Subjective, personal and highly vulnerable to disruption, autobiographical memory has proven unusually intractable for researchers.

Nonetheless, a handful of people have tried to systematize its study.

In 1879 Englishman Francis Galton, a half-cousin of Charles Darwin, attempted an empirical description by creating a list of 75 words and then, using a timer, recording the associations that came to his mind when he read each word back to himself. Galton repeated the procedure over several months. And after completing a statistical analysis, the 57-year-old scientist noted a curious result: Thirty-nine percent of his associated memories were from before he was 22.

Galton was the first to demonstrate what later came to be known as the reminiscence effect: As people approach 60, they increasingly reminisce about their youth.

Two decades after Galton, psychologist F.W. Colegrove asked subjects about their memories of Lincoln’s assassination and discovered that even though the event had taken place 34 years earlier, in 1865, more than three-quarters of the participants were able to recall where they were and what they were doing when they heard the news. A century later, Roger Brown and James Kulik of Harvard University termed this phenomenon a “flashbulb memory.”

The modern era of research into autobiographical memory happened almost accidentally 50 years ago, when a Yale neurosurgeon, hoping to relieve a patient of epileptic seizures, removed much of the man’s medial temporal lobe. H.M., as the patient has been referred to in the medical literature ever since, suffered profound memory loss. Because the hippocampus was the primary structure removed from the medial temporal lobe, memory researchers turned their attention to that small, sea horse-shaped bulge of brain tissue.

In the 1970s, a Canadian cognitive psychologist, Endel Tulving, further refined the concept of autobiographical memory to include not only the recollection of the events and facts of one’s life but the actual experience of recollecting, which involves a sort of voluntary mental time travel.

A breakthrough in understanding the molecular basis of memory occurred in 1973, when Norwegian scientist Terje Lomo and colleague Tim Bliss published a major paper identifying the process of “long-term potentiation” in the hippocampus. LTP results from the simultaneous stimulation of two neurons and enhances the synaptic connection between them.

A quarter-century earlier, the Spanish anatomist Santiago Ramon y Cajal had proposed something similar, and in 1949 Donald Hebb, a Canadian psychologist, hypothesized that memory was created at the synapse. The discovery of LTP by Lomo and Bliss provided the biological mechanism and launched the study of neural plasticity, the ability of cellular circuits in the brain to undergo activity-induced changes.

This year, University of California-Irvine scientists Christine Gall and Gary Lynch reported they had taken the first images of memory formation. The experiments were conducted using the brain tissue of rats and mice, which has led some scientists to question the validity of the results.

“We don’t have a biochemical marker yet to show a single synapse is undergoing LTP,” said Johns Hopkins neuroscientist David Linden. “Is there a way you can see synapses changing? Not where you kill the animal and look at a slice of brain tissue. … We’re getting closer to the Holy Grail explanation, but it’s going to take techniques that allow us to reveal electrical activity from millions of neurons simultaneously. Right now, we’re only at the earliest stages of being able to do that.”

Amy Ellis Nutt is a staff writer for The Star-Ledger of Newark, N.J. She can be contacted at [email protected].