My grandmother spent the last twelve years of her life in a nursing home. The place was a twenty-minute drive from my parents’ house, so my mother visited regularly, even after my grandmother’s dementia reached the point that memories of her daughter had vanished into the tangled circuits of her brain.

This greatly disturbed my mother. She complained that my grandmother believed my mother to be her sister—my mother’s aunt—and persisted in this fantasy even after my mother repeatedly corrected her. This clearly made my mother sad, frustrated, and maybe even a little angry. I suggested that she just go along with whatever my grandmother believed, but my mother, who was normally receptive to advice, couldn’t imagine accepting what must have felt like an erasure of the long, detailed story of their relationship.

Shortly before my grandmother died, I saw the Oscar-nominated documentary, “Complaints of a Dutiful Daughter.” The film concerns a daughter’s relationship with her Alzheimer’s-afflicted mother, and early on the younger woman struggles with her mother’s confusion, which includes forgetting, as did my grandmother, her daughter’s identity. But after many hours of frustration over her inability to straighten our her mother’s faulty memory of people, places, and events, she has an epiphany. If her mother believed her daughter was a distant cousin or long-ago sorority sister, so be it. Instead of persisting in mostly-futile attempts at correction, she would step inside her mother’s reality, no matter how unsettling those empty spaces in her mother’s perceptions might be.

This seems eminently sensible. On the other hand, losing your identity in the eyes of someone you’ve been intimately related to must be profoundly disturbing, as it appeared to be to my mother. What would I feel if my wife no longer recognized me? If she believed I was her brother, or some casual acquaintance or even a stranger. What if she bestowed her portion of our matrimonial affection upon someone else?

That last question is at the heart of Alice Munro’s 1999 short story, “The Bear Came Over the Mountain.” The story, first published in The NewYorker and adapted for the 2006 film, “Away From Her,” concerns the emotional conflicts that arise when a man who has reluctantly moved his wife to an Alzheimer’s facility discovers that she and a male patient have embarked upon a romantic relationship. It’s easy, at least for me, to imagine this happening in real life, not just in fiction. But if I were the one to visit my wife and find her and another man holding hands and exchanging endearments, how would I feel? What would I do?

When my father was in a nursing home, he didn’t—to my knowledge—have any dalliances with women there, but his Parkinson’s related dementia manifested itself in a fantasy that my mother was running around having fun while he was stashed away in his dreary little room, out of sight and out of mind. I never heard him accuse her of infidelity, but I inferred from the tone of his complaints that he suspected her entertainments to involve, in some manner, other men. Given my mother’s prim and religiously pious nature, this idea would have been amusing if not for the fact that, no matter the precise details of what he imagined my mother doing, my father was obviously jealous and resentful and deeply hurt.

Because visits to my parents involved a four-hour plane trip, I didn’t see him regularly in the nursing home. Still, it was jarring when I did make a visit and found him looking gaunt and feeble. He also called me by the name of his younger brother, and before I could stop myself I corrected him. He didn’t address me by name again, so I don’t know how he processed this information, but I made a point of repeatedly calling him “Dad,” hoping to imprint upon his disease-afflicted brain the fact of our identities. It occurred to me that he might think I was his oldest son, but he didn’t utter my brother’s name or, indeed, the names of any of his three children even though I talked about siblings as well as myself. Unless I asked a question, he was silent. When I did ask a question, he would deliver a wandering answer that faded back into the silence before it reached a natural conclusion.

After my father died, my mother told me and my sister that she had a confession to make. Her gravely serious tone had us both thinking she was going to shatter our notions about her by confessing to an affair. I knew she had read the bestselling “Bridges of Madison County,” and although she told me at the time that even at her unhappiest in her marriage to my father she wouldn’t have fallen into bed with a stranger, as did the farm wife in the book, I thought maybe she was parsing language. Maybe she had fallen into bed with someone she knew well, a fellow churchgoer I imagined, an oily figure slipping into the picture knowing that my mother and father were emotionally estranged.

But no. My mother didn’t confess to surrendering to extra-marital passion, but only to having had feelings of desire for another man. Like Jimmy Carter, she had committed adultery in her heart. I stopped myself from laughing out loud. How many men and woman can truthfully say that fantasies of passion haven’t arisen when thinking about someone who isn’t their spouse or partner? If it turns into obsession, that’s obviously a problem, but feeling guilty about moments of lustful fantasy seems as unnatural to me as feeling remorseful about drinking water and eating food.

I wouldn’t have thought badly of my mother if she had sought respite from the dark cloud that hung for a few years over our house. She must have been lonely, with my father’s unhappiness manifested in long, deep silences. The woman in “The Bear Comes Over the Mountain” must have been lonely, too, finding herself detached both physically and mentally from the husband who loves her and all the other familiar parts of her life. To be attracted to another man, to consider him her boyfriend, to find simple, basic pleasure within the cruel prison of disease seems not only understandable, but right.

For anyone struggling with a loved one’s dementia, the subject is awash with books and articles, ranging from the scientific to memoir to the advice-giving variety. But sometimes fiction can illuminate a subject more brilliantly than the scholarly findings and opinions of medical experts and psychologists. In “The Bear Comes Over the Mountain,” Munro treats a subject of great emotional complexity with characteristic deftness and nuance. She humanizes the characters and steers clear of any moral judgments, the result being a rich narrative of genuine emotional power. If someone told me they were dealing with a spouse or loved one with Alzheimer’s or other dementia, I’d say, read this story.