TR AVAILABLE

[Virginia Marchman]

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	      CENTER FOR RESEARCH IN LANGUAGE
            UNIVERSITY OF CALIFORNIA, SAN DIEGO
		Technical Report #9201
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       LANGUAGE LEARNING IN CHILDREN AND NEURAL NETWORKS:
  	  PLASTICITY, CAPACITY AND THE CRITICAL PERIOD

	  	   Virginia A. Marchman
  Department of Psychology, University of Wisconsin, Madison

			ABSTRACT
This paper investigates constraints on dissociation and plasticity using
connectionist models of the acquisition of an artificial language
analogous to the English past tense.  Several networks were "lesioned"
in varying amounts both prior to and after the onset of training.  In
Study I, the network was trained on mappings similar to English regular
verbs (e.g., walk ==> walked).  Long term effects of injury were not
observed in this simple homogeneous task, yet trajectories of development
were dampened in relation to degree of damage prior to training, and
post-natal lesions resulted in substantive short term performance
deficits.  In Study II, the vocabulary was comprised of regular, as well
as irregular verbs (e.g., go ==> went).  In intact nets, the acquisition
of the regulars was considerably slowed, and performance was increasingly
susceptible to injury, both acutely and in terms of eventual recovery,
as a function of size and time of lesion.  In contrast, irregulars were
learned quickly and were relatively impervious to the effects of injury.
Generalization to novel forms indicates that these behavioral
dissociations result from the competition between the two classes of
forms within a single mechanism system, rather than a selective disruption 
of the mechanism guiding the learning of regular forms.  Two general
implications for research on language development and breakdown are
discussed: (1) critical period effects may derive from prior learning
history in interaction with the language to be learned ("entrenchment"),
rather than endogenously determined maturational change, and (2) selective
dissociations in behavior CAN result from general damage in systems that
are *not* modularized in terms of rule based vs. associative mechanisms
(cf. Pinker, 1991).

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