This is the second part of William Matchin's paper. Thanks again to William for putting this down on paper and provoking discussion.
One (very rough)
sketch of a possible linking theory between a minimalist grammar and online
sentence processing
I am going to try and sketch out what I think is a somewhat
reasonable picture of the language faculty given the insights of syntactic
theory, psycholinguistics, and cognitive neuroscience. My sketch here takes
some inspiration from TAG-based psycholinguistic research (e.g., Demberg &
Keller, 2008) and the TAG-based syntactic theory developed by Frank (2002)
(thanks to Nick Huang for drawing this work to my attention).
Figure from Frank (2002). The
dissociation between the inputs and operations of basic structure building and
online processing/manipulation of treelets is clearly exemplified in the
grammatical framework of Frank (2002).
The essential qualities of this picture of the language
faculty are as follows. Minimalism is essentially a theory of the objects of
language, the syntactic representations that people have. These objects are TAG
treelets. TAG is a theory of what people do with these objects during sentence
processing. TAG-type operations (e.g., unification, substitution, adjunction,
verification) may be somehow identifiable with memory retrieval operations,
opening up a potentially general cognitive basis for the online processing
component of the language faculty, leaving the language-specific component to Merge. This proposal severs any inherent
connection between Merge and online
processing – although nothing in the proposal precludes the online
implementation of Merge during
sentence processing, much of sentence processing might proceed without having
to implement Merge, but rather TAG operations
operating over stored treelets.
I start with what I take to be the essential components of a
Minimalist grammar – the lexicon and the computational system (i.e., Merge). Things work essentially as a
Minimalist grammar says – you have some lexical atoms, Merge combines these elements (bottom-up) to build structures that
are interpreted by the semantic and phonological systems, and there are some
principles – some of them part of cognitive endowment, some of them “third
factors” or general laws of nature or computation – that constrain the system
(Chomsky, 1995; 2005).
The key difference that I propose is that complex derived
structures can be stored in long-term memory. Currently, Minimalism states that
the core feature of language, recursion, is the ability to treat derived
objects as atoms. In other words, structures are treated as words, and as such
are equally good inputs to Merge.
However, the theory attributes the property of long-term storage only to atoms,
and denies long-term storage to structures. Why not make structures fully
equivalent to the atoms in their properties, including both Merge-ability AND long-term
store-ability?
These stored structures or treelets can either be
fully-elaborated structures with the leaves attached, or they might be more
abstract nodes, allowing different lexical items to be inserted. It seems
important from the psycholinguistic literature to have abstract structural
nodes (e.g. NP, VP), so this theory would have to provide some means of taking
a complex structure created by Merge and modifying it appropriately to
eliminate the leaves (and perhaps many of the structural nodes) of the
structure through some kind of deletion operation.
Treelets are the point of interaction between the syntactic
system (essentially a Minimalist grammar) and the memory system. It may be the
top-down activation of memory retrieval operations that “save” structures as
treelets. Memory operations do much of the work of sentence processing –
retrieving structures and unifying/substituting them appropriately to
efficiently parse sentences (see Demberg & Keller, 2008 for an
illustration). Much of language acquisition amounts to refining the
attention/retrieval operations as well as the set of treelets and the
prominence/availability of such treelets) that the person has available to
them.
I think that there are good reasons to think that the
retrieval mechanisms and the stored structures/lexical items live in language
cortex. Namely, retrieval operations live in the pars triangularis of Broca’s area and stored structures/lexical
items live in posterior temporal lobe (somewhere around the superior temporal
sulcus/middle temporal gyrus).
This approach pretty much combines the Minimalist generative
grammar and the lexicalist/TAG approaches. Note also that retrieving a stored
treelet includes the fact that the treelet was created through applications of Merge.
So when you look at structure that is finally said by a person, it is both true
that the syntactic derivation of this structure is generated bottom-up in
accordance with the operations and principles of a minimalist grammar, AND that
the person used the thing by retrieving a stored treelet. We can (hopefully)
preserve both insights – bottom-up derivation with stored treelets that can be targeted
by working memory operations.
One remaining issue is how treelets are combined and lexical
items inserted into them – this could be a substitution or unification
operation from TAG, but Merge itself might also work for some cases (suggesting
some role for Merge in actual online processing).
I think this proposal starts to provide potential insights
into language acquisition. Say you’re a person walking around with this kind of
system – you’ll want to start directing your attentional/working memory system
to all these objects being generated by Merge and creating thoughts. You’ll
also (implicitly) realize that other people are saying stuff that connects to
your own system of thought, and you’ll start to align your set of stored
structures and retrieval operations to match the patterns of what you’re seeing
in the external world. This process is language acquisition, and it creates a
convergence on the set of features, stored structures, and retrieval operations
that are used within a language.
This addresses some of the central questions I posited
earlier:
When processing a
sentence, do I expect Merge to be
active? Or not?
- Not necessarily, maybe minimally or not at all for most
sentences.
What happens when
people process things less than full sentences (like a little NP – “the dog”)?
What is our theory of such situations?
- A little treelet corresponding to that sub-sentence
structure is retrieved and interpreted.
Do derivations really
proceed from the bottom up, or can they satisfactorily be switched to go
top-down/left-right using something like Merge
right (Phillips 1996)?
- Syntactic derivations are bottom-up in terms of Merge, but sentence processing occurs
left-to-right roughly along the lines of TAG-based parsing frameworks (Demberg
& Keller, 2008).
What happens mechanistically
when people have to revise structure (e.g., after garden-pathing)?
- De-activate the current structure, retrieve new
treelets/lexical items that fit better with what was presented. Lots of
activity associated with processing lexical items/structures and memory
retrievals, but there may not be an actual activation/implementation of Merge.
Are there only
minimal lexical elements and Merge?
Or are there stored complex objects, like “treelets”, constructions or phrase-structure
rules?
- Yes, there are treelets, but we have an explanation for why there are treelets – they were
created through applications of Merge
at some point in the person’s life, but not necessarily online during sentence
processing.
How does the
syntactic system interact with working memory, a system that is critical for
online sentence processing?
- The point of interaction between syntax and memory is the
treelet. Somehow certain features encoded on treelets have to be available to
the memory system.
Now that I have these answers, I can proceed to do my
neuroimaging and neuropsychology experiments with testable predictions
regarding how language is effected in the brain:
What’s the function
of Broca’s area?
- Retrieval operations that are specialized to operate over
syntactic representations.
- Which is why when you destroy Broca’s area you are still
left with a bunch of treelets that can be activated in comprehension/production
that you can use pretty effectively, although you have less strategic control
over them.
- We expect patients with damage to Broca’s area to be able
to basically comprehend sentences, but really have trouble in cases requiring
recovery/revision, long-distance dependencies, prediction, and perhaps second
language acquisition
What’s the function
of posterior temporal areas?
- Lexical storage, including treelets.
- We expect activation for basic sentence processing, more
activation for ambiguity/garden-path sentences when more structural templates
are activated.
- We expect patients with damage to posterior temporal
damage to have some real problems with sentence comprehension/production).
Where are fundamental
structure building operations in the brain, e.g. Merge?
- Merge is a
subtle neurobiological property of some kind.
- It might be in the connections between cortical areas,
perhaps involving subcortical structures, or some property of individual
neurons, but regardless, there isn’t a “syntax area” to be found.
What are the
ramifications of this proposal for the standard contrast of sentences >
lists that is commonly used to probe sentence processing in the brain?
- This contrast will highlight all sorts of things, likely
including the activation of treelets, memory retrieval operations, semantic
processing, but it might not be expected to drive activation for basic
syntactic operations, i.e. Merge
Here I have tried to preserve Merge as the defining and simple feature of language – it’s the
thing that allows people to grow structures. It also clearly separates Merge from the issue of “what happens
during sentence processing”, and really highlights the core of language as
something not directly tied to communication. Essentially, the theory of syntax
becomes the theory of structures and dependencies, not producing and
understanding sentences. On this conception of language, there is this Merge machinery creating structures,
perhaps new in evolution that can be harnessed by an (evolutionarily older)
attentional/memory system for the purposes of producing and comprehending
sentences through storing treelets in long term memory. Merge is clearly separate from this communication/memory system,
and an engine of thought. Learning a language then becomes a matter of refining
the retrieval operations and what kinds of stored treelets you have that are
optimized for communicating with others over time.
If this is a reasonable picture of the language faculty,
thinking along these lines might start to help resolve some conundrums in the
traditional domain of syntax. For example, there is often the intuition that syntactic
islands are somehow related to processing difficulty (Kluender & Kutas
1993; Berwick & Weinberg, 1984), but there is good evidence that islands
cannot be reduced to online processing difficulty or memory resource demands
(Phillips, 2006; Sprouse et al., 2012). One approach might be to attribute
islands to a processing constraint that somehow becomes grammaticalized
(Berwick & Weinberg, 1984). The present framework provides a way for
thinking about this issue, because the interaction between syntax and the
online processing/memory system is specified. I have some more specific
thoughts on this issue that might take the form of a future post.
At any rate, I would love any feedback on this type of
proposal. Do we think this is a sensible idea of what the language faculty
looks like? What are some serious objections to this kind of proposal? If this
is on the right track, then I think we can start to make some more serious
hypotheses about how language is implemented in the human brain beyond Broca’s
area = Merge.
Many thanks to Nick Huang (particularly for pointing out
relevant pieces of literature), Marta Ruda, Shota Momma, Gesoel Mendes, and of
course Norbert Hornstein for reading this and giving me their thoughts. Thanks
to Ellen Lau, Alexander Williams, Colin Phillips and Jeff Lidz for helpful
discussion on these topics. Any failings are mine, not theirs.
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