Perceptual adaptation to a novel accent: Phonetic category expansion or category shift?

Listeners can rapidly adapt to an unfamiliar accent. For example, following exposure to a speaker whose /f/ sound is ambiguous between [s] and [f], they categorize more sounds along an [s]-[f] phonetic continuum as /f/. We investigated the adaptation mechanism underlying such perceptual changes-do listeners shift the target sound in phonetic space (category shift), or do they adopt a more general mechanism of broadening the category (category expansion)? In experiment 1, we trained listeners on an accent containing ambiguous /θ/ = [θ/s] and then tested them on categorizing phonetic continua spanning [θ]-[s] or [θ]-[f]. Listeners tested on the [θ]-[s] continua showed a significant increase in proportion of /θ/ responses vs controls, while those tested on [θ]-[f] did not. Experiment 2 investigated how acoustic-phonetic similarity may modulate the mechanism underlying recalibration. Listeners were trained on the same /θ/ = [θ/s] accent as in experiment 1 but were tested on a different continuum, [θ]-[ʃ]. This time, trained listeners showed a significant increase in proportion of /θ/ responses with the novel phonetic contrast. This suggests that phonetic recalibration involves some degree of non-uniform category expansion, constrained by phonetic similarity between training and test sounds.