Cracking the Dual Code: Toward a Unitary Model of Phoneme Identification.

The results of five experiments on the nature of the speech code and on the role of sentence context on speech processing are reported. The first three studies test predictions from the dual code model of phoneme identification (Foss, D. J., & Blank, M. A. , 1980, , 1-31). According to that model, subjects in a phoneme monitoring experiment respond to a prelexical code when engaged in a relatively easy task, and to a postlexical code when the task is difficult. The experiments controlled ease of processing either by giving subjects multiple targets for which to monitor or by preceding the target with a similar-sounding phoneme that draws false alarms. The predictions from the model were not sustained. Furthermore, evidence for a paradoxical nonword superiority effect was observed. In Experiment IV reaction times (RTs) to all possible /d/-initial CVCs were gathered. RTs were unaffected by the target item's status as a word or nonword. but they were affected by the internal phonetic structure of the target-bearing item. Vowel duration correlated highly (0.627) with RTs. Experiment V examined previous work purporting to demonstrate that semantic predictability affects how the speech code is processed, in particular that semantic predictability leads to responses based upon a postlexical code. That study found "predictability" effects when words occurred in isolation; further, it found that vowel duration and other phonetic factors can account parsimoniously for the existing results. These factors also account for the apparent nonword superiority effects observed earlier. Implications of the present work for theoretical models that stress the interaction between semantic context and speech processing are discussed, as are implications for use of the phoneme monitoring task.