Regardless of ethical issues, you couldn't logically do an fMRI study of a meltdown. Most meltdowns require stimming and/or throwing things and/or movement of some kind, and you have to be perfectly still inside an fMRI. The ever-so-slight natural movement from our heartbeats can throw off fMRI data, as can a head movement of merely a millimeter. There is no way that you could have a person sustain stillness during a meltdown. This is also likely why there are so few fMRI studies of acute bipolar I mania, my research interest. With psychomotor hyperactivity being such a key symptom of mania, fMRI experiments during this state are pretty much useless.
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Helinger: Now, what do you see, John?
Nash: Recognition...
Helinger: Well, try seeing accomplishment!
Nash: Is there a difference?