Recent research has shown that certain learners' responses to feedback, specifically repair and modified output, are predictive of subsequent second language (L2) development. Yet, little is understood about why these responses are associated with second language acquisition (SLA). The current study investigated this question by exploring the cognitive processes underlying learner responses. Learners of Japanese (n = 24) engaged in task-based interactions during which they received recasts of their errors. Each learner then watched video clips of the recast episodes and commented on them. The learners' stimulated recall reports were analyzed in relation to their responses to the recasts: uptake, repair, and modified output. In recast episodes where they produced uptake, their reports indicated that they perceived the recasts as corrective feedback significantly more frequently compared to cases where they did not produce uptake. In episodes where learners correctly repaired their errors, they were significantly more likely to report not only recognizing corrective recasts but also noticing the interlanguage-L2 mismatch. Modified output was also significantly related both to learners' recognition of corrective recasts and to their noticing of the gap. Given the developmental benefits commonly associated with noticing the gap, these findings may partly explain why repair and modified output have been found to be predictive of SLA.