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Review of  Models of Language Acquisition: Inductive and Deductive Approaches


Reviewer: Christophe Parisse
Book Title: Models of Language Acquisition: Inductive and Deductive Approaches
Book Author: Peter Broeder Jaap Murre
Publisher: Oxford University Press
Linguistic Field(s): Psycholinguistics
Book Announcement: 12.2216

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Review:

Broeder, Peter, and Jaap Murre, ed. (2000) Models of Language Acquisition:
Inductive and Deductive Approaches. Oxford University Press, hardback ISBN:
0-19-829989-3, ix+291pp., $85.00.

Christophe Parisse, INSERM (National Research Institute for Health in France),
Paris, France


GENERAL DESCRIPTION OF THE BOOK
The book is intended for research specialists from various domains:
linguistics, computational linguistics, psychology, cognition, language
acquisition, and behavioural science. It composed of 11 papers and an
introduction for a total of 291 pages and presents recent advances in
computational modelling of language acquisition by leading researchers. The
general aim of the book is to try to show what interesting lights may be
thrown on fundamental issues in language acquisition when powerful
computational techniques are combined with real data.

1) Introduction (Broeder & Murre)
The introductory text is a bit disappointing in its shortness -5 pages- and
does not really help the non-specialist to understand the papers that
follow. It sketches a rapid description of the computational study of
language acquisition and divides it into three parts (i- Implementation:
theory and experiment; ii- Discovery of existence proof; iii- Testing on
empirical data) which can be found scattered throughout the book as they
somewhat confusingly do not correspond to the book structure. Implementation
goes with chapters 2, 3, 11, and 12. Existence proof goes with chapters 4,
5, 6, and 7. Empirical data goes with chapters 8, 9, and 10.

Part 1:
2) Lexical connectionism (MacWhinney)
This paper starts with a short presentation of the differences between the
traditional symbolic approach and connectionism. The limits of the
connectionism approach are then underlined and an extension of this approach
is proposed using Self-Organizing Features Maps, which allow to implement
lexical access within the connectionism framework. The author describes how
this solves several problems such as suffix extraction, word compounding and
syntax learning, but how far the proposals are actually implemented is not
quite clear.

3) Are SRNs sufficient for modelling language acquisition (Sharkey, Sharkey
& Jackson)
The authors of this paper attempt to evaluate whether SRNs (Simple Recurrent
Nets) are powerful enough to model language acquisition by trying to get an
answer to four questions: Q1) Is a specific initial state required for SRNs
learning to be successful? Q2) Can SRNs transfer old knowledge to new
lexical items? Q3) Will SRNs forget information about old items? Q4) Are
SRNs able to represent more than they can learn? The results for Q1 is Yes,
for Q2 No and for Q3 Yes, which makes limit the chances of SRNs being good
candidates for modelling language acquisition. However, the answer to Q4
being Yes means that the problems with SRNs might be due to inadequate
learning algorithm (Answer to Q4 is Yes because is it possible to find node
weights which, set manually, obtain results that cannot be achieved using
the automatic learning procedure). Improving the learning algorithm may
allow future SRNs to provide better answers to the first three questions.

4) A distributed, yet symbolic, model for text-to-speech processing (van den
Bosh & Daelemans)
A model of text-to-speech processing (SPC: Subword-Phoneme Correspondence)
is presented, a single-route model that does not use a connectionist
implementation but a procedure akin to lazy-learning (learning using a
straightforward storage of examples and an example-based similarity matching
procedure). It nevertheless allows good generalization and automatic
learning through examples and to define a consistency metric to characterize
word pronunciation. The algorithm is implemented and has been tested in
three languages, English, Dutch and French.

5) Lazy learning: Natural and machine learning of word stress (Gillis,
Daelemans & Durieux)
This paper presents another application of the lazy-learning principle, used
for implementing a single-route model of word stress assignment in Dutch.
The results are compared to those of 3- and 4-year-olds. The comparison
shows that the model tends to display the same characteristics as the
children, e.g. to regularize irregular words. The model is also able to
explain how regular words can sometimes be irregularized. This model
represents another alternative to rule-based models, different from the
connectionist models discussed further on in the book.

Part 2:
6) Statistical and connectionist modelling of the development of speech
segmentation (Shillcock, Cairns, Chater & Levy)
This paper describes a succession of strategies that follow a developmental
path and can be used to develop an efficient algorithm to segment speech
into words. The first algorithm uses connectionist modelling and pure
bottom-up processes. It is based on the idea that, the lower the probability
of predicting the following phoneme, the higher the probability of being at
the end of a word. This very simple algorithm already yields results above
chance level. The next algorithm is based on the fact that low frequency
n-phones are more likely to contain a word boundary. The next algorithm uses
the principle of Metric Segmentation Strategy (Cuttler & Noris, 1988), which
states that children take advantage of the fact that, in English, strong
syllables are more likely to occur at the beginning of a word than weak
syllables. Finally, the results can be improved again by storing lexical
level representations. All these algorithms -except the first one- are
implemented with non-connectionist statistical procedures. All are bottom-up
procedures and yield results that provide children with the means to learn
how to segment their language. Later on, children will improve their
performance by using syntactic, semantic, and pragmatic, top-down
information.

7) Learning word-to-meaning mappings (Siskind)
This paper attempts to offer a solution to the word-to-meaning mapping
problem. The goal of the algorithm presented here is to find the
correspondences between all the elements of a string of words and a
representation of the world semantics -for example which elements in "quux
bleen plugh baz" correspond to which elements in "RUN(Bill, TO(Mary))". This
problem is made more complex in that initially none of the correspondences
are known, that more than one semantic structure may correspond to the same
string of words, and that the action referred to may not correspond to the
present action -for example in most past, or future tense utterances, or
conditionals. A final complication comes from homonymy- one word can have
more than one correspondence. First, a categorical algorithm for
non-ambiguous and non-noisy learning is described and, second, a statistical
algorithm for noisy and ambiguous learning. These algorithms are tested
using a synthetic corpus, which allows for varying percentages of
uncertainty, noise and homonymy. Performances are good until noise levels of
70% are reached. Results for large homonymy rates are also good.
Note: There is an error in Table 7.3, which makes it useless. The appendix
which should have contained the description of the pruning strategy is
missing.

8) Children's overregularization and its implications for cognition (Marcus)
The author presents a "rule and memory" model for the formation of the past
tense of verbs. The argument is that pure connectionist models of the
formation of verb morphology are unable to simulate the production of past
tense by children correctly. When these models manage to do it, they do so
by resorting to features that implement -in some way or another- the
existence of a default rule. Markus then presents an argumentation about the
existence of rule processes in cognitive domains other than language. He
concludes with the proposal that the cognitive system is better explained
with two separate basic mental mechanisms, rule and associative memory,
instead of one single one -rule or associative memory.

9) A recurrent network with short-term memory capacity learning the
German -s plural (Goebel & Indefrey)
In this paper, a connectionist implementation of the formation of the German
plural is presented. The authors compare the results of the connectionist
model and those of human native speakers at length and show that the -s
ending is not the only default plural in German. In many cases, model and
humans produce a -e ending for masculine words and a -n ending for feminine
words. The authors argue then that the -s ending plural of German cannot
correspond to a language universal because default plurals in other
languages do not correspond to the same features. They propose that the -s
ending appears when several phonetic cues are present, and that otherwise
children learn to produce it when certain semantic cues are present -as is
true in many other languages. However, models that implement only
phonological rules cannot use semantic cues and cannot answer this question,
which leaves the issue open and to be tested further using more complex
models.

10) Single- and dual-route models of inflectional morphology (Nakisa,
Plunkett & Hahn)
This paper presents computer implementations of single- and dual-route
models of inflectional morphology. These models are then tested with three
languages: Arabic, German, and English. The results demonstrate that
single-route models always perform better than dual-route models. This
surprising result comes from the fact that dual-route models have an
unfortunate weakness, which is that they have to decide whether a word is
regular or not before being able to apply the default rule. This leads to
results worse than directly producing a derivation with a single-route
model. All which shows how fundamental implementing models and putting them
to the test is, because this leads to discover hidden weaknesses overlooked
when devising theoretical structure.

Part 3:
11) Formal models for learning in the principles and parameters framework
(Nyogi & Berwick)
This paper gives a formal presentation of an algorithm for learning the
parameters of the principle and parameters framework. The authors present a
Markov model formulation of the Triggering Learning Algorithm of Gibson and
Wexler (1994). This gives them a tool for a better analysis of the
characteristics of the learning algorithm (e.g. convergence or number of
exemplars needed to attain the result with high confidence) and allows to
propose then better variants of the algorithm.

12) An output-as-input hypothesis in language acquisition (Elbers)
This paper presents a theoretical model of language acquisition where the
child's own output serves as primary material for the building of her
linguistic knowledge. Five main arguments are presented which justify the
construction of such a model. These arguments are mainly based on Levelt's
(1989) model of speech production. A model is then described which contains
three phases: a) Intake and production of incompletely analyzed fragments;
b) Analysis of own production and hypothesis formation; c) Testing of
hypothesis against adult input. Examples from case studies are presented to
give some evidence of the validity of the model. No specific implementation
of the model is yet proposed.


DISCUSSION OF THE BOOK
The book is of very good quality, having been written by specialists of the
field that all describe research which has been ongoing for a long time. For
this reason, most of the information in the book could have been found
elsewhere, but the authors were able to take advantage of their mastery of
the subject to produce clear and thought-provoking presentations. Thus the
book is perhaps best for PhD students that want to find a far-ranging
presentation of computational linguistics applied to language acquisition.
The book will also be a good read for people already working in the field of
language acquisition or computer linguistics who want to discover how these
two domains can profit from one another. Implementations are not limited to
connectionist models, which attests the wide coverage of the book.
The only reservation I could make is that I find the introduction a bit
insufficient and that chapter 7 has its annex missing. Moreover, the
technical complexity of the papers is not homogeneous an might surprise some
readers. However, it remains nonetheless a more than worth reading, offering
a comprehensive and valuable idea of this field of research.


REFERENCES
Cutler, A., & Norris, D. (1988). The role of strong syllables in
segmentation for lexical access. Journal of Experimental Psychology: Human
Perception and Performance, 14, 113-121.

Gibson, T., & Wexler, K. (1994). Triggers. Linguistic Inquiry, 25, 407-454.
Levelt, W.J.M. (1989). Speaking: From intention to articulation. Camdridge,
MA: MIT Press.


BIOGRAPHICAL SKETCH
My main research interests are in language development. my main work is on
the initial development of syntax (children aged one to four). The tools I
use include computer simulation as well as psycholinguistic experiment. I
work with children with language disorders as well as normally-developing
children.


 
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