[rosser]

Why would someone on SSRIs not notice a thing?

In the case of ayahuasca, quite possibly because they're dead. You don't mix SSRIs and MAOIs unless you have a very good idea what you're doing, and even then probably not without a reasonably well-equipped clinic to hand.

Serotonin Syndrome is not something to fuck around with.

[spangry]

Not that I'd advise anyone take the risk, but there are lower risks associated with mixing SSRIs and MAO-B inhibitors (like ayahuasca). It's the older class of anti-depressants which are both MAO-A and MAO-B inhibitors that you don't want to mix with SSRIs (and lots of other things too). MAO-A oxidises/metabolises seratonin, noradrenaline and dopamine, whereas MAO-B is mostly just dopamine. MAO-A inhibition also causes the 'cheese effect' (consumption of tyramine causing hypertensive crisis).

As an aside, it's a real pity that the side effects of MAO-A inhibition loom so large in people's imaginations. They're extremely effective anti-depressants and many of the risks can be easily mitigated, but GPs and even shrinks are now too afraid to prescribe them.