As a speaker of a fairly standard North American English accent, I occasionally find it difficult to understand people who speak with a heavy accent. I've always been curious about what exactly makes accents difficult to understand, and it occurred to me that intelligibility could either be an absolute property (for example, driven by the brain's ability to distinguish phonemes) or relative (determined by the brain's ability to 'pattern match' to known phonemes).
If it were true that intelligibility is absolute, then I would expect that any listener would tend to have an easier time understanding speech in a "clear" or "common" accent, even if they natively speak with a heavier accent. On the other hand, intelligibility being relative would suggest that what is a "clear" accent to one person would be a "heavy" accent to another. Has this sort of experiment been done, and what does it tell us about intelligibility?
Also, I realize that this could also possibly be different between first- and second-language speakers - presumably people growing up in a community naturally understand other speakers from that community clearly, but perhaps two speakers with the same native language who are conversing in a second language may struggle to understand each other more than they would a native speaker.