Placeholder Image

字幕表 動画を再生する

  • The following content is provided

  • under a Creative Commons license.

  • Your support will help MIT OpenCourseWare continue

  • to offer high-quality educational resources for free.

  • To make a donation or to view additional materials

  • from hundreds of MIT courses, visit MIT OpenCourseWare

  • at ocw.mit.edu.

  • PROFESSOR: This is our first introductory meeting

  • of the course, which is 9.04.

  • And we are going to cover vision and audition in this course,

  • and there are going to be two of us lecturing.

  • My name is Peter Schiller, and this is Chris Brown.

  • And I will be talking about the vision portion,

  • and Chris will be lecturing about the auditory portion.

  • Now, what I'm going to do is I'm going

  • to hand out the syllabi that we have, in this case,

  • for the first half of the course.

  • And that we are going to discuss in some detail

  • today for the first half of the lecture,

  • and Chris is going to discuss it for the second half.

  • So that is the basic plan for today.

  • And I will go through some of the basic procedures and issues

  • that we may want to deal with at this very introductory portion.

  • So first of all, let me talk about the reading assignments.

  • If you have the handout, they are ready for you.

  • If you look at the second page, that's

  • where we have the assigned readings for the vision

  • half of the course.

  • Now, for that half of the course,

  • the top eight assignments are all articles

  • in various journals.

  • We don't have a textbook for this portion of the course.

  • And then in addition to the assigned readings,

  • we have recommended readings that are listed there.

  • And then another important factor

  • that is listed there-- let me first

  • say that the lectures will be put on Stellar, in most cases,

  • after each lecture.

  • And in addition, the videos that we are now recording

  • will also become available, but they will not

  • be available until well after each lecture.

  • So I would advise each of you to come to the lectures

  • rather than hoping to read the assigned material only

  • or to eventually look at the videos.

  • The reason I'm telling you this is

  • that our analysis has shown that those students who

  • attend the lectures regularly get

  • much better grades on the exams than the students who do not.

  • So I strongly will urge all of you

  • to come to as many lectures as you possibly can.

  • Now, the additional requirement that you're

  • going to have for this course is to write two research reports,

  • one for vision and one for audition.

  • And the assigned written report that you need to put together

  • is in a paper at the bottom of the second page.

  • In this case, it's going to be a paper that

  • was written quite some years ago,

  • a very important and remarkable paper that has been published

  • by Oster and Barlow, as you can see.

  • And the task for you will be to not just report

  • what they had reported, because that's repetitious,

  • but to do a bit of research and write

  • about what has been discovered since the remarkable findings

  • that these two people had made at the time.

  • All right.

  • So that's the research report.

  • And then I want to specify the exams.

  • We are going to have a midterm exam,

  • and the exact date for this has already been set on October 23.

  • All right?

  • But as I say, you can find this, and I

  • will specify that in more detail in just a minute.

  • And then we are going to have a final exam

  • at the end of the term.

  • The exact date for this, as always at MIT,

  • will not have been set until probably sometime in November.

  • So now let me also specify the grade breakdown.

  • I think that's important for all of us.

  • The written report for each half of the course--

  • there's going to be one report, as I've already said,

  • for vision and one for audition--

  • and that will constitute 10% of the grade for each.

  • And the midterm exam, this constitutes 25%.

  • The final exam constitutes 55% of the overall grade.

  • And in that, 15% will be on vision and 40% on audition.

  • So if you add that up, you can see that vision and audition

  • are set up to be exactly equally weighed for the exams.

  • MICHELLE: Hi.

  • I'm Michelle.

  • I'll be helping the professors, especially with [INAUDIBLE].

  • PROFESSOR: So I'm Chris Brown, and I'm one of the instructors.

  • I'll be teaching the second half.

  • And my research is on two areas, brain stem auditory reflexes,

  • like the startle reflex and the middle ear muscle reflex.

  • And I also work on animal models of the auditory brain stem

  • implant, which is a neural prosthesis that's

  • used in deaf individuals.

  • PROFESSOR: All right.

  • And I'm Peter Schiller, and I work on the visual system.

  • And I'm a professor here in our department.

  • So that's very nice.

  • Thank you for the introductions.

  • And I hope, you guys, we all get to know each other.

  • I'm very impressed that there's so many seniors here.

  • That's actually unusual.

  • I don't remember having this high a percentage

  • of seniors in the class.

  • That's really very nice, very nice.

  • OK.

  • So now we are going to talk, for the first part

  • of today's lecture, about what aspects of visual processing

  • we are trying to understand and, therefore,

  • what we are going to try to cover

  • in this course in terms of topics.

  • OK?

  • So first of all, what we are going

  • to do for several lectures is to talk about the layout

  • and organization of the visual system itself.

  • Most of it we will discuss as it applies to higher mammals,

  • in particular monkeys and primates and humans.

  • Then we are going to talk about specific aspects

  • of visual processing.

  • We're going to try to understand how we adapt in vision,

  • and, very interestingly, how we are

  • able to perceive colors and process them accurately.

  • Another fascinating topic is how we

  • are capable of analyzing motion.

  • That's a complex, very interesting topic,

  • as is depth perception.

  • And the reason depth perception is particularly

  • interesting is because, as you know,

  • the retinal surface is essentially

  • a two-dimensional arrangement.

  • And yet from whatever falls on these two dimensions

  • in the left and right eyes, somehow the brain

  • needs to convert to be able to see the third dimension.

  • And as a result, several mechanisms

  • have evolved to accomplish that, and we

  • are going to discuss them.

  • Then, again, another very complex topic

  • is how we can recognize objects.

  • Perhaps the most complex of those

  • is our incredible ability to recognize faces.

  • And that is highlighted, of course,

  • by the fact that if you look at more simple organisms,

  • like, I don't know, monkeys, they all look the same to you.

  • But human beings, who are actually

  • more similar to each other than perhaps monkeys are,

  • we are really capable of telling them

  • apart and readily recognize them over long periods of time.

  • So it's a very interesting topic.

  • And yet another topic that we will discuss

  • is how we make eye movements.

  • As you probably know, or you're aware of,

  • that we are constantly moving your eye.

  • You make saccadic eye movements about three times a second,

  • thousands of times a day, hundreds of thousands of times,

  • to be able to see things clearly in the world.

  • So we are going to try to understand

  • how that incredible ability has evolved

  • and how it is realized by the brain.

  • OK.

  • So now to look at exactly how we are going to cover this,

  • let me go through this.

  • During the next lecture, which is September 9,

  • we are going to look at the basic layout of the retina

  • and the lateral geniculate system,

  • as well as how the visual system in general is wired.

  • Then on September 11, we're going

  • to look at the visual cortex, then

  • at the ON and OFF channels, so-called,

  • that you'll realize what they are once we talk about it.

  • And then there's another set of channels

  • that originates in the retina, which

  • are the midget and parasol channels.

  • We'll discuss those, try to figure out why did they evolve

  • and what is their role in being able to see

  • the world in realistic fashion.

  • Then we're going to talk about adaption and color, depth

  • perception, form perception.

  • And then we're going to have a lot of fun on October 2,

  • and we're going to look at illusions and also

  • visual prosthesis, because one of you,

  • in particular, is interested in that topic.

  • Then we are going to talk about the neural control of visually

  • guided eye movements.

  • That's going to consist of two sessions.

  • And then we're going to talk about motion perception

  • and another aspect of eye movements

  • when we pursue something with smooth eye movements.

  • And then we're going to have an overview.

  • And then, lastly, on October 23rd,

  • we are going to have the midterm exam.

  • That's going to cover questions from all of these lectures.

  • I should tell you right now that the midterm exam is going

  • to consist of multiple-choice questions.

  • So you're not going to, maybe, asked to write anything.

  • You're going to have to just pick

  • from each of the questions the correct answer.

  • All right.

  • So now what I would like to talk about next in a summary fashion

  • are what we call the tools of the trade.

  • What has happened over the many years

  • that scientists tried to understand

  • how the visual system and, for that matter the brain, works,

  • what kinds of methods have been employed.

  • And so I'm going to talk about each of these just very briefly

  • this time, and then they will come up repeatedly

  • during all of the lectures.

  • Now, the first method I'm going to talk about

  • is called psychophysics.

  • I'm sure most of you know what that is.

  • It's a scientific way to study behavior of humans and animals

  • to determine how well they can see.

  • Now, there are several procedures with this.

  • I'm going to describe one that's used

  • both in humans and monkeys.

  • And what you can do nowadays, you can use a color monitor,

  • and I will describe that in just a second.

  • After that, I will talk about anatomy.

  • I will talk about electrophysiology,

  • pharmacology, brain lesions, imaging, and optogenetics.

  • So now let's start with psychophysics in more detail.

  • So here is a color monitor, and either monkeys or humans

  • can be trained to first look at a fixation spot.

  • And that's important because we want to always

  • be able to present stimuli in selected

  • locations of the visual field or selected

  • locations along the retina.

  • This is particularly important, because when

  • you study the brain, different regions of the visual field

  • representation are located in different areas,

  • for example, in the visual cortex.

  • So what you do then is you can present a single stimulus

  • like this, and the task of the human or the monkey

  • is to either make a saccadic eye movement to it,

  • say that's where it is, or to press

  • a lever that's in front of them.

  • And then on each trial, it appears someplace else.

  • You can present it in many different locations,

  • and maybe one of those locations will

  • be relevant to what part of the brain you are studying.

  • And then what you do, you can systematically

  • vary all kinds of aspects of the stimulus.

  • You can vary the color.

  • You can vary the contrast.

  • You can vary the size.

  • You can vary the shape.

  • And by systematically varying this,

  • you can create curves to describe exactly how well you

  • can see any particular thing like,

  • for example, just how much contrast you

  • need to perceive something.

  • All right.

  • So that's called the detection task.

  • Now, a related task, which has also been used extensively,

  • is called the discrimination task.

  • In this case, you present a fixation spot again.

  • The person or the monkey fixates,

  • and then you present a whole bunch of stimuli, one of which

  • is different from the others.

  • And you have to select where that one had appeared,

  • the one that's different, by making an eye movement

  • or pressing the appropriate lever.

  • Now, when you do this, you systematically

  • can vary the difference between the so-called distractor

  • stimuli, which are all identical,

  • and the target until the person is

  • no longer able to tell the difference.

  • And that way you can, again, generate a curve.

  • And you can specify just what is the amount of difference

  • that you need to put in this, say,

  • how good are you at perceiving slightly different colors.

  • All right?

  • And by doing that systematically,

  • you can generate these functions using

  • these psychophysical procedures to determine

  • pretty well how you're able to see.

  • And now this particular approach is also very useful

  • when it comes to studying individuals, humans

  • in this case, who have some problems with vision.

  • So if they have a problem in seeing colors,

  • you can readily determine, well, what's

  • the magnitude of that problem?

  • And thereby it can tell you what procedures

  • might be used to try to ameliorate their shortcoming.

  • Now, another method that has been used extensively

  • in not only vision, but in many, many different areas

  • of studying the brain, including audition of course, is anatomy.

  • Numerous methods have evolved in the course of anatomists

  • working on these problems, and the first is a very simple one.

  • You just look at the whole brain.

  • And I'm showing this because this is a monkey brain,

  • and you will encounter the monkey brain repeatedly.

  • And it so happens that after people

  • have studied this extensively, they

  • were able to give names to just about

  • every gyrus or every brain area and also relate it

  • to what the function is of those areas.

  • And so, for example, just to name a few of these,

  • this is called the central sulcus.

  • I need to do one more thing here.

  • OK.

  • So this here is the central sulcus.

  • All right?

  • Just for you to remember, humans also

  • have a central sulcus, of course.

  • And this is the lunate sulcus.

  • And this region back here-- let's see, did I label this?

  • Oh, here's a couple more, the arcuate and the principalis.

  • You will encounter these repeatedly.

  • And this region back here is the one

  • which is the primary visual cortex in monkeys that

  • has been extensively studied and has

  • yielded remarkable discoveries about the way it works.

  • So that is called area V1, or primary visual cortex.

  • All right.

  • So now just another example, if I

  • can show you just a few examples of anatomy.

  • Here's another example showing what the eye looks like.

  • And the remarkable thing about the human eye,

  • and the eye of monkeys as well and primates,

  • is it has become highly specialized.

  • There's a region here which is called the fovea.

  • And in that region, you have a very, very dense distribution

  • of photoreceptors and other cells in the retina.

  • And because of that, you have very high acuity there.

  • Now, because of that, the eye movements

  • have to become effective for you to be able to see fine detail.

  • So even, for example, when you read,

  • what do you do when you read?

  • You make saccadic eye movements across a line.

  • Then you go down the next line three or four saccadic eye

  • movements, and so on down the page.

  • And you do that because you cannot make out the details

  • of letters in the periphery because, there,

  • the distribution of the photoreceptors and the cells

  • in the retina in general become less and less dense.

  • So that is a high degree of specialization

  • that we will discuss in much more detail next time.

  • Now then, all the fibers from the retinal ganglion

  • cells course across the inner surface of the retina

  • and go to the so-called optic nerve through which

  • over a million fibers from the retina project

  • into the nervous system.

  • And how they project exactly, what that's

  • like I will discuss in considerable detail

  • the next time.

  • Now, this area here is often also called

  • the blind spot, and that you don't

  • see even if you close one eye.

  • But if you do a careful experiment--

  • I'll explain that the next time-- you can actually map out

  • this little region where you don't see anything.

  • They're in different locations in the two eyes,

  • so the two blind spots do not overlap.

  • And so, consequently, when you look with both eyes,

  • you don't have a, quote, blind spot.

  • So that's an example of what the human retina looks like,

  • and this has been studied extensively

  • using a whole array of anatomical procedures.

  • Now, the third anatomical procedure I want to tell you

  • is labeling individual cells.

  • Now, the way this was done, or still is being done,

  • is that you slice the brain into very, very thin sections.

  • And you put them on a glass, and then you

  • can look at them under a microscope.

  • Now, here's an example of a cross-coronal section

  • of the monkey lateral geniculate nucleus.

  • That's one region in the brain to which the retinal ganglion

  • cells project.

  • And it's a beautifully layered structure,

  • which I'll describe in detail the next time.

  • And these little spots that you see here

  • are actual cells, which are labeled using a so-called Nissl

  • stain.

  • Now, another method used in staining cells

  • is the famous Golgi stain, which was discovered, invented,

  • perhaps, you could say, by Golgi, for which he received

  • the Nobel Prize in 1906.

  • The remarkable quality of those productions

  • is that this label-- it's a silver label-- stains not only

  • the cell bodies, as the Nissl stain you have just seen,

  • but also all the processes, the dendrites as well as the axons.

  • So you see a whole cell as a result

  • of that staining procedure.

  • Yet another way to do this, which

  • is more sophisticated nowadays, is

  • to record intracellularly from a cell and then inject a label.

  • This happens to be the so-called Procion Yellow labeling

  • substance.

  • You inject in the eye, and then you process the tissue again

  • in thin layers.

  • And this is an example of what that looks like.

  • So this also stains all the processes of the cell.

  • And the advantage here is that you

  • can study this cell electrophysiologically

  • and determine what it is like, and then

  • stain it so you can establish the relationship what

  • the cell does and what the cell looks like.

  • All right.

  • So now let's turn to the electrophysiological method,

  • which is a consequence of this, logical consequence of it.

  • Once again, here we have a monkey brain.

  • And what you do here, we put microelectrodes into the brain.

  • Now, this was a discovery that was

  • made around the turn of the century, little bit after.

  • Initially, microelectrodes were made

  • from very thin tubes of glass, which were heated

  • and then pulled so that the tip became smaller than a micron.

  • So it was very, very small.

  • Subsequently, other methods were developed.

  • They etched fine pieces a wire until the tip was very, very

  • small, and then they corded it.

  • And they then we were able to put these electrodes

  • into the brain and record single cells just like with glass

  • pipettes.

  • So the example here then is that you take a microelectrode,

  • put it into the brain, and then that is connected

  • to an amplifier system and a computer.

  • And when you do that, you can record from a single cell.

  • Now, as you well know, single cells

  • generate action potentials.

  • And that is shown here on an oscilloscope

  • in a schematic fashion.

  • And what some clever people did is

  • that they decided that an easy way to process information

  • about the manner single cells generated

  • action potentials is to put this signal onto a loudspeaker

  • system.

  • And so every time a cell fired, what you would literally hear

  • is beep, beep, like that.

  • OK?

  • And so if you shown a light on it several times,

  • it will go like brrrp, like that.

  • And the big advantage of this was

  • that many cells see only a tiny portion of the world.

  • And if you don't know where it is,

  • you have to take some projector or something

  • and move it around.

  • And if the receptive field is here,

  • you go, brrrp, brrrp, brrrp, brrrp, like that.

  • OK?

  • And so then you can map it out very accurately.

  • Instead of having to like with the oscilloscope, you hear it,

  • and that enables you to do all kinds of things, experiments,

  • and you don't have to look at the oscilloscope.

  • But you can perform all sorts of experiments,

  • and you can hear the responses of the cell.

  • Now, another method that is used extensively

  • in electrophysiology is called electrical stimulation.

  • It's a similar process.

  • You take a microelectrode, for example, you

  • put it in the brain, and then you pass electric current.

  • Typically, that electric current is passed in such a way as

  • to mimic action potentials.

  • So if you are to listen to when it's activated, again, brrrp,

  • you hear that.

  • But this time, instead of the cell firing,

  • you are firing the area there, and that then

  • can elicit a response, all right, all kinds of responses.

  • If you stimulate here, for example-- remember now,

  • this is the visual system in the monkey-- as

  • has been shown in humans, electrical stimulation elicits

  • a percept, a small spot, a star-like image.

  • In the auditory system when you stimulate,

  • you can hear something.

  • And if you stimulate in the areas that

  • are related to moving your eyes, then the simulation

  • causes a saccadic eye movement.

  • So all those methods are very, very good

  • in trying to understand better the organization of the brain

  • for, in this case for vision, and for eye movements.

  • Now, yet another method that is used--

  • several methods I should say-- is pharmacology.

  • And when you do pharmacological experiments,

  • one procedure is-- and many different procedures,

  • I'll just describe one here-- once again,

  • you can stick a glass pipette into the brain,

  • and then you can inject it.

  • You can inject it either with actually a syringe

  • or using several other methods, and you

  • can inject all kinds of agents.

  • For example, you can inject a neurotransmitter analog

  • or a neurotransmitter antagonist to determine what effects it

  • has in various parts of the brain and, in our case,

  • in the visual system or the ocular motor system.

  • Now, yet another method that has been used

  • is brain inactivation.

  • Several procedures are available to inactivate the brain.

  • Once again, here's a monkey brain.

  • And this region here that I already

  • told you a little bit about because

  • of naming the gyri there, OK, is called the frontal eye fields.

  • This region has something to do with moving your eyes.

  • So what you can do if you want to study and find out

  • just what does this area do, one procedure

  • is to-- again, here is V1-- is to make a lesion.

  • Now, sometimes you actually do that in a monkey.

  • But sometimes in some experiments,

  • humans may have an accident, some event when

  • you served in Vietnam or something,

  • and a region of the brain has been

  • removed by virtue of a bullet or something.

  • And that way you can find out what

  • is the consequence of having a lesion like that.

  • You can study, use a psychophysical procedure,

  • as I described to you, to determine just

  • what is the consequence of having lost this area.

  • This is a huge region of research.

  • A great many types of experiments have been done.

  • One of the famous individuals who had done this kind of work

  • is Hans-Lukas Teuber, who used to be

  • the chairman of our department starting way, way, way

  • back in 1962.

  • And his specialty was to study Second World War veterans who

  • had sustained various kinds of brain injuries.

  • And the basis of that, studying them

  • using psychophysical procedures, to assess what

  • various areas in the brain, what kind of functions they have,

  • what kinds of tasks do they perform.

  • Now, yet another method, which has some major advantages,

  • is to make reversible inactivations rather than

  • permanent ones.

  • And this you can do by, for example,

  • using a method of cooling.

  • There's a device, the so-called Peltier device,

  • that you can put on top of the brain.

  • And then electronically, you can cool the surface that

  • is in touch with the brain, and then

  • you can see what happens when you cool it.

  • And then when you warm it up again,

  • you can see what happens to recovery, which, in this case,

  • in almost all cases like this, leads

  • to full recovery and the same performance

  • as prior to starting the cooling.

  • Yes?

  • AUDIENCE: Can you only use this method for surface structures

  • or?

  • PROFESSOR: No.

  • They have now developed methods where you can actually

  • lower them into the brain.

  • And they're usually much finer, and very often they're

  • sort of a loop type device.

  • And you can lower that into the gyri or wherever you like,

  • and, again, do the reversible cooling.

  • Yes.

  • That's a fairly recently developed device,

  • and it works extremely well.

  • Now, yet another approach is to inject substances

  • into the brain that anesthetize, if you will,

  • a particular region after the injection,

  • but only for a limited time.

  • And then you can see how the behavior is affected.

  • Now, a variant of that that I referred

  • to before, of course, is that you can use agents that

  • are selective, that don't inactivate the whole area,

  • but for example, only affect excitatory neurotransmitters

  • or only affect inhibitory ones.

  • So you can do kinds of selective things.

  • And so you can establish the role

  • of various neurotransmitters in the role

  • they play in various brain areas.

  • Now, yet another method very recently developed,

  • which has been incredibly successful, is imaging.

  • As you probably all know, you all

  • know we do have an MRI facility here

  • in our department on the ground floor, and some of you

  • may even have been subjects in it.

  • What that does is when you put a subject

  • into this-- the variant of that is called

  • fMRI, functional Magnetic Resonance Imaging.

  • And so if you put a person in the magnet and you present

  • a certain set of stimuli repeatedly, OK,

  • or differential ones, whatever, the brain areas

  • that are active performing the analysis

  • that you ask them to do light up.

  • So that method-- here's a complex picture of that.

  • This is one that is in Freiberg, Germany.

  • This is done with monkeys again.

  • You have this device and you put the monkey in there.

  • You lower the device on it, and then you

  • can have the subject perform trained tasks.

  • And then you can analyze the brain

  • and look at the nature of activation.

  • And here's an example of that, whatever

  • this task is, doesn't matter.

  • You can see that a particular brain

  • region has been very heavily activated

  • as a result of whatever manipulation they did.

  • Now, there are lots and lots and lots of experiments

  • of this sort, and we will talk about several of those

  • as we look at various aspects of the visual system.

  • Now, the last method I want to just mention very, very briefly

  • is optogenetics.

  • That's a new method, and we have actually several people

  • in our department here who are using this method.

  • Now, this particular method-- let

  • me explain just very quickly what this is all about.

  • Does everybody know what an opsin is?

  • OK.

  • How about if I say rhodopsin?

  • OK.

  • Most of you will know that.

  • A rhodopsin is a set of molecules

  • that are in the rods in the photoreceptors,

  • and they are sensitive to light.

  • Now, there are all kinds of variants.

  • That's why I mentioned opsins rather than rhodopsins.

  • And what can be done is that you can selectivity

  • place these various substances into selected types of cells

  • in the brain.

  • And then, because these cells become light sensitive just

  • like the photoreceptors are, then when you shine light

  • onto the area where you had placed these opsins in cells,

  • you can drive them.

  • You can make them respond by turning the light on.

  • Now, the amazing thing about that

  • is that makes it a more powerful technique

  • than electrical stimulation, that you can set this up

  • in such a way that, for example, if you use a rhodopsin

  • substance that is sensitive to red light,

  • it will be excited by the red light.

  • But if you have a slightly different substance that

  • would be inhibited by blue light,

  • then you can see what happens if you excite those cells,

  • and then you inhibit those cells.

  • So this gives you two sides to the coin,

  • whereas electrical stimulation provides only one side.

  • So that's a wonderful technique.

  • And here's sort of a good example of that.

  • Here we have injected-- I shouldn't

  • say injected-- but genetically labeled

  • cells with channelrhodopsin, so-called.

  • And when that's done, when you shine in a blue light,

  • OK, you excite the cell.

  • And then instead, if you put in so-called halorhodopsin, OK,

  • halorhodopsin, then if you use yellow light

  • you inhibit the cells.

  • So that's a remarkable technique.

  • It's just at the very beginning of things,

  • so we won't talk too much about this technique

  • in studying the visual system yet.

  • But I bet you that in another 10 years

  • this is going to be a central topic.

  • All right.

  • So to summarize these techniques,

  • other than the psychophysics, just remind you again,

  • number one, we have electrical recording

  • using microelectrodes.

  • OK?

  • Then secondly, we have electrical stimulation.

  • Thirdly, we have injection of pharmacological agent.

  • Then we have methods to inactivate regions,

  • either permanently by lesions or reversibly by cooling

  • or by injecting various substances.

  • And lastly, we have optogenetics that

  • enables you to activate cells or inhibit cells

  • by shining light onto the brain.

  • So these are quite a remarkable set of techniques.

  • And individuals who want to become neuroscientists,

  • they're going to have to learn to master not maybe

  • all of these techniques, but certainly some of them

  • so that they can carry out new and original experiments

  • in determining how the brain works

  • and, in our case, of course, how the visual system works.

  • So that is, in essence then, what I wanted to cover.

  • And now we are going to move on and have Chris tell you

  • about his portion of the course, which

  • will be taught during the second half of this semester.

  • So as I said, next time we are going

  • to start talking about, first of all,

  • about the wiring of the visual system, the basic wiring.

  • And then we're going to talk about the retina in detail

  • and a little bit about the lateral geniculate nucleus.

  • That's the next lecture.

  • Please.

  • AUDIENCE: So for the eigtht readings, the eight assigned

  • readings that you have on here, how

  • will we know when to read what?

  • PROFESSOR: Well, that's a good question.

  • The sections that have to do with eye movements

  • that you can see, that you don't have to read until we

  • get to the eye movements, the latter part.

  • Initially, when we talk about the retina, for example,

  • you definitely want to read the [INAUDIBLE] paper

  • and the Schiller paper on parallel information processing

  • channels.

  • Then the ones that have to do with the Hermann grid

  • illusion and visual prosthesis, that you

  • don't have to cover until you come

  • to the section on illusions and visual prosthesis.

  • PROFESSOR: OK.

  • Welcome, everybody.

  • I'm Chris Brown.

  • And I just, in the remaining time,

  • wanted to give you a synopsis of what's

  • going to happen during the second half of the term.

  • So I'll be giving the lectures during the second half

  • on the topic of audition, or hearing.

  • And there's my email, chris_brown@meei.harvard.edu.

  • So as you can see by my email, I'm

  • associated with Harvard, in fact, Harvard Medical School.

  • And I'm in the so-called ENT department at Harvard Med

  • School, and that stands for Ear, Nose, and Throat.

  • So some of you who are going to be going to medical school

  • will certainly do an ENT rotation,

  • where you learn about the various aspects of ENT.

  • And much of it, of course, is the subject

  • of otology, what happens when people have disorders

  • of hearing, problems with their hearing.

  • And in addition, many ENT doctors

  • also operate on people who have head and neck cancers.

  • So surgeries of those two types go

  • on at my hospital, which is Massachusetts Eye and Ear

  • Infirmary.

  • And that's across the river in Boston,

  • and it is, of course, one of the main teaching hospitals for ENT

  • as well as ophthalmology.

  • There's a big Ophthalmology department

  • where the ophthalmologists deal with disorders

  • of sight and vision.

  • So I have an introductory reading,

  • which is a book chapter that I wrote, and also

  • with Joe Santos-Sacchi, which is actually

  • now in the fourth edition of a textbook

  • called Fundamental Neuroscience.

  • And I believe this book chapter is on the course website now.

  • And it summarizes pretty much what

  • I'll cover during the semester in a reading

  • that you could probably do in an hour or less,

  • and it has many of the figures that I'll use.

  • So if you're shopping around for a course

  • and want to know what's going to happen in the second half here,

  • you can look at that book chapter.

  • There's a nice textbook that I'll also

  • be assigning a number of readings

  • from throughout the term, and it's

  • called Auditory Neuroscience, Making Sense of Sound

  • by Schnupp, Nelken, and King.

  • And these fellows are, in the case of the first and the last,

  • at Oxford University, and they work on psychophysics, that is,

  • how we perceive hearing.

  • And they test humans, and they also

  • do a fair amount of animals psychophysics.

  • And in the case of Israel Nelken,

  • he's at Hebrew University in Israel.

  • And he does a lot of electrophysiological

  • recordings-- you heard about electrophysiology

  • from Peter's talk just now-- and recordings

  • especially from the auditory cortex.

  • But this is a very nice book for coverage especially

  • of the central auditory pathway in psychophysics.

  • And it's pretty cheap.

  • I think it's $30.

  • And I believe I was told earlier that you can get it,

  • as an MIT student, online free.

  • And what's good about the online edition

  • is there are lots of demonstrations,

  • each indicated by the little icon in the margin of the text.

  • And when you click on that demonstration,

  • if you have your earbuds in you can

  • hear what the sound demonstration is.

  • And I'll be doing quite a lot of sound demonstrations

  • through the course of the semester because I think

  • it livens up the class a little bit,

  • and this book is especially good for sound demos.

  • So I encourage you at least to get

  • the online edition of that textbook.

  • So also on the course website is the syllabus

  • for what the audition lectures will cover,

  • and I just put a couple here to give you a flavor.

  • In the first lecture on October 28th,

  • we'll talk about the physical characteristics of sound

  • and what happens to that sound when

  • it strikes your external, and then

  • your middle, and inner ears.

  • And associated with each lecture is an original research article

  • that, in this case, is Hofman, et al.,

  • and the title is "Relearning Sound Localization

  • with New Ears."

  • And so, in a nutshell, what they did in that research report

  • was they took little pieces of clay,

  • and they inserted them into their external ears or pinna,

  • and therefore distorted quite a lot your pinnae.

  • They couldn't do what van Gogh did,

  • which was cut off the pinna, but they certainly

  • distorted their pinnaes quite a lot on both sides.

  • And then they tested, using themselves

  • as subjects and other volunteers,

  • how good their ability to localize sound was.

  • And especially in terms of when the sound varied in elevation,

  • they found that there were huge differences,

  • that they couldn't localize sounds that were straight ahead

  • versus sounds that were coming from above themselves

  • with these distortions.

  • But what was funny and what harks back

  • to the title of the article is that when

  • they had the volunteers go out and live their normal lives

  • with these pinna distortions in for a few weeks,

  • then they came back into the lab and tested them again,

  • they found that they could now localize sounds in elevation

  • with these new ears.

  • They relearned how to localize sound.

  • So this is a nice demonstration of learning or plasticity,

  • at least in psychophysical responses.

  • And it also emphasizes the function

  • of your external ear, which helps you localize sounds

  • in space.

  • In the second lecture, we'll be talking about the receptor

  • cells for hearing, which are called hair cells because they

  • have these little appendages at their top

  • that looked to the early neuroanatomists like hairs.

  • And of course, sound is a mechanical energy

  • that moves the fluid in which these hair cells are immersed

  • and moves these little hairs or appendages

  • at the top of the cell, and that's

  • how the cell can respond then to sound.

  • And so hair cells are the very important receptor cells

  • for hearing, which are, of course, the analogs of the rods

  • and cones in the visual system.

  • And the research report associated

  • with our talk about hair cells will

  • be how a special protein called prestin is required

  • for electromotility, which is a function of outer hair

  • cells, one of the two types of hair cells,

  • which allows them to actually move and flux and change

  • their membrane links when they sense

  • these mechanical disturbances by their stereocilia.

  • So it's a pretty interesting paper

  • in what we call a knockout animal.

  • So the prestin is genetically knocked out

  • in this particular animal, and the sense of hearing

  • is then tested again in these knockout animals.

  • So we have a whole bunch of lectures.

  • I haven't indicated all them.

  • They're on the course website.

  • Toward the end of the semester, we'll have, as Peter indicated,

  • a written assignment for audition.

  • So I haven't actually thought it up yet,

  • but let me just give you an example.

  • Last year and this year, we'll be talking a lot

  • about neural prostheses for audition.

  • And the most famous neural prosthesis for audition,

  • of course, is the cochlear implant,

  • which I'll talk a little bit about later,

  • and it works quite well.

  • There's also a neural prosthesis that

  • goes into the auditory brainstem.

  • It's called the auditory brainstem implant,

  • and it's used in some other types

  • of individuals who are deaf.

  • And it doesn't work anywhere near as well as the cochlear

  • implant.

  • It's sometimes called a lip reading assist device

  • because people who have the brainstem implant

  • usually can't understand speech over the telephone.

  • They need to be facing you and looking at your lips

  • as you're speaking.

  • They need additional cues.

  • And so there'll be a lot of discussion

  • this term about the differences between these two

  • types of the implant, where they're

  • put in the auditory pathway, and why

  • one works much better than the other.

  • And that was the written assignment

  • for last year, which was a discussion of why the cochlear

  • implant works a lot better than the auditory brainstem implant.

  • So I'll have something along those lines that

  • uses the material from our course

  • that you can take to answer a question.

  • OK.

  • Let me just go through in a half a dozen slides or so

  • what I consider to be the high points

  • of the auditory part of the course.

  • About the first third of the auditory part of the course

  • will be a discussion of the auditory periphery.

  • And the periphery is usually divided

  • into these three basic divisions, the external ear,

  • which most of us think about as the ear, the pinna and the ear

  • canal, which leads to the tympanic membrane

  • here in yellow, or eardrum.

  • And at that point begins the middle ear.

  • The middle ear is an air-filled space.

  • If you've been on a recent plane flight

  • and your ears are a little bit stuffed up,

  • it can be very uncomfortable, especially when

  • the plane is coming down in altitude.

  • And your eardrum bulges because the eardrum is just

  • a very thin layer of skin, and it can bulge very easily.

  • But it's painful when the eardrum

  • bulges when the change in pressure

  • happens as you're descending in a plane.

  • In the air-filled space of the middle ear

  • are three small bones, the malleus, the incus,

  • and the stapes.

  • If you remember from high school biology, the hammer, the anvil,

  • and the stirrup.

  • The stirrup looks like what a cowboy has on his saddle

  • that he puts the cowboy boots through.

  • It's a very, very small bone.

  • In fact, it's the smallest bone in the body.

  • These bones are very small because they

  • have to vibrate in response to sound,

  • and so they can't be big and massive.

  • Massive things don't vibrate very well.

  • And so the stapes is sometimes encompassed by bony growths

  • around it and prevented from vibration

  • in a disease called otosclerosis.

  • And so at my hospital, the Massachusetts Eye

  • and Ear Infirmary, they do an operation

  • to cure that type of deafness called the stapedectomy.

  • So what's an -ectomy?

  • You medical types, what does that mean?

  • AUDIENCE: Removal.

  • PROFESSOR: It means removal, right.

  • So they take the stapes out.

  • And that's because if they just loosen it up,

  • the bone regrows and re-adheres the stapes from vibration.

  • So they replace the stapes, the natural stapes,

  • with a stapes prosthesis, either a little piston or a tube,

  • that they hook on with a wire to the incus,

  • and they put into the so-called foot plate area or oval

  • window of the cochlea, which is the next structure I'll

  • talk about.

  • And that very nicely restores the sense of hearing.

  • In fact, when I was a postdoc fellow at Mass Eye and Ear,

  • I could go watch the surgeries, and I watched a stapedectomy.

  • And the patient was anesthetized, but not

  • so much that she was really out.

  • She was more sedated.

  • And at the end of the operation, the surgeon

  • was adjusting the artificial prosthesis,

  • and the surgeon said, well, can you hear me?

  • And the patient didn't respond.

  • So he moved it around a little bit or adjusted the wire,

  • I don't know which.

  • He says, can you hear me now?

  • And there was no response from the patient.

  • And he did a little more manipulation and adjustment.

  • And he finally said, can you hear me?

  • And the patient said, why are you yelling at me?

  • I can hear you just fine.

  • So usually at the end of the operation,

  • the patient has become more light.

  • The anesthesiologist turns off the anesthesia.

  • And they adjust the stapes prosthesis so it works well.

  • So that type of operation is fairly common

  • and very successful to restore the so-called types

  • of conductive hearing loss.

  • These bones conduct the acoustic sensation into the cochlea.

  • Now, in the cochlea-- this is the structure here.

  • The "cochlea" is the word for the inner ear.

  • It comes from the Greek word "kokhlias," which means snail,

  • and it is certainly a snail shell-shaped capsule.

  • The cochlea looks like a coiled snail shell.

  • And inside it are the receptor cells

  • for hearing, the hair cells and the dendrites

  • of the auditory nerve fibers.

  • OK?

  • And the cochlea is a bony-filled capsule

  • filled with fluid and membranes and cells inside.

  • And this anatomy is a little bit complex,

  • so I brought in a model here of the auditory periphery.

  • So we have the external ear, the long ear canal

  • here, the eardrum.

  • It's kind of slanted here.

  • And this part here that I'll lift out here

  • is the cochlea or the inner ear, the snail shell-shaped area.

  • And leading from it is the yellow-colored, in this case,

  • auditory nerve, which sends messages from the cochlea

  • into the brain.

  • And these funny, loop-shaped structures that I'm grasping

  • are the semicircular canals, which

  • mediate the sense of balance or angular acceleration.

  • When you rotate your head, those hair cells in there

  • are sensitive to those rotations.

  • OK.

  • So I'll pass this model around.

  • You can take a closer look at it.

  • And in our hospital, the surgeons practice

  • on real, live specimens like that from postmortem material

  • because in otologic surgery there's

  • a lot of drilling to access, for example, the middle ear

  • or the inner ear.

  • And there's a lot of important structures

  • that you don't want to run into with your drill bit,

  • like the jugular bulb is that red thing there.

  • The facial nerve goes right through the middle ear.

  • And so the surgeons need to know their way around the middle ear

  • so that they can avoid important structures

  • and go to the right structure that they intend to operate on.

  • OK.

  • So you heard Dr. Schiller talk about electrophysiology

  • and recordings from individual neurons.

  • And a lot of what we know about how the inner ear works

  • comes from such types of experiments.

  • And this is an experiment at the top here

  • that gives the responses in the form of action potentials.

  • Each one of these blips is a little action potential,

  • or impulse, or response from one single auditory nerve

  • fiber recorded in the auditory nerve of a cat, which

  • is the very common model for auditory neuroscience,

  • or at least it was in years past.

  • So this response area is a mapping of sound frequency.

  • So this axis is tone frequency.

  • And the frequency of a sound wave form

  • is simply how many times it repeats back and forth

  • per second.

  • Frequencies that are common in human hearing

  • are, for example, 1,000 hertz.

  • This is a graph of kilohertz, tone frequency in kilohertz.

  • And the upper limit of human hearing

  • is approximately 20 kilohertz.

  • The lower limit of human hearing is down around 50 or 100 hertz.

  • In terms of smaller animals like the cat,

  • they're shifted up in frequency of perhaps an octave, maybe

  • a doubling of the frequencies that they're most sensitive to.

  • This auditory nerve fiber responded

  • to a variety of frequencies, except at the very lowest

  • sound level.

  • The y-axis is a graph of sound level.

  • This is very low level or soft sound,

  • this would be a medium sound, and this

  • would be a very high level sound.

  • At the lowest levels of sound, the auditory nerve fiber

  • only gave a response to frequencies

  • around 10 kilohertz, or 10,000 cycles per second.

  • There are some spontaneous firings from the nerve fiber,

  • and those can happen even if there's no sound presentation.

  • These neurons can be spontaneously active.

  • If you outline this response area,

  • you can see that it's very sharply tuned

  • to sound frequency.

  • It only responds around 10 kilohertz.

  • And this exquisitely sharp tuning of the auditory nerve

  • is the way, perhaps, that the auditory nerve sends messages

  • to the brain that there is only 10 kilohertz that the ears are

  • hearing and not 9 kilohertz and not 11 kilohertz.

  • OK?

  • If you increase the sound level to higher levels,

  • this auditory nerve fiber, like others,

  • responds to a wide variety of sound frequencies,

  • but it has a very sharp cut off at the high frequency edge.

  • Maybe at 11 kilohertz, it responds,

  • but 11.1 kilohertz there's no response.

  • So the tuning becomes broader, but there's still

  • a really nice, sharp, high-frequency cut off.

  • So what good is this for?

  • Well, the ear is very good at resolving frequency, saying

  • there's 10 kilohertz but not 9 kilohertz.

  • And that's very important for identification of sounds.

  • For example, how do we know, if we're talking on the telephone

  • or not seeing the subject who's talking to us,

  • that it's a female speaker or a male speaker

  • or an infant speaker?

  • Well, male speakers have lower frequencies

  • in their speech sounds.

  • And so right away, if we hear a lot of low frequencies

  • in the speech sounds, we assume we're

  • talking to a male speaker.

  • And that's, of course, a very important identification.

  • How do we know we're hearing the vowel A and not

  • the vowel E, ah or eh?

  • Because of the different frequencies in the speech

  • sounds for those two vowels.

  • So frequency coding is a very important subject

  • in the auditory pathway for identification

  • and distinguishing different types of sounds.

  • And one way we know what frequencies we're listening to

  • is if the auditory nerve fiber's tuned

  • to a particular frequency [INAUDIBLE]

  • responding and not the others.

  • Now, some very elegant studies have

  • been done to look at the mapping of frequency

  • along the spiral of the cochlea.

  • And what those show is that way down

  • at the base of the cochlea, the very highest frequencies

  • are sensed by the hair cells and the auditory nerve fibers

  • there.

  • And as you go more and more apically,

  • you arrive at first middle, and then lower frequencies.

  • So there's a very nice, orderly mapping

  • of frequency along the receptor epithelium,

  • or along the cochlea in the sense of hearing.

  • And so, obviously, the hearing organ

  • is set up to distinguish frequencies and identify

  • sounds.

  • OK.

  • So that's not the only code for sound frequency that we have.

  • We'll talk extensively about another code

  • that uses a time-coding sense.

  • And this comes from the way that auditory nerve fibers

  • so-called phase lock to the sound's waveform.

  • Here's a sound stimulus, and here's auditory firing, first

  • with no stimulus.

  • It's a fairly random pattern.

  • And here is with a stimulus turned on,

  • and you can see that the spikes line up during a certain phase

  • or part of the stimulus waveform.

  • And that's not that impressive until you

  • look at the time scale here.

  • As I said before, sounds that are important in human hearing

  • are as high in frequency as 1 kilohertz.

  • So if this is going back and forth 1,000 times per second,

  • then the scale bar here for one period would be 1 millisecond.

  • And so the auditory nerve is keeping track and firing only

  • on a certain or preferential phase of the stimulus waveform

  • with the capability of milliseconds.

  • OK?

  • And this is a much better phase-locking pattern

  • than you get in other senses.

  • For example, in the visual system,

  • when you flash the light on and off just even 100 flashes

  • per second, everything sort of blears out,

  • and you sort of don't have any phase locking

  • the way you do in the auditory nerves firing.

  • So this is a very nice coding for sound frequency that

  • is sort of a secondary way to code.

  • This is a very important coding for musical sounds.

  • Musical sounds, for example, like an octave,

  • 1 kilohertz and 2 kilohertz, a doubling of sound frequency,

  • have very similar patterns in their temporal responses

  • to those two frequencies that probably

  • makes an octave a very beautiful musical interval to listen to.

  • And it appears in music of many different types of cultures.

  • So one of the demonstrations that I'll

  • play for you is an A for 40 hertz

  • and an octave above that, 880 hertz, and you'll

  • hear how beautiful the two sound together.

  • And then I'll mistune them, which is easy for me

  • to do because I'm an amateur violinist,

  • and I'll be doing this on a violin.

  • And it's pretty easy to have a mistuned octave,

  • and it sounds so awful and very dissonant

  • when you listen to it.

  • And one of the reasons for that, the reason for that,

  • is the difference in phase locking

  • for the two dissonant sounds versus the two consonant

  • sounds.

  • OK.

  • So we will talk about what happens

  • when you have problems with your hearing.

  • One of the main problems with hearing

  • that causes loss, complete loss of hearing,

  • is when the receptor cells are attacked

  • by various types of insults, diseases, drugs,

  • the aging process, stimulation, or listening

  • to very high-level sounds.

  • These can all kill the hair cells.

  • And in the mammalian cochlea, once the hair cells are lost

  • they never grow back.

  • And there's very active interest in trying

  • to get hair cells to regenerate by using

  • stem cells or various growth factors,

  • but so far that can't be achieved.

  • Luckily, in the auditory periphery,

  • even if you lose the hair cells, which

  • is the major cause of deafness, you

  • retain your auditory nerve fibers.

  • So these blue structures here are the auditory nerve fibers

  • coming from the hair cells.

  • And even if the hair cells are killed

  • by these various insults, the auditory nerve fibers,

  • or at least many of them, remain.

  • So you heard Dr. Schiller talk about electrical stimulation.

  • You can put an electrical stimulating electrode

  • in the inner ear and stimulate those remaining auditory nerve

  • fibers to fire impulses that are sent to the brain.

  • And if you hook that system up right,

  • you have a so-called cochlear implant.

  • The cochlear implant has a microphone that detects sound.

  • It has a processor that converts that sound into various pulses

  • of electrical stimulating current, which

  • can be applied to a system of electrodes

  • that is inserted into the cochlea.

  • The cochlea is a beautiful bony capsule.

  • You can snake this electrode in.

  • It doesn't move away.

  • You can glue it in place.

  • You can lead the electrode out to the processor that

  • activates it when the subject hears a sound that's

  • detected by the microphone.

  • And this cochlear implant is the most successful

  • neural prosthesis that's been developed.

  • It's implanted all the time at Mass Eye and Ear Infirmary.

  • It's paid for by insurance.

  • These days, insurance pays for a cochlear implant

  • in your left cochlea if you're deaf,

  • and it will also pay for another cochlear implant

  • in your right cochlea if you're deaf.

  • So the metric for how successful this is,

  • is whether the subject who's using the cochlear implant

  • can understand speech.

  • And so you can have various tests of speech.

  • A speaker can give a sentence, and the person can respond.

  • The speaker can give various simple words,

  • and the cochlear implant user can respond.

  • You can test these individuals in a very straightforward

  • manner.

  • And we will have a demonstration by a cochlear implant user.

  • I have an undergraduate demonstrator who's here at MIT.

  • And she'll come in and she'll describe and show you

  • her cochlear implant, and you can ask her questions.

  • And there, I guarantee you with this particular demonstration

  • that I have in mind, that she won't always

  • understand your questions.

  • I have had really great cochlear implant users

  • who are superstars, that understand every word.

  • But the more norm is they understand much of what you say

  • but not everything.

  • And this particular room is a little bit challenging.

  • There's some noise in the background.

  • It's not one-on-one, so the implant user

  • won't know exactly who's speaking at once.

  • And in this case, the person just has one ear implanted,

  • so her ability to localize sounds is compromised.

  • And she won't know who's asking the question until she sort

  • of zeroes in a little bit on it.

  • So you'll see the cochlear implant is not perfect,

  • but it's pretty good in its metric

  • for speech comprehension.

  • OK.

  • Now, I said this particular cochlear implant user only

  • has one implant in one ear.

  • To really be good at localizing sound,

  • we need to have two ears, and there

  • are the so-called binaural cues for sound localization.

  • Here's a subject listening to a sound source.

  • The sound source is emitting sound here,

  • and it gets to the subject's left ear first.

  • And a short time later, it gets to the subject's right ear.

  • And that's one of the cues for binaural localization of sound,

  • which is interaural time difference.

  • The velocity of sound in air is 342 meters per second.

  • And depending on how big your head is,

  • you can calculate the Interaural Time Difference.

  • And of course, that ITD is maximal

  • if the sound source is located off to one side,

  • and it's exactly zero if the sound source

  • is located straight ahead.

  • OK?

  • So that is a cue for localization of sounds.

  • We'll listen to sounds that differ in interaural time

  • difference.

  • We can play these through headphones for you.

  • And we'll have some demonstrations of that.

  • The other binaural cue for sound localization

  • is the interaural level difference.

  • Here is the same sound source, same subject.

  • This ear will experience a slightly higher sound level

  • than the other ear because sound can't perfectly

  • bend around the head.

  • The head creates a sound shadow.

  • And this is especially important for high frequencies of sound,

  • like above 3 kilohertz.

  • So that is a second binaural cue for sound localization.

  • We'll listen to those cues.

  • And we'll talk a lot about the brainstem processing

  • of those binaural cues for sound localization.

  • If you compare the visual and the auditory pathways--

  • this is a block diagram of the auditory pathway,

  • and there are a lot of brainstem nuclei.

  • We have the cochlear nucleus, the superior olivary complex,

  • and the inferior colliculus.

  • And these latter two get input from the two ears on the two

  • sides, and they probably do the bulk

  • of the neural processing for sound localization.

  • You don't have to do that so much in the visual system

  • because the visual system, in a sense,

  • has a mapping of external space already on the retina.

  • But remember, the inner ears map sound frequency.

  • And so the inner ears themselves don't know

  • or don't have a good cue for where

  • the sound is localized in space.

  • Instead, you need input from the two sides,

  • and you need a lot of neural processing

  • to help you determine where that sound source is

  • coming from in space.

  • OK.

  • So we'll talk about the neural processing

  • of those binaural cues.

  • We'll talk toward the end of the course

  • about the various auditory cortical fields.

  • These are the cortical fields in the side of the cat's brain.

  • So this is the front of the brain.

  • This is looking at the left side of the brain.

  • This is the rear of the brain where the occipital lobes are,

  • where V1 is.

  • And on the side or temporal cortex,

  • you have the auditory fields, including

  • A1 or primary auditory cortex.

  • And we'll talk, at least touch upon a little bit,

  • toward the end of the course the human auditory-- primary

  • auditory cortical field is right here on the superior surface

  • of the superior temporal gyrus.

  • And just near it, it could be called an auditory association

  • area, is an area called Wernicke's area

  • that's very important in processing of language.

  • And of course, connected with that

  • is the Broca's area, which is another important language

  • center, in the dominant hemisphere at least, of humans.

  • So we'll touch upon that at the very end of the course.

  • And we'll also have a special topic called bat echolocation,

  • how bats use their auditory sense to navigate around

  • the world at night even without their vision.

  • And finally, at the very last class period before the review

  • sessions, we'll all go over for a tour of the research lab

  • where I work at the Massachusetts Eye and Ear

  • Infirmary.

  • So it's across the Longfellow Bridge right at the end

  • there where the arrow is.

  • And we'll have some demonstrations there.

  • I think last year we had a demonstration

  • on single-unit recording from an awake animal that's

  • listening to sound.

  • We had measurements from humans of the so-called otoacoustic

  • emissions.

  • These are sounds that can be detected

  • in the ear canal with a very sensitive microphone.

  • They're used in tests of hearing.

  • And I also think we had a discussion

  • of imaging of the auditory system.

  • And of course, if you've ever listened to an MRI machine,

  • it's sort of described sometimes as being

  • as loud as being inside a washing machine.

  • And it's very challenging for people

  • that image subjects who are listening

  • to especially very low-level sounds

  • when there's all this background noise coming from the imaging

  • machine.

  • So there's some special things that

  • are done to minimize the noise coming

  • from the imager in auditory studies.

The following content is provided

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B1 中級

1.イントロダクション、視覚システム (1. Introduction, the visual system)

  • 105 4
    1namip に公開 2021 年 01 月 14 日
動画の中の単語