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

Do you mean "outside" or "outdoors"? You use both these words in your question. I'll assume the former.

The bracketed element is a non-restrictive relative clause. Unlike restrictive relatives, non-defining ones are not modifiers; rather, they are supplements that refer to a semantic 'anchor' (in supplementary relatives, the anchor and the antecedent are the same). In your example the anchor is "outside". Note also that non-restrictive relatives can have virtually any element as antecedent, from a noun phrase to an entire clause.

"Where" is a relative adverb (or preposition in some grammars); it functions as a locative adjunct in the relative clause. Adjuncts in relative clauses are mostly associated with PPs, and it would be plausible here to interpret "outside" as roughly meaning "in the open". 

Trad grammar takes this "outside" an adverb, but there are good reasons for calling it a preposition.