Last month's Oscar ceremony was a reminder that the predictable tribe that are the Academy voters can't resist disability.

Eddie Redmayne's Best Actor gong was given for a depiction of motor neurone disease, Julianne Moore's Best Actress for her portrayal of someone in the grip of Alzheimer's.

On the one hand, this signifies a simple-minded assumption that extreme physical conditions require greater acting virtuosity; to my mind, Redmayne was a little fortunate to beat Michael Keaton's warts-and-all but able-bodied turn in Birdman. On the other, Moore's performance in Still Alice has a terrifying authenticity that is beyond doubt or compare.

It was also about time this fine actress received an Oscar. What a career she's had: the porn superstar of Boogie Nights, the avant-garde artist intent on having the Dude's child in The Big Lebowski, the wartime adulteress of The End Of The Affair, the 1950s housewife daring to fall in love with a black man in Far From Heaven, the lesbian whose relationship and family life are threatened by her fling with a man in The Kids Are All Right, the Hollywood diva who will do anything to rescue her career in black comedy Maps To The Stars. She might not be a showy chameleon, like Meryl Streep or Cate Blanchett, yet she has a vigour, boldness and emotional integrity that make her one of the best actresses of the past 20 years.

And so to Still Alice. In truth, for storytelling prowess this is an average, even clunky film, which could just as easily have been made for television. Yet in conveying the devastating effect of Alzheimer's on both sufferers and their families, it is remarkably effective, due to two astute decisions by writer-directors Richard Glatzer and Wash Westmoreland: the first, to adapt the novel by Lisa Genova, with its focus on a highly intelligent woman suffering from early onset of the disease (rather than the usual sufferer, who will be in their sixties or older), thus accentuating the horror when confronted with losing your mental faculties; the second, the casting of their lead actress.

Moore is Alice Howland, a successful linguistics professor with a comfortable and loving family life, who begins to experience an unwelcome fraying of her memory - forgetting her place in a lecture, suddenly not knowing where she is when running, repeating introductions over dinner. She thinks she may have a tumour, but a neurologist confronts her with a different and irreversible diagnosis.

At first it's difficult for Alice and her husband John (Alec Baldwin) to accept that such a vibrant mind can simply disappear. But with early onset the effects can be speedier, and all too quickly the lapses in Alice's memory become endemic, and her very awareness of the everyday deserts her.

The film charts this process and its diverse consequences, from the humiliating experience of forgetting where the bathroom is, to Alice and John enjoying what they both understand will be their last visit to their favourite place, to another particularly awful one involving the potential legacy of Alice's disease for her children.

Baldwin and Kristen Stewart (as the youngest of the three kids) are both excellent, as their characters are forced to make difficult decisions about their own lives, as well as Alice's. Genova certainly loaded the dice for her protagonist, whose love of language is mocked by the flood of words and thoughts from her mind. And as this once remarkable woman, now fading before our eyes, Moore is heartbreakingly real - never saccharine, never exaggerated, simply truthful and fearless.