Neurological experiments do not scale very well from small to large species like ours due to the huge differences. One can do as with the OP’s rats and jig the human nervous system to induce motor reactions. However, advanced neurosurgery against epilepsy and parkinson has recently shown that even this is hard to predictably perform in the human brain. The rough motor centers in the brain are indeed rough and the detailed wiring varies from individual to individual. The OP mentions drug induced emotional manipulations, this is however also very unpredictable as anyone with even the most innocent experience of recreational drugs will know. Different day, different trip – Different human, different rip. Free will, if it exists (I believe it does) would have mostly to do with our frontal lobes in the upper brain were abstraction and logic seems to mostly take place and our lower brain, with which we intuit, remember, coordinate and emote. Much of the rest of the upper brain seems to be busy working motor skills, language comprehension and such everyday tasks that seem mundane, but when you think of for instance the human hand or the English language are rather remarkable (I’m being real rough here BTW, this is the most complex thing you can imagine and we only know a small fraction of what there is to know about our brains).
The frontal lobes and the lower brain are where the larger part of our differences to the rest of the world’s animals reside. The frontal lobes in humans are extremely large, but elephants, other primates and some whales come pretty close. However, in the lower brain we have up to 200 times as many neurotransmitters per cubic centimeter as our closest other, namely chimps.
Evidence has it that the combined work of the brain is processed into awareness in the back end of the frontal lobes.
That it takes a while (1.4 seconds on average) for a nervous signal to travel through our central nervous system to awareness does some freaky stuff to us that could give you the illusion that we have no free will. Try this for instance; next time you converse with someone, see if you know what you were going to say word by word or if you became aware only after you said it. Don’t worry, it has to be that way for the sake of speed, you still decided what you were going to say as a concept before you said it, it’s only the wording that happened without you knowing it. Likewise if you, for reasons beyond logic place your hand on a hot stove you’ll pull it off before you become aware of the burning pain. This is easy to try as well; ask someone to prick your upper hand gently with a pin while you have your eyes closed. Say ‘now’ when you feel the pain and you’ll find that your hand has already jerked away. Your antagonist will be able to verify this with his/her own eyes.
This all has to do with the fact that the lower brain `and reptilian brain where reflex happens, as stated earlier largely function subconsciously and faster than awareness. All that being said, you can easily reverse the process and actively decide to not say something for instance or sit still and let someone prick you with needles (which many a tattoo artist are grateful for).
‘The Man Who Tasted Shapes’ by neurologist Richard E Cytowic is a book that illustrates the way this all works on a layman’s level in a pretty fun way. Cytowic also makes a case for the existence of free will.