When we hear or see language, our brains have to quickly react to whatever just happened, so we know what's being said to us. Linguists are naturally interested in what's going on with all those neurons, but that activity can be hard to pin down, because your brain is such a noisy, active place all the time. That's why when we go to make our measurements, we time-lock the changes we see to the appearance of the stimulus we're looking at, so we know it has to do specifically with our test. This technique is known as Event-Related Brain Potentials, or ERPs.
ERPs let us know what steps the brain takes to make sense of the linguistic input that it's receiving. We talk about three of the different ERP reactions that show up in response to different kinds of stimuli: the N100 / P200 complex, which is a response to hearing something new in the environment that your brain thinks you should pay attention to; the N400, which is the brain's reaction to integrating the meanings of words into the sentences that it's building, particularly when they have weird or unexpected words; and the P600, which is what the brain does when something goes wrong with the syntax of the sentence it's compiling. These are all separate effects, but you can get all of them together in the same sentence, and they provide us with markers for how the brain takes care of its linguistic duties.
We talk in the video about the P600, the brain's response to syntactic violations like incorrect case marking, as in He hit they with a stunning spell, or wrong agreement between a noun and a verb, such as with The wizard with the lightning scar are eating now. When the brain processes problems like these, we see a large positive shift in brainwave patterns around 600 milliseconds (ms) after hearing the offending word. That's the brain's way of signalling something just plain wrong happened with the syntax.
The P600 isn't the only ERP component that's been associated with syntax getting messed up, though. There's another, earlier part of the wave that's often been argued to represent an initial response to syntactic problems. It's known as the Left Anterior Negativity, or LAN. The LAN occurs about 300-500 milliseconds after exposure to the problematic word, and it happens further forward in the brain, under more frontal electrodes on the left side of the skull. It's elicited by the same sorts of grammatical mistakes that get P600s, and it's been viewed as part of a two-part response to the syntax going haywire: first the LAN, and then the P600.
There's been a number of studies that have argued for an even earlier LAN effect: the Early Left Anterior Negativity, or ELAN. The ELAN occurs in a similar part of the left frontal region of the brain, but it happens earlier: usually 100-300 milliseconds after getting the stimulus word, with a peak around 200 ms. That's in a time region that's usually associated with phonological processing, so finding syntactic problems this early is really surprising! We've only been looking at the ELAN since Angela Friederici put out the first publication that integrated it in 2002 and made it a new player on the ERP component scene.
Not all syntactic problems can elicit an ELAN, though. It's only ones like in the examples below, with the offending words italicized:
- Hermione's of wand
- The textbook fell in the shrunken
- His intone
In each of these cases, the problem is that there is no possible way that the italicized word can be fit grammatically into the sentence. Of showing up after a possessive in (1) is just irredeemably bad, and its appearance throws the whole sentence in the mental trash. Because it's such a basic word category error, it breaks the structure of the sentence immediately, and the brain reacts very rapidly to the problem.
So why didn't we touch on these effects in the video itself? Well, for the LAN, it's not as clear that it really represents its own separate component. This has been a point of contention for at least a decade now, but to illustrate why, consider the points recently argued by Darren Tanner in an article on what best represents the neural response to syntactic lapses. The P600, on the other hand, is a very robust measure that shows up reliably around the same time and in the same region. Whenever there's a syntactic problem or a sentence that's grammatical but very challenging to process, that P600 is there for you around 600 milliseconds later, around the centro-parietal area.
What about the LAN? Well, it doesn't show up all the time, whenever there's an error and across all speakers. And it's not always reliably in the left frontal area; sometimes, it's more spread across to the other side of the brain, as well. So the LAN looks suspicious as an independent ERP response to syntactic problems; if it really were a response to syntax issues, it would be more reliable in how it presents itself.
Tanner's paper, along with another article by Tanner and Janet van Hell, advance the argument that the LAN, which has looked like an independent ERP effect, is actually an illusion. Instead, it's generally a combination of multiple factors, including a combination of an N400 and P600 effects that shows up in a subset of participants when they see syntactic faults, and issues with how the EEG data was read with respect to a baseline referent. They argue that the LAN isn't generally a real syntactic effect - it's a testing issue.
This helps illustrate an important fact: ERPs are a young science. If you looked at what linguists thought all the different effects meant 10-15 years ago, you'd get a different sense of how humans process language than we do now. We're still sorting all this out. We know a lot more than we used to, but we still have a long way to go! Makes you want to get down and work on some brains, doesn't it?
So how about it? What do you all think? Let us know below, and we’ll be happy to talk with you about ERPs and the brain: what it can tell us about processing, what steps we go through, and how to get the best measurements. There’s a lot of interesting stuff to say, and we want to hear what interests you!