We model word substitution errors made by normal and phasic speakers with an interactive activation model of lexicalization. This comprises a three-layer architecture of semantic, lexical, and phonological units. W e test four hypotheses about the origin of aphasic word substitutions: that they result from pathological decay, loss of within-level inhibitory connections, increased initial random noise, or reduced flow of activation from the semantic to the lexical level. W e conclude that a version of the flnal hypothesis best explains the aphasic data, but with random fluctuations in connection strength rather than a uniform decrement This model accounts for aspects of recovery in aphasia, and frequency and imageability effects in paraphasias. Pathological lexical access is related to transient lexical access difficulties in normal speakers to provide an account of normal word substitution errors. W e argue that similar constraints operate in each case. This model predicts imageability and frequency effects which are verified by analysis of our normal speech error data.

We study speech production difficulties in speakers with dementing illnesses by inducing tip-of-the-tongue (TOT) states. W e found that dementing speakers experienced TOTs but were unable to supply any information about the target, unlike an age-matched control group. W e distinguish between items generated by the subjects as relatives of the targets, subjects' own target words, and what we call "constructive search" words that subjects use in their search for the target. When related words came to mind, they were almost all semantic relatives of the target, whereas in nondementing adults, phonological relatives are also reported. W e interpret the results in terms of a three level interactive account of lexicalization. W e propose that the retrieval deficit in dementia occurs in the first stage of lexicalization, that of retrieving abstract lexical forms from a semantic specification, rather than in a second stage of retrieving phonological forms.