Age of acquisition effects in adult lexical processing reflect loss of plasticity in maturing systems: Insights from connectionist networks

Early learned words are recognized and produced faster than later learned w ords. The authors showed that such age of acquisition effects are a natural property of connectionist models trained by backpropagation when patterns are introduced at different points into training and learning of early and late patterns is cumulative and interleaved. Analysis of hidden unit activa tions indicated that the age of acquisition effect reflects a gradual reduc tion in network plasticity and a consequent failure to differentiate late i tems as effectively as early ones. Further simulations examined the effects of vocabulary size, learning rate, sparseness of coding, use of a modified learning algorithm, loss of early items, acquisition of very late items, a nd lesioning the network. The relationship between age of acquisition and w ord frequency was explored, including analyses of how the relative influenc e of these factors is modulated by introducing weight decay.