Quote From eloise567:
Supervisor B doesn’t really want me to meet or interact with any of the other researchers who have expertise in disorder Y at the university. I think he might have had a negative experience with a researcher there who has overlapping research interests.
My PhD supervisor was something similar. She was paranoid about someone else stealing reserach ideas because of one bad experience. Long story short, when she was a postdoc, she talked with an eminent professor about an idea she was working on and about a year later he published work far superior to what she was doing at the time. Therefore she thought he had stolen her idea and was thus paranoid. The funny thing is I have recently started a postdoc with a sperate group that was part of the eminiment professor's project. I got a copy of the report at my new uni and found they had been working on the idea long before her. So there was no idea "theft" and my supervisor became overly protective of ideas/data over something trivial. Personally I think that you are deluding yourself to think that you have a wholey unique idea that no-one else in the field has thought about before. The limitations in research are usually time and money not who has the better idea. So telling someone else your idea is far far more likely to create an interesting conversation than idea theft.
Back on topic. You don't have to do everything your supervisor says. They can be paranoid but you don't have to be. I would talk freely with other people but be quite to supervisor B about it.
