The act of two or more individuals engaging in erudite, intellectually stimulating conversation that causes an orgasm-like reaction which includes 'symptoms' such as a boost in serotonin/dopamine and makes the individual(s) happy and enthusiastic to converse more, physical reactions (e.g accelerated heart rate, flushed, shivering/shaking).
Brain sex examples: a conversation between a couple where they have enthusiastic philosophical ramblings
A brain fart consists of verbalising the most incoherent, incongruent, inexplicable sentence one could ever hear, at any giving moment, usually resulting in the act of 'facepalming', proceeded by everyone who had the misfortune of being there to witness such unfathomable stupidity
Person A: Oh my god I've just had the most incredible brain fart...
Person B: That's so Maria Marcos de Macedo Sempiterno redacted of you!