So let’s talk about order. People love putting things together in a way that’s neat and
pretty and easy; arranging things is literally part of being alive. And it’s no different
when it comes to language. Our brains go through the same steps every time. From hearing the
sounds to understanding what language things just happened, our lobes up here follow an
orderly path to do their magic. So let’s follow them down! I’m Moti Lieberman, and
this is the Ling Space.
So every time we hear some language, every time we read something, our brains have to
really quickly work out what we’re doing. Speech comes at us fast, and if we don’t
respond just as fast, we won’t understand what’s happening. Just take a look at our
videos. We’re talking at an average of 165 words a minute - that’s 2.75 words per second!
Our brains have to be flying to deal with that fast talk, and that’s why when we talk
about neural responses to language, we discuss it in terms of milliseconds - that’s thousandths
of a second.
But before we go traveling down those neural pathways, we have to talk about a problem
that we can’t avoid when we go poking around the brain looking for any particular thing
that it does. And that’s that the brain is a super busy place.
It’s not just waiting idly by, hoping some stray sounds will swing its way. It’s got
to do everything for managing your continued life. Your breathing, your heart, your background
thoughts, your brain helpfully and thanklessly manages all of that for you. You might think
that your brain would be a Ravenclaw, but it’s really more of a Hufflepuff.
With all of this frantic activity going on all the time, though, we have to be super
careful that any changes that we see happening have to do with language, and not,
like, worrying about Quidditch or something. If we want to look at the progress the brain
is making with processing sentences, we need to take a continuous measurement, just look
at what it’s doing all the time.
Which means we can use electroencephalograms, or EEGs. EEGs track the brain’s activity
by measuring small changes in the electric field the brain emits through the skull, and
we can look at those in real-time. But just that ongoing measurement isn’t enough - after
all, your brain is abuzz all the time. So what linguists do is time-lock the presentation
of any given sound or word to a few hundred milliseconds of brain activity.
Within the recording of everything that’s happening brain-wise, we measure the small
changes that happened because of the stimulus the person heard or read. These changes are
so tiny - we measure them in microvolts. That’s a millionth of a volt. And since you could
still be thinking about your Potions homework rather than our stimulus, we average across
a large number of examples, to strip away that variation.
Because we’re measuring changes in the brain's electric field that are responses to specific things
that happened, these are known as Event-Related Brain Potentials, or ERPs. So what sort of
effects do we find? What happens before what else in the brain? We probably expect that
the first thing we register is that we heard some sounds that were language-y. And that’s
what the first ERP effects are thought to show us.
They happen really soon after you hear a sound: a negative bump 100 milliseconds after, known
as the N100 effect, and then a positive spike, known as the P200. These effects are centered
around the centro-parietal region, like over here. They make a combined reaction that tells
you that you’ve just heard a sound, and particularly a meaningful and unexpected sound.
And these effects are really attuned to phonology. For example, if you’ve been hearing a lot of words
that start with [b], like buck, beak, and broomstick, and then suddenly you get
a word that start with a nice puffy [pʰ], like potion, you’ll get a much bigger
N100. The N100 even happens if you’re asleep! It’s a little bit
slower, but it means that you’ll be able to register the change in the sounds around
you and know if a magical creature is coming for you in the night.
After we get past the effects for just sounds, the next step down our orderly path is to
get the meanings of our words. We can find how the brain deals with integrating semantics
into our sentences another couple tenths of a second down the line. This shows up again
in the centro-parietal region, about 400 milliseconds after you hear the word. It’s known as the
N400.
The N400 was first found in 1980 by looking at sentences that were completely normal,
except for this one weird word that would throw people off. Like, let’s consider how
we could end a sentence like “Dumbledore attended a party wearing a hat and... blank."
So, what should go in the end there? Well, there’s probably all sorts of words you
could think of, but you’d probably never come up with “cake”. It’s not that it’s
wrong, exactly.
It’s still a well-formed sentence - that’s a place for a noun like cake to go, so when
you put it in there, no warnings go off to tell you that the whole thing is unacceptable
now. But it’s just ridiculous; a man like Dumbledore is no place for a cake to be worn.
And your brain struggling to fit in this randomness gets shown to us as a big ol’
N400 spike.
What if you have something a bit more normal than that? If it’s at least something you
could wear, but not something that you would expect, like, say, “stilts", you still get
an N400, but it’s not as big. And the most normal thing, like the word you’d predict to
go in there, say, something like “robe”, that doesn’t get much of an N400
at all. The harder it is to fit the word in, the bigger the N400. So the N400 is argued
to reflect the cost of semantic integration - like, fitting in a word. You find it all
over the place, but it only gets big when things get weird.
So finally, the last step down the path is when you have your sounds and your meanings,
and you want to make sure that you checked off all the boxes right for the syntax
of your sentence. If you missed something or you mucked about with the sentence, you
get the last of the effects we’ll talk about today, the P600. The P600 is a positive shift
about 600 milliseconds after the word in question. It’s in the same brain region as the other
effects, and it’s HUGE. It’s strong, and it lasts for a long time after the problem passes by,
like the better part of a second.
What kind of syntactic issues make your brain explode like that? Well, these are sentences
that are missing the right syntactic markers. Take “Our hero Neville pull the sword from
the hat.” Now, for English, that pull is just wrong. It has to be pulls or pulled or
something, but just plain pull is right out. When your brain slides past pull and checks
the syntax, it notices there’s nothing on the verb, and it flares right up.
We can also see P600s occurring for grammatical violations of case marking: the pieces around
nouns that tell you what role they fill in the sentence. So maybe you want to say “Ron
drinks the butterbeer” in German. That’d be “Ron trinkt das Butterbier.” And the
das marks the Butterbier as the object in the sentence. If you swap out that das for a different
case, like “Ron trinkt dem Butterbier,” German brains can’t take the bad syntax.
It’s not that you can’t work out what the meaning here is: in both cases, it’s
100% clear what’s trying to be said. But the syntax doesn’t work, and we really do
care about syntax, down to our brains. That’s what the P600 tells us.
So ERP effects can show us the way we impose order on our linguistic world, from sound
to meaning to syntax. By looking at what’s going wrong when we mess with sentences, we
can get a sense of how we approach language when it’s going right. Even though we still
have a long way to go to work out all the intricacies of language and the brain, the
knowledge we’ve gained from ERPs so far lights up the way.
So we’ve reached the end of the Ling Space for this week. If you weren’t thrown off
by the sudden cake, you learned that we can see how the brain deals with language
by time-locking our stimuli to neural reactions; that the N100-P200 complex responds to sounds
in the environment and their phonology; that the N400 stems from your brain trying to deal
with weird words and fitting them into the sentence; and that the P600 is your brain’s
way of telling you that something went horribly wrong with the syntax.
The Ling Space is produced by me, Moti Lieberman. It’s directed by Adèle-Elise Prévost,
and it’s written by both of us. Our production assistant is Georges Coulombe, our music and
sound design is by Shane Turner, and our graphics team is AtelierMuse. We’re down in the comments
below, or you can bring the discussion back over to our website, where we have some extra material
on this topic. Check us out on Tumblr, Twitter and Facebook, and if you want to keep expanding
your own personal Ling Space, please subscribe. And we’ll see you next Wednesday. Annyeong!