First, I know Reagan is to this day, still a political lightning rod, but I want to take the risk of asking, anyway.
My mother's husband was diagnosed with Alzheimer's last year, from my own experience in hindsight, there were indication of memory issues, confusion, etc., from much earlier on that were simply neglected for years until the degenerative effects become problematic and eventually unmanageable. In fact, his diagnosis explained much of the things that struck me as "off" about him when I met him in the first place, more than ten years before his official diagnosis.
From my tangential reading of political and popular commentary of the 1980's, Reagan was lampooned by satirists and critics for seeming slow-witted and absent-minded, or appearing to have almost a childlike innocence. This caricature of Reagan played by Phil Hartman in an SNL skit about the Iran-Contra scandal is one such example.. In the skip, the slow, unwitting, nice-old-man public Reagan is portrayed as an act juxtaposed with a secretly vigorous and dynamic mastermind behind closed doors. Forgive me if I am miss-characterizing his perception, but is there any basis for the speculation that Reagan was already suffering from the mental deterioration of his disease while in the White House, much earlier than his diagnosis? If so, is there any reason to believe that this impacted the decisions and outcomes of his administration?
Edit: I misattributed /u/Nadarama and what he said, for that I apologize! I've amended my statement to reflect that.
/u/Nadarama suggests that the consistent "I don't recall" or "I don't remember" statements Reagan made during the Iran-Contra hearings could be due to Alzheimer's, though it's not definitive. This is a legitimate hypothesis, I would like to reiterate, and one I agree with. As noted in "Ronald Reagan's Presidency: The Impact of an Alcoholic Parent":
Some may attribute Reagan's words [in the hearings] to early signs of the Alzheimer's Disease that was diagnosed in 1994; others to the self-delusions that so commonly characterized his behavior throughout his life.
Reagan had a complex life, and as the title implies, a quite impactful childhood. There's no way to really know if this was a manifestation of that, or if he had early signs of Alzheimer's. Doing some more research into the subject, I did manage to find a thing or two. The best source I found was a November, 1994 article revealing his diagnosis and details about it, in the New York Times.
In November of 1994, Reagan's doctors revealed that they had been considering a diagnosis of Alzheimer's for a year. It was also revealed by an unnamed associate of Reagan that he'd been having a definite memory problem for two years. That would place the memory issues as definitively being noticed in 1992 by close friends of Reagan.
In 1994, a statement from his doctors said he was "entering the early stages of this disease."
It's definitely plausible that he was suffering to some degree during the 1990 hearings (those were the Iran-Contra ones), considering he couldn't remember the name of his chief-of-staff. However, the chances of it manifesting during his Presidency were a bit lower. While he did have memory "farts" and did have some issues with memory, he also was very public and careful about "senility". He announced publicly in 1980 that he wanted to be watched for "senility", and reportedly got checked by his doctors very often.
While some experts believe that Alzheimer's was not usually diagnosed unless all other possibilities were ruled out, which was very rare with public, important, or well-liked patients, that isn't to say that they didn't check. And they did, often, according to the New York Times article as well.
Looking at the stages of Alzheimer's, it's very possible to say that by 1990 he had entered the second stage, since he couldn't remember some things like names, but it was really noticeable only in 1992 to his friends, and I think it's safe to say that's probably when he entered the third stage (where it appears he progressed to stage 4 when diagnosed in 1994 and continued degenerating). Because of his status and public attention, it's likely that people simply seized on early signs of it immediately, since so much of what he said and did was under intense scrutiny. However, the chances of it affecting his decisions and planning are (at least in my opinion) pretty low. Did he probably have some momentary lapses in memory that would've been roughly the same for many people his age? Yeah. However, he was flanked by aides and constantly discussion issues, as Presidents tend to. It doesn't seem very likely that by 1988 he was so far down that hole that he began making decisions based on those lapses in memory, especially since family/friends said they didn't notice a definite memory problem until 1992.
Hope that sheds some light on it!
Source:
M.D., L. K. A.,. (1994, Nov 08). DOCTOR'S WORLD; reagan and alzheimer's: Following path his mother traveled. New York Times.
Ronald Reagan's Presidency: The Impact of an Alcoholic Parent Robert E. Gilbert Political Psychology, Vol. 29, No. 5 (Oct., 2008), pp. 737-765
The only source I can think of for this is Edmund Morris’ biography of Reagan. Morris is very clear that Reagan had a certain “blankness” to him throughout his adult life:
"Decades before Alzheimer's clouded Reagan's mind, he showed a terrifying lack of human presence. "I was real proud when Dad came to my high school commencement," reports his son, Michael Reagan. After posing for photos with Michael and his classmates, the future president came up to him, looked right in his eyes, and said, "Hi, my name's Ronald Reagan. What's yours?" Poor Michael replied, "Dad, it's me. Your son. Mike."
Somewhere in the book Morris also claims that Reagan’s mental capacities slowly began to decline after the assassination attempt in 1981; but in this article, Morris is pretty emphatic that he feels Reagan did not have any symptoms of Alzheimer’s while in office:
http://www.newsweek.com/edmund-morris-reagan-and-alzheimers-66943
This press conference from 1987: http://youtu.be/n8gN-GxiRD8 is a good example of the type of behavior that Reagan would sometimes fall into when making non-scripted public appearances. On one level it all kind of makes sense, but there is a kind of slipping in and out of a logical response to the questions that began to trouble people.