(CNN) - Former President Ronald Reagan's youngest son suggests in a new book that his father showed signs of Alzheimer's disease while he was in the White House.
In the book titled "My Father at 100," which is due out next week, Ron Reagan writes, "Three years into his first term as President … I was feeling the first shivers of concern that something beyond mellowing was affecting my father."
He writes about watching his father's first debate with Walter Mondale, the 1984 Democratic presidential nominee.
"I began to experience the nausea of a bad dream coming true," Ron Reagan writes. He adds: "My heart sank as he floundered his way through his responses, fumbling with his notes, uncharacteristically lost for words. He looked tired and bewildered."
But the younger Reagan also admits, "I've seen no evidence that my father (or anyone else) was aware of his medical condition while he was in office." He then questions: "Had the diagnosis been made in, say, 1987, would he have stepped down? I believe he would have."
Former president Reagan disclosed he had Alzheimer's disease in 1994, five years after he left the White House. Questions have been raised in the past about whether he developed the disease while he was still in office, but suggestions that he did have been widely dismissed.
In an effort to set the record straight, four of the president's White House doctors spoke to the New York Times in 1997 to say the president didn't show evidence of the disease until 1993 and that he was mentally sound while in office. The newspaper reported the doctors said, "they had taken the unusual step of discussing their former patient's medical history publicly because neither they nor Mr. Reagan had covered up any illness, and because they did not want history to see them as having done so."
Reagan biographer Lou Cannon who interviewed the former president more than 100 times said he interviewed him after he left the White House and did not see a difference.
“I think I would have noticed,” Cannon said in a statement. “There’s simply no evidence he had Alzheimer’s while in the White House.”
Cannon said Reagan was “lucid” during their last interview in 1991.
Some people who served in the Reagan administration also are dismissing Ron Reagan's new claims.
Kenneth Duberstein, who served as Reagan's chief of staff at the end of his second term, told CNN Chief National Correspondent John King: "I think Ron these days is mostly in the business of trying to sell books. That's what I think." About the older Reagan, Duberstein said: "Day in, day out, from beginning to end, he was in command. He was fully in command."
Bill Bennett, a CNN contributor who served as education secretary under Reagan said, "In all my interactions with the president, I never witnessed anything in him to give me any concern."
CNN's Senior Political Analyst David Gergen, who was director of communications for Reagan, told Wolf Blitzer on CNN's "The Situation Room:" "I'm sure Ron has written a good book. I look forward to reading it, but I'm surprised he's revived these claims." Gergen added, "I think (Ron Reagan) got this wrong, but he's a good fellow."