Why are the Catalan contrasts between /e/-/ɛ/ and /o/-/ɔ/ so difficult for even early Spanish-Catalan bilinguals to perceive?

Previous research has shown that Catalan mid-vowel contrasts remain difficult for native Spanish adults to perceive, even those who began using Catalan as an L2 as young children. Here we tested 82 Spanish-Catalan bilinguals first exposed to their L2 by school age (3-6 years) who differed in self-reported use of Catalan. All participated in categorization and AXB discrimination tasks using four 10-step vowel continua. The endpoints of 2 high-mid vowel continua (/i/-/e/, /u/-/o/) were vowels that contrast in both Spanish and Catalan whereas those in the remaining 2 mid-mid vowel continua (/e/-/ɛ/, /o/-/ɔ/) were mid vowels that do not contrast phonemically in Spanish. We hypothesized that Catalan mid-vowels contrasts are difficult not only because of cross-language phonological differences between Spanish and Catalan, but also because these phonemic contrasts are phonetically “weak” as a result of several factors. All participants perceived the high-mid contrasts less categorically than the mid-vowel contrasts but the magnitude of the difference seemed to depend on frequency of Catalan use. While not excluding possible effects of even brief delays in exposure to vowel contrasts found in an L2, these findings suggested the importance of variations on overall amount of L2 exposure on the eventual L2 performance of bilingual adults.