1.2 Aims of the Study
In recent years，there has been an increasing amount of literature on successful incidental vocabulary learning through watching short video clips（Montero Pérez，Peters＆Desmet，2018）as well as through single，full-length TV programs（Ahrabi Fakhr et al.，2021；Peters＆ Webb，2018）.However，more studies have yet to investigate the potential effects of TED-talk videos on incidental vocabulary learning.TED-talk videos have received less attention and have yet to be explored as the audiovisual input material.If incidental vocabulary learning through viewing TED-talks can enhance vocabulary growth，as Rashtchi，Khoshnevisan，and Shirvani（2021）suggest，studies that show the value for incidental learning through viewing TED-talks are needed.The present study provides great evidence to show to what extent watching TED-talk videos could contribute to incidental vocabulary learning.Besides， a comparative study was carried out investigating the effect of word learning between viewing TED-talks and reading transcripts of TED-talks，determining which input mode stands in a better position for promoting incidental vocabulary learning.
According to Rodgers（2013），vocabulary gains from audio-visual input proved relatively small.Some factors have been cited to account for the reason，such as the challenging nature of real-time viewing，limited capacity of human working memory，complicated meaning inferencing processes， all of which have contributed to the unpredictability of incidental vocabulary learning from audio-visual input，thus leading to minimal vocabulary gains（Laufer，2003；Nguyên，2017）.It is suggested that language teachers should resort to some pedagogical interventions to enlarge the vocabulary gains. Therefore， this study decided to optimize the effectiveness of incidental vocabulary learning by providing two input enhancement techniques：① captioning and② repeated viewing，with the purpose of shifting learners'attention to target words and make form and meaning connections for facilitating greater vocabulary uptake（Schmitt，2001）.However，most studies only examined the effects of different enhancements on vocabulary gains from written input（Baleghizadeh，Yazdanjoo＆Fallahpour，2017；Han，Park＆Combs，2008；Loewen＆Inceoglu，2016）.Only a few researches investigated the effect of captioning and repeated viewing on word uptake through movies or TV series（Peters，2019；Peters，Heynen＆Puimège，2016；Teng，2019b）.The effect of captioning or repeated viewing on incidental vocabulary learning through TED-talk videos was even scarcer.This gap needs to be addressed in the present study.
What is more，words that are learned vary greatly among learners（Webb＆Chang，2015）.Some learners with high motivation work harder than others，thus reaching their vocabulary goals easily.Some learners with a relatively larger vocabulary may make larger word gains while viewing captioned videos（Teng，2019b）.Moreover，since words have their learning burden（Puimège＆ Peters，2019；Schmitt，2010），incidental learning gains may depend on item-related factors，such as：corpus frequency（e.g.，Vidal，2003），part of speech（e.g.，Campoy，2008）etc.However，the research into the role played by both the learner-related variables and word-related variables related to audio-visual studies is still insufficient.Hence，the current study attempts to explore how word-related variables and learner-related variables might contribute to incidental vocabulary learning via viewing TED-talk videos.