US President Donald Trump received a perfect score in a cognitive function test used to detect a mental decline in someone who's otherwise healthy.

White House doctor Ronny Jackson said Mr Trump did "exceedingly well" in the Montreal Cognitive Assessment (MoCA). But what did the President have to do?

What's a MoCA?

It's a relatively simple test that helps health professionals tell whether someone has an abnormal cognitive function — whether their ability to think is disturbed.

Cognitive function refers to memory, speech, reading comprehension and the ability to learn new information.

The one-page test takes about 10 to 12 minutes to complete.

The test was created in 1996 by Ziad Nasreddine in Montreal, Quebec.

But it wasn't until 2005 that McGill University researchers found it had the potential to detect early Alzheimer's disease and mild cognitive impairment (MCI).

It's now available in 55 languages and dialects.

Sorry, this video has expired White House doctor says Trump in "excellent" health

What do you actually have to do?

MoCA assesses orientation, short-term memory, executive function, language abilities, attention and visuospatial ability.

The test is worth 30 points, with each task allocated a value between one and six points.

Here's what the tasks consist of:

Short-term memory — learning five nouns and recalling them five minutes later

— learning five nouns and recalling them five minutes later Visuospatial ability — drawing a clock and a three-dimensional cube

— drawing a clock and a three-dimensional cube Attention — reading digits forwards and backwards; a serial subtraction task — counting backward from 100 in sevens; reading a list of letters and tapping each time at a specific letter comes up

— reading digits forwards and backwards; a serial subtraction task — counting backward from 100 in sevens; reading a list of letters and tapping each time at a specific letter comes up Language — naming three low-familiarity animals; repeating two sentences with a complex syntax; naming as many words as you can in one minute that begin with a certain letter, say, F

— naming three low-familiarity animals; repeating two sentences with a complex syntax; naming as many words as you can in one minute that begin with a certain letter, say, F Orientation — asking the subject for the date and city in which they are doing the test

— asking the subject for the date and city in which they are doing the test Executive functions — an alternation task adapted from a trail-making task; a phonemic fluency task testing sound comprehension; and a two-item verbal abstraction task — for example: what is the similarity between bananas and oranges? Fruit

In the clock-drawing task participants are asked to draw a clock showing a specific time. In this illustration, AD stands for Alzheimer's disease. ( WikiMedia: M Mattson )

What's a good score?

A score of 26 or over is considered to be normal.

The initial study by McGill University researchers showed that people with good cognitive function scored an average of 27.4; those with MCI got an average of 22.1 and people who had Alzheimer's had an average score of 16.2.

Mr Trump scored 30 out of 30.

He has previously declared himself "a very stable genius".

The test did not assess his psychiatric fitness and his doctor said he didn't undergo a psychiatric evaluation.

What do doctors say?

An expert in memory disorders says patients with good or average memory forget one of the five words but can still be within the "normal" range.

Dr James Mastrianni from the University of Chicago Medicine says MoCA is routinely used in clinics.

"If they [patients] score poorly on that assessment, then usually there is more detailed evaluation that follows. But if they score well that usually indicates there is pretty good cognitive function. They are essentially intact," he said.

But an Alzheimer's disease expert, Dr Ronald Petersen from Mayo Clinic in Rochester, Minnesota, says while the test is "pretty good" it's "not definitive".

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ABC/Reuters