Turns out Voyager also established an area of the Vulcan brain that’s responsible for suppressing emotions:

EMH: There’s a definite neurochemical imbalance in the mesiofrontal cortex.

JANEWAY: Which means what?

EMH: That’s where the Vulcan psycho-suppression systems are located. This may be the result of an incompatibility with the Betazoid telepathic neural centre, I’m not sure.