When learning a second language, many people find that they have occasional difficulty remembering words from their native tongue. This phenomena is called first-language attrition, and is currently being studied by University of Oregon researchers Benjamin Levy and Dr. Michael Anderson.
They found that it's not really "attrition" of the native tongue that's going on so much as "inhibition" as the brain suppresses the first language to facilitate access to the new one.
Studying college-level Spanish learners, they found that the more students repeated a list of Spanish words, the more trouble they had recalling their English equivalents. This was particularly true for earlier learners, while more fluent students had an easier time transitioning between the two languages.
Levy and Anderson suggest these differences are due to learning needs:
[W]hen first learning a new language, we have to actively ignore our easily accessible native language words while struggling to express our thoughts in a novel tongue. As a speaker achieves bilingual fluency, native-language inhibition becomes less necessary, accounting for the better performances of fluent bilingual speakers in the study.
Read more: A New Language Barrier: Why Learning A New Language May Make You Forget Your Old One
FULL TEXT: Inhibiting Your Native Language (PDF)