But a couple of years ago, the aging English setter - he'll turn 15 tomorrow - stopped heeding that barrier. "He started wandering out of it," his owner, Vicki Sayles, recalls. "The shocks no longer seemed to bother him."

Tests by the veterinarian ruled out various medical conditions that might have explained the behavior. Her diagnosis: cognitive dysfunction syndrome, more commonly known as dementia.

First recognized in dogs in the early 1990s, the disease causes progressive cognitive and behavioral decline. Changes in the canine brain mirror those seen in people suffering from dementias such as Alzheimer's disease.

Cats get dementia as well, though they are diagnosed less frequently, perhaps because felines are less social. Similar deterioration has been seen in the brains of aging rodents, sheep, goats, bears and primates.

The changes in Matthias, subtle at first, became more pronounced. A lean, muscular blue belton bred from championship bloodlines, he was losing weight, urinating in the house, sleeping excessively, and withdrawing socially.

"He'd just lie there. We'd pet him and there would be no response," says Sayles, an addictions counselor who lives in Doylestown.

Animals with dementia appear generally confused. The main abnormalities, known by the acronym DISH (Disorientation, Interaction declines, Sleep-wake disturbances, Housetraining lapses), can take varied forms: the rottweiler that stops responding to its name. The poodle that paces all night and sleeps all day. The domestic short-haired cat that stares into space, howling for no apparent reason. The boxer that waits at the hinge side of the door to go outside.

"The affected dog may come in from a walk and then randomly urinate next to its owner's shoe," says Ilana Reisner, director of the behavior clinic at the University of Pennsylvania's Matthew J. Ryan Veterinary Hospital.

Medications can help shift some behaviors. But there is no cure for dementia, which is surprisingly common.

Recent research published by the Journal of the American Veterinary Medical Association found that 28 percent of 11- and 12-year-old dogs - and two-thirds of 15- and 16-year-old dogs - had behavioral impairment linked to dementia. Another study noted at least one such problem in more than half of cats age 15 or older.

There is no evidence that specific breeds are more vulnerable, although dementia may be more common in the longer-lived canine breeds, which tend to be smaller.

But dog or cat, Pekingese or pug, animal or human, the process is the same. Beta-amyloid plaques - the hallmark of both cognitive dysfunction syndrome (CDS) and Alzheimer's disease - form a sort of protein sludge that accumulates in the brain, eventually damaging the tissue. To make matters worse, levels of the crucial neurotransmitter dopamine drop.

The upshot is the blunting of nerve conduction. This leads to mental deterioration ranging from memory lapses to impaired processing of visual cues.

"The brain just stops talking to itself," says John Phillips, technical services veterinarian for Exton-based Pfizer Animal Health, which makes the only drug approved for CDS in canines.

"An owner may say, 'I walk into the room and my dog doesn't see me come in.' It's not that the dog didn't see its owner come in," he explains. "It's just that its brain did not process it."

There is no definitive test; veterinarians diagnose dementia primarily by excluding other conditions that could cause the same behaviors.

But many pets with the disease never make it to the diagnosis stage because their mental decline is viewed as a normal age-related event.

"A lot of owners say to themselves, 'He's just slowing down because he's getting older,' " says Melissa Bain, assistant professor of clinical animal behavior at the University of California, Davis, School of Veterinary Medicine.

Older and even middle-aged dogs and cats displaying any of the telltale signs should be taken to the vet. Conditions that can look like dementia include a brain tumor, blindness, deafness, osteoarthritis, diabetes and heart disease.

Some can be found in a simple physical exam. Others require a thorough workup, which may include a neurological exam, blood work, urinalysis and imaging studies that could total $2,000.

For pets diagnosed with cognitive dysfunction syndrome, there is help. The drug Anipryl (selegiline HCL) produces an amphetaminelike response. More critical, it enhances dopamine levels and boosts overall neurotransmission in the brain. Possible side effects include vomiting and diarrhea.

Though it can take up to 60 days to deliver its full effect, Anipryl has been shown in studies to alleviate at least one clinical sign of dementia after a month of treatment in about 70 percent of dogs.

"All of a sudden, the dog is interacting and becoming part of the household once again," says Pfizer's Phillips.

Matthias' owner can attest to this. After six weeks on Anipryl, the black-and-white setter perked up. "He was bouncing around the house again," recalls Sayles, who pays $49 for a two-month (one-pill-a-day) supply. "He seemed happy."

No drugs have been approved specifically for dementia in cats, although adjunctive treatments include antidepressants, pheromones and tranquilizers to reduce CDS-associated anxiety.

Foods fortified with antioxidants may also counteract cognitive damage. Studies show that older dogs fed Hill's Prescription Diet Canine b/d exhibited improved alertness and enthusiasm, as well as increased attentiveness to problem-solving tasks.

To keep in step with mental decline, adds Reisner, of Penn's behavior clinic, pets with dementia should be maintained in a stable environment:

Adhere to a regular schedule.

Keep sleeping areas comfortable and easy to access.

Make litter boxes readily available.

Minimize furniture rearrangements.

Use baby gates, fences and hand-held leads to prevent injury.

She also suggests avoiding punishments.

Perhaps most important is a regular staple of human interaction and training, to sharpen the mind.

Says Reisner, "It's a good time to challenge the old adage 'You can't teach an old dog new tricks.' "