Lexical Tone Perception in Chinese Mandarin Anna Björklund
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Lexical Tone Perception in Chinese Mandarin Anna Björklund University of Florida 1 ABSTRACT Perceptual asymmetry, where Stimulus A is more often confused for B than B is for A or vice versa, has been observed in multiple lexical contexts, such as vowels (Polka & Bohn, 2003) and consonants (Dar, Mariam & Keren-Portnoy, 2018). Because historically perceptual space was assumed to be Euclidean, perception of stimuli was in turn assumed to be symmetrical, and observations of asymmetry were explained as simply response bias (Polka & Bohn, 2003). However, further examination of such biases has suggested that they are a much more fundamental occurrence. This study examines the effects of memory load and native language on perceptual bias in lexical tone perception of Mandarin Chinese for native speakers of Mandarin and English. Native speakers of both languages were given an AX categorical discrimination task with each combination of the four lexical Mandarin tones. To examine the effects of training on this bias, they were then trained on one of two tones (1 and 4) and given the same sequence of discrimination tasks again. Each pre- and post-training discrimination task featured both 250 ms and 1000 ms ISI intervals. 2 0. INTRODUCTION TO PERCEPTUAL ASYMMETRY It is often assumed that perception is symmetrical: that is, when presented with two stimuli, A and B, it is equally as easy to discriminate going from stimuli A to B as going from B to A. Although there had long been data that conflicted with such a model, the aberrant data was explained as response bias (Polka and Bohn, 2003), and subsequently ignored. It was not until the mid-1970s, when research began challenging the assumption of perception as ‘Euclidean’ (Polka and Bohn, 2003)—that is, straightforwardly one-to-one— that new models of perception were considered and previously anomalous data re-contextualized within this new framework. Whereas the ‘Euclidean’ model assumed perception to be symmetrical, this new model of perception posited that perception was asymmetrical. In this paradigm, given stimuli A and B, it is not assumed that distinguishing stimulus A from B is equally as easy as distinguishing B from A. That is, participants may more accurately categorize stimuli when A is the target stimulus, rather than when B is. Although this paper is concerned with asymmetrical perception in the auditory domain (specifically, within speech perception), perceptual asymmetry has been reported across many domains, with the best documented being perhaps the visual one. Biases in the visual system have been noted as far back as the 1960s, when Campbell and Kulikowski observed that the visual system is better at horizontal and vertical discrimination than it is at oblique discrimination—that is, objects are more easily discriminable at vertical and horizontal angles than they are at angles, of, for example, 45 degrees (Campell and Kulikowski, 1966). Likewise, perceptual asymmetries have been noted for phenomena such as a preference for foveal (frontal) stimuli over peripheral stimuli (Karim and Kojima, 2010). Even within the strict domain of 3 horizontal/vertical visual perception, studies have shown that contrast sensitivity and spatial resolution are better along the horizontal midline (Carrasco, Talgar, and Cameron, 2001). Visual asymmetries have also been demonstrated in the perception of texture --it is easier to find a ‘broken’ circle in an array of closed circles versus discriminate a closed circle in an array of ‘broken’ circles (Williams and Julesz 1992)-- as well as in the recognition of faces. In a 1973 study by Gilbert and Bakan, multiple volunteers’ faces were photographed, and then ‘reassembled’ into two new photographs, such that each new photo was comprised only of one half of the original face (a ‘left’ face would, for instance, be comprised of the normal left half of the original photograph and a reversed left half to serve as the ‘right’ side of the face). When participants were asked to compare these new photos to the original photo and rank which one best resembled the original, participants overwhelmingly chose the photo made up of the right half of the volunteers’ faces (Gilbert and Bakan, 1973). The robustness of the literature in regards to visual asymmetrical perception also provides important speculation as to the reason for why such asymmetry exists in the first place. Notably, the above literature primarily cites biology as the driving force. For example, foveal vision is believed to be favored over peripheral vision due to the lower presence of cones in the periphery of the cornea (Karim and Kojima, 2010). Perhaps even more helpful to the present project is Williams and Julesz (1992)’s discussion of the asymmetrical perception of visual texture, in which they state that their “results suggest that visual processing of textures is not limited to simply the collective responses of isolated elements. Instead, it appears that an active visual process encodes the stimulus without necessarily keeping the individual elements intact. The completion of local boundaries into global structures (subjective contours) is fundamental to the process of texture perception” (emphasis mine, pp. 6533). That is, rather than encoding every feature of every stimulus, the visual system ‘sorts’ stimuli into broader super-perceptual categories. 4 1. PROTOYPE THEORY AND THE PERCEPTUAL MAGNET EFFECT Before prototype theory was introduced in the 1970s, the traditional definition of a category was essentially an item that matched a certain set of definitions. An item was either a member of a category or it was not, with little room for gradation (Rosch, 1973). However, beginning in the 1970s, Eleanor Rosch and other researchers began to suggest a new categorization paradigmː that of prototypicality. In this paradigm, there is not necessarily a strict in-/out-of-category binary. Rather, “categories [are] typically composed of distributions of attributes, and some instances of categories [are] designed to be more ‘typical’ members of the category than others” (Rosch, 1973, pp. 329). In such a paradigm, category members are not defined as merely having a certain list of features. Rather, they may individually possess those features to a greater or lesser degree. Therefore, some category members may be considered more “prototypical” than others, even though they all are de facto members of the category. One of the most well-known theories of perceptual categorization and application of prototype theory is the perceptual magnet effect, introduced by Patricia Kuhl in a 2001 study. The study consisted of four experiments. In Experiment 1, Kuhl asked adult English-speaking participants to “rank” a series of vowels from “best” to “worst” in a given category. She discovered that these rankings were not random, but rather that the “best” example of a category was consistently accorded to a specific area of the vowel space. Experiment 2 used the results of this survey to create an AX study where participants were presented with a once-per-second stream of sounds that alternated randomly between a “referent” speech sound and a “comparison” speech sound. Participants were then asked to press a button when they heard a change in the “referent” sound—that is, when the sound had changed to the other, “comparison” 5 sound. The study found that participants were more willing to overgeneralize (and thus give inaccurate button-pushes) when the “prototype” sounds from Experiment 1 were used as the “referent” sound. Experiment 3 replicated the conditions of Experiment 2, except with infants conditioned for the head-turn procedure. These infants were presented with the same “prototype” and “non-prototype” vowels as the adults, and their results were highly correlated with the adults’ as well. In light of this data, Kuhl 2001 proposes the presence of a “perceptual magnet effect”: not only are certain stimuli within a category considered “better” or “more prototypical” than others, but that these prototypical stimuli have a tendency to “assimilate” the other, less- prototypical stimuli. As Kuhl 2001 puts itː “Surrounding members of the category are perceptually assimilated to it to a greater degree than would be expected on the basis of real psychophysical distance. Relative to a non-prototype of the category, the distance between the prototype and surrounding members is effectively decreased; in other words, the perceptual space appears to be ‘warped,’ effectively shrunk around the prototype” (pp. 99). In other words, prototypes hold greater perceptual weight than non-prototypes in a speaker’s mind. Thus, although two vowels may differ equally with respect to a given stimuli, they may not be perceived by the listener as being equally different if one of these vowels is a prototype. Listeners are more willing to over-generalize non-prototypes and categorize them as variants of the prototypical category than they are willing to sort prototypes into non- prototypical categories. Prototypes could then be said to exhibit a sort of masking effect on non-prototypes, making them harder to distinguish when compared to the prototype. This is a fundamental theory used in explaining perceptual asymmetry, which will be explored later in this paper. 6 Another fundamental observation noted in Kuhl 2001 is that prototypes do not merely exist within one person’s perception. Rather, they appear to have a measure of universality. The adults in Experiment 1 consistently and collectively grouped certain stimuli as “better” than others, and these “better,” more prototypical prototypes were then demonstrated to be easier to distinguish against non-prototypes than the reverse, across multiple adults and infants. 2. PERCEPTUAL ASSYMMETRY OF VOWELS The study of symmetrical vowel perception began ostensibly with Polka and Werker 1994’s study of infant vowel perception across native/non-native vowel contrasts. The study’s initial purpose was not related to asymmetrical perception at all, but rather examining if and when infants’ phonetic perception of vowels and consonants began to be affected by native language sound inventories.