"Can you make sure it has lots of stuff?" my mum would regularly ask the server at her favourite Italian restaurant.

Apparently, the minestrone soup was mostly broth with just a couple of noodles, which was unacceptable. "Really scoop it up," she instructed, adding her nervous, friendly laugh.

With her salmon, she demanded all sauces on the side, and an additional plate of lemon slices. Sometimes she brought her own dessert from the place she liked down the street. Her fastidiousness made me want to shrink into the jar of Parmesan cheese, but what could I do? That was my mum.

Today, I'm sitting with Mum as she happily eats everything on her plate - chicken, potatoes, pie, things she had elaborate reasons for avoiding for as long as I can remember. My mum, Dorothy, is 79 and one of the 5.3 million Americans who have Alzheimer's disease. Mum is in one of the middle stages. She can't string a sentence together.

But while her disease is devastatingly sad, there's a weird and welcome side effect: She's often more fun to be around.

My mother lives in Cherrywood Village, in Louisville, Colorado, an assisted-living facility that provides "memory care" for some 50 residents, most of whom have Alzheimer's or some other form of dementia. Cherrywood is built to look like a small American town, with a red brick square where residents regularly sing songs, bounce large rubber balls to one another and enjoy singers who reproduce the Frank Sinatra and Glenn Miller hits of their youth. This is where Julianne Moore's character in "Still Alice" moves in after the movie ends.

My mum's new personality skews busy social butterfly. Her old laugh is intact. It is a nervous "heh heh heh" that she once wielded to soften negotiations - after ordering food to her exacting standards, or negotiating with colleagues in her uncompromising way, the laugh would say: "I'm still nice and let's be friends." Today, it contains no self-consciousness: She laughs when she's happy.

She likes to hang around the Cherrywood director's office, shuffling papers and talking to the staff as if closing business deals. When we walk through the halls, she frequently reaches past me to connect with other residents, touching them lightly on their wrists or hands. She responds "sure!" to every activity I suggest, whether it's sitting on a bench, getting food or posing for a selfie.

One Sunday morning, I arrived for a visit and found Mum enthusiastically singing along to Christian hymns, although she was raised an Orthodox Jew and spent most of her life as an agnostic.

After lunch, as we walk, she says: "I always wait for Steve." For a moment I think she remembers my name - something that hasn't been true for at least a year. "What do you mean, Mum?" I ask. Maybe she is sending a message that I don't visit frequently enough (which is probably true).

But the short-term memory loss kicks in within seconds, and she loses the thread, as always. "The bagels are float for J.J.," she says. Another frustrating mystery.

I ask whether she prefers walking or sitting. "No - like, petting," she says. I think I know what she means. She craves contact.

This is strange for me, since my mum was never much of a hugger. I lean over and wrap my arms around her. She holds the embrace for several seconds.

"Titter," she says later.

"What, Mum?" "Titter. Titter!" She flaps the top of her blouse so there is no doubt what part of her anatomy she is referring to. In lieu of asking what she means, I laugh. She gives up and laughs, too.

My oldest brother Mark's shorthand for this behaviour is "wacky". He finds it hilarious. He lives in Michigan, and talks to our mum by phone every day. He enjoys trading gibberish with her in their incomprehensible conversations. "She's completely unplugged from reality and disoriented to time and space," Mark says. "It doesn't make any sense, so why not have a good time?"

At first, I worried Mark's antics were making fun of Mum - laughing at her, rather than with her. Then I tried it. I stopped concentrating on the content of her words and, instead, went along with every twist. Once she said she wanted to go somewhere for a picnic. "Why, Mum?" I asked. She said: "So we can boil the chicken." Rather than grilling her about what she meant, I asked what kind of chicken we should boil and where we should hold this picnic.

Please don't misunderstand. Alzheimer's is terrible. I don't like the way Mum can no longer brush her teeth, and how her smile has become a sad brown-yellow. The light in her blue-grey eyes is intermittent. She wears whatever her care- givers put on her, and this Saturday afternoon it's a sparkly pink-and- white blouse, nothing special, with beige slacks and matching slippers.

Mum of just three or four years ago would have been mortified by Mum of today. She used to be a force, running three miles every day and overseeing an entrepreneurial empire of quarterly gifted- education journals and a yearly conference providing resources for children, parents and teachers. God help the hapless school administrator who dared to cut funding for these students.

It's impossible to pinpoint when Alzheimer's begins. I wonder if an early-onset form of the disease changed my mum's personality long before any of us suspected it, possibly in the 1980s, when she was in her 40s and I was beginning my teens. Shortly after my parents followed a long-time dream of living in the Rocky Mountains, and moved from Michigan to Colorado, she downsized almost everybody out of her life. Only Dad, my brothers, Mark and Doug and I, and, later, our own families were allowed inside her protective circle.

When Dad died in 2008, of a rare blood disease that resulted in leukaemia, Mum's behaviour became more noticeable. She insisted, repeatedly, that everything was fine. But her finicky food habits and lifestyle routines became ironclad - she refused to make any meal for herself other than a concoction of pita, yogurt, spinach and nuts. She scribbled Post-it speeches about why she was fine and that we should all leave her alone. Then she read them to us over the phone, in a stiff and uncompromising voice. For a few hard years, Alzheimer's divided my brothers and me.

Eventually, it became impossible for any of us to ignore. We hired a Boulder caregiving company specialising in dementia. With their advice, and fighting off guilt, my brothers and I used trickery to drag Mum away from her car, house and twisty mountain roads. She visited Doug's family one weekend in Palo Alto, California. While she was gone, we'd furnished an apartment at an assisted living centre with her belongings. I'd never seen Mum so furious.

That was three years ago. She lasted about six months at the apartment, until she attempted to microwave a metal bowl. We had to move her to Cherrywood.

At first, she complained bitterly about the place, but something unexpected happened. Instead of secluding herself, she reached out, connecting with other people and making new friends - something she rarely did in her "real" life. One friend was a docile woman who spent long hours relaxing silently on the Cherrywood couches. Sometimes the two of them cuddled on the couch together. I'd find them this way on visits and think: This is my mum? She doesn't cuddle.

INTOXICATED

Mum's private caregiver, Ms Ellen Knapp, has a term for this type of behavioural change: disinhibition. It means, essentially in this context, that your brain forces you to let your guard down, like someone who's intoxicated. "Perhaps these were qualities in her life that she was suppressing," says Ellen, a Boulder geriatric care manager with a master's degree in psychology. "Your previously held rules diminish."

It was Ellen who taught me how to talk to Mum in this new phase of her life. The technique is called validation, and Ellen learned it from the veteran author and psychologist Naomi Feil. As Dr Feil explains it to me in a phone interview, this translates to "accepting whatever behaviour the person has and trying to become a part of it". For example, a carpenter with Alzheimer's might pound his fist against a wall. The wrong way to talk to him would be to say: "Why are you pounding the wall? Stop it!" The right way would be to ask: "Is that wood made out of oak or pine?"

I'm not exactly a Jedi knight of this technique, but I find it relaxes Mum and makes her more easygoing and less frustrated. The key, for me, is to stop judging or trying to analyse or change her behaviour. That means fewer questions about her past, even if that subject is difficult for me to avoid - in the past, she was my mum! That's what I really want to talk to her about, not chickens and picnics. Occasionally I can't resist and I ask whether she remembers Dad, her husband of 54 years. Sometimes she says yes. Sometimes she says no. It's a dead end either way.

The photo outside Mum's apartment door at Cherrywood is a familiar one - she is crossing the finish line of the Bolder Boulder, a face in a crowd of exhilarated runners, raising her arms in victory. I ask if she remembers the race she'd run more than 25 years in a row, every Memorial Day without exception. "Yeah," she says, and her eyes flicker briefly with recognition. I follow up excitedly. "What do you remember, Mum? The running? Or brunch afterwards?" In lieu of answering, she says: "So I went home, and bome de bome!" For an instant, I'm disappointed. A memory from Mum is too much to hope for. So I scale down my expectations, and put my arm around her shoulders. We laugh together for a long time. "Bome de bome" - that's pretty funny, when you think about it. Of course, she hugs me back.

NEW YORK TIMES