The central result of this simulation is that a simple connectionist network when simultaneously trained on adjective-like and noun-like categories learns the nouns faster, just as children do. Yet this difference is not due to any built-in preferences on the part of the network nor to any pre-training representation of a difference between nouns and adjectives. It is due entirely to the similarity structure inherent in the learning task --- that is, to the nature of the categories which the network learns and the linguistic input which specifies which of several classes of overlapping categories is the relevant one. In brief, a learner can show a marked advantage for the learning of one kind of category over another without any built-in distinction between them. The developmental precedence of nouns over adjectives in children thus need not derive from a priori conceptual distinctions, as commonly assumed, but rather from quite general similarity-based learning mechanisms.
During the course of learning, the network, like young children, also exhibits a structured pattern of errors --- dimensional terms are confused with each other and not with nouns. This distinction emerges as a consequence of simultaneously learning not a single adjective class but several different adjective categories. The most likely possibility is that this is accomplished by the rapid learning of noun categories. That is, what the network ``really knows'' may essentially be that adjectives are ``not nouns.'' The implication is that this may be all that young children know too (see Smith, 1995 for a similar suggestion based on empirical evidence from children).
The network did not show strong learning of the connection between pairs of terms on a single dimension. This is also consistent with the evidence from children. With the exception of color terms, between-dimension rather than within-dimension confusions characterize children's initial errors [BS93,Car94,SS92].
This experiment thus demonstrates the viability of a similarity-based approach to the noun advantage in children's early lexical acquisitions. In the following experiments, we examine the specific contributions of the volume and shape of category extensions, overlap and word-word associations in creating the noun advantage by examining unnaturally structured classes of categories that differ only in their volume, shape, overlap, or associations between linguistic context inputs and outputs.