Washington DC - As Joe Biden's verbal gaffes, shaky voice, and other troubling signs have brought an intense focus on the president's mental acuity, health experts are calling on him and rival Donald Trump to pass additional cognitive tests, even while warning against leaping to conclusions.

Is Biden truly compos mentis or not? That one is a matter for the experts.

Cognitive tests, experts said, could either help repudiate speculation that the 81-year-old president's mental state is in worrying decline – or else confirm it – and could enlighten voters on the mental abilities of Trump, who has had his own share of verbal lapses.

But reliable diagnoses, they caution, cannot be made from afar.

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Since Biden's disastrous performance in his debate with Trump two weeks ago, the Democrat's campaign has faced fierce opposition. A growing number of officials in his own party are questioning his ability to lead the country for four more years.

Even longtime supporter Barack Obama has seemingly joined the din of voices calling for Biden to step down.

It doesn't help matters that Biden recently tested positive for Covid.

Dennis Selkoe, a neurologist at Harvard Medical School, said the fundamental issue is whether Biden is suffering from "a normal age-related process" or "something that represents a neurologic disease."

"Making a mistake with a name is not automatically a sign of dementia or of Alzheimer's," he said, referencing Biden's recent flub in accidentally calling Ukrainian President Zelensky the name of his arch-rival, Vladimir Putin.

But Selkoe, who sees many patients with neurodegenerative problems, said Biden does seem to have the "appearance of an early Parkinson patient" – including his slow, stiff gait and his low, sometimes barely audible voice, which could be a condition known as hypophonia.

In February, Biden underwent a complete physical exam. A published summary of its results indicated that an "extremely detailed neurologic exam" had ruled out Parkinson's.

No detail was provided on the exact nature of the tests or their results, however.

Could a neurological illness have taken root over just the past five months? If the exams in February had been comprehensive, Selkoe said, there should have been early signs of a nascent condition.

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In an editorial in March, the scientific journal Lancet called for standardized procedures to examine the health of sitting and prospective presidents so as to insulate American voters from a "pestilence of speculation, misinformation, and slander."

Absent such reliable testing, "the US public remains beholden to voluntarily released reports from politicians' personal physicians," the journal said.

Biden has said that he effectively passes a cognitive test every day, simply by carrying out his presidential duties.

At a news conference on July 11, the president said he would be willing to take a new neurological exam if his doctors recommended it, but that "no one is suggesting that to me now."