2.9 – Event-Related Potentials (ERPs)

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“Let s start with one of the oldest ones scalp electrodes. So here s an an example of a net of electrodes you can put over your head. This method been around in various forms for decades. I think they were doing this even in the 50s certainly by the 60s there was lots of this stuff and ideas you can have you know from from one.

Which would be sort of trivial to hundreds of electrodes just sitting on the gets against your scalp. You don t have to make a hole or anything. Sometimes you d like scratch a little bit to get a good contact or you put some goo in there to make nice contact to the scalp and so and from that you just record electrical potentials at each of these positions over the scalp. Okay and the idea is that neural activity that s going on underneath through the skull.

You know maybe half a centimeter away is producing some net potential from you know millions of neurons underneath. There that you re picking up outside okay. So this is very very crude this method. But and it has very bad spatial resolution right because electrical potentials as you guys will know if you ve taken 802 will diffuse over the scalp.

So even if you had neurons firing all and only right underneath. One of those several hundred electrodes and you had some little tiny local potential. So i m tiny little bit there it s going to diffuse a bit and lots of electrodes are going to pick it up and you re gonna get a blurry measure of that underlying activity okay so the spatial resolution is pretty crappy right the old analogy is that if you imagine putting a microphone on the outside on top of a football stadium. What could you did you recorded audio during a game.

What could you tell well you could probably tell when there was a touchdown. There d be a lot of noise. And maybe you could tell a few other things. But not a whole lot okay.

That s the analogy. We re listening from the outside outside the roof of the stadium to whatever net kind of gross activity. Happens and you know maybe if you move the microphone around to different parts of the roof or you had 300 microphones all over the roof. You might get a little bit of information about you know.

Which side that i don t know anything about football and stop making this up which side of the stadium the touchdown. Happened honor or something right but beyond that you wouldn t get very far okay. But that s changing and actually the whole attitude about this kind of charming old fashioned low tech. Method is kind of flipping around and it s suddenly like bell bottoms occasionally become trendy and hot.

I guess not at the moment..

But it s happened several times in my lifetime and erps are right now scalp electrodes are now undergoing a little kind of trendy phase. And we ll get later to why that is pretty cool okay so let me tell you a little bit more. I ve used different words. Electroencephalography or eeg traditionally people would use this to just measure waves in different frequency.

Spectra alpha waves delta waves. You know all these oscillations change with your level of awareness. They change and sleep. They change with this and that that s all very interesting.

But less relevant for today. The version of this is more relevant. It s what s what s called event related potentials and the idea. There is we have the same net of electrodes.

I m the subject. I m looking at a screen you flash something up on the screen. An electrical activity does various stuff for example. We might have an electrode here here s time so one of those electrodes are just making a potential measured at the scalp.

Okay. When we present a stimulus right there. Okay. Now the key to event related potentials is since those data are weak and noisy.

What you do is you repeat. That experiment that little measurement. Lots of times like say 50 times or something. Thirty hundred whatever lots of times and you signal average.

So here s the key idea. This is an important idea. So here would be the response of one electrode. Someplace on the scalp in a subject stimulus comes on say.

It s a face for example..

And here s what happens at that electrode over time maybe. This is a second or something okay. And i think microphones. So then we just do it again and here.

It is again you see that looks different from this and then we do it again and we see that and it looks different again. But if you kind of squint well maybe. There s a family resemblance so instead of squinting you signal average. And the crucial idea is that most of that stuff is noise.

And by average and over many events aligning them to the onset of the stimulus. All the stuff that isn t phase locked to the stimulus will average out and you ll be left only with the systemic electrical changes at that part of your scalp. That are systematically replica bleah related to processing that stimulus does everybody get that idea if not ask a question let me try it again. Because it s actually really important that makes sense okay.

That s why it s called event related is it has a systematic temporal relationship to the onset of the stimulus. So that s called an event related potential and so there s a whole field where people have been doing this stuff for as i said decades where for various kinds of visual presentation. You can get the systematic set of bumps. So this is really weird and and and i don t know intriguing.

But kind of bizarre field where people label. These things these people they re people who spend their whole lives. Saying. Oh yeah.

That i study the p1 that s a bump that goes in that direction about a hundred milliseconds after you present a visual stimulus. Oh. There s another bump that and one that happens. There.

And there s a p2 and an n2 and a p3 and they re people who spend their whole life studying these things and some of them are pretty cool like for example that p1 pretty clearly responds. It s an early visual cortex response to a visual stimulus. So from dozens maybe hundreds of studies. It s clear that you present any visual stimulus to a subject you re going to get a p1 around 80 milliseconds like less than a tenth of a second after that stimulus flashes on when the visual information is coming up here to visual cortex ok.

So that s kind of cool this one is more intriguing the p3 sometimes called the oddball response..

I m going to give you all a p3 right now beep beep beep beep beep beep boop. You all had a p3 when i said boo anything that changes in the stimulus will give you that systemic wave. Okay. It s pretty interesting you can think of lots of ways you could use that study all kinds of cool stuff.

And people have been doing that for 50 years or so okay so anyway. So there s a version of erps. Where people kind of get obsessed with one or the other bump and you can spend a whole lifetime studying it and that s you know one way to spend a lifetime and some cool stuff has been studied. But there are other more general ways to use it okay so let s get back to the question about what can we learn anything about face perception from scalp ear piece.

Now. This is one of these cases. Where there s no obvious answer in advance. It just turns out empirically that if you stick those electrodes on people s heads and show them faces and other things what you ll find is there are certain certain waves.

Like that p1 and p3 and whatever i just described p1 and p3 that happen systematically and pretty much every subject at more or less the same latency the same duration after stimulus. Onset and that are selective for faces. Okay. So this is an old study it s a crappy slide.

But i like using the actual original slide when it was first discovered by anyway you don t care by who i care about anyway. This was discovered in a paper in 1996. And it s hard to see. But here is the recording of one electrode.

T5 and t6 are approximately here t. Is for temporal doesn t you don t need to know this i m just helping you read this line. They re just positions on the scalp. That s a totally irrelevant sidebar.

I have all there s a whole system for putting these positions on the scalp that was invented decades ago. You measure around here and then you take the inter ear distance. And you bisect. It and you set up a whole coordinate system on the scalp.

So you would have a way to specify where is that electrode on another person s scalp and i have the hole..

It s called the 10 20 system i have the whole 10 20 system tattooed on my scalp color coded. I ll show you later if you re interested anyway so it s for a failed experiment. But anyway they re still there 20 years after the experiment failed anyway so t5 which we could find on me. If i could remember what color.

It was which i don t is somewhere around here and t6. Is on the other side over the temporal lobe. That s what the t is and so this is another one of those bumps like that p1p3 that i described called the n170 because it s negative going and because it happens at 170 milliseconds. After stimulus onset and what you see if you look at it is that bump is deeper for phases than it is for scrambles phases cars or scrambled cars scrambled meaning you chop it up and move the bits around okay and that s even bigger over the right hemisphere and the left okay.

So that s a you know you re just measuring scalp electrodes and it just so it didn t have to be. But it just so happens that a hundred and seventy milliseconds after you present a face not after you present a car or a scrambled face or it turns out pretty much anything else you get this particular electrical response. Okay everybody got that so that s cool. And what does it tell us well.

It gives us a little bit of a hint that there may be some different brain mechanisms involved in face perception compared to object perception. Okay everybody get how you can sort of infer that it s very indirect. We have very little spatial information. It s just somewhere over here at around 170 milliseconds something happens in the brain.

Two faces not two objects. That s basically what you can infer. So that still pretty low tech. Finding already tells us there s something a little bit different going on with face perception and object perception.

But it also tells us another new thing that the other ones didn t and that is this tells us when we have that information right. It tells us that by a hundred and seventy milliseconds your brain has distinguished a face from a non face okay. That s important because if we want to understand that whole sequence of processing steps those processing steps unfold over time okay and if we want to try to someday write the code that the brain is implementing when it recognizes faces we need to know what that sequence of processing steps is and this is our first clue that of them discriminates faces versus objects and in humans. That happens.

At 170 milliseconds. Okay. Everybody got that okay so that is the old fashioned. ” .


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