Concepts
From what I know, Japanese people can’t generally distinguish the sounds of the letters L and R, simply because they aren’t usually exposed to that variation in their “phonological environment.” (This doesn’t mean, though, that this ability can’t be gained by training.)
I’ve also read that before words for other than primary colors entered our languages, we couldn’t really “see” the difference between colors we had no words for. For instance, in mediaeval times, both black and blue were called “blue”. (What’s also interesting is that the words for colors seem to emerge in the same order in different languages.)
(We register that for which we have concepts, and learn to distinguish variations we’re exposed to. One wonders what is going on that we can’t yet see. I don’t know why I came to think about this.)