In this two-part study, we conducted both cross-sectional and longitudinal investigations on the relative weights of experiential, cognitive, and sociopsychological factors in adult L2 speech learning. In the cross-sectional phase (Study 1), speech was elicited from 73 Japanese speakers of English via a picture description task, and rated for accentedness and comprehensibility. These scores were linked to scores on a range of tests designed to measure aptitude, motivation, and anxiety. The results showed that comprehensibility was exclusively linked to experiential variables (e.g., the amount of L2 use outside classrooms), while accentedness was linked to phonemic coding ability and anxiety. In the longitudinal phase (Study 2), we tracked the same participants' L2 comprehensibility and accentedness development when they received four weeks of explicit pronunciation instruction. According to the results of pre- and post-tests, participants significantly improved the comprehensibility and accentedness of their speech regardless of cognitive and sociopsychological differences.

In this paper, I first provide a brief review of how scholars have conceptualized, tested, and elaborated aptitude frameworks relevant to second language (L2) speech learning. Subsequently, I introduce an emerging paradigm that assigns a fundamental role to domain-general auditory processing (i.e., having a good ear) in L1 speech acquisition and proposes that the same faculty acts as a cornerstone of L2 speech learning (i.e., the Auditory Precision Hypothesis-L2). This hypothesis predicts that learners with more precise auditory processing ability will be able to make the most of every input opportunity, which will result in more advanced L2 speech proficiency. To close, I will provide suggestions on how scholars can assess L2 students’ auditory processing ability (e.g., our team's offline test deposited at L2 Speech Tools for Researchers & Teachers [http://sla-speech-tools.com/]) and discuss how the results can be used to maximize learners’ L2 speech learning opportunities via optimal, profile-matched training programs (e.g., explicit vs. incidental training; naturalistic vs. classroom learning; phonetic vs. auditory training).

This article proposes a conceptual and measurement framework for postpubertal, L2 speech learning aptitude that is centered around domain-general auditory processing (i.e., representing spectral and temporal characteristics of sounds). To this end, we examine the construct and reliability of a battery of auditory processing tests by presenting the results of an empirical study wherein 100 participants took the tests twice. The findings showed (a) that the tasks tapped into multiple dimensions of auditory processing ability (e.g., perceptual acuity, audio-motor integration); and (b) that test-retest reliability ranged from “fair” to “excellent” (ICC (2,2) = .4–.8). Last, we introduce an offline version of the tests (deposited at L2 Speech Tools for Researchers & Teachers; http://sla-speech-tools.com/), provide a brief user manual, and invite researchers to use these tests to examine the role of auditory processing in various aspects of L2 speech learning (naturalistic vs. classroom, perception- vs. production-based practice).

The current study set out to examine to what degree age of acquisition (AOA), defined as a learner’s first intensive exposure to a second language (L2) environment, mediates the final state of postpubertal, spoken vocabulary attainment. In Study 1, spontaneous speech samples were elicited from experienced Japanese users of English (n = 41) using storytelling and interview tasks. The samples were analyzed using a range of corpus- and rater-based lexical measures and compared to the speech of inexperienced Japanese speakers (n = 40) and native speakers of English (n = 10). The results showed that most experienced L2 learners tended to demonstrate nativelike proficiency for relatively easy lexical dimensions of speech (i.e., richness), but that AOA appeared to play a key role in predicting the ultimate attainment of relatively difficult lexical dimensions (i.e., appropriateness). In Study 2, the findings were successfully replicated with experienced L1 Polish users of English (n = 50).

Extending the paradigm in L1 acquisition, scholars have begun to investigate whether participants’ domain-general ability to represent, encode, and integrate spectral and temporal dimensions of sounds (i.e., auditory processing) could be a potential determinant of the outcomes of post-pubertal L2 speech learning. The current study set out to test the hypothesis that auditory processing makes a unique contribution to L2 speech acquisition, for 70 Japanese classroom learners of English with different levels of L2 proficiency when biographical backgrounds (length of instruction and immersion) and memory abilities (working, declarative, and procedural memory) are controlled for. Auditory processing loaded onto modality-general capacities to represent and incorporate anchor stimuli (relative to target stimuli) into long-term memory in an implicit fashion, but dissociated from explicit abilities to remember, associate, and elaborate sensory information. Auditory processing explained a small-to-medium amount of variance in L2 speech learning, even after the other potentially confounding variables were statistically factored out.

Whereas many scholars have emphasized the relative importance of comprehensibility as an ecologically valid goal for L2 speech training, testing, and development, eliciting listeners’ judgments is time-consuming. Following calls for research on more efficient L2 speech rating methods in applied linguistics, and growing attention toward using machine learning on spontaneous unscripted speech in speech engineering, the current study examined the possibility of establishing quick and reliable automated comprehensibility assessments. Orchestrating a set of phonological (maximum posterior probabilities and gaps between L1 and L2 speech), prosodic (pitch and intensity variation), and temporal measures (articulation rate, pause frequency), the regression model significantly predicted how naïve listeners intuitively judged low, mid, high, and nativelike comprehensibility among 100 L1 and L2 speakers’ picture descriptions. The strength of the correlation (r = .823 for machine vs. human ratings) was comparable to naïve listeners’ interrater agreement (r = .760 for humans vs. humans). The findings were successfully replicated when the model was applied to a new dataset of 45 L1 and L2 speakers (r = .827) and tested under a more freely constructed interview task condition (r = .809).

Recently, scholars have begun to explore the hypothesis that individual differences in domain-general auditory perception, which has been identified as an anchor of L1 acquisition, could explain some variance in postpubertal L2 learners’ segmental and suprasegmental learning in immersive settings. The current study set out to examine the generalizability of the topic to the acquisition of higher-level linguistic production skills—that is the appropriate use of diverse, rich, and abstract vocabulary. The speech of 100 Polish-English bilinguals was elicited using an interview task, submitted to corpus-/rater-based linguistic analyses, and linked to their ability to discriminate sounds based on individual acoustic dimensions (pitch, duration, and amplitude). According to the results, those who attained more advanced L2 lexical proficiency demonstrated not only more relevant experience (extensive immersion and earlier age of arrival), but also more precise auditory perception ability.

Eighty Japanese learners of English as a foreign language encountered 40 target words in one of four experimental conditions (three encounters, six encounters, three encounters with talker variability, and six encounters with talker variability). A picture-naming test was conducted three times (pretest, immediate posttest, and delayed posttest) and elicited speech samples were scored in terms of form-meaning connection (spoken form recall) and word stress accuracy (stress placement accuracy and vowel duration ratio). Results suggested that frequency of exposure consistently promoted the recall of spoken forms, whereas talker variability was more closely related to the enhancement of word stress accuracy. These findings shed light on how input quantity (frequency) and quality (variability) affect different stages of lexical development and provide implications for vocabulary teaching.

Precise auditory perception at a subcortical level (neural representation and encoding of sound) has been suggested as a form of implicit L2 aptitude in naturalistic settings. Emerging evidence suggests that such implicit aptitude explains some variance in L2 speech perception and production among adult learners with different first language backgrounds and immersion experience. By examining 46 Chinese learners of English, the current study longitudinally investigated the extent to which explicit and implicit auditory processing ability could predict L2 segmental and prosody acquisition over a 5-month early immersion. According to the results, participants’ L2 gains were associated with more explicit and integrative auditory processing ability (remembering and reproducing music sequences), while the role of implicit, preconscious perception appeared to be negligible at the initial stage of postpubertal L2 speech learning.

In this study, we propose a hypothesis that domain-general auditory processing, a perceptual anchor of L1 acquisition, can serve as the foundation of successful post-pubertal L2 learning. This hypothesis was tested with 139 post-pubertal L2 immersion learners by linking individual differences in auditory discrimination across multiple acoustic dimensions to the segmental, prosodic, lexical, and morphosyntactic dimensions of L2 proficiency. Overall, auditory processing was a primary determinant of a range of participants’ proficiency scores, even after biographical factors (experience, age) were controlled for. The link between audition and proficiency was especially clear for L2 learners who had passed beyond the initial phase of immersion (length of residence > 1 year). The findings suggest that greater auditory processing skill benefits post-pubertal L2 learners immersed in naturalistic settings for a sufficient period of time by allowing them to better utilize received input, which results in greater language gains and leads to more advanced L2 proficiency in the long run (similar to L1 acquisition).

In this study, we propose a hypothesis that domain-general auditory processing, a perceptual–cognitive anchor of first language (L1) acquisition, can serve as an important deciding factor for successful postpubertal second language (L2) pronunciation learning. To examine this hypothesis, samples of spontaneous speech were elicited from a total of 30 L1 Chinese L2 English learners at two points (outset and endpoint) during an 8-month study-abroad period in the United Kingdom. The participants were tested on three different components of auditory processing ability (formant, pitch, and duration discrimination) using behavioral instruments. The auditory processing scores were then linked to the segmental, prosodic, and fluency dimensions of their L2 pronunciation proficiency development throughout the project. Overall, most learners’ speech became smoother, faster, and more fluent (fewer pauses, faster articulation rate, and more optimal perceived tempo). Certain learners with high-level auditory processing ability (more precise formant discrimination) appeared to further attain more correct pronunciation of individual sounds and words (greater segmental and word stress accuracy), leading to more advanced L2 phonological skills (fluent and accurate). The findings suggest that auditory processing abilities can be a root of language learning throughout the life span and may apply to the initial- to midphase of naturalistic L2 pronunciation learning in adulthood.

Following the trends established in psychology and emerging in L2 research, we explain our support for an Open Science approach in this paper (i.e., developing, analyzing and sharing datasets) as a way to answer controversial and complex questions in applied linguistics. We illustrate this with a focus on a frequently debated question, what underlies individual differences in the dynamic system of post-pubertal L2 speech learning? We provide a detailed description of our dataset which consists of spontaneous speech samples, elicited from 110 late L2 speakers in the UK with diverse linguistic, experiential and sociopsychological backgrounds, rated by ten L1 English listeners for comprehensibility and nativelikeness. We explain how we examined the source of individual differences by linking different levels of L2 speech performance to a range of learner-extrinsic and intrinsic variables related to first language backgrounds, age, experience, motivation, awareness, and attitudes using a series of factor and Bayesian mixed-effects ordinal regression analyses. We conclude with a range of suggestions for the fields of applied linguistics and SLA, including the use of Bayesian methods in analyzing multivariate, multifactorial data of this kind, and advocating for publicly available datasets. In keeping with recommendations for increasing openness of the field, we invite readers to rethink and redo our analyses and interpretations from multiple angles by making our dataset and coding publicly available as part of our 40th anniversary ARAL article.

For a character of the absolute Galois group of a complete discrete valuation field, we define a lifting of the refined Swan conductor, using higher dimensional class field theory.

The current study examines how second language (L2) users differentially assess the comprehensibility (i.e., ease of understanding) of foreign-accented speech according to a range of background variables, including first language (L1) profiles, L2 proficiency, age, experience, familiarity, and metacognition. A total of 110 L2 listeners first evaluated the global comprehensibility of 50 spontaneous speech samples produced by low-, mid-, and high-proficiency Japanese speakers of English. The listeners were categorized into two subgroups according to a cluster analysis of their rating scores: lenient and strict. Results showed that while the lenient listeners appeared to rely equally on many linguistic areas of speech during their judgments, the strict listeners were strongly attuned to phonological accuracy. Analysis of the background questionnaire data revealed that more lenient listeners likely had higher levels of awareness of the importance of comprehensibility for communication (metacognition); regularly used L2 English in professional settings (experience); and had L1s more linguistically close to the target speech samples, Japanese-accented English (L1-L2 distance).

Building on the extensive work conceptualizing, developing, and validating foreign language aptitude for successful second language acquisition (e.g., Skehan, 2015, in Applied Linguistics), the current project expounded the cognitive abilities relevant and instrumental to adult Japanese classroom learners’ pronunciation attainment of English /ɹ/. The speech production of 50 Japanese participants was elicited, acoustically analyzed, and linked to different aspects of their aptitude profiles (phonemic coding, associative memory, and sequence recognition). Whereas those with higher phonemic coding demonstrated better performance in a relatively easy dimension of English /ɹ/ pronunciation (lower F2 for tongue retraction), those with greater associative memory demonstrated more advanced performance in the relatively difficult dimensions of English /ɹ/ pronunciation (longer transition duration for phonemic length; lower F3 for labial/alveolar/pharyngeal constrictions). The role of incidental learning aptitude (i.e., sequence recognition) remained unclear. The findings here indicate that explicit aptitude related to phonological analysis and memory may play a key role in predicting the incidence of advanced second language segmental proficiency attainment in classroom settings.

The current study examines the role of cognitive and perceptual individual differences (i.e., aptitude) in second language (L2) pronunciation learning, when L2 learners’ varied experience background is controlled for. A total of 48 Chinese learners of English in the UK were assessed for their sensitivity to segmental and suprasegmental aspects of speech on explicit and implicit modes via behavioural (language/music aptitude tests) and neurophysiological (electroencephalography) measures. Subsequently, the participants’ aptitude profiles were compared to the segmental and suprasegmental dimensions of their L2 pronunciation proficiency analyzed through rater judgements and acoustic measurements. According to the results, the participants’ segmental attainment was associated not only with explicit aptitude (phonemic coding), but also with implicit aptitude (enhanced neural encoding of spectral peaks). Whereas the participants’ suprasegmental attainment was linked to explicit aptitude (rhythmic imagery) to some degree, it was primarily influenced by the quality and quantity of their most recent L2 learning experience.

The current study longitudinally examined the influence of aptitude on second language (L2) pronunciation development when 40 first-year Japanese university students engaged in practice activities inside and outside English-as-a-Foreign-Language classrooms over one academic year. Spontaneous speech samples were elicited at the beginning, middle, and end points of the project, analyzed for global, segmental, syllabic, prosodic, and temporal aspects of L2 pronunciation, and linked to their aptitude and experience profiles. Results indicated that the participants generally enhanced the global comprehensibility of their speech (through reducing vowel insertion errors in complex syllables) as a function of increased classroom experience during their first semester, and explicit learning aptitude (associative memory, phonemic coding) appeared to help certain learners further enhance their pronunciation proficiency through the development of fluency and prosody. In the second semester, incidental learning ability (sound sequence recognition) was shown to be a significant predictor of the extent to which certain learners continued to improve and ultimately attain advanced-level L2 comprehensibility, largely thanks to improved segmental accuracy.

In the context of 90 adult Japanese learners of English with diverse second language experience and 10 native speakers, this study examined the linguistic characteristics and learner profiles of low-, mid- and high-level fluency performance. The participants’ spontaneous speech samples were initially rated by 10 native listeners for global fluency on a 9-point scale (1 = dysfluent, 9 = very fluent), and then divided into four proficiency groups via cluster analyses: low (n = 29), mid (n = 30), high (n = 31), and native (n = 10). Next, the data set was analyzed for the number of pauses within/between clauses, articulation rate, and the frequency of repetitions/self-corrections. According to the results of a series of analyses of variance, the frequency of final-clause pauses differentiated low- and mid-level fluency performance; the number of mid-clause pauses differentiated mid- and high-level performance; and articulation rate differentiated high-level and nativelike performance. The analyses also found that the participants’ second language fluency was significantly associated with their length of residence profiles (0–18 years), but not with their age of arrival profiles (19–40 years).