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