The defense was on referential structures in sign languages. Many of my readers may know that sign language establishes person references ("pronouns") by establishing a "pronoun zone" in which (or toward which) the signer can "inflect" signs to refer to that person.
The argument of this thesis was that in fact, sign languages (especially ASL, which is the best studied) don't have pronouns with person at all: even though it's easy to maintain as many as seven distinct "pronoun" areas, they're all fundamentally of the same type -- they're all demonstrative pronouns, not the usual personal pronoun. Our brave thesis defender made a fairly convincing case that this "point-to-a-region-and-then-refer-to-it"
She suggests that a reason that sign languages, as a group, don't have personal pronouns but instead use demonstrative pronouns. She points to the richness and discriminability of the spatial medium as a possible cause -- it's never become necessary to grammaticalize pronouns in sign, since pronouns usually begin their history as a verbal hint that the speaker is performing deixis. Instead, in sign language, there is no such "hint" -- the language is itself deictic, and remains so because it is so concrete.