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Preparing words in speech production is normally a fast and
accurate process. We generate them two or three per second in fluent
conversation; and overtly naming a clear picture of an object can
easily be initiated within 600 msec after picture onset. The
underlying process, however, is exceedingly complex. The theory
reviewed in this target article analyzes this process as staged and
feedforward. After a first stage of conceptual preparation, word
generation proceeds through lexical selection, morphological and
phonological encoding, phonetic encoding, and articulation itself. In
addition, the speaker exerts some degree of output control, by
monitoring of self-produced internal and overt speech. The core
of the theory, ranging from lexical selection to the initiation of
phonetic encoding, is captured in a computational model, called
weaver++. Both the theory and the computational
model have been developed in interaction with reaction time
experiments, particularly in picture naming or related word production
paradigms, with the aim of accounting for the real-time processing in
normal word production. A comprehensive review of theory, model, and
experiments is presented. The model can handle some of the main
observations in the domain of speech errors (the major empirical
domain for most other theories of lexical access), and the theory
opens new ways of approaching the cerebral organization of speech
production by way of high-temporal-resolution imaging.
Levelt et al.: Lexical access in speech production
Levelt et al. describe a model of speech production in which
lemma access is achieved via input from nondecompositional conceptual
representations. They claim that existing decompositional theories
are unable to account for lexical retrieval because of the so-called
hyperonym problem. However, existing decompositional models have
solved a formally equivalent problem.
Though weaver has knowledge that gets activated
by words and pictures, it is incapable of responding appropriately to
these words and pictures as task demands are varied. This is because
it has a most severe case of attention deficit disorder. Indeed, it
has no attention at all. I discuss the very complex attention demands
of the tasks given to weaver.
Language production and comprehension are intimately
interrelated; and models of production and comprehension should, we
argue, be constrained by common architectural guidelines. Levelt et
al.'s target article adopts as guiding principle Ockham's
razor: the best model of production is the simplest one. We recommend
adoption of the same principle in comprehension, with consequent
simplification of some well-known types of models.
Levelt, Roelofs & Meyer present a comprehensive and
sophisticated theory of lexical access in production, but we question
its reliance on binding-by-checking as opposed to binding-by-timing
and we discuss how the timing of retrieval events is a major factor
in both correct and errorful production.
Things should be made as simple as possible, but not simpler.
The theory of lexical access in speech production reported by
Levelt, Roelofs & Meyer is exciting, well-described and
well-organized, but because it relies mainly on the principle of
simplicity (Ockham's razor), I argue that it might not be
true. In particular, I suggest that overapplying this principle is
Any complete theory of lexical access in production must
address how words are produced in prosodic contexts. Levelt, Roelofs
& Meyer make some progress on this point: for example, they
discuss resyllabification in multiword utterances. I present work
demonstrating that word articulation takes into account overall
prosodic context. This research supports Levelt et al.'s
hypothesized separation between metrical and segmental
The theory of lexical selection presented by Levelt, Roelofs
& Meyer addresses the mechanisms of semantic activation that
lead to the selection of isolated words. The theory does not appear
to extend naturally to the referential use of words (particularly
pronouns) in coherent discourse. A more complete theory of lexical
selection has to consider the semantics of discourse as well as
I examine four core aspects of weaver++.
The necessity for lemmas is often overstated. A model can incorporate
interaction between levels without feedback connections between
them. There is some evidence supporting the absence of inhibition
in the model. Connectionist modelling avoids the necessity of a
nondecompositional semantics apparently required by the
The use of lexical concepts in Levelt et al.'s model
requires further refinement with regard to syntactic factors in
lexical choice, the prevention of pleonasm, and the representation
of near-synonyms within and across languages.
Levelt et al. attempt to “model their theory”
with weaver++. Modeling theories requires a model
theory. The time is ripe for a methodology for building, testing, and
evaluating computational models. We propose a tentative, five-step
framework for tackling this problem, within which we discuss the
potential strengths and weaknesses of Levelt et al.'s modeling
According to the theory of lexical access presented by
Levelt, Roelofs & Meyer, processing of semantic–syntactic
information (i.e., lemma information) and phonological information
(i.e., lexeme information) proceeds in a strictly discrete, serial
manner. We will evaluate this claim in light of recent evidence from
the literature and unpublished findings from our laboratory.
The weaver++ model discussed by Levelt et
al. assumes incremental encoding and articulation following complete
encoding. However, many of the response latency results can also be
accounted for by assuming incremental articulation. Another temporal
variable, initial segment duration, can distinguish weaver++'s incremental encoding account from the incremental
Lexemes supposedly represent phonological but not grammatical
information. Phonological word substitutions pose problems for this
account because the target and error almost always come from the same
grammatical class. This grammatical congruency effect can be explained
within the Levelt et al. lexicon given that (1) lexemes are organized
according to phonological similarity and (2) lexemes from the same
grammatical category share phonological properties.
(1) Reaction time (RT) studies give only a partial picture of
language processing, hence it may be risky to use the output of the
computational model to inspire neurophysiological investigations
instead of seeking further neurophysiological data to adjust the RT
based theory. (2) There is neurophysiological evidence for differences
in the cortical representation of different word categories; this
could be integrated into a future version of the Levelt model. (3) EEG/MEG coherence analysis allows the monitoring of synchronous
electrical activity in large groups of neurons in the cortex; this is
especially interesting for activation based network models.
The insistence on strict seriality and the proscription of
feedback in phonological encoding place counterproductive limitations
on the theory and weaver++ model. Parsimony cannot
be stipulated as a property of the language system itself. Lifting the
methodological prohibition on feedback would allow free exploration of
its functionality in relation to the substantial content of this major
The following questions are addressed concerning how
a theory of lexical access can be realized in the brain: (1) Can a
brainlike device function without inhibitory mechanisms? (2) Where
in the brain can one expect to find processes underlying access to
word semantics, syntactic word properties, phonological word forms,
and their phonetic gestures? (3) If large neuron ensembles are the
basis of such processes, how can one expect these populations to
be connected? (4) In particular, how could one-way, reciprocal, and
numbered connections be realized? and, (5) How can a neuroscientific
approach for multiple access to the same word in the course of the
production of a sentence?
We discuss our concerns associated with three assumptions
upon which the model of Levelt, Roelofs & Meyer is based: assumed
generalisability of decontextualised experimental programs, assumed
highly modular architecture of the language production systems, and
assumed symbolic computations within the language production system.
We suggest that these assumptions are problematic and require further
Recent aphasiological findings, not mentioned in the target
article, have been accounted for by Levelt et al.'s theory and
have, in turn, provided it with empirical support and new leads. This
interaction is especially promising in the domain of complex word
retrieval. Examples of particular categories of compounds are
Despite several positive features, such as extensive
theoretical and empirical scope, aspects of Levelt, Roelofs &
Meyer's theory can be challenged on theoretical grounds
(inconsistent principles for phonetic versus phonological syllables,
use of sophisticated homunculi, underspecification, and lack of
principled motivation) and empirical grounds (failed predictions
regarding effects of syllable frequency and incompatibility with
observed effects of syllable structure).