What are cognitive tests and what can they tell us about Biden and Trump?

Age and mental fitness recently have become a leading issue in the US presidential race.

While the issue has loomed over President Joe Biden, 81, and his opponent Donald Trump, 78, in the past, things reached a tipping point after Biden's weak debate performance late last month.

Mr Biden is the oldest president in US history and Trump, if elected, would be the second oldest.

Mr Biden, during an interview with ABC News, refused to commit to taking a cognitive test, saying he has "a cognitive test every day" and his doctors say he doesn't need one.

Trump has said he has completed cognitive tests, one while he was president, and another more recently. He said he aced both.

Here's what the tests are, their purpose and how hard they are to pass.

What does a cognitive test do?

There are different tests and screenings to measure how well the brain is functioning.

They don't determine a specific disease, but help signal if additional tests are needed for diagnosis, according to the Cleveland Clinic.

Typically, a cognitive screening could be warranted if a person is having problems with memory, personality changes, or balance, or if they are repeating themself, forgetting parts of their past or having problems understanding information.

One of the most widely used tests is the Montreal Cognitive Assessment (MoCA), a quick way to "assess cognitive skills in people with suspected deficits," according to Sanford Medicine.

The assessment tests orientation, memory, attention and the ability to name objects, and follow verbal and written commands. It's also available online.

It's typically easy for someone without cognitive impairment, but is harder for those declining mentally.

The creator of the test, Canadian neurologist Ziad Nasreddine, told the BBC that he thought the test could be good for Biden - both as a way to reassure Americans and in case there is a problem.

What does a cognitive test look like?

In brief cognitive tests, medical professionals ask patients a variety of learning and memory questions.

Longer clinical evaluations include cognitive tests plus a physical and neurological exam and a full history of a patient.

The latter could paint a clearer picture of the cognitive abilities of both Mr Biden and Trump, said Dan Mungas, an associate director of the University of California's Davis Alzheimer’s Disease Research Center.

Often medical professions will start with a brief test, such as the MoCa, and if their score is lower than expected, they move to a more comprehensive examination.

More in-depth tests assess language, executive function and visual-spatial abilities.

For example, a medical professional may read a story to a patient then ask them to recall parts of the story to test memory and learning abilities.

A patient might be asked to recall a list of words, name objects in pictures or name items that start with a specific letter.

In addition to questioning the patient, Dr Mungas suggests speaking with people who spend time with the patient regularly to determine symptoms of cognitive decline.

Dr Mungas said it is important to see if someone's abilities have changed over time, adding that a one-time assessment can be misleading.

"You have to understand where a person started out, if they're declining from where they've been before, that's a bad sign," he said.

But he notes, cognitive tests are not everything.

"The idea that there’s a simple cognitive test that says this person's going to be a good president doesn’t make any sense and I’ve been doing cognitive testing all my career,” Dr Mungas said.

What does Biden and Trump's age tell us about their likelihood of passing a test?

The American Academy of Neurology (AAN) recommends doctors assess people older than 65 for their cognitive abilities.

That determination was made because as age increases, so does the likelihood of impairments, Dr Nasreddine told the BBC.

By age 75, 25% of patients will have a cognitive disorder of some kind, he warned.

"It becomes extremely frequent that cognitive impairment is present, and sometimes it’s present without people realising it’s present,” said Dr Nasreddine, who has neither treated nor met the president.

He said he has noticed a change in Mr Biden in the past year. In public appearances, the president moves slowly, his speech has slowed, his voice is very low, and he mumbles and slurs some words, he said.

Not many people have such an intense job at Mr Biden's age, he noted, and it is difficult to describe the normal function of someone at that age.

"I haven't seen this [decline] in previous years, only in the past year I have seen this," he said.

Dr Nasreddine noted that, despite only being three years younger, Trump seems more vigorous.

25th Amendment: What if a president has dementia or Alzheimer's disease?

The 25th Amendment of the US Constitution lays out the line of succession and procedure if a president dies or is "unable to discharge the powers and duties of his office".

The amendment notes it should be used if a president is removed from office, incapacitated, dies or resigns.

The amendment was ratified in the aftermath of the assassination of President John F. Kennedy but has become a topic of debate in recent years.

Congressional lawmakers eyed legislation during Trump's presidency that would alter the amendment to enable a panel of medical experts to determine a president's fitness for office.

Democrats also approved a House resolution after the US Capitol riots in 2021 that called for then-Vice-President Mike Pence to invoke the 25th Amendment to remove Trump from office.

Neither move went anywhere.

Following Mr Biden's debate performance, some Republicans have called on members of his cabinet to invoke the clause.

Section 4 of the amendment states if the vice-president and a majority of the cabinet or Congress deem the president is "unable to discharge the powers and duties of his office," then the vice-president would take office.

It has never been used or tested in the case of mental decline.

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