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Having an old dog is akin to having a newborn baby, only worse. Babies, at least, become easier to care for and more fun to be with as they get older. The dog gets harder and less amusing. No owner can get a continuous night's sleep when every whimper may be an announcement that the dog, no longer continent, needs to go out in a hurry.

New babies remain confined to cradle or crib and rarely rustle the bedding. But owners of old pets are often awakened by nighttime pacing and the click, click, click of nails on the floor, or by the painful yelp of an arthritic dog trying to lie down.

And while caring for an old pet gets harder, the rewards decline as it becomes increasingly unresponsive and immobile. Loss of hearing keeps a sleeping pet (and they do sleep most of the day) from noticing its owner's return and bestowing a loving greeting. Arthritis turns a walk into a crawl and forces many owners to carry their dogs up and down stairs and lift them into cars.

An aging animal's interests are likely to wane — even favorite toys are often ignored — and memory fades, as well, with some dogs forgetting whether to go through a door on the hinge side or the knob side.

Hearing a description of these symptoms, an older friend remarked, "So how is this different from an elderly person?" And indeed, scientists who study canine brain aging have found many close parallels to the human experience, close enough to warrant research financing from the National Institute on Aging.

And happily for older people as well as for pet owners, scientific studies in dogs strongly suggest that some of the more debilitating effects of age on the brain may be averted or at least eased by consuming more substances that protect the brain's cellular mechanisms from the ravages of oxidation.

Dr. Norton W. Milgram, a behavioral neuroscientist at the University of Toronto, and Dr. Carl W. Cotman, a neurochemist in the Institute for Brain Aging and Dementia at the University of California at Irvine, have been studying old dogs as models of human aging.

"It has become clear as we've looked at how cognitive changes occur over time," Dr. Milgram said, "that antioxidants are the best suggestion of a possibly useful intervention, because the evidence indicates that oxidative stress is the main factor in brain aging."

Dr. Cotman agreed. "Oxidative damage is a key feature in the aged brains of animals and people," he said, "and the brains of individuals with Alzheimer's disease show greater damage."

He suggested that antioxidant supplements like vitamins E and C might "improve cognitive function and reduce age-associated cognitive decline" in people as well as in pets, since dogs, as they grow older, develop the same pathological changes in the brain as aged people.

Clinical trials testing the value of antioxidants in treating Alzheimer's are under way. In one completed study, vitamin E supplements delayed the need for institutionalization among moderately to severely demented people, Dr. Cotman found.

Dr. Barbara Shukitt-Hale, a research psychologist at the Department of Agriculture's Human Nutrition Research Center on Aging at Tufts University, is among those who have demonstrated benefits in an antioxidant-enriched diet for the aging mammalian brain. By feeding extracts of blueberries, strawberries and spinach to rats, she has found that "we can stall or even reverse the effects of aging on the brain, both behaviorally and chemically."

She added, "It's very plausible that antioxidant-rich diets would also help older dogs." As for people, she said, "we all should be doing this — eating at least five servings of fruits and vegetables a day to forestall or even improve the effects of aging."

Pet food companies and producers of animal pharmaceuticals have taken a strong interest in the research. Dr. Milgram said at least one company, Hill's Pet Nutrition, had formulated an antioxidant-rich food associated with significant improvement in canine brain function in well-designed studies in both the laboratory and the homes of older dogs. The studies were jointly supported by the company, the National Institute on Aging and the United States military.

These studies, and the background research supporting them, were described last month at a symposium on brain aging in dogs sponsored by Hill's in conjunction with the annual meeting of the North American Veterinary Association in Orlando, Fla.

For many reasons, the brain is highly vulnerable to oxidative damage. It is loaded with polyunsaturated fats, which are easily attacked by oxidizing agents. The brain is a metabolic powerhouse that uses as much as 20 percent of the body's oxygen supply.

The oxygen, in turn, generates free radicals, highly reactive chemical fragments that can damage the membranes of cells, making them leaky and vulnerable to toxic agents. The free radicals can also destroy critical cellular ingredients like DNA and enzymes, including the enzymes needed to process metabolic debris.

Under the stress of oxidation, synapses that enable brain cells to communicate with one another are lost, and there is a buildup of a toxic peptide called beta amyloid, resulting in brain cell dysfunction. Yet the brain has relatively low levels of antioxidant enzymes to protect it from free-radical damage. And unlike other organs, it cannot repair itself from oxidative damage.

Yet as with people, some of whom remain mentally sound into their 90's and beyond, the brains of some old dogs continue to function well, which suggests that it may be possible to improve the fates of those that do not.

Working with a colony of 48 beagles, ages 9 to 14 owned by the military, Dr. Milgram and his colleagues spent a year studying their cognitive function, then divided the animals into four groups that were cognitively equal.

Two groups were fed the antioxidant-rich dog food, now marketed by Hill's as Canine b/d, and the other two were fed an ordinary supermarket food that looked identical. One group in each of the two diet categories also received "cognitive enrichment": five to six days a week they were given puzzles like finding hidden food rewards. At the end of a year, the four groups were retested for cognitive function. The group that had both the antioxidant-rich food and the enrichment scored significantly higher than the other groups.

Dr. Milgram concluded while the animals receiving the fortified food performed better than the others, the special diet was most effective when combined with mental stimulation, which owners naturally provide for their pets by playing with them, walking them, talking to them and petting them.

In a similar study in the homes of older dogs, those receiving the antioxidant-rich diet also did significantly better on cognitive tests, with differences showing up in some cases in as little as a month, said Dr. Gary Landsberg, a veterinary behaviorist in Thornhill, Ontario.

The special diet, formulated as a dry dog food, contains antioxidant- rich fruits and vegetables, vitamins E and C, the fatty acids DHA and EPA, carnitine and alpha lipoic acid, which helps to promote the health of mitochondria, the powerhouses of cells, Dr. Landsberg said.

Dr. Landsberg lamented that most pet owners waited too long to bring age-related behavioral changes to the attention of their vets, since preventing cognitive decline is preferable to trying to slow or reverse it.

"The effects of brain aging can be subtle and progress slowly, which makes them difficult to detect," he noted. "Dogs are typically more than 11 years old when owners first detect clinical signs of cognitive dysfunction syndrome. However, in the laboratory, we can often see subtle changes in cognitive function at 8, 9 or 10."

There are also differences in breeds and in the demands placed on the animals, with early signs of brain impairment more obvious in dogs trained to perform tasks, like assisting people who are blind or hard of hearing. An estimated 10 million dogs in the United States are at ages that render them vulnerable to cognitive dysfunction.

"Owners should start looking at their pets for early signs of brain aging," Dr. Landsberg said. "Does the dog recognize people, places and pets it knows and greet you in the same way? Does it remember previously learned behaviors, like house- training? Has it acquired new fears and anxieties?"

The hallmarks of brain aging in dogs are summarized by the acronym DISH — disorientation; interaction changes; sleep or activity changes; and house-training is forgotten.

If any such behavioral changes are noted, Dr. Landsberg said, the first step is to have the animal fully examined by a veterinarian for a treatable underlying health problem, like thyroid, kidney or adrenal gland disease, that may cause symptoms that mimic those of cognitive decline. Behavioral changes could also be a result of arthritis or of failing hearing or vision or of changes in the environment like the arrival of a new pet or baby, marriage, divorce or a new home or schedule.

"Many owners fail to discuss geriatric-onset behavioral changes with their veterinarians because they incorrectly assume that these problems are an unfortunate and untreatable result of age," Dr. Landsberg told the Orlando symposium. Testing old dogs that may seem to the owner and the vet to be clinically healthy, he said, often reveals undiagnosed medical problems at earlier and more treatable stages.

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