This is, of course, a human study but I thought it was very interesting because pet parrots, living not only under unnatural conditions but also incredibly 'unsteady' ones [and by this I not only mean that they are rehomed all the time but also that most people who keep a parrot is simply not very consistent in their routines which is very unnatural to a parrot psyche] that they must 'change their opinion' of things much too often. And they found that it's dopamine that opens the door for the change - dopamine being the 'reward' [aka the 'happy'] hormone and also being the medium for 'reward learning' which is Michael's method of training.
https://www.zmescience.com/science/dopa ... -242789930