"The Interface Hypothesis was put forward by Sorace and colleagues as an attempt to account for patterns of non-convergence and residual optionality found at very advanced stages of adult second (L2) acquisition. The IH originally proposed that language structures involving an interface between syntax and other cognitive domains are less likely to be acquired completely than structures that do not involve this interface."
Sorace, A. (2011). Pinning down the concept of “interface” in bilingualism. Linguistic Approaches to Bilingualism, 1(1), 1-33.