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.”
Sensory Transduction
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Do curso por Hebrew University of Jerusalem
Sinapses, Neurônios e Cérebros
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Na lição
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
I'm going to talk with you today about things the brain does.
Together with Dan, we decided to call this lecture Sensations, Perception,
Actions, and Emotions but really, the sub-title is the Story of a Sound because
I'm going to emphasize the auditory system, which is my research subject.
Now what are neuronal processing mechanisms for?
So what is sensation? Sensation is the transformation of
external events into neural activity. Perception, has to do with the processing
of the sensory information. And we believe that the end result of the
conception is the useful representation of the central information in terms of
external object, objects that produce the sensations and I want to illustrate these
starting with a well know quote of David Marr.
From his book Vision David Marr was one of the premiers of computational
neuroscience. in his book he defines vision as seeing
is knowing what is where by looking. The idea being that you have a two,
2-dimensional image, which represents the whole visual world on the retina, and the
brain has to reconstruct the three-dimensional representation of the
visual scene. Today, we will say it a little bit
differently. We have a 2-dimensional image on the
retina and what we want to do is recover the latent variables.
Latent variables as the course is for the things that fall on the retina.
So for example if this is the, the, the image that falls on the retina you have
the eye and you have the brain. Behind the eye, that processes this image
and eventually should come out with a list of latent variables like the fact
that the image included a house, the sun, a tree, and maybe that it was drawn by a
bad artist. They result the representation that
produced by perception is a base for actions and the idea is that organisms
use the representation a of that, of the world that was produced by the sensory
systems. in order to act on it, and the goal is to
produce a lot of reward, rewards and to avoid as much as possible punishment.
The view of a linear order, sensation, perception, actions, is actually a little
bit naive and probably wrong. About the years, people helped in
discussing the concept of a perception action loop were found that going to the
brain and produce back options that changed the world, changed world and
changed perception and these actions and perceptions.
Are not separate things. They act in a, in a loop that and they
modify each other. Eh, this beautiful illustration comes
from a meeting in which eh, we tried to eh, bring up eh, people who were eh,
studying perception and action a few years ago.
but the idea is therefore, has been there for many years and there are many people
who study it. In this lecture, I cannot go into the
complexity of the perception action loop, so, we leave it at that.
The last subject I'm going to discuss is emotions and within the context of these
sensation perception actions, a pathway or loop.
Emotions are a little bit strange and we'll discuss them at the end.
So, at this point, I would like to start the main subject of the talk.
And what I will, the subject that I will deal with, in this lecture and Sensory
Transduction. Okay, how do sound become electrical
activity. I will discuss one example of the early
processing of sensory formation and this example is the case of auditory
localization and more specifically, the calculation of the minute.
A difference between time arrival to the two ears.
I will show how this sensory information guides motion.
And in this case again it will be a simple, I will discuss a simple test
case, the case of addition guiding head turns to an auditory target.
Then, I will discuss a little bit, try out all the processing of sensory
formation specifically, what surprised does to neurons in brains.
And finally, I'll discuss a little bit emotions that are sound induced.
Okay, so lets start with sensory transduction.
How do sound become electricity? As the, the, scri, the standard
description of the way that sound become electricity is simple.
We have hair cells. Hair cells are specialized cells, cells
they sit in the ear, and in a moment I will show you where they sit in the ear.
And these cells have hills, this is why they are called hill cells.
The notion is that when sounds come, they cause movements of the hairs.
Easily said, the mechanic sensitive ocean signals the hills and the motion of the
hills open and close. These mechanosensitive ion channels okay,
this cause currents into the neuron and the currents, into the hair cell, and the
currents into the hair cell cause depolarization in the hair cell.
This depolarization, like in all neurons, acts on a synapse.
It modulates the amount of transmitter that is released in sinups between the
hair cell and the auditory nerve fibers. And this cause changes in the spiking
activity of the auditory nerve fiber. Now this auditory nerve fiber has one
end. contacting the hair cell in the ear, and
its other end is inside the brain. And so the brain gets to know about the
motion of the hairs. Okay, so up to here, this is, this sounds
pretty simple, but this begs the question of how does sound, sound cause vibration
of the hair, of the hairs? So, the sounds cause vibrations of the
hairs because the hair cells themselves sit on top of a membrane, called the
baslar membrane, again in the ear. The hair cells are embedded In a
structure which is very stiff. It's called the organ of corti, the name
is not important but the structure is very stiff and vibration in the ear
caused by vibration of the basilar membranes.
So, the basilar membranes sometimes goes up and sometimes goes down.
So, the hair cells are embedded in the organ of corti and when the, the basilar
membrane go up or down, where this causes a relative motion of the hair we suspect
to this huge huge copula this kind of a roof that lies above them.
And, this causes a [UNKNOWN] forces that moves the, moves the hairs around.
So the motion of the basilar membrane is translated into motion of the hair bundle
and this causes the opening and closing of ion channels and the polarization, the
transmitter release and spikes. So next, the next question is, why does
the baslar membrane vibrate in response to sound?
So it turns out that the baslar membrane itself sits inside a long tube.
This long tube eh, is shown here in cross section So it the, the tube is a divided
in the middle by the basilar membrane so here is the basilar membrane and the
organ of quality. Now the hair cells are so small that you
can hardly see them in this illustration and, and the, this whole tube.
This whole tube is so long that whoever designed this sensory apparatus, coiled
round, coiled this tube around itself, forming a snail like structure which is
called the Cochlea. So, we have the Cochlea.
Its a long tube and if I cut the cross section to the cochlea, I see here this
tube at the number of, of location along its length and at each one of these cross
sections I see the basilar membrane with the organ of coat on top of it.
And in this illustration you can see also this all of these fibers that contacted
the hair cells. Which joined together to form the
cochlear nerve onto nerve that goes out of the ear and into the central nervous
system. Now how do sound cause vibrations of the
basilar membrane? We have the external ear that sits
somewhere here and then we have the ear canal and the ear canal ends in the
tympanic membrane. Vibration in the ear causes vibration of
this tympanic membrane and this vibrations are.
It transfers to the fluid that fills up the, the cochlea, through a chain of
three bones. These are the tiniest bones in the human
body. The vibrations, of this bone into the
fluid cause pressure waves. And these special waves in the fluid,
cause the vibration of the [UNKNOWN] membrane.
And then the basilar membrane vibrates, the organ of corti moves with it, the
hairs move because of the vibration of the organ of corti, ion channels open,
the polarization transmitter release and so on.
One important feature of the Cochlea that has been discovered already in the 19th
century is the fact that it is mechanically homogeneous.
Close to the middle ear, close to the bones of the middle ear.
The basilar membrane is narrow and stiff, whereas close to the end of the, of the
tube the basilar membrane is wider and less stiff.
And this led researchers in the Nineteenth century, mostly Hermann von
Helmholtz, Helmholtz. So to suggest that the basilal membrane
vibrates best when excited by different frequencies along its length.
So if this is the whole tube of the cochlea, uncoiled, and this is the
representation of the basilal membrane, if you are going to play a low frequency
sound. It will cause vibrations best close to
end, to the far end of the basilar membrane.
If you use a high frequency sound, then it will cause vibrations with the loudest
amplitude close to the beginning of the of the basilar membrane, close to the
middle ear. And the frequency in between.
For example, 1,000 Hertz in this case, will cause vibrations somewhere in the
middle. And finally, when you have a complex
sound, a complex sound will include a lot of different frequencies and the
different frequencies that are included in the complex sound will Will cause
vibrations of the baslar membranes at different places along its length.
This turned out to be more or less correct.
I'm going to show, to show to you here now a short, a movie.
That is based on actual measurements of the movement of a real basilar membrane.
This axis is the length of the basilar membrane and this represents its width
and the stimulation here its the middle ear by a full sine wave by a full
frequency, whose frequency correspond to the best.
Frequencies that excite, that vibrates best the middle of this part of the
basilar membrane here. And you can see the way that the
vibrations increase in size as they reach the best location and then they start to
decre, to decrease in size again. So you can imagine that the wave,
traveling, this peak traveling along the basilar membrane, becoming larger and
larger until it reach the, its best, the location that best fits its frequency,
and from there on it is reduced in amplitude as it progresses beyond that
point. so you see that the you can, you can see
here that the way that energy travels in the basilar membrane is by so-called
traveling wave. This peak of activity shifts around,
okay, it moves From the middle here to the edge of the, to, to the far end of
the cochlear max, with maximal size, at its best location.
The movie that's shown here was also recorded from the same cochlea except
that this is a dead cochlea. This movie comes from a cochlea that's
alive and well, and this movie was taken from the same cochlea except that it got,
it was damaged, and the living tissue in this cochlea is dead.
What you can notice, is that the amplitude of vibrations here, is
substantially smaller than here. So the fact that the cochlear includes a
living tissue, something in the living tissue of the cochlear increases
substantially the amplitude of the of the vibration of the, of the basilar membrane
and for many years. They, there was a, a serious search for
what are the mechanisms that cause these, same, eh, these, say, additional increase
in amplitude the so called action of the cochlea amplifier and it turned out in a
that they a, the main source of a, a energy to the cochlea amplifier actually
the hair cells themselves. The hair cell come into a, in 2 types of
flavel and, and flavels and one of these flavels is so called outer hair cells are
piezoelectric. When they change their electricity, they
change their length and this transformation of energy of the
polarization into mechanical energy, supplies the energy to the cochlea
amplifier. So, the question of how the hairs of the
hair cells move down the out to include many, many, many details that will worked
out over many years as I said initially Hermann von Helmholtz as suggested that
the cochlea frequency, the composition of sound.
But eh, he thought that the cochlea is basically like eh, a piano.
So you have a lot of strings that are eh, connected eh, eh, from, from one side of
the tube to the other, and they vibrate with eh, with the sounds like The
sympathetic vibration of eh, of the strings of a string instrument, with eh,
with sound. Early in the 20th century, Georg von
Bekeshy discovered that this is not a correct eh, representation of what
happens in the, in the cochlea. And he discovered the traveling wave He
got a Nobel Prize for it in 1961, and he is the only auditory scientist to ever
who ever won the Nobel Prize. And finally in the late 20th century, and
the research is still ongoing by many researchers, the cochlear amplifier was
discovered and its relationship to the outer hair cells.
This is Herman von Helmholtz. The drawing is by Hans [UNKNOWN] and this
is Georg von Bekesy, the picture from the Nobel Prize website.
To summarize, auditory transduction is performed by specialized cells, that sit
in a specialized organ. With many different specialization the
organ of quality its extreme a, a, extreme stiffness the basilar membrane
with its mechanical properties the change as the function of its length.
Along its length and finally this specialized organ is coupled to the
stimulus to the, to the middle ear to the external ear and the a lot of fascinating
details that I couldn't touch upon in this lecture.
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