Does perceptual learning in speech reflect changes in phonetic category representation or decision bias?

Recent studies show that perceptual boundaries between phonetic categories are changeable with training (Norris, McQueen, & Cutler, 2003). For example, Kraljic and Samuel (2005) exposed listeners in a lexical decision task to ambiguous /s-∫/ sounds in either s-word contexts (e.g., legacy) or ∫-word contexts (e.g., parachute). In a subsequent /S/-/∫/ categorization test, listeners in the /s/ condition categorized more tokens as /s/ than did those in the /∫/ condition. The effect-termed perceptual learning in speech-is assumed to reflect a change in phonetic category representation. However, the result could be due to a decision bias resulting from the training task. In Experiment 1, we replicated the basic Kraljic and Samuel (2005) experiment and added an AXB discrimination test. In Experiment 2, we used a task that is less likely to induce a decision bias. Results of both experiments and signal detection analyses point to a true change in phonetic representation.