According to Ellis (1997, p.23) language “acquisition follows a U-shaped course of development; that is, initially learners may display a high level of accuracy only to apparently regress later before finally once again performing in accordance with target-language norms”. This definition is similar to that found in Krashen’s (1982) concept of affective filter, which claims that feelings such as anxiety, stress or low-self-esteem pose barriers on learning.
Ellis, R. (1997) Second Langage Acquistion. Oxford University Press: Oxford.
Krashen, S.D. (1982) Principles and Practice in Second Language Acquistion, Oxford: Pergamon Press