h, those were the days. At least that's what the nostalgic among us feel. And those hard-nosed realists who are immune to its siren call dismiss it — wrongly, of course — as cheap sentiment.

I remember during a particularly grueling time of medical internship joking with a colleague that one day we would look back on that year with fondness. Clearly, he thought I must be crazy. But two years later, he was bragging to me about our having lived through the days of the giants, his bittersweet and almost nostalgic label of rough days gone by.

Nostalgia now is not what it used to be. As a word, it was first coined by a Swiss medical student in the 17th century to describe mercenaries who suffered from severe homesickness. Now, we see it as a special form of memory.

Of course, there is nothing fair or necessarily accurate about human memory. Human recall — whether eyewitness accounts of a crime or retelling of emotionally neutral events — is subject to bias and error.

What about early childhood memories, the stuff of which nostalgia is made?

Well, most people can recall little about their lives before the age of 3 or so. Freud suggested that these early memories were kept out of consciousness by the process of repression to safeguard our sanity. In other words, the memories existed but were unavailable.

But it turns out that there may be a far simpler explanation for the amnesia of early childhood: one memory circuit of the brain, a region called the hippocampus, is not yet mature.

Unfortunately, the brain has no trouble recalling traumatic events from a very young age because they are encoded by a different system, the amygdala, that processes fearful and threatening experiences and matures — at least in animal studies — before the hippocampus.

Now you needn't have been traumatized to understand the powerful effect that emotions can have on the formation of memory. In fact, it has been known for a while that adrenaline, the hormone released during stress and anxiety, enhances memory. Dr. Jim McGaugh at the University of California at Irvine showed that rats injected with adrenaline just after learning a task had enhanced retention.

In a study with humans, Dr. Larry Cahill, also at Irvine, showed that blocking the effects of adrenaline could prevent emotional arousal from enhancing memory. Subjects were given propranolol, which blocks adrenaline receptors, before reading either an emotionally upsetting or closely matched but emotionally neutral story. Blocking adrenaline impaired memory only for the emotionally arousing story. The clear implication is that any emotionally charged situation that causes adrenaline release will produce stronger memories.

Nostalgia is nothing if not powerful memory formed during emotionally charged moments. But that alone doesn't guarantee accurate encoding or recall of memory. In fact, people get nostalgic all the time about experiences that were really never all that wonderful.

A patient of mine once fondly reminisced about his days in sleep-away camp. One memory stuck in his mind: a counselor had spun him around by his feet and hurled him headfirst into the lake off a pier. It was, he said, thrilling and fun.

But a decade later he happened to run into his camp counselor who was surprised at his warm memories. Why? His former counselor reminded him that he had been miserable enough to insist almost daily that he wanted to go home.

Not only that, but he was teased by his peers for being an egghead and poor athlete. Yet this revelation did not change my patient's nostalgia. That's because nostalgia serves a useful defensive and reparative function; it took the sting from a painful episode in his life by shading the script.

Sometimes, though, complex mental states like nostalgia have surprising origins. A patient once revealed that she episodically had the strange and overwhelming sensation of familiarity in places she had no conscious recollection of having been before. And this experience was always coupled with a visceral sense of foreboding for no apparent reason.

In search of a hidden meaning behind her déjà vu experience, she entered psychotherapy for several years but came up empty-handed.

Finally, her astute family physician referred her to a neurologist who did an EEG, which showed abnormal electrical activity in the temporal lobe of her brain. She had been having temporal lobe seizures.

The uncanny sense of déjà vu concealed no special meaning after all; it was just an illusion of familiarity created by a group of neurons that misfired from time to time.

Of course, this is not to say that our fondest memories or our explanations of our own mental experiences are all suspect. They're not. But if anything marks us as human, it's more our bent for making sense of things than for discovering the essential truth about them.