Say that you've been glued to coverage of the U.S. Open tennis tournament for the last two weeks. Today, you go to hit with a friend, and are incredulous when you blow a cross-court volley you felt sure you had mastered. You're flustered by your inability to serve with the sizzle you distinctly remember you brought to the court when you last played.

A group of German and Canadian psychologists have another possibility for you to consider: Perhaps your glowing assessment of your tennis talent is the result of a false memory, induced by watching Roger Federer's five-set duel with Novak Djokovic or the epic match between Venus Williams and Kim Clijsters. As you watched intently, their shots looked so fluid, so familiar, so good. You felt for all the world like they were yours -- that you could hit those shots.

And voila -- you convinced yourself that you, too, could hit like that! No wonder, given your distinct memory of having played so well before, you're finding your present performance a little lackluster.

In a series of experiments published in the September issue of the journal Psychological Science, researchers assert that they have found a "hitherto undiscovered source of false memories that is ubiquitous in everyday life: observations of other people's actions." Their research expands on a long-established psychological phenomenon: that imagining in fine detail the performance of an action often creates false memories of having actually performed that action. (You distinctly imagined yourself telling your husband that the company picnic is this weekend; why is he looking at you as if this is the first he's heard of it?)

In the current set of experiments, researchers found that watching a video of someone else shake a bottle seems to "create" a personal memory for many subjects of having shaken the bottle themselves. The false memory was slightly more likely to take hold when a subject was asked to imagine vividly the act of shaking a bottle than when he or she saw someone else do it. But it was still strong.

Whether it was induced by imagining or watching another person doing it, the false bottle-shaking memory was not short-lived: Two weeks after the experiment was over, subjects who had watched a video of someone else shaking the bottle were still far more likely to report that they themselves had shaken the bottle than were subjects who only read aloud the action statement "shake the bottle," or who played word games with the phrase "shake the bottle." Subjects were vulnerable to such false memories even when researchers warned them of the possibility and cautioned them to take care to distinguish between actual and observed actions.

It's a reminder (we need one?) that our memory isn't always reliable, says co-author Gerald Echterhoff of Jacobs University in Bremen, Germany. Having some "informed doubt" about your memory performance is a good thing, he says, since it can readily be tricked. "Don't just easily trust whatever comes to your mind as true," he cautions.

The four experiments that explored this trait build upon a growing body of evidence showing that when we watch or imagine someone else doing something familiar, or when we interact socially with another person, our brain unconsciously pantomimes the action or mimics the emotion we imagine or see before us. When research subjects in brain-imaging scanners watch a video of another person throwing a ball, researchers typically detect a burst of activity in the areas of the brain that govern the motion of throwing. When a subject is shown pictures of a person laughing or crying, the areas that process emotions such as happiness or sadness light up. This has led some neuroscientists to suggest that the human brain reflexively mirrors much of what it sees and experiences in the world, helping us to internalize and make sense of our social and physical environment.

But the "mirror neurons" that such scientists believe are at work may also be engaged in a bit of mental trickery.

"When, on a memory test, participants reactivate mirrored action representations, they could -- erroneously -- remember having performed the action," wrote the researchers.

-- Melissa Healy / Los Angeles Times