This study investigated intra-learner variation in the effects of implicit and explicit focus on form (FonF) on second language (L2) acquisition of the English passive, an interface construction. The study employed a pretest-posttest-delayed posttest design, with five treatment sessions between the pretest and posttests. Implicit FonF was operationalized as textual enhancement (TE), and explicit FonF as consciousness-raising (C-R). Five different outcome measures were utilized to examine the respective treatment effects on the form, meaning, and function of the passive. Data were collected with four Korean high school students, two engaging in TE and the other two in C-R, whose developmental routes through the experiment were analyzed quantitatively and qualitatively. The results of the analysis showed that implicit and explicit FonF led to differential variation within each learner, not only across the outcome measures but also over time, yet were suggestive of a potential benefits to each for the different domains encompassed in the passive. These results are further discussed in relation to the learners’ proficiency levels and the types of errors they produced.

The Cognition Hypothesis postulates that more cognitively complex tasks can trigger more accurate and complex language production, thereby advancing second language (L2) development. However, few studies have directly examined the relationship between task manipulations and L2 development. To address this gap, this article reviews, via an analytic approach, nine empirical studies that investigated the impact of task complexity on L2 development in the domain of morphosyntax. The studies are categorized into two groups based on if they include learner-learner interaction or a focus on form (FonF) treatment provided by an expert interlocutor. The results indicate that the findings of the studies, albeit partially mixed, tend to support the predictions of the Cognition Hypothesis. More importantly, a further analysis reveals seven key methodological issues that need to be considered in future research: target linguistic domains, different types of FonF, the complexity of the target structure, task types, outcome measures, the use of introspective methods, and the need of more empirical studies and replicable study designs.