"[Negishi] has intervened in some areas of my paintings, but the intellectual authorship of the works is mine."

Via Clancco, news of a co-authorship claim in Spain by a former studio assistant of the artist Antonio de Felipe.  I don't know what the story is in Spain, but the problem with this sort of claim under U.S. law is that co-authorship requires that the parties intended to be recognized as co-authors, and that's obviously not the case in most artist-studio assistant situations.