The study you link to doesn't take into consideration the Overton window of opinions. Perhaps there's some dimension along which you could say that one ideology lies 'opposite' to another political persuasion, but that doesn't necessarily mean that the two ideologies are equally acceptable to support in a given society.

I don't think calling defining neutral a 'challenge' does the question justice - neutral will always be context-dependent, and what may be in the center of the Overton window of one society may be unpopular or even highly illegal in a different society.