| Different speakers may pronounce the same sounds very differently (with native or non-native accents, or atypical variants like lisps); and yet listeners have little difficulty perceiving speech accurately, especially after a bit of experience with a particular speaker. Recent research on perceptual learning suggests that listeners adjust their preexisting phonetic categories to accommodate speakers' pronunciations of those phonemes (Norris, McQueen & Cutler, 2003). But the underlying processes that enable such learning, and the implications for linguistic representation, are poorly understood.; In a series of experiments, I used variations on the perceptual learning paradigm to examine these issues. Listeners were exposed to a particular person's speech and later tested on their perception of critical aspects of that speech. The first three experiments investigated the nature of perceptual learning: Is learning specific to the speakers and phonemes heard during exposure? Does it result in temporary adaptations or enduring representational changes? Can listeners adjust to several speakers simultaneously? Taken together, Experiments 1-3 suggest that perceptual experience leads to very different learning for different types of phonemes. For phonemes that vary primarily on a spectral dimension, perceptual learning is very robust (shifts of approximately 15%), long-lasting (the shifts are just as large 25 minutes later, even with intervening speech input), and speaker-specific. For such phonemes, the perceptual system is able to maintain several different representations simultaneously. In contrast, for sounds that vary on a timing dimension, perceptual learning is less robust (shifts of 4-5%), generalizes to new speakers and new phonemes, and results in representations that need to be re-adjusted whenever a new pronunciation is encountered.; In Experiments 4 and 5, I extended the study of perceptual learning to dialectal variations in pronunciation, and to production. The results suggest that the perceptual system handles dialectal variation quite differently than idiosyncratic variation: dialectal variation does not result in perceptual learning. Further, while listeners' productions can change to reflect speech they have just heard, their productions do not appear to change to reflect speech they have previously adjusted to (perceptually). This suggests that phonemic representations may not be shared by the perceptual and production systems. |
