Perceptual adaptation to an artificial accent: Phonetic category shift or expansion?

Abstract

Listeners can rapidly adapt to a novel 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/. Wetested the adaptation mechanism underlying such changes in category structure — do listeners adjust their phonetic boundary for the target sound (category shift), or simply relax their categorization criteria (category expansion)? We trained listeners on a pronunciation containing ambiguous /θ/ [θ / s] and then tested them on categorizing either [θ]-[s] or [θ]-[f] phonetic continua. If listeners simply shift their /θ/ boundary, we predicted an increase in /θ/ responses for the [θ]-[s] continuum, but not [θ]-[f]. By contrast, if listeners expand the category, we predicted a categorization change in both continua, indicating a general broadening of the /θ/ category in phonetic space. Consistent with the boundary shift hypothesis, we found that listeners tested on the [θ]-[s] continua showed a significant increase in proportion of /θ/ responses versus controls [χ2(1) = 4.80, p < 0.05), while those tested on [θ]-[f] did not [χ2(1) = 1.30, p = 0.25). [Work supported by an NSF GRFP Fellowship to YVM.]