Lord Tullibardine was a good-looking bright young man but highly excitable one moment and reflective the next. When aged twenty, he went with his regiment to Portugal accompanied by Dr. Alexander Menzies to watch over him.
There he began to show alarming symptoms as though something had happened to him, perhaps struck on the head. Dr. Menzies thought it could also be the heat of the sun, and decided to write to the family physician, Sir Walter Farquhar, advising him of appearances of mental derangement. Consequently, the Marquess was placed in a garrison under military command and later sent back to England. He alternatively seemed lost in thought or became violent.
Despite never recovering his senses, he nevertheless still became Duke of Atholl in 1830, though probably never knew it. He was kept in a padded room in St.John's Wood where there were no windows, but where he had one female and two male servants to keep him out of harm's way.
 

Source: Leo van de Pas