Did ancient neurotypical people dismiss mental illness by saying "it's all in your chest"?
It's all in your liver
Short answer: Mostly, no!
Long answer: We have a lot of evidence that, from prehistoric times, disabled folks of all kinds were largely cared for. The Celts even had codified laws that made sure disabled, children, and elderly people were cared for by SOMEBODY, even if they didn't have the usual relatives available or even willing to care for them. It was one of the reasons the Romans hated them so much and genocided them out of existence!
Indigenous cultures are very largely humane and prosocial, across the world, especially ancient prehistoric folks--we have a lot of evidence that they cared very deeply for disabled people of all kinds, even without any resources or knowledge. They had love, because humans have always had love. We love one another, like most social animals we have a great love in our hearts for everybody. That's what being a social animal MEANS.
Now, the ROMANS would leave unwanted babies at the city dump, and do all kinds of violence to anybody as entertainment; and largely, cultures that followed that throughline--such as the Roman Catholics, which is what the Roman Empire turned into lbr--tended to be more violent toward the vulnerable or the foreign. The other Indigenous cultures of Europe--Goths, Celts, Picts, Gauls--were much more humane, as were the pre-Roman-contact Persians. The Greeks could go either way, depending on the city-state, but were largely less violent than Rome, particularly because they weren't an Empire. Empires are violent and dehumanise, that's what they do; they're very antisocial.
I'm gonna mostly talk about Christendom in the middle ages now, because that's what I know most about and that's where the humours come from.
Madness, in that time and place, was seen as a) contagious and b) God-given. There was wisdom in the ravings of a madman, as well, so it behooved you to listen, for his madness was because he had seen the face of God or heard the voice of angels.
Depending on the way you were neurodiv, and depending on how it manifested, you might see cases of "autism be damned, my girl can spin!" or "well he don't talk much but he knows every sheep by name" or "well she speaks in tongues and the Lord has sent her visions, let's get her to a convent so she can have all that holy knowledge recorded" or "well his mind wanders off sometimes but if you set him pulling a fleece or somebody minds him that doesn't matter too much" or "well the baby is behavin mightily odd, must be a fairy baby" (this last one could go very badly or just fine depending on the parents, but people generally wanted to keep babies alive--remember, half of them tended to die and regular people don't really like that to happen, even if the baby is colicky or bitey; especially considering about a quarter of all mothers died in childbed).
And remember, sanity is defined by culture--some things we think are insanity now would seem normal then, and some things we think of as perfectly normal would seem insane to a medieval person. Things like hallucinations and delusions are also informed by culture--you can't hallucinate something you have no context for, you will naturally try and put it INTO context. That's just what humans do, we think in narratives and we try and put any kind of data we are receiving into some kind of context or narrative that makes sense to us at the time. So, naturally, a lot of people who had hallucinations thought they were visions and saw angels and God and things of this nature. As well, the medieval mind put everything in the whole world--animals, plants, etc--into context as symbols of Christianity. Every animal was a parable. The world itself was symbolic, full of messages from God. That means there's a baseline level of what WE in the modern day might call imaginary that a medieval person would think of as normal. And that brings me back to: Sanity is defined by culture!
Now health, in the middle ages in Europe, was seen as a quantifiable thing you had, and getting sick meant you had lost it, so a doctor's job was to make sure to put health back inside you! Medicine was quite a bit more sophisticated than leeches; doctors learned from Muslim doctors as well as from centuries of studying the natural world (much of observational science was done by monks and clergymen, since studying Creation was a way of appreciating God's work!), and herb gardens at monasteries were exquisitely detailed. We're finding all kinds of information nowadays that indicates people had knowledge of herbs that could be used in surgical procedures, as anaesthesia or painkillers or even to prevent and treat infection, even outside the Muslim world! And no, I don't just mean mercury and other actual poisons, though mercury was one of the first working antibiotics!
Bonus silly fact: nobody ever has liked paying taxes, and often a village would organize to all behave like madness had come and afflicted the town when the tax man was coming (remember, madness was considered contagious!). The tax man would record something like "town mad, couldn't collect taxes" and move on quickly so he wouldn't catch the madness; and after he left everybody would just drop it and go back to business!
Further Reading: Terry Jones' Medieval Lives (book and docu series) Terry Jones' Barbarians (book and docu series) Bedlam: London and its Mad by Catharine Arnold (I haven't read this yet but I love her other books: City of Sin and Underworld London, about sex work and crime respectively) (Sorry I don't remember all my sources, this is knowledge I've collected up over years and years of reading books, talking to medieval scholars, and watching documentaries; I'm not an academic, just a disabled person in the SCA who has been interested in this from childhood)
