In a previous post I mentioned Nigidius Figulus' discussion of the intonation and/or stress of the vocative and genitive of Valerius. I didn't say anything about Nigidius' odd term casus interrogandi, in this context, apparently meaning the genitive. This term and the interpretation of this passage and others are the subject of a book-length treatment by Walter Belardi and P. Cipriano, Casus interrogandi. Nigidius Figulus e la teoria stoica della lingua. They argue that casus (plural) interrogandi refer to the oblique cases in general, a term originating in Stoic grammatical theory.
The authors specifically lament the fact that English-speaking scholars, especially on the subject of ancient Grammar, don't read works written in Italian (p. 159). I try very hard to read what Italian scholars produce, but I missed this one.
Belardi W. and P. Cipriano, 1990. Casus interrogandi. Nigidius Figulus e la teoria stoica della lingua. Rome: Libreria Herder.