[James]

I was looking at the Wikipedia page for Memento and it has this graphic about the film's structure.

Spoiler:

The film is structured with two timelines: color sequences are alternated with black-and-white sequences. The latter are put together in chronological order. The color ones, though shown forward (except for the first one, which is shown in reverse), are ordered in reverse. Chronologically, the black-and-white sequences come first, the color sequences come next.

Using the numbering scheme suggested by Andy Klein—who took numbers from 1 to 22 for the black-and-white sequences and letters A–V for the color ones in his article for Salon magazine[4]—the plotting of the film as presented is: Opening Credits (shown "backward"), 1, V, 2, U, 3, T, 4, S, ..., 22/A, Credits.

There is a smooth transition from the black-and-white sequence 22 to color sequence A and it occurs during the development of a Polaroid photograph.

The chronological order of the story can be viewed as a "Hidden feature" on the 2-Disc Limited Edition Region 1 DVD and the 3-Disc special Edition Region 2 DVD. In this special feature the chapters of the film are put together into the chronological order and is shown: Ending Credits (run in reverse), 1, 2, 3, ..., 22/A, B, ..., V, then the opening title runs "backward" to what was shown (the opening title sequence is run in reverse during the actual film, so it is shown forward in this version).

Stefano Ghislotti wrote an article in Film Anthology which discusses how Nolan provides the viewer with the clues necessary to decode the plot as we watch and help us understand the chronology. The color sequences include a brief overlap to help clue the audience into the fact that they are being presented in reverse order. The purpose of the fragmented reverse sequencing is to force the audience into a sympathetic experience of Leonard's defective ability to create new long-term memories, where prior events are not recalled, since the audience has yet to see them.

Also this is what it says about the portrayal of memory loss.

Scientific response
Many medical experts have cited Memento as featuring one of the most realistic and accurate depictions of anterograde amnesia. Caltech neuroscientist Christof Koch called Memento "the most accurate portrayal of the different memory systems in the popular media", while physician Esther M. Sternberg, Director of the Integrative Neural Immune Program at the National Institute of Mental Health, identified the film as "close to a perfect exploration of the neurobiology of memory."

Sternberg concludes:
This thought-provoking thriller is the kind of movie that keeps reverberating in the viewer's mind, and each iteration makes one examine preconceived notions in a different light. Memento is a movie for anyone interested in the workings of memory and, indeed, in what it is that makes our own reality.

Clinical neuropsychologist Sallie Baxendale writes in The BMJ:

The overwhelming majority of amnesic characters in films bear little relation to any neurological or psychiatric realities of memory loss. Apparently inspired partly by the neuropsychological studies of the famous patient HM (who developed severe anterograde memory impairment after neurosurgery to control his epileptic seizures) and the temporal lobe amnesic syndrome, the film documents the difficulties faced by Leonard, who develops a severe anterograde amnesia after an attack in which his wife is killed. Unlike in most films in this genre, this amnesic character retains his identity, has little retrograde amnesia, and shows several of the severe everyday memory difficulties associated with the disorder. The fragmented, almost mosaic quality to the sequence of scenes in the film also reflects the 'perpetual present' nature of the syndrome.