Constraints upon word substitution speech errors

We explore the features of a corpus of naturally occurring word substitutio n speech errors. Words are replaced by more imageable competitors in semant ic substitution errors but not in phonological substitution errors. Frequen cy effects in these errors are complex and the details prove difficult for any model of speech production. We argue that word frequency mainly affects phonological errors. Both semantic and phonological substitutions are cons trained by phonological and syntactic similarity between the target and int rusion. We distinguish between associative and shared-feature semantic subs titutions, Associative errors originate from outside the lexicon, while sha red-feature errors arise within the lexicon and occur when particular prope rties of the targets make them less accessible than the intrusion. Semantic errors arise early while accessing lemmas from a semantic-conceptual input , while phonological errors arise late when accessing phonological forms fr om lemmas. Semantic errors are primarily sensitive to the properties of the semantic field involved, whereas phonological errors are sensitive to phon ological properties of the targets and intrusions.