I’m technically ADD. The Diagnostic Medical Guides explain them well if you interpret them as is. They explain them in terms of a behaviors that are not productive to expected social goals. They are not necessarily brain dysfunctions so much as they are chemical differences in brain operation which cause people to act or not act in a way that is unconventional and usually detrimental to functioning in some given social tasks.
I function completely normal and best sitting behind a computer typing out messages at my will and convenience. But if I was forced into certain social interactions, I can behave awkward and inappropriate which may give off a false impression of my actual capabilities. Everyone can understand this to some degree. You can be forced to do a television interview on the spot without a script by some interviewer who speaks at 100mph and you do not have time to think of a response of a hard question. Before you can answer, the interviewer may all of sudden get too impatient and go on to a different question which now you must change your focus onto. ADD is like that…a sort-of stage fright because you may hear something but your physical emotions are fighting against. So you need more time to register what you heard to transfer to thought.
Emotional arousal tends to be the likely cause of this chemically. So they give us drugs relating to chemicals that try to balance these out. Sometimes they are more effective, sometimes not. The brain still has a lot of unknown details for psychiatrists to know what to do yet. So it’s trial by error for now.