As Glen Campbell transformed from a country music legend to an Alzheimer’s patient, Dr. Ronald Petersen was by his side.

Petersen, director of the Mayo Clinic Alzheimer's Disease Research Center, was the late entertainer’s physician. He saw first-hand how music and personality affected the course of the disease in Campbell, robbing him of the ability to remember lyrics, but not guitar solos or social graces.

The musician and his family were extraordinarily open about his diagnosis in 2011, going on a goodbye tour and starring in the documentary, "Glen Campbell: I'll Be Me," released in 2014.

With 5.5 million Americans living with Alzheimer’s, Campbell's journey was painfully familiar to many families. Petersen talked with TODAY about his famous patient’s experience and legacy.

What do you think about Glen Campbell’s decision to be so open about his diagnosis?

Petersen: The impact on Alzheimer’s awareness was tremendous. It let the general public know anybody can get this disease, regardless of their background, intelligence, or occupation. It really cuts across all sectors.

Glen not only came out and said, “This is what I’ve been diagnosed with,” but he and his family said, “We’re willing to have a documentary made to characterize how we deal with this.” That is unique.

It’s comforting for other families who are dealing with this to know they’re not alone. Having somebody like Glen Campbell make that announcement — I think it makes people feel more comfortable.

Campbell didn’t forget how to play guitar — what do you make of that?

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Petersen: He needed a teleprompter to help him with the lyrics that he’d been singing for four or five decades. Yet, you give him a cue — it might say on the teleprompter, “Glen, play a long guitar solo here” — and he does a perfect guitar solo for five minutes.

He was a striking example of the dissociation between some memory and verbal skills, and then motor skills that go along with playing a guitar and remembering the music. It does show that different regions of the brain are affected differently by the disease process.

What that tells us is maybe we shouldn’t dwell on what we cannot do because of the disease, but focus on what we still can do. One interpretation of the documentary is exactly that. Here is a person who really was moderately affected by the disease, yet he continued doing what he loved to do, he was surrounded by his family and friends and maintained a pretty high quality of life well into the disease.

How does music affect Alzheimer’s patients in general?

Petersen: Music stored in the brain from a bygone era can actually stimulate some memories and some motor functions.

If you think about songs that you knew when you were a teenager, those memories — like old memories of families and photographs — are relatively retained far into the disease and can be a source of comfort for the person.

Music therapy can have a calming effect, reducing the need for medications. If an Alzheimer’s patient becomes agitated later in the day, so-called sundowning, rather than giving them a sedative medication that may have side effects, you can give them a musical experience. It’s beneficial and enjoyable.

Campbell struggled with alcohol and drug abuse. Could that have caused Alzheimer’s?

Petersen: Saying it would cause Alzheimer’s disease is probably stretching it, because it would mean that alcohol or drug usage actually leads to the development of plaques and tangles in the brain, and we clearly don’t know that.

On the other hand, severe alcohol use and certain drug behaviors can compromise the brain’s resilience. If we’ve had certain insults over our life — be it alcohol, drugs or head injuries — that may reduce the ability of the brain to compensate for the development of either aging or Alzheimer’s-type pathology.

Other people who have not had those kinds insults might be able to compensate — their brains might be more plastic, more resilient — and stay functional at a higher level for a longer period.

Campbell seems to have stayed sweet and upbeat. Do people’s personalities stay the same after a diagnosis?

Petersen: In general, I think most people remain consistent with their previous personality characteristics.

If a person was a nice individual throughout his or her life, that probably will continue and that certainly was exactly the case with Glen.

On the other hand, there are instances where people do undergo a change. The plaques and tangles that cause the disease may be distributed in the brain in different regions in different people. If you get more of the frontal lobes involved, which to a certain extent dictate our behavior, personality, social appropriateness, you may get these behavioral personality changes. It’s variable.

Glen remained the nicest person — always socially appropriate, cordial, polite well into the disease. He may not know who you are, but he still treats you with respect and you could have a reasonable — somewhat superficial — but a very nice conversation with him. I never saw him irritable, impolite or rude at any point in the disease process.

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