact that each location in the ICX which is determined by its IT interval
time difference in sensitivity. Project to the optic tectum to a location
which corresponds, to the, correct azimuth of the visual field.
So, neurons whose best ITD is zero microseconds would project to neurons
whose visual field is straight ahead. And neurons with ITD sensitivity center
is 50 microseconds. 50 microseconds in the barn owl is about
20 degrees to the side. So they will project the location in the
optic tectum where the visual fields are 20 degrees to the side, and so on and so
forth. So we have a projection of the space map.
which is defined in term of interaural time differences of the level of the ICX.
Into a space map which is both visual and auditory and the two maps match.
We have a matching spatial representation of the auditory and visual field in the
optic tectum. This match between the spatial maps in a,
the optic tectum in, is found also in its mammalian homolog, the superior
colliculus. this can be shown in this has been shown
in a number of mammalian species including cats primates, and ferrets.
So there too if you go and record from the appropriate location in the superior
collicullus, the homologue of the, of the optic tectum you can find neurons that
are by model respond both to vision and audition and they have spatial
sensitivity. They respond only to visual stimuli from
a pitch of world and they respond to sound from a pitch of world and these two
pitches match. Okay so now.
What happens when when the prisms are attached to the eyes of the, of the barn
owl chick? It turns out, that somehow the visual
and, and. The, the visual information from the
optic tectum instructs the neurons in the ICX to change both of these.
The optic tectum, the visual fields and the optic tectum got shifted, okay?
They are now the place that will, that was, that corresponded to a straight
ahead and now sees locations which are shifted in space.
So we have a shift in the map in the optic tectum.
We have this point to point projections from the ICX to the optic tectum.
It turns out that the neurons in the ICX now change the properties.
Neurons that set at the location that was sensitive to zero microseconds before
prisms experience will respond now. To 50 micro seconds after prisms, prism
experience. So the whole ITD axis in the ICX gets
shifted and how is it shifted? It is shifted because the neurons in the
[UNKNOWN] station, in the. Main part of the IC a a now grow new
axons. So for example the neuron that was
sensitive to 50 micro seconds from all frequencies sent the axons to the
corresponding location in the ICX it went to the 20 degrees location in the optic
tectum. But after prism experience you see that
the same neurons generated now new axons and these new axons grow to the location
that was previously zero microseconds and these new axons now provide the input to
the neurons that sit in the ICX in this location and makes them now sensitive.
To a, a to an ITD of 50 micro seconds. So that they now project correctly.
They correspond correctly to the visual field that got shifted to 20 degrees in
the optic tectum. So, the question is how does the auditory
system know about the shift to the visual system, since the plasticity occurs in
the auditory system? This lead to the hypothesis that there is
a visual signal coming from the optic tectum and a instructing the ICX and the
all the circuit here instructing it how to behave.
However, recording in the ICX reveal mostly auditory activity.
If you just present a a visual stimuli, you don't see a response in the, you
don't, you don't elicit responses in the ICX.
So the nature of this instructive signal was unknown for many years.
People knew that it has to be there but people could not demonstrate it
explicitly. About ten years ago the instructive
signal, I mean, activity that might correspond to the instructive signal has
been discovered. This is a paper of Johan Guttfreund with
Erik Knudsen from the time he was a post-doc in Erik Knudsen's lab.
what Guttfreund and Knudsen did was to position two electrodes, one in the ICX,
the other one in the optic tectum and they presented visual stimuli, okay.
So they presented visual stimuli and they saw responses in the optic tectum but no
responses in the ICX. So, you see the responses here the
experiment start with this point. And every, every time period, time
interval there's a flash from the corresponding direction from, from, the
correct direction in space that corresponds to the, to the, to the
position of the two electrodes. And you see in the optic tectum, there's
a response and the ICX, there's no response, okay?
So just presenting flashes that excites the neurons in the optic tectum does not
cause any response in the, in the ICX and this is what's, this was well-known,
that's, this is what's the that was usually resolved.
Then Guttfreund and Knudsen, reasoned that if the signal should have been there
but is not there, presumably it's inhibited, okay?
Presumably, there's some inhibitory activity in the optic tectum that causes
the signal not to be transmitted to the ICX.
So, in addition to stimulating the optic tectum, they also put near the
electrode's tip a material that is that antagonizes inhibition so they removed
inhibition in the optic tectum around the recording electrode.
And they continued to present the flashes, so since they removed they
removed inhibitions, the responses in the optic tectum became larger.
So you see that here each flash, evoked some spikes and they after we started,
putting this material. this a inhibitory antagonist in the optic
tectum these responses become more intense and time went by minutes, minute
by minute, and suddenly. Responses started appearing in the ICX as
well. So these are responses in the ICX for
light flashes these are visual responses in a station that's usually auditory.
At that point, after they got these responses they stopped putting
antagonist, inhibitory antagonist in the optic tectum and went on recording.
And as this material got washed by the standard mechanism of the brain, okay,
the inhibition got went back. And the responses in the ICX disappeared.
So, these experiments shows that their responses, that there is some transfer of
information back from the optic tectum to the ICX and that there's, this transfer
of information is inhibited, there's a break on it in, under standard
conditions. And that's if under non standard
conditions. In this case, when the inhibitions is
antagonized by a a and removed. this projections back from the optic
tectum to the ICX become active and you can see then [UNKNOWN] responses, the
visual responses in the ICX. So, the current model would say that when
does the mismatch between the auditory response properties to the optic tectum
and the visual responses in the optic tectum.
something elicits this inhibition from the optic tectum.
It sends the instructive signal to the ICX and then the.
ICX rearrange itself to fit the, to fit the visual responses in the optic tectum.
So, again what did we learn from this story?
We saw interacting systems and motor systems.
In this case two sensory systems interacting vision and audition and multi
model interaction of a multiple sensors are extremely important.
There are a lot of known illusions that are created by this type of interactions.
One of the best known is the talking heads type of illusion where you see
someone moves its head, the sound source is from the side but you assign the sound
source to the, you perceive the sound source as coming from the talking, from
the talking head. The other small subtle illusions that
come from the integration of vision and audition and similarly, a vision and
somatosensation or audition and somatosensation.
These interactions are there because objects in the world usually produce a
stimuli for more than one, for more than one sense.
You see and hear, you see and smell, you hear and maybe touch.
So, when you create the complete representation of the world, you want
the, all of these sensory inputs to contribute together to the formation of,
and representation of an object out there.
And again, the multi model interactions interact cause a, a, coherence in the
guided movement towards the sensory event.
Here we saw it in birds. But as I said, in mammals there are
similar although not identical mechanisms.
For example, in ferrets, they had been, they been studied in detail by Andy King
and Schnupp. And as usual, this is just the tip of an
iceberg and there's a lot of such stories with a lot of important and fascinating
details. So, this is what I wanted to tell you
about the interaction, between the sensory and motor systems.
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