Baggio and Hagoort: "The balance between memory and unification in semantics" (2011)

This paper contains some interesting anatomical details about how the N400 component might be produced.

Once Upon a Time

The theory proposed is essentially this:

First, the temporal regions of the brain get their cues from the visual about 200ms after reading a word. Then, they go on to pull the right semantic building blocks out of the drawer. After that, they send a message to the inferior frontal parts of the cortex, which then helps construct the right sentence-level meaning out of the building blocks.

The idea is then that if this construction process runs into trouble, an excess of negative charge is observed after another 200ms.

Cycles and Loops

The theory also spells out the directionality of the communication: The visual cortex can talk to the temple, but the temple can't talk back; the temple can talk to itself as well as to the forehead, but the forehead can only talk back to the temple, not to itself. This allegedly explains the timeline of activation in these two areas.

In schematic form, the feedback loops thus look as follows (their Fig. 3, p. 18):

In terms of the anatomy involved, these are the underlying brain structures (Fig. 2, p. 16):

The different little blobs have names, but they aren't much differentiated in the larger narrative.

Wheres, Whats, and Whens

It's a little surprising that the N400 should be attributed to the construction of meaning rather than simply to the matching of data with expectations.

The argument for this relies crucially on the assumption that the frontal brain regions are instrumental in provoking the N400: Much of the evidence cited on page 13 assumes that this is true and uses this to link the timeline constructed on the basis of EEG studies to the topological map constructed on the basis of fMRI studies.

This data doesn't bear on the question if increased activation in the frontal areas aren't related to the N400. Although ambiguity does indeed activate the frontal cortex, this does not necessarily imply that ambiguity is related to the N400.

Contents

Rather than go through the article step by step, I thought it might be good to provide an annotated table of contents of its six sections. Now that I see how long my comments became, I'm not so sure. At any rate, here it is:

1. INTRODUCTION: We can understand "combinatorics" like syntactic analysis as a last resort to make sense of inputs that are not seen before in the exact same form (p. 2).
1. Memory, unification, and control: Memory is located in the temple, "unification" (i.e., the construction of complex forms out of simpler ones) in the frontal areas (p. 3).
2. Unification the formal way: Think of lexical-functional grammar representations and the like. "This morning, I worked all night" does not unify.
3. Semantics processing as constraint satisfaction: Memory supplies the building blocks, executive control stacks them together (p. 4)
4. What a theory of semantics processing should do: It should both account for the ability to comprehend for novelty (e.g., "The ship sailed into the living room") and for the averse reactions to it.
2. MEMORY AND TEMPORAL CORTEX: Simple recall is not enough even for cliches, because context matters too (p. 7).
1. Memory and the N400: Based on what you've understood so far, you've built up an expectation; if that expectation is violated, you have an N400 response.
2. The N400 and temporal cortex: "The N400 priming effect has been shown to have primary neuronal generators in temporal cortex" (pp. 8–9) The reference is to a paper by Ellen Lau et al.: "A cortical network for semantics" (2008).
3. UNIFICATION AND FRONTAL CORTEX: In order to get "a much processing mileage as possible" out of recall routines, the brain looks for "the nearest attractor state at each word-processing step" (pp. 9–10). A footnote explains that this can either be explained with reference to Hopfield networks or predictive coding, but "The theory presented in Sections 4 and 5 is more consistent with the former framework, which is also a more standard approach to language in the brain than Bayes" (note 3, p. 10).
1. Unification and Integration: Integration basically means selection from a menu based on converging evidence; unification means constructing new menu items in accordance with stated constraints (p. 10).
2. Unification and the N400: The N400 cannot only track expectation or retrieval, since it is also evoked by sentences like "The journalist began the article," which require a post-hoc reconstrual of the verb "began" (p. 12–13). It is, they say, "hard to see how a noncombinatorical account could explain these data" (p. 13), which are attributed to Baggio et al.: "Coercion and compositionality" (2010) and Kuperberg et al.: "Electrophysiological correlates of complement coercion" (2010). An additional discussion takes up the role of the inferior frontal gyrus in "unification."
3. Unification and selection: Conventional wisdom has it that the frontal cortex is involved in "controlled retrieval" (p. 14). But word-association tasks show that it is more active when you try to find a word related to "wheel" than when you are looking for a word related to "scissors" (which is much easier and less ambiguous). Moreover, the voice experiments by van Berkum et al. (2008) show that other types of hard-to-combine stimuli activate the inferior frontal gyrus (pp. 14–15).
4. THE BALANCE BETWEEN MEMORY AND UNIFICATION: The temporal areas do retrieval, integration, and prediction; the frontal areas do unification (p. 15).
1. Connectivity matters: There are several pathways between the temple and the forehead, including some we had forgotten about (p. 16).
2. Neurotransmitter dynamics: The temple talks to the forehead using the rapidly decaying transmitters AMPA and GABA; the forehead talks back using a much more slowly decaying transmitter called NMDA (pp. 17–18).
3. A typical processing cycle: After about 200ms, the temple knows what the visual cortex sees; it then talks to itself to built predictions and to the forehead to trigger competitive selection (p. 19). The forehead cannot talk to itself, but only talk back to the temple (pp. 19–20).
5. A DYNAMIC ACCOUNT OF THE N400: One first sweep retrieves relevant representations, and a second sweep back checks consistency.
1. The N400 component: The wave itself is always present and reflects the feedback within the temple (p. 21).
2. The N400 effect: If a word is unexpected in the sense of having lower "semantic relatedness" to the preceding context, the wave is higher (p. 22). This should allegedly be the result of the forehead talking back to the temple.
3. Testability and falsifiability: This is all very scientific and stuff. Specifically, "patients with focal lesions in [the frontal areas] BA 45 or BA 47 are expected to show at most the onset of an N400 response [not a full-fledged one,] corresponding, in the theory, to the feed-forward spread of activation from sensory areas and inferior temporal cortex to MTG/STG." (p. 23).
6. CONCLUSIONS: Thus, the N400 is a reaction to a message from the font of the class: "The theory explains the N400 as the result of the summation of currents injected by frontal cortex due to the local spread of activation to neighbouring neuronal populations (pre-activation). In our theory, pre-activation and unification are not independent step-like processes, suggesting mutually exclusive accounts of the N400" (p. 24).