By JAY WEISSBERG

A high-ranking retired cop with Alzheimer's suspects there's a killer loose in his nursing home in Nicolas Boukhrief's unusual, cerebral thriller "Cortex." In "Memento," the protag's brain is wiped clean every few minutes; here, the detective's senility floats in and out, leaving auds guessing whether he's deliberately fooling the staff or simply heading swiftly toward oblivion. Fascinating concept is occasionally crowded out by unnecessary characters, though Andre Dussollier's complex performance keeps sympathy in the plus column. Late January opening in Francehas seen modest returns.

Dussollier plays Charles Boyer -- a detective, not the star of yesteryear -- who knows his memory is deteriorating and doesn't object to moving into "the Residence." The surprise death of a patient puts Charles back in investigator mode, though the staff assures him it was a natural death. But when fellow resident Carole (Marthe Keller, reuniting with Dussollier 34 years after Claude Lelouch's "And Now My Love") follows suit after a night together with Charles, he's convinced nature isn't to blame.

Part of pic's cleverness is that auds don't know if Charles is simply suffering from Alzheimer's-related dementia or if he's really on to something. The staffers are certainly a grumbly bunch, caught up in their own petty jealousies, but only at the finale is it clear whether someone's really offing the residents.

Unfortunately, Boukhrief spends so little time building up the side characters that when they do suddenly matter, it's difficult to sense any motivation. Too many weak links in the chain, including Carole, result in a sense of anticlimax, especially when the build-up has been so methodically paced. Claire (Anne-Marie Faux), a mute patient who seems to be a key figure, is simply a MacGuffin with little reason for even being in the movie.

Still, the always impressive Dussollier brings the full weight of experience to the role, delivering a multilayered characterization that captures both Charles' interior rigidity and the confusion that's quickly gripping his brain. His ability to disguise his forgetfulness makes for an impressively accurate portrayal of the disease.

Though d.p. Dominique Colin also lensed Boukhrief's "Cash Truck," there's little stylistic similarity between the two, and certainly no big action moments. Instead, the overall mood is tamped down, full of long silences reflecting the nursing home's closed-off atmosphere. Charles' brief escape to Paris is nicely lensed with appropriate handheld movements, signaling the additional confusion imparted by the urban jungle.

Camera (color), Dominique Colin; editor, Lydia Decobert; music, Nicolas Baby; production designer, Laurent Allaire; costume designer, Annie Thiellement; sound (Dolby SRD), Lucien Balibar, Aymeric Devoldere, Nicolas Becker, Philippe Amouroux; line producer, Mathieu Bompoint; assistant director, James Canal; casting, Anne Pham. Reviewed at Berlin Film Festival (market), Feb. 8, 2008. Running time: 104 MIN.