Stimulants are a popular drug of use / abuse for people who need to focus for extended periods. I used low levels of methamphetamine (a kind of stimulant) to assist my one week of 20 hours of writing per day in the week before I submitted my thesis, and it isn't uncommon for students to use it to help them cram before an exam. Not the wisest move, perhaps. Risk of addiction, risk of taking too much and developing symptoms that are indistinguishable from paranoid schizophrenia...

But that being said, some people simply are able to pull a week of studying / writing for 20 hours a day and sleeping for 4. They can do that without stimulants. I can't. I need stimulants to do that. Does that mean I'm disordered or dysfunctional or impaired somehow? Maybe I have ADD... A prescription for stimulant medication would be a great deal more socially acceptable than my hunting out methamphetamine from the dodgey guy down the road...

I'm just saying that aside from the studies that have been done on both humans and animals it is common knowledge that stimulants assist learning and memory. Like anything... Too high a dose wrecks havoc. But lower doses... Common knowledge these days and the science backs that. People with ADD have improved performance but then so do people without ADD. So... Response to medication simply isn't diagnostic. The medication works on dopamine so you might think that differences in the production, release, uptake, or metabolization of dopamine would be forthcoming either in living patients or upon autopsy. Once again... Nope. There are individual differences to be sure but you can't diagnose any more reliably from autopsy than you can from the behavioral symptoms or from the response to medication.

Many diagnoses are like that... Depression, schizophrenia, anxiety, and so on and so forth.