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\begin{center}
{\em 
Andreas van Cranenburgh 0440949 \\
Mechanisms of Meaning \\
\today}

\section*{Linguistic meaning and perceptual symbols}
\end{center}

%\abstract{
%What underlies the meaning of language in the mind? In cognitive science
%it has long been assumed to be a Language of Thought (Fodor 1975),
%an innate variety of first-order logic. Another possibility, which has a longer
%history, is that perceptual imagery is the main form of mental representation.
%}

% RQ: Is all meaning grounded in perceptual content?
% versus Deacon's more context-free symbols

% RQ: is cognition perceptual all the way down?

% blah blah blah blah blah blah blah blah blah blah
% words words words words
% I'm making flowers out of paper. a darkness taints the afternoon
% I know that they won't last forever. the real ones fade away too soon
% I still cry sometimes when I remember you

\subsection*{Introduction}

What underlies the meaning of language in the mind? In cognitive science
it has long been assumed to be a Language of Thought (Fodor 1975),
an innate variety of first-order logic. Another possibility, which has a longer
history, is that perceptual imagery is the main form of mental representation.
A recent version of such a theory is the one presented by Barsalou et al. 
I will first present this theory and then assess whether it can really account
for the meaning of language, in light of reflections on symbolic reference.

\subsection*{Perceptual symbols}

Barsalou (1999), a target article in the Behavioral and Brain Sciences journal,
presents a sweeping vision fully integrating perception and cognition.

Up until the demise of introspectionism it was commonplace to grant
a major role to imagery and perceptual representations in the mind. Influential
philosophers such as Locke, Hume, Berkeley, and Kant all subscribed to such
ideas. However, since behaviorism's rejection of the mental (i.e., any form of
mentalism), perception had to go as well. Mentalism returned with cognitivism,
which replaces the perceptual and imagistic representations with so-called
amodal representations. Amodal representations are like the symbols in logic
and programming languages in that they can be operated on without any regard to
their content.

Take the example of a chair, cited in Barsalou (1999). A modal representation
of a chair is derived from seeing any number of chairs, after which an average
chair, or perhaps a silhouette, is extracted. Although this image or image
schema of a chair will contain less detail than the original, it will clearly
share structural features, such as having four feet. 

An amodal symbol, on the other hand, will share no such structural features.
There needs to be a translation step, also called transduction, to get from
a perception to an amodal symbol such as ``chair.'' The use of an English word
here is misleading, because from a symbol system's perspective it might as well
have been \texttt{G778}, i.e., amodal symbols are arbitrary. This might not
seem disconcerting, if only one accepts that natural language semantics works
like that of formal languages; that assumption is far from obvious, however.
Examples of such amodal representations are feature lists, frames, schemata,
semantic networks, and production systems. Clear advantages of these symbols
are their ability to represent propositions and abstract concepts, to
recombine productively, and to produce inferences.

Important problems with amodal symbols are the problem of transduction
and the grounding problem. The first is about connecting the information
from the senses to the internal symbols that are assumed to be the real
mental representations of cognition. This problem is usually not dealt with
as perception has been viewed as rather separate from the rest of cognition.
The second problem is about where the ultimate meaning of symbols derives from.
Symbols are of course defined in terms of other symbols, just as dictionary
entries contain definitions made up of words. However, some words like `pain,'
`red' and `warm' seem to be very experiental in nature and hard to describe
or define. This problem can be solved by letting meaning derive from the
external world through sensory experience, instead of only from the web of
internal relations of symbols; in effect, by allowing for modal information
after all.

Another problem with amodal symbols is that there is simply no direct evidence
to support them, which is all the more troubling given that they form an
unfalsifiable hypothesis which can explain any data as being the result of
underlying amodal symbols in a \emph{post hoc} fashion (Barsalou 1999,
sect.~1.2.3). The perceptual symbol system hypothesis, on the other hand, makes
\emph{a priori} predictions which can be tested.

%---

Perceptual symbols can be both unconscious and conscious, in contrast to
the classical empiricist theories which were only about conscious imagery,
seen in the mind's eye. Furthermore, perception is often characterized as
analogue but also holistic, taking in the whole of a situation without focus on
any specific area. If this were true, perception would be like a recording
device such as a photo camera or tape recorder, which does not interpret
or compress information but reproduces an attenuated copy of experience. This
view of perceptual representations as holistic is one of the most important
reasons that they are rejected by most cognitive scientists. This leads to a
received view where the perceptual and conceptual are mutually exclusive.

Another important point is that perceptual is taken to be rather broad by
Barsalou, as it includes not only information from the five senses, but also
proprioception (the state of one's body and muscles) and introspection
(what goes on in the mind's eye). Together these sources of information are
stored and reproduced in simulations, which is a top-down activation of
sensory-motor areas to re-enact perceptual experience. Once a particular object
has been witnessed repeatedly as being relevant, a simulator is formed. A
simulator consists of a frame and simulations associated with the simulator. A
frame is what adds structure to a perception. In the case of perceiving cars,
subregions may come to be identified, through selective attention, after which
doors and wheels may become their own simulators. Each new car that is seen is
matched against the existing simulator with the highest activation. This
process of framing always happens relative to other concepts in a situation, in
other words meaning is background-dependent.

Up till now perception can be parsed into components, which are integrated
into frames. This amounts to a basic conceptual system. A full conceptual
system also has (1) productivity, (2) propositions, and (3) abstract concepts.

Productivity can be seen as reverse of the just sketched symbol formation
process. Given a number of distinct simulators, a complex simulation can be
formed, such as the plane flying next to a cloud in Barsalou (1999, sect.~3.1). On the one hand productivity relies on schematization, where perceptual
input is filtered out to arrive at a more general concept; on the other hand
it relies on the complementary notion of specialization, where for example
the concept of `ball' can be specialized into `red ball.' Aside from filling in
schematic features, structures can also be transformed, replaced and deleted.
Productivity explains the power of imagination to surpass experience.

Once there is productivity, there can be propositions that conceptualize
a situation in a certain way. To form a proposition is a creative act,
because it is one of an infinity of possible construals. The simplest type
of proposition is a type-token mapping, such as ``that is a car'' when
recognizing a car, where a certain simulator produces a simulation matching
a situation. With the help of productivity, propositions can become arbitrarily
complex.

An important feature of propositions is that they should have intentionality, 
in other words they should have the ability to be about something. 
Intentionality for perceptual propositions is determined through both the
content and external factors such as causal relationships, whereas for amodal
symbols it can only be through external factors related through form, since
they are arbitrary symbols and thus have no content. Since the content of
perceptual symbols plays a role they can be recognized with heuristics, such as
noting a resemblance; however, content is neither necessary nor sufficient for
establishing a referent.

% ambiguity of images
% variable embodiment
The fact that perceptual symbols can work with analogical reference implies
that embodiment is crucial to their functioning. The result is that different
perceptual systems also lead to different conceptual systems, a phenomenon
referred to as variable embodiment.


The final feature of a full conceptual system is its ability to represent
abstract concepts. An approach popular in cognitive linguistics, with a similar
emphasis on the sensory-motor system, argues that all abstract thought
derives from metaphors (e.g., Lakoff \& Jonhson 1980). Barsalou argues
against this on two grounds. Using the example of `anger' with  the purported
underlying metaphor `liquid exploding from a container,' he argues that this
cannot form a sufficient representation for the concept of `anger,' because it
does not supply a direct, underlying concept; this concept should firstly 
supply a basic understanding of the domain, and secondly it should guide the
mapping to concrete domains such as the exploding liquid in this example.
Another issue is that in frequently used metaphors it is very reasonable to
suppose that they function through polysemy, rather than through a metaphorical
mapping.  This last point reveals the tension between conceiving of
metaphor as conceptual versus linguistic. In the example of anger this would
mean that ``he exploded with anger'' is no longer metaphorical, but instead
relies on an emotional sense for ``exploded.''

Barsalou's proposal of representing abstract concepts is to do it directly
with perceptual symbols. The proposal has three features:
(1) an abstract concept is framed against a background of a simulated event
sequence, (2) selective attention highlights the core content, (3)
introspective states play a central role, especially in the formation of a
propositional construal.

According to these three features the formation of an abstract concept in
a perceptual symbol system can be explained. The concepts of truth and
disjunction are explained in this manner (Barsalou 1999, section 3.4.3--3.4.4).
For the concept of truth this proceeds as follows. In one situation an agent
simulates a balloon above a cloud, perhaps after hearing someone talk about it.
Then this simulation is mapped to an actual scene that is perceived, and a
correspondence of a balloon actually being above a cloud is noted. After
repeated occurrences of such a sequence, an abstract concept of truth can
emerge; at least the everyday sense of truth.

% the car is on fire and there's no driver at the wheel.
% and the sewers are all muddle with [...]
% we're trapped in the belly of this horrible machine,
% and the machine is bleeding to death

\subsection*{Language and Situated Simulation}

In Barsalou et al.\ (2008), the perceptual symbol system hypothesis returns
in extended form. The principal difference is the introduction of a separate
language system responsible for shallow processing of word forms. This allows
for explaining differences in reaction times in priming effects with words and
pictures. The language system is described as a distributional semantic model
(DSM), which can generate associated linguistic forms for a given word. 

Language comprehension is equated with the construction of a perceptual
simulation. The words are taken as instructions in guiding the construction
of the simulation. The resulting conceptualization then functions as the gist
of the sentence. The gist is what people remember after the exact formulation
of the sentence has been forgotten.

The language and simulation system act in parallel, though not in the matter of
independent modules but with extensive interaction. To produce an utterance
for a situation in mind, the situation is scanned for particular regions
which can be categorized, after which associated linguistic forms are
activated; finally these are integrated and translated into a motor program
to pronounce the utterance.

Barsalou et al.~(2008) note the following on the relation between language
and thought:

\begin{quote}
Whereas simulations represent the content of thought, words provide tools for
indexing and manipulating this content [...].  

In general, we assume that linguistic forms provide a powerful means of
indexing simulations (via simulators), and for manipulating simulations in
language and thought.
\end{quote}

It is clear that the role of language in the LASS theory is a rather shallow
one, and most of the work is assumed to be done by simulations. On top of that
it is puzzling why attention should be restricted to linguistic forms.
Additionally, it is unfortunate that seemingly all of the empirical evidence
relates to single words, without any strategy to generalize to sentences.
Thus it seems to be rather presumptious to use the term ``language system,''
when actually the results seem to be restricted to ``a system of word forms.''

\subsection*{Criticism of perceptual symbols}

%Perceptual symbols are (1) modal, (2) analogical, they resemble their referents
%(3) reside in sensory-moter areas of the brain
%
%ad 2:
%``structure of a perceptual symbol corresponds, at least somewhat, to the
%perceptual state that produced it'' (sect. 1.1).
%perceptual symbols resemble their referents (sect. 3.2.8),

%Grounded in perceptions yet transformed into amodal symbols
%Liane Gabora (peer commentary)

% perhaps the challenge to define `perceptual' in perceptual systems is based
% on the previously held categorical distinction between perception and
% cognition, which this approach seeks to do away with. Rather there might be
% a continuum of perceptual information and abstractions thereof

The two core properties of perceptual symbols are:

\begin{itemize*}
\item neural representations in perceptual systems (sect. 2.1)
\item schematic perceptual symbols (sect. 2.2).
\end{itemize*}

Both of these are problematic. The first because it violates the
personal-subpersonal distinction (as noted by Dennett in the peer commentary).
On the one hand there are subpersonal neurons, wired together in intricate
patterns and firing according to the laws of causality. On the other hand there
is the personal level at which meanings play a role and people report on
thoughts. %intentionality and normativity.
%normativity arises.

%FIXME
The second point refers to the claim that perceptual symbols consist of
schematized perceptions. The question is, what grounds are there to call them
`symbols,' if they are merely schematized associations --- in other words signs
correlated with objects. This is problematic because in that case symbols
cannot rise above the level of indexical reference to the level of conventional
and arbitrary reference.  Arbitrary reference can be viewed as a \emph{symbolic
leap} (more on this in the next section), when one can summarily declare ``from
now on this squiggle will stand for [...].'' 

Tomasello (2003) makes this point by arguing that what distinguishes 
linguistic symbols is that they are intersubjective and perspectival. His
hypothesis is that communication and cognitive representation are closely
interrelated:

\begin{quote}
As the young child internalizes a linguistic symbol or construction --- as she
culturally learns the human perspective embodied in that symbol or
construction --- she cognitively represents not just the perceptual or motoric
aspects of a situation, but also one way, among other ways of which she is also
aware, that the current situation may be attentionally construed by ``us,'' the
users of the symbol. The intersubjective and perspectival nature of linguistic 
symbols thus creates a clear break with straightforward perceptual or
sensorimotor cognitive representations. \\
 \hspace{3em}-- Tomasello (2003)
\end{quote}

This interpersonal aspect of symbols is completely neglected by Barsalou.
The productivity of perceptual symbols in imagination allows for
displaced reference --- an oft cited feature distinguishing human from
animal communication. However, the perspectival and intersubjective nature
presents a much bigger challenge. 
For example, to represent a situation with multiple participants,
first person perceptual experience is not enough; to represent a concept such
as ``hide and seek'' requires a combination of first and third person
perspective so that each role can be envisioned.
Sensory-motor interactions with the world undoubtedly form the bedrock
for cognitive development. However, for higher cognition and especially
language, social interactions appear to be crucial as well.

That every abstract concept must derive from perceptual content seems to me an
unnecessary dogma. The explanations of how truth and disjunction derive from
perceptual experience seem similar to Lakoff \& Johnson's (1980) insistence
that abstract concepts must derive from bodily metaphors such as `more is
better.' Such explanations are \emph{ad hoc} at best. The main problem is that
they both rely on the inventors of the theory to come up with plausible stories
for each concept, because it is not at all obvious how it is supposed to work.
Although there is the advantage that there is a lot of literature on the
sensory-motor system,\footnote{``Much more is known about how brains implement
perception than about how they implement cognition.'' (Barsalou 1999,
sect.~4.3)} the thesis that cognition is perceptual all the way down seems even
more implausible than the idea that language-like symbols underlie
all of cognition. A better theory should combine the grounding in perception
with definitions in terms of other symbols.


\subsection*{The triadic theory of symbols}
%``Language comprehension can be viewed as the construction of a perceptual
%simulation to represent the meaning of an utterance or text.'' Barsalou (1999)

Deacon (1997) presents an account of the co-evolution of language and the
brain. Compatible with Barsalou's emphasis on the use of the sensory-motor
system, he argues that the brain evolved to deal with concrete sensory-motor
tasks (recognizing enemies, acquiring food, etc.). In contrast to Barsalou,
however, he stresses the special nature of language in contrast to other forms
of cognition. Although language and the brain co-evolved in parallel, the
evolution of language occurs at a cultural-historical pace (as opposed to
phylogenetic or ontogenetic). This implies that it is very likely that language
is thoroughly adapted to suit to human brain, as opposed to the other way
around. This explains why the attempts to isolate a language module or a
Language Acquisition Device (LAD) have failed, and also why human brains are 
structurally so similar to those of other primates.

The two areas that are often cited as the language areas, Broca's and
Wernicke's area, are not the sites of concrete features of language such as
grammar or semantics, because often patients recover and the symptoms of brain
damage to these area differs greatly depending on the language of the patient
(e.g., inflected versus analytic). Deacon notes that a better explanation of
these areas is that they serve as two bottlenecks of language, speech
production and perception, respectively. Instead of looking in the brain and
mapping specific language functions to it, we should be trying to map language
functions on existing brain functions. This idea seems very compatible to
Barsalou's framework in which the perceptual is the groundwork for all
cognition, except that Barsalou does not grant a special status to words --- it
seems he views them as mere labels to summon, index and manipulate simulations.

Deacon argues that the most important feature that sets humans apart from
other primates is symbolic reference. Symbolic reference is more than just
an association of a word and an object (or concept). Such an association
would need to be maintained by reinforcement continually, say a dog that
expects to eat when it hears the phrase ``let's eat.'' If the phrase would
be uttered in other situations, it would start to lose its `meaning.' For
humans, the phrase is {\em understood}, whether it is applicable to the current
situation or not. Another distinguishing feature of symbolic reference is
that it is indirect, mediated by a web of other symbols.

Following Peirce's triadic theory of signs, there are three
levels of reference:

\begin{description*}
\item[iconic:] relies on similarity to referent (analogical)
\item[indexical:] a physical or temporal correlation \\
        An index is an association of two icons, for example sound images of a
        word and percepts of an object (Saussurean signs).
\item[symbolic:] conventional and arbitrary \\
        A symbol arises from a web of indexical relations to other symbols;
	similarly, the system of symbols bears an iconic relation to the
	collection of relationships among the objects symbolized. 
\end{description*}

None of these modes of reference is causal, rather they arise when something
is interpreted as a sign. Thus not everything with a resemblance to something
is automatically an icon, it is only when a sign is designed or clearly used
that way that it is an icon. Also, while some gestures in American Sign
Language (ASL) may seem `merely' iconic, they actually function symbolically.
The triadic account of reference represents {\em levels} of interpretation,
each building upon the previous one hierarchically.

However, to cross the symbolic threshold requires unlearning indexical
correlations (e.g., expecting to see an apple upon hearing the word `apple'),
and integrating a large number of relations into a coherent web of signs and
relations. Crossing of the symbolic threshold is described by Deacon as
as symbolic insight in which previously learned indexical relations are
restructured after they are seen in a new light. Once a pattern is discovered
lots of individual correlations can be reduced to a single symbolic meaning,
in effect it is about discovering a more efficient representation.
The so-called critical period of language acquisition could be viewed as a
period of receptivity to the patterns implicit in the vocabulary of natural
languages, presumably because the prefrontal cortex, which is associated
with abstract thought and inhibiting behavior in adulthood, is idle and
available to detect these patterns at this time.

What emerges is a system of signs whose reference is not determined directly
but derived ``from combinatorial possibilities and impossibilities'' (Deacon
1997, p.~83). Because of this words only have meaning in context, in
utterances. Both learning and use depend on combinations of words. On this 
view syntax and semantics are inextricably linked. What the brain has
co-evolved for is not a language module but the ability of symbolic reference,
driven by the great advantages conferred by bigger brains capable of handling
more complex language.
%The idea of word meaning is thus a somewhat misleading abstraction from the meaning of words in phrases; misleading 

\subsection*{Synthesis}

Barsalou argued that there is simply no evidence for amodal symbols. Does this
mean that Deacon's symbols cannot exist, because they are not directly 
associated with perceptual experience? I think not. To cross the symbolic
threshold surely requires grounding in perceptual experience, but crossing
it results in qualitatively different forms of reference.

On the one hand there is the position that there is a continuum from concrete
to abstract, which allows abstract concepts to be derived from concrete ones
through gradual abstraction.
On the other hand there is the more orthodox position that there is a
categorical difference between concrete and abstract, e.g., perceptual versus
propositional representations. I believe that Deacon's account shows that
these positions need to be synthesized. Abstract concepts and other symbols
are grounded in concrete ones, but only indirectly through a web of relations
to other symbols that are collectively linked with a web of signifiers. Yet
still, because of the distinction between indexical and symbolic relations,
there is a break along this continuum where further abstraction yields new
properties, which explains why language yields so much cognitive benefits
(Clark 1998; 2006, Schaller 1995).

This break thus hints at a distinction between linguistic meaning and other
conceptual representations, or at least a special way in which linguistic
meaning is mediated to produce Barsalou's simulations. As Clark (1999) suggests
in a position paper on the relation of language and thought, it is very likely
that the features of particular languages determine semantic representations,
such as through mandatory grammatical markings; but when not using language, we
should not be so constrained, for example imagination and memory probably do
not require language. It is plausible to assume that general conceptual
representations are universal (or at least not influenced by the particular
language we speak), but that semantic representations differ for each language.
Conversely, any given utterance will only have a single semantic
representation (or a few in case of ambiguity), yet the simulations it evokes
in hearers might be as unique as snowflakes. Although I believe simulation is
constitutive of linguistic understanding, I disagree with Barsalou that a
simulation \emph{represents} linguistic meaning of an utterance or text
(Barsalou 1999, sect.~4.1.6).

Thus, \emph{pace} Barsalou who posits that in language comprehension an
utterance leads directly to a simulation, or to the construction grammarians
who similarly believe that linguistic understanding is conceptualization, there
must be an intermediate level of semantic representation. This level is
intimately linked with word forms, which themselves form a web such as that of
a distributional semantic model --- the difference is, though, that at the
same time the semantic representations are grounded in something like
Barsalou's perceptual symbols.

% poverty of language forms vs richness of perceptual symbols
% but: semantics

% bourbaki: pictures misleading.
% language as compression
% languageless people

%\subsection*{Clark's complementarity view}
%The superficial language system of LASS adequately explains
%the experimental data cited in Barsalou (2008), by viewing it
%as a distributional semantic model that keeps track of word frequencies.
%
%However, it seems obvious that language grants humans much greater cognitive
%benefits than merely making rapid associations. Clark (2006) lists three ways
%in which language benefits cognition:

%\begin{enumerate*}
%\item 
%\item
%\item
%\end{enumerate*}


% abstract concepts
% imagination
% emotions
\subsection*{Conclusion}

The general idea that perception and cognition should be more integrated seems
to have merit and much plausibility. However, that all of meaning and
especially abstract concepts can be explained as implemented by perceptual
symbols seems problematic. There seems to be a clear break between the general
sensory-motor cognition shared with other animals, and the linguistic cognition
based on symbolic reference. This symbolic reference calls for a triadic
theory of signs in which symbols represent a distinct level from perceptual
associations, while still being grounded in such perceptions.
In sum, the perceptual symbols and simulations seem to be a necessary
component of meaning; however, neither the language system of language and
situated simulation, nor perceptual symbols seem to be sufficient to
explain language, because of the qualitative difference between non-symbolic
and symbolic reference.

% inadequacy of semantics in construction grammar (conceptualization)

% The language system in LASS is a caricature of language, because it only
% aims at explaining the single word reaction time tasks described in
% Barsalou et al.~(2008). Other than that, the theory of simulations and
% perceptual symbols seems very much compatible for a synthesis with Deacon's
% account of symbolic reference.
% A synthesis would form a welcome voice agains the over-emphasis on grammar
% and rules, be it in the form of a language of thought or in natural language.
% 

\begin{center}
$\infty$
\end{center}
\newpage

\subsection*{Bibliography}
\begin{description}

\item[]
Barsalou, Lawrence (1999), Perceptual symbol systems,
	Behavioral and brain sciences, 22:4

\item[]
Barsalou, L.W., A. Santos, W. K. Simmons, \& C.D. Wilson (2008),
	Language and simulation in conceptual processing. 
	In `Symbols, embodiment, and meaning,' pp.\ 245--283,
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\item[]
Clark, Andy (1998), Magic words: How language augments human cognition.
        In: Language and thought: Interdisciplinary themes, ed. P. Carruthers \&
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\item[]
Clark, Andy (2006), Material Symbols,
        Philosophical Psychology, Vol. 19, No. 3, June 2006, pp.~1--17

\item[]
Clark, Eve (1999), Languages and representations,
	In `Language in mind: advances in the study of language and thought,'
	pp.\ 17--24, eds.\ Dedre Gentner \& Susan Goldin-Meadow, MIT Press,
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% languages affect thinking because of diffferent grammatical features that
% are obligatory or what is lexically encoded; however, this does not affect
% thinking when not using language.

\item[]
Deacon, Terrence W. (1997). 
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\item[]
Fodor, Jerry A.~(1975), The language of thought, Harvard University Press.

%\item[] Pinker, Steven (1994), How the mind works. %FIXME

\item[]
Lakoff, George, Mark Jonhson (1980),
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	University of Chicago press.

\item[]
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\item[]
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%E. Clark (2004) How language acquisition builds on cognitive development, Trends in Cognitive Science, 8(10):472-477.


\end{description}

\end{document}


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\item[Smolensky,] Paul (1996).
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\item[Tomasello,] Michael. (2000). 
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	Trends in cognitive sciences, 4(4):156–163.


- dilemma occurs because of competence theories which focus on mechanism, but
  a symmetric mechanism should not be assumed.
  NB: article talks about comprehension-production difference in competence,
      but this is an oxymoron!
- for memory-based theories there is no dilemma: the gap is a side-effect of
  not having enough critical mass to generalize linguistic input into units
  fit for productive use.
- macwhinney: rote learning
- cite van kampen: no innate syntactic categories

- perhaps pronoun interpretation problem (de hoop & hendriks) is not an actual
  comprehension-performance gap but occurs because grammaticality judgment is
  not developed.

- problem: optimality theory abstracts from actual language usage by
  positing constraints and mechanism operating on them. while this is
  useful in an effort to parsimoniously describe language, it does not
  explain acquisition because
- what is the methodology for constraints? can they be posited arbitrarily,
  can they be arbitrarily complex? eg. "must have evidence for claim if main
  verb is marked as such" in languages that mark source of information. is it
  possible for new constraints to be learned?

- asymmetry in dual code: linguistic code is compact versus perceptual code
  which is very sparse. 10 realisations of "dog" are much more similar to each
  other than seeing 10 different dogs.

- hypothesis: asymmetry greater for more difficult languages?

