This article discusses the conceptualization, measurement, and validity of a recently emerged construct in the field of second language acquisition (SLA)—implicit language aptitude (alternatively “implicit aptitude”). Implicit aptitude is a set of cognitive abilities that enable learners to make unconscious computations of the distributional and transitional probabilities of linguistic input. Implicit aptitude is key to an accurate understanding of the cognitive foundation of language learning and contributes significantly to the advancement of SLA theory and pedagogy. The article starts by clarifying the concept and components of implicit aptitude, elaborating its role in SLA theories, identifying its attributes, and discussing its measurement. It then synthesizes the empirical evidence on its divergent, convergent, and predictive validity, which refers to whether it is distinct or separable from explicit aptitude, whether measures of implicit aptitude are correlated, and whether it is predictive of learning outcomes, respectively. Next, the article provides an overview of the seven empirical studies included in this special issue that examined implicit aptitude from various perspectives. The article concludes by identifying future directions.

This study examines the associations between implicit and explicit language aptitude and the effects of the timing of corrective feedback (CF). A total of 112 seventh-grade EFL learners were assigned to three groups: Immediate CF, Delayed CF, and Task Only. The three groups underwent three treatment sessions during which they performed six focused communicative tasks eliciting the use of the English past tense. The Immediate and Delayed CF groups received CF treatments in the first and final sessions, respectively, and the Task Only group performed the communicative tasks without receiving any feedback. Treatment effects were measured through an untimed grammaticality judgment test and an elicited imitation test. Implicit language aptitude was operationalized as procedural memory and explicit language aptitude as working memory and declarative memory. Multiple regression analysis showed that procedural memory was significantly predictive of the effectiveness of Immediate CF, declarative memory was significantly associated with Delayed CF and Task Only, and working memory was a significant predictor of Immediate CF and Delayed CF. The results were interpreted by consulting the methodological features of the treatments and the mechanisms of the three cognitive abilities.

This aptitude–treatment interaction study investigated the extent to which explicit and implicit cognitive abilities are differentially related to learning outcomes under two corrective feedback conditions. One hundred and thirteen intermediate English learners of Spanish were randomly assigned to an implicit feedback (recast), explicit feedback (explicit correction), or control group after completing tests from two aptitude batteries (High-Level Language Aptitude Battery [Hi-LAB] and LLAMA). Linguistic improvement on noun-adjective gender agreement and Differential Object Marking was assessed using grammaticality judgment and oral production tasks. Results showed that implicit but not explicit abilities were relevant for the acquisition of gender agreement under implicit feedback as measured by grammaticality judgments. In contrast, explicit but not implicit abilities were relevant for the acquisition of object marking under explicit feedback as measured by oral production. These results lent support to a double dissociation, but they also suggested higher-order interaction effects between the type of cognitive ability, outcome measure, and target structure.

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.

This study investigates the validity of syntactic priming as a measure of implicit language aptitude. Syntactic priming refers to the tendency to reproduce a linguistic structure due to a previous exposure to the structure. The validity of the construct was verified by collating evidence for divergent validity—whether it is dissociable from explicit aptitude; convergent validity—whether it is correlated with other measures of implicit aptitude; and predictive validity—whether it is predictive of learning attainment. One hundred sixty-six university EFL learners completed three tests of implicit aptitude: syntactic priming, sequence learning, and LLAMA_D; three tests of explicit aptitude: LLAMA_B, _E, and _F; and three tests of L2 proficiency: untimed grammaticality judgment, metalinguistic knowledge, and elicited imitation. The results showed that syntactic priming was dissociable from explicit aptitude, but it failed to converge with the other measures of implicit aptitude, and it also failed to predict L2 proficiency. The results also showed that priming was negatively correlated with sequence learning and that sequence learning was a negative predictor of learners’ metalinguistic knowledge. On the other hand, the construct validity of explicit aptitude was strong. The results suggest the multidimensionality of implicit aptitude and the need for more research into the construct validity of syntactic priming as a cognitive ability for implicit learning.

This study addresses the role of domain-general mechanisms in second-language learning and knowledge using an individual differences approach. We examine the predictive validity of implicit-statistical learning aptitude for implicit second-language knowledge. Participants (n = 131) completed a battery of four aptitude measures and nine grammar tests. Structural equation modeling revealed that only the alternating serial reaction time task (a measure of implicit-statistical learning aptitude) significantly predicted learners’ performance on timed, accuracy-based language tests, but not their performance on reaction-time measures. These results inform ongoing debates about the nature of implicit knowledge in SLA: they lend support to the validity of timed, accuracy-based language tests as measures of implicit knowledge. Auditory and visual statistical learning were correlated with medium strength, while the remaining implicit-statistical learning aptitude measures were not correlated, highlighting the multicomponential nature of implicit-statistical learning aptitude and the corresponding need for a multitest approach to assess its different facets.

Evidence for the role of procedural memory in second language (L2) acquisition has emerged in our field. However, little is known about the reliability and validity of the procedural memory measures used in this research. The present study (N = 119) examined the reliability and the convergent and discriminant validity of three assessments that have previously been used to examine procedural memory learning ability in L2 acquisition, the dual-task Weather Prediction Task (DT-WPT), the Alternating Serial Reaction Time Task (ASRT), and the Tower of London (TOL). Measures of declarative memory learning ability were also collected. For reliability, the DT-WPT and TOL tasks met acceptable standards. For validity, an exploratory factor analysis did not provide evidence for convergent validity, but the ASRT and the TOL showed reasonable discriminant validity with declarative memory measures. We argue that the ASRT may provide the purest engagement of procedural memory learning ability, although more reliable dependent measures for this task should be considered. The Serial Reaction Time task also appears promising, although we recommend further consideration of this task as the present analyses were post hoc and based on a smaller sample. We discuss these results regarding the assessment of procedural memory learning ability as well as implications for implicit language aptitude.

A subtest of the LLAMA test battery (LLAMA_D) has been proposed as a potential test of implicit learning aptitude. To improve its construct validity, in the present study, the original LLAMA_D (a) instructions for incidental learning were modified, and (b) confidence ratings of test responses and (c) reaction time (RT) measurements were added. This revised LLAMA_D was administered along with the other LLAMA subtests (LLAMA-B, -E, and -F). Unconscious knowledge that may (not) result from the exposure was assessed through the relationship between the accuracy/RT and confidence ratings. The results suggest that LLAMA_D accuracy largely reflects conscious retrieval of previously heard sound sequences. However, an index derived from the LLAMA_D RT measure (coefficient of variance) was associated with an aspect of oral fluency, which is presumably dependent on proceduralization. Several recommendations are proposed to redesign and extend LLAMA_D as a potential aptitude test for proceduralization.

Although this special issue reveals some promising achievements, most of the contributions show that tasks of implicit learning are not or are only weakly correlated with each other, and they have inconsistent predictive power on L2 acquisition. This commentary examines four possible explanations for this surprising pattern: The (suboptimal) selection of tasks, the low reliability of measures, the deep influence of the starting level even for nominally “new” implicit tasks, and the fact that the mastery of L2 may involve other implicit processes than implicit learning measured through laboratory tasks.

In this commentary, we summarize the findings of the seven included studies that examined implicit language aptitude from various perspectives and highlight issues to be resolved in the validation of this new construct in second language research. We start by providing an overview of the contributions of the studies. We then identify the lack of convergent validity of the measures of implicit aptitude reported in the included studies and problematize the equally varied nature of the measurement of implicit knowledge—the outcome variable of aptitude research—and related concepts. In particular, by drawing on empirical evidence and theoretical claims, we attempt to clarify the relationships between implicit and explicit knowledge, implicit and explicit learning, and implicit and explicit instruction. Next, we draw attention to the interactions reported by the included studies between aptitude and outcome measures and between aptitude and instruction type, emphasizing the value and importance of interactional research. We conclude by making recommendations for future research.