This is an extremely interesting topic and I've been looking into it quite a bit recently, mostly looking at deformity. My research was mostly on the physical side of things, so I'm not too sure about the treatment of those with mental disorders. Nevertheless, there are some interesting laws about lunacy in Rome that it might be worth looking into though.

In Rome, the treatment of people with medical issues really depends which period you look at. There is actually a real difference in social constructions of disability that develops from the Roman republican era to the imperial period. In the Republic, those with disabilities, be they mental or physical, are seen as prodigies, those that are divergent from the norm. Livy (and Julius Obsequens' book based on Livy) has some great examples of prodigies, which under the republic were displayed publicly and viewed by the Senate as a religious problem to be expiated (It might be difficult to get your hands on, and unfortunately its in French as well, but check out Allely's study on prodigies). Prodigy, and its variant synonyms (prodigium, ostentum, potentum, miraculum) was anything so far outside the norm that it was considered to be remarkable, a message from the gods that some problem needed to be resolved. Even in law, such as the Twelves Tables (which unfortunately is not an extant source), there are references to mandatory exposure of deformed children (Cicero, On the Laws). This period was extremely harsh to those considered less than ideal. Most likely, the majority were exposed. Those displayed as prodigies were most likely killed in order to expiate the perceived problem that they posed to the Roman state. It was less a social stigma and more a death mark. Nevertheless, in the archaeological record there are examples of those with physical deformities growing to adulthood, in what Laes calls "evidence for compassion".

However, under the empire, there was a different view. In the conceptual gap of social constructions of the disabled left by the political sidelining of the Roman Senate, there was a movement towards viewing disabilities as 'marvels' or even 'trendy', and those with disabilities might even gain a position of entertainer in the imperial courts or in the households of the upper class. There's an amusing account in Martial where he complains that the fool for whom he paid extra turned out to be no fool at all. Under the empire, law also becomes much more permissive in allowing such people to exist (Digest of Justinian). However, there were certainly vocational restrictions, at least de facto. Claudius is a success story for someone with disability, but he certainly didn't have it easy. Check out Seneca's Apocolocyntosis if you want to see a brutal attack on disability.

Here's a few sources.

Primary:

Republic: Livy, particularly 31.12; 27.37; 39.22; 27.37; 34.45; 35.21; 41.21; 41.9; Also Julius Obsequens - Book of Prodigies 14; 14; 20; 21; 50.

Empire: Pliny, Natural History, particularly chapter 7.

Secondary:

Allély - "Les enfants malformés et considérés comme prodigia à Rome et en Italie sous la République" and "Les enfants malformés et handicapés à Rome sous le Principat”

Barton - The Sorrows of Ancient Romans.

Garland - The Eye of the Beholder.

Laes - Learning from Silence: Disabled Children in Roman Antiquity.

Disabilities in Roman Antiquity, edited by C. Laes, C. F. Goodey and M. L. Rose.

You might want to read on the Emperor Claudius. He was somewhat physically disabled (he had a speech impediment and a condition that affected his walking. The wiki can give you a decent overview of his life.

Suetonius in his [Life of Claudius] (http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Claudius*.html) says "throughout almost the whole course of his childhood and youth he suffered so severely from various obstinate disorders that the vigour of both his mind and his body was dulled, and even when he reached the proper age he was not thought capable of any public or private business. For a long time, even after he reached the age of independence, he was in a state of pupillage and under a guardian, of whom he himself makes complaint in a book of his, saying that he was a barbarian and a former chief of muleteers, put in charge of him for the express purpose of punishing him with all possible severity for any cause whatever. It was also because of his weak health that contrary to all precedent he wore a cloak when he presided at the gladiatorial games which he and his brother gave in honour of their father; and on the day when he assumed the gown of manhood he was taken in a litter to the Capitol about midnight without the usual escort. Yet he gave no slight attention to liberal studies from his earliest youth, and even published frequent specimens of his attainments in each line. But even so he could not attain any public position or inspire more favourable hope of his future."

Not many in his family were hugely fond of him - (Suet. Claud 3.2) "When his sister Livilla heard that he would one day be emperor, she openly and loudly prayed that the Roman people might be spared so cruel and undeserved a fortune". Augustus also apparently said (Suet. Claud 4.2) "if we realize that he is wanting and defective in soundness of body and mind, we must not furnish the means of ridiculing both him and us to a public which is wont to scoff at and deride such things." He was denied most actual public offices (he held a few ceremonial offices) until Gaius Caligula who made him consul. It's only when Caligula was assassinated that he was made emperor by the Praetorian Guard.

He was actually a reasonably popular emperor - his infirmities were only physical and over time he managed to control them to a certain degree. His mind was quite sound and he spent much time during his life studying and writing (Suet. Claud 41-42). In fact his infirmities allowed him to grow on the fringes of power and thus escape the recurrent cullings made by successive emperors desirous to eliminate opposing claims to the imperial throne.

Obviously, this example relates to the imperial family, and to an unknown degree the rest of the aristocracy, let alone the rest of the population.

For more information, I would recommend you research on medical history in ancient Rome and disabilities (a better search term than stigma for your purposes). Some recommended readings:

• Roy Porter, The Greatest Benefit to Mankind: A Medical History of Humanity (London, 1997;1999).

• Christian Laes, Disabilities in Roman Antiquity http://www.brill.com/disabilities-roman-antiquity Review of book and preview/chapter abstracts

• Have a look at list of contributors here