Talker information is not normalized in fluent speech: Evidence from on-line processing of spoken words

Talker information is not normalized in fluent speech: Evidence from on-line processing of spoken words Sarah C. Creel (creel@cogsci.ucsd.edu) University of California, San Diego, Department of Cognitive Science La Jolla, CA 92093-0515 Melanie A. Tumlin (mtumlin@cogsci.ucsd.edu) University of California, San Diego, Department of Cognitive Science La Jolla, CA 92093-0515 Abstract one assumes that humans are intrinsically equipped to detect particular speech sounds, listeners still must discern which subset of those speech sounds is relevant to word identity in her or his own language. Consistent with this, infants are more sensitive than adults to phonemically-irrelevant variability, including phoneme categories outside their own language (Werker & Tees, 1984), and acoustic variability among talkers (Houston & Jusczyk, 2000). Over development, the learner discovers that his or her spoken language input has certain dimensions (such as F0 variation, phoneme variation, rate variation), and further, that certain dimensions (phoneme variation, but not rate variation) are linked to changes in word meaning. Gradually, then, the learner would reweight attention toward phonemic variation and away from other types of variation. On this account, adult native speakers attend primarily to phonemic information, even in learning new words in their native language, as long as they are able to account for nonphonemic variation coming from another source (properties of the talker). Recent work demonstrates that talker characteristics can be used as predictive cues for spoken word recognition. However, abstractionist accounts suggest that talker information is usually stripped away or “normalized” based on preceding speech material. A contrasting account is that listeners never normalize, instead storing detailed, acoustically-varied instances of words. These varied instances then facilitate recognition of words in a vast variety of voices and accents. We present data suggesting that such “irrelevant” acoustic characteristics of word forms (talker-varying acoustic attributes) are not normalized, but are instead encoded, when learned in fluent speech context. Experiment 1 replicates recent demonstrations of talker specificity in word recognition. Using the same set of words in carrier sentences, Experiment 2 finds that learners still encode talker information even though in principle they could easily normalize away talker-based acoustic variability. Keywords: talker variability; spoken word recognition; eye tracking; normalization; exemplar theory Introduction This approach contrasts with recent findings that talker information can be used to disambiguate words prior to their phonemic divergence point. Creel, Aslin, and Tanenhaus (2008) examined the time course of talker effects on word recognition by presenting listeners with known or novel words, where each word was perfectly correlated with a single talker. Creel et al. found that listeners showed less competition between different-talker pairs (male maid, female maze) than between same-talker pairs (male maid, male maze). More specifically, in an eye tracking paradigm, listeners hearing “maid” in a male voice showed fewer looks to the maze when it had consistently had a different talker than when it had the same talker as “maid. ” These data imply that adult listeners do not ignore talker information in learning and recognizing words. One central question in spoken language processing has been how we recognize nonidentical or novel instances of known linguistic categories. The problem faced by a listener is that word forms exhibit a large amount of acoustic variability, but only a subset of that acoustic variability actually discriminates one word from another. The remaining variability in the speech signal has typically been regarded as useless noise. On this account, the presumed goal of the human language processing system is to remove irrelevant variation, leaving only that variability which cues word identity. Problematically, there is a different set of parameters to filter out depending on the utterance. For instance, fundamental frequency (F0) and accent characteristics can vary from talker to talker, and rate of speech can vary even within a talker (Van Lancker, Kreiman, & Emmorey, 1985; Van Lancker, Kreiman, & Wickens, 1985). Nonetheless, adult native speakers of a language are good at recognizing novel instances of a familiar word, and seem robust to acoustic variability among talkers. Note here that talker information is not necessarily a conscious percept that there is a particular talker. By talker information, we refer to whatever set of acoustic attributes vary between talkers (such as F0, pitch variability, or speech rate, among other things). Of course, there are other types of acoustic variation that occur in speech as well, such as dialectal (accent) or idiolectal differences. In principle, listeners should be able to take advantage of any sort of Much research has been devoted to how listeners strain out or normalize over variability in the speech signal. Even if