In light of mixed findings in existing input enhancement research, Issa and Morgan-Short in a 2019 article urged researchers to compare the relative effects of input enhancement that taps into learners’ attention to the external format of second language (L2) target forms (e.g. through capitalizing or boldfacing the forms) and input enhancement that taps into learners’ attention to the internal attributes of L2 target forms (e.g. via increasing the frequency of the forms). In response to this call, the study described in this article drew on a pretest-treatment–posttest-experimental-design to explore whether working memory (WM) capacity modulates the extent to which L2 learners benefit from input enhancement engaged by internal and external attentional manipulations for partially-acquired L2 vocabulary. Analyses of these learners’ lexical gains under different experimental conditions showed that although compound input enhancement engaged by internal attentional manipulations did indeed lead to better lexical gains, such manipulations did not unequivocally lead to greater gains than the external manipulations in all cases. Furthermore, simple input enhancement engaged by internal attentional manipulations (i.e. varying the contextual supports for the target words) could be as effective as compound input enhancement. Importantly, we found that the aforementioned pedagogical effects of internal and external manipulations were both modulated by differences in WM capacity, albeit to differing extents. Insights from this study provide important pedagogical implications for differentiated input enhancement theory and practice.
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