AMNA NAWAZ: If he is reelected, former President Donald Trump, now 78 years old, would be the oldest president ever elected.

After a number of appearances where his remarks were rambling or incoherent, and one event in which he swayed silently to music on stage for close to 40 minutes, questions are being raised about possible cognitive decline.

Here are a couple of recent events that sparked concern, the first one in which he began talking about electric vehicles, then switched to a story about an electrically powered boat.

DONALD TRUMP, Former President of the United States (R) and Current U.S. Presidential Candidate: Let's see your boat goes down, and I'm sitting on top of this big powerful battery, and the boat's going down.

Do I get electrocuted?

(LAUGHTER) DONALD TRUMP: And he said: "You know what?

Honestly, nobody's ever asked me that question."

But if I'm sitting down and that boat's going down and I'm on top of a battery, and the water starts flooding in, I'm getting concerned.

But then I look 10 yards to my left and there's a shark over there.

So I have a choice of electrocution or a shark.

You know what I'm going to take?

Electrocution.

I will take electrocution every single time.

Let's listen to Pavarotti sing "Ave Maria."

Can you hear that?

(MUSIC) AMNA NAWAZ: Mr. Trump has dismissed any speculation about mental decline, describing his rambling rhetoric as him weaving together different topics and saying his supporters get it.

Let's take a look at some of these questions with an author and clinical psychologist who's raised some of them.

That's Dr. Ben Michaelis.

Doctor, welcome to the "News Hour."

Thanks for joining us.

DR. BEN MICHAELIS, Clinical Psychologist: Thank you for having me.

AMNA NAWAZ: So, Doctor, both his political opponents, but also some academics, look at his speech patterns, they look at his stories and some odd behavior and they say it's all evidence of mental decline.

Do you see it that way?

Are those concerns valid?

DR. BEN MICHAELIS: So, obviously, I can't diagnose someone in absentia, but, certainly, there is ample evidence.

And, look, he's 78 years old.

Just to be fair, all of us tend to decline over time.

Some of what we're seeing is pretty extreme elements of his decline that are certainly suggestive of more serious impairment.

AMNA NAWAZ: Well, tell us a bit more about what you see that says that to you, especially in terms of who he is today versus a year ago or even to Trump of 2016.

DR. BEN MICHAELIS: So, I did an analysis for STAT News about seven years ago, looking at the complexity of his speech from when he was in his 40s to when he was running for president back in 2016.

And at that time, there was definitely a difference both in his sentence structure and in the complexity of his vocabulary between from when he was, say, in 40s to when he was in his earlier later 60s or earlier 70s.

What's happened between 2016, 2017 and now is not so much a difference in the vocabulary.

The vocabulary is not significantly less complex than it was, but it's in the thought patterns.

So he's not staying linear.

So, right now, we're having a conversation.

We're staying on topic.

You're asking questions.

I'm responding.

And what we're saying kind of has a sort of hand-in-glove quality to it.

But what we're seeing with Trump is, basically, he's moving further and further away from linearity, from linearity to tangentiality, so he's sort of weaving together ideas, and then it's moving further afield towards circumstantiality.

And that's really where you sort of start talking about a topic, and then you just really sort of lose the thread entirely.

And there's a lot of that we're seeing in his speech patterns.

AMNA NAWAZ: Now, as you know, there are others who see this differently, and they think that his speech patterns, his remarks, his behavior don't necessarily suggest anything, especially cognitive decline.

Among them are Dr. Jamie Reilly, he's director of a cognition lab at Temple University.

Here's part of what he had to say.

DR. JAMIE REILLY, Temple University: There are people who count syllables, and they count the speech rate, and they count the number of things like how much he curses and things like that, and have noted changes.

They have noted changes in his syntax, the grammatical complexity of his language.

The question of whether that is a marker of a cognitive -- of a disease process is really, really tricky.

So, when you read that literature, you will see that people note that there are changes, but they don't take that next step of saying, this is a marker of something, right?

We just know that there are some changes happening, and sometimes people run with that, and they say this is evidence for something, but I'm not sure you can make that leap at all.

AMNA NAWAZ: Dr. Michaelis, what do you make of that?

Is that a leap too far to make?

DR. BEN MICHAELIS: So, again, not making a formal diagnosis of dementia -- you need to actually assess him formally and face-to-face in order to do that.

You can't diagnose anyone in absentia.

But what we're seeing is real, and it's -- part of the challenge is, so, look, again, he's 78 years old.

If this was your grandfather, you probably wouldn't think twice about it.

Again, we all decline with age.

What I'm seeing is more challenges with the thought processes, that, again, I'm not saying that he has dementia, but they are suggestive of, if you sort of took this all in totality, right?

So, problems with memory loss, communication difficulties, changes in mood, poor judgment, personality changes, those in total are what really the symptoms of dementia.

And there's certainly evidence to suggest, and I think it is a reasonable thing to think about this.

I mean, this is a person that is running for the most powerful office in the world.

And I think we should be very thoughtful about who we're putting in that office, because these are real concerns.

His decisions were he to be reelected affect all of us.

AMNA NAWAZ: You mentioned dementia specifically.

There's a family history I want to ask you about, because his father, Fred Trump, was diagnosed with dementia.

Does that mean anything for a former President Trump's likelihood of also potentially developing dementia?

DR. BEN MICHAELIS: Again, it just increases the odds.

We know these things run in families.

There's certainly evidence of that.

And there are sort of genetic markers that you can certainly take tests for to see.

But all that does is increase the odds.

So you take that information, which -- as well as all the different things that you're seeing, and it paints a picture.

And what I'm hoping to do is just put a little bit of a spotlight on this, so that people make an informed decision about who they are putting in the Oval Office.

That's all.

AMNA NAWAZ: Separate and apart from any formal diagnosis, there are those who say, look, this is just who President Trump is and has always been, right, that he occasionally speaks this way and tells incoherent stories and uses more brash and aggressive language.

It's just Trump being Trump and has nothing to do with cognitive decline.

Could that be it?

Could it just be personality?

DR. BEN MICHAELIS: I mean, I suppose, but, look, he -- the fact of the matter is that he is an entertaining -- he's an entertainer fundamentally, and people -- he's entertaining to watch for people.

So if you're not trained in this, you may be focused only on -- and he has a lot of vigor.

Let's be very clear.

For a 70-year-old man, he has a lot of vigor.

And so you're focusing on those things, but not the sort of total picture of what we're seeing.

And this isn't -- from my perspective, this is not just sort of Trump being Trump.

Like, if you look at footage for him over the last few years, he seems to be becoming more and more circumstantial, I mean, this idea about sort of starting talking about electric cars and then moving to sharks, and then this sort of digression with music the other day, where he was swaying to music for 30 minutes, and then using more and more sort of derogatory language.

Again, it just paints a concerning picture.

AMNA NAWAZ: What would it take to definitively answer or quiet these concerns?

DR. BEN MICHAELIS: Look, I think that anyone that is running for office, and certainly the highest office in the land, should be evaluated by an independent evaluator, not someone that is appointed by his team or by the opposition, by someone that is truly independent.

And maybe it's three independent evaluators.

But that seems appropriate to me, right?

We license people to drive.

When you're behind the wheel of a two-ton death machine, you could harm people.

And think about the power that is invested in the presidency.

So it just seems appropriate to do a formal evaluation of him.

AMNA NAWAZ: That is clinical psychologist Dr. Ben Michaelis joining us tonight.

Dr. Michaelis, thank you for your time.

We appreciate it.

DR. BEN MICHAELIS: Thank you.