The extent to which individual differences in cognitive abilities affect the relationship among task complexity, attention to form, and second language development has been addressed only minimally in the cognition hypothesis literature. The present study explores how reasoning demands in tasks and working memory (WM) capacity predict learners' ability to notice English question structures provided in the form of recasts and how this contributes to subsequent development of English question formation. Eighty-one nonnative speakers of English completed three interactive tasks with a native speaker interlocutor, one WM task, and three oral production tests. Prior to the fi rst interactive task, participants were randomly assigned to a task group (simple or complex). During task performance, all learners were provided with recasts targeting errors in question formation. The results showed that learners' cognitive processes during tasks were in line with the cognitive demands of the tasks, at two complexity levels. The fi ndings suggest that WM was the only signifi cant predictor of the amount of noticing of recasts as well as of learners' question development. With regard to interaction effects between WM and task complexity, high WM learners who carried out a complex version of the tasks benefi tted the most from task-based interaction.Since Long's ( 1996 ) updated interaction hypothesis, there has been a surge in research conducted on the effects of conversational interaction on second language (L2) learning, to the degree that "a robust connection" between interaction and learning is now commonly accepted (Gass & Mackey, 2007 , p. 176; see also Mackey, Abbuhl, & Gass, 2012 , for a review). Learner internal and external factors mediating the positive relationship between interaction and L2 learning have been identifi ed, and the role of task design features in both L2 performance and interactiondriven language learning is being increasingly examined in the fi eld of instructed SLA. Researchers have explored the effects of task complexity on L2 development by testing the predictions of Robinson's ( 2011 ) cognition hypothesis and Skehan's ( 1998 ) trade-off hypothesis. However, very little research has looked into the ways individual learners' cognitive characteristics, such as working memory (WM) capacity, mediate the effects of task complexity on L2 learning. Moreover, from a methodological standpoint, researchers have yet to systematically document evidence of task complexity validation (e.g., Baralt, 2013 ;Gilabert & Barón, 2013 ;Norris, 2010 ;Révész, 2014 ). To address these issues and to test the predictions of the cognition hypothesis, the current study (a) assesses whether task complexity manipulations are in line with learner perceptions and cognitive processes during task completion; (b) examines the relationship between task complexity, WM, and noticing of recasts; and (c) investigates the development of English language questions by L2 learners.
BACKGROUND