EFFECTS OF TOKEN VARIABILITY ON OUR ABILITY TO DISTINGUISH BETWEEN VOWELS

Even when the speaker, context, and speaking style are held fixed, the physical properties of naturally spoken utterances of the same speech sound vary considerably. This variability imposes limits on our abili ty to distinguish between different speech sounds. We present a concep tual framework for relating the ability to distinguish between speech sounds in single-token experiments (in which each speech sound is repr esented by a single wave form) to resolution in multiple-token experim ents. Experimental results indicate that this ability is substantially reduced by an increase in the number of tokens from 1 to 4, but that there is little further reduction when the number of tokens increases to 16. Furthermore, although there is little relation between the abil ity to distinguish between a given pair of tokens in the multiple-and the 1-token experiments, there is a modest correlation between the abi lity to distinguish specific vowel tokens in the 4- and 16-token exper iments. These results suggest that while listeners use a multiplicity of cues to distinguish between single tokens of a pair of vowel sounds , so that performance is highly variable both across tokens and listen ers, they use a smaller set when distinguishing between populations of naturally produced vowel tokens, so that variability is reduced. The effectiveness of the cues used in the latter case is limited more by i nternal noise than by the variability of the cues themselves.