In the chaos that is my mind at this hour —
Brain damage such as stroke can take away the symbolic processing ability known as language. I wonder: does the most obvious kind of sign-language, pointing and miming, go with it? And do Deaf signers so afflicted lose more or less of the naïve sign-language than the hearing do? In one sense these questions have no meaningful answer, because every stroke is different; but there must be interesting correlations.
Does stroke ever take away the ability to process conditional or relative clauses?
Depends on the location of the lesion: spatial (sign language) and abstract (spoken language) understanding are handled by separate parts of the brain.
Much fruitful research on strokes has been done on Japanese victims with alexia, some of who lose only kana (syllabic-abstract) understanding, some only kanji (ideographic-spatial) understanding, and some both. I believe the same is true of sign: that some lesions will leave sign intact even when standard orthography is lost. I’m not so clear on aural understanding.
Oliver Wills relates the story of a woman who lost only the ability to name vegetables, but not fruits. I wonder how she dealt with tomatoes?