Two experiments were carried out to extend Logan et al.'s recent study [J. S. Logan, S. E. Lively, and D. B. Pisoni, J. Acoust. Soc. Am. 89, 874-886 (1991)] on training Japanese listeners to identify English /r/ and /l/. Subjects in experiment 1 were trained in an identification task with multiple talkers who produced English words containing the /r/-/l/ contrast in initial singleton, initial consonant clusters, and intervocalic positions. Moderate, but significant, increases in accuracy and decreases in response latency were observed between pretest and posttest and during training sessions. Subjects also generalized to new words produced by a familiar talker and novel words produced by an unfamiliar talker. In experiment 2, a new group of subjects was trained with tokens from a single talker who produced words containing the /r/-/l/ contrast in five phonetic environments. Although subjects improved during training and showed increases in pretest-posttest performance, they failed to generalize to tokens produced by a new talker. The results of the present experiments suggest that variability plays an important role in perceptual learning and robust category formation. During training, listeners develop talker-specific, context-dependent representations for new phonetic categories by selectively shifting attention toward the contrastive dimensions of the non-native phonetic categories. Phonotactic constraints in the native language, similarity of the new contrast to distinctions in the native language, and the distinctiveness of contrastive cues all appear to mediate category acquisition.

Native speakers of Japanese learning English generally have difficulty differentiating the phonemes /r/ and /l/, even after years of experience with English. Previous research that attempted to train Japanese listeners to distinguish this contrast using synthetic stimuli reported little success, especially when transfer to natural tokens containing /r/ and /l/ was tested. In the present study, a different training procedure that emphasized variability among stimulus tokens was used. Japanese subjects were trained in a minimal pair identification paradigm using multiple natural exemplars contrasting /r/ and /l/ from a variety of phonetic environments as stimuli. A pretest-posttest design containing natural tokens was used to assess the effects of training. Results from six subjects showed that the new procedure was more robust than earlier training techniques. Small but reliable differences in performance were obtained between pretest and posttest scores. The results demonstrate the importance of stimulus variability and task-related factors in training nonnative speakers to perceive novel phonetic contrasts that are not distinctive in their native language.

The susceptibility of xenografts to hyperacute rejection is postulated to reflect in part failure of complement regulatory proteins (CRPs) to control activation of heterologous complement on graft endothelium. To test this concept, transgenic swine expressing the human CRP decay accelerating factor and CD59 were developed using a novel expression system involving transfer of the proteins from erythrocytes to endothelial cells. Hearts from transgenic swine transplanted into baboons had markedly less vascular injury and functioned for prolonged periods compared to hearts from nontransgenic swine. These results indicate that expression of human CRPs in xenogeneic organs may contribute to successful xenografting and suggest that intercellular protein transfer might be a useful approach for expression of heterologous proteins in endothelial cells.

In a recent study, Martin, Mullennix, Pisoni, and Summers (1989) reported that subjects' accuracy in recalling lists of spoken words was better for words in early list positions when the words were spoken by a single talker than when they were spoken by multiple talkers. The present study was conducted to examine the nature of these effects in further detail. Accuracy of serial-ordered recall was examined for lists of words spoken by either a single talker or by multiple talkers. Half the lists contained easily recognizable words, and half contained more difficult words, according to a combined metric of word frequency, lexical neighborhood density, and neighborhood frequency. Rate of presentation was manipulated to assess the effects of both variables on rehearsal and perceptual encoding. A strong interaction was obtained between talker variability and rate of presentation. Recall of multiple-talker lists was affected much more than single-talker lists by changes in presentation rate. At slow presentation rates, words in early serial positions produced by multiple talkers were actually recalled more accurately than words produced by a single talker. No interaction was observed for word confusability and rate of presentation. The data provide support for the proposal that talker variability affects the accuracy of recall of spoken words not only by increasing the processing demands for early perceptual encoding of the words, but also by affecting the efficiency of the rehearsal process itself.The perception of spoken language is a skill that requires the listener to extract stable linguistic content from a physical signal that is notoriously unstable. The acoustic realization of speech is simultaneously modulated by numerous variable phonetic, prosodic, and semantic characteristics inherent to the particular message. The speech signal is further modulated by variable source characteristics. Indeed, it has frequently been suggested that if the perceptual system is required to extract canonical units of meaning from spoken language, various idiosyncracies related to different talkers must be somehow "normalized" during an early stage of processing the sensory input (e.g., Joos, 1948). Such talker-specific sources of variability include differing individual dialects, vocal tract sizes and shapes, and speaking rates (Mullennix, Pisoni, & Martin 1988;Verbrugge, Strange, Shankweiler, & Edman, 1976).Sources of variability in speech, whether from phonetic context, spoken stress, or talker variations, have typically been considered "perceptual problems" to be solved by listeners, just as they must be solved for the design of adequate speech-recognition systems (e.g., Gerstman, 1968;. Although the communicative importance of talker-specific information has long been recognized (e.g., Ladefoged & Broadbent, 1957), only recently have questions regarding the encoding of talker-specific information been considered alongside questions regarding the perception of the phonetic content of speech. By assessing the effects ...

A continuing controversy concerns whether speech perception can be best explained by singlefactor psychoacoustic models, single-factor specialized linguistic models, or dual-factor models including both phonetic and psychoacoustic processes. However, our recent cross-language speech perception research has provided data suggesting that a three-factor model, including auditory, phonetic, and phonemic processing, may be necessary to accommodate existing findings. In the present article, we report the findings from three experiments designed to determine whether three separate processing factors are used in speech perception. In these experiments, English and Hindi subjects were tested in a same-different (AX) discrimination procedure. The duration of the interstimulus interval, the number of trials, and the experimental context were manipulated when testing the English-speaking subjects. The combined results from the three experiments provide support for the existence of three distinct speech-perception factors.