Question: Basically, relative adverbs modify their antecedent and antecedents should be nouns, which I learned. 

However, in the following sentence, 'where', which I think is a relative adverb, seems to modify 'outdoor,' which is an adverb:

Visual awareness is also heightened outside, where the eyes are exercised by motion. 

Also, if 'where' is relative adverb, then it is used as a nonrestrictive relative adverb. Does 'where' modify 'outdoor,' despite the fact that it is an adverb, or is its antecedent the whole sentence before the comma? Or isn't 'where' a relative adverb at all?

Thank you.

Answer: 
I think "outside" is a noun modified by a non-restrictive relative clause. It is part of a PP with understood preposition "at", and it is that PP which is the adverb,. This doesn't make the relative clause an adverb, because the relative clause doesn't modify the adverb.
*Outside* or *0utdoors* can be considered substantives (considered to be nouns) and therefore eligible to be modified by the adjective clause introduced by the relative adverb. A substantive is an adjective or adverb whose idea can be substantiated with essence, as in I like green plants because green (substantive) is my favorite color. Likewise, the great outdoors is the substantive for anything that is beyond the door, so to speak.