In 2 experiments, the authors examined the electro-physiological auditory responses of monolingual French listeners to American English vowel contrasts as a function of the surrounding vowel context. The context was determined on the basis of behavioral results (C.Meunier, C. Frenck-Mestre, T. Lelekov-Boissard & M. Le Besnaris, 2003, 2004). In the 1st experiment, where the vowel /I/ was placed in a context in which it could easily be discriminated from the surrounding vowels (82% /i/ and 3% /ae/), the electro-physiological response to this vowel showed both acoustic and phonemic responses in line with behavioral results. In the 2nd experiment, where the same vowel /I/ was placed in a "difficult" context (82% /e/ and 3% /ae/), the electro-physiological response of French participants to this vowel revealed a greatly reduced phonemic response, showing assimilation of the vowel to the surrounding context, again in line with behavioral results. The results of a 3rd control experiment with American participants showed both an acoustic and a phonemic response to the vowel /I/ in the difficult context (82% /e/ and 3% /ae/). This pattern demonstrates the fluctuations in perception as a function of context, and hints at a supple system that may be modified through experience. (Contains 5 figures, 1 table and 3 footnotes.)