While frequency and predictability are known to effect phonetic form in word production, there are few models of the mechanisms underlying this kind of variation. In addition, because most models of lexical production have focused on content words, little is known about the similarities and differences between content and function word production.
In a regression study of conversational speech, we show that content and function word pronunciations are affected differently by their frequency and predictability. Content words are shorter when more frequent, and shorter when repeated (verbs and modifiers as well as nouns), while function words are not so affected. Function words have shorter pronunciations, after controlling for frequency and predictability. While both content and function words are strongly affected by predictability from the word following them, only very frequent function words show sensitivity to predictability from the preceding word.
The results support the view that content and function words are accessed by different production mechanisms. We suggest a lexical-access based model of our results, in which frequency or repetition lead to shorter or longer word durations by causing faster or slower lexical access, mediated by a general mechanism that coordinates the pace of higher-level planning and the execution of the articulatory plan.