Noted neuroscientist Eric Kandel ’52 looked to his audience to illustrate his lecture on the molecular basis of memory.

“If you remember anything about this lecture, it’s because genes in your brain will be altered,” said the Columbia University professor, who shared the 2000 Nobel Prize in physiology or medicine for his studies on memory. “If you remember this tomorrow, or the next day, a week later, you will have a different brain than when you walked into this lecture.”

Kandel’s standing-room-only talk in Science Center D on Monday (Feb. 8) was organized by the Harvard’s Center on the Developing Child and sponsored by the Faculty of Arts and Sciences and Harvard Medical School.

“Memory, as you know, makes us who we are,” Kandel said. “It’s the glue that binds our mental life together. Without the unifying force of memory, we would be broken into as many fragments as there are moments in the day.”

Kandel described what researchers have learned in recent decades about the molecular underpinnings of memory. Among other things, he said, neuroscientists have found that short-term memory — the ability to recall things for minutes or hours — is fundamentally different from long-term memory, which holds information for weeks, months, even a lifetime.

“Long-term memory differs from short-term memory in requiring the synthesis of new proteins,” Kandel said, adding that there’s a high threshold for information to be entered into long-term memory.

“Something really has to be important to be remembered,” he said.

Long-term memory stimulates protein syntheses, Kandel said, by altering gene expression. While the genes themselves remain unchanged, their activity levels are tweaked by the molecules involved in the creation of long-term memory.

“Many of us are accustomed, naively, to thinking that genes are the determinants of our behavior,” he said. “We are not accustomed to thinking that genes are also the servants of the mind.”

The genes affected, he said, lead the brain’s 100 billion neurons to grow new synapses, or connections with other neurons. A typical neuron, he said, connects to about 1,200 others. But neurons that are subject to repeated stimuli have been found to have much denser networks, with up to 2,800 synapses.

The brain is especially susceptible to forming such new connections early in life, he said, when its structure is highly malleable, or plastic.

“This is why almost all great musicians, all great basketball players, all great anything, all get started very early in life,” Kandel said.

But Kandel’s host, Jack Shonkoff, director of Harvard’s Center on the Developing Child and a faculty member at the Harvard School of Public Health, Harvard Medical School, and the Harvard Graduate School of Education, said the young brain’s plasticity also can be detrimental to children.

“Significant trauma, significant stress, may have some adverse effect on these circuits that makes it more difficult for children to learn,” Shonkoff said.

Kandel said better understanding of how the biology of the brain relates to individual behaviors and how complex behaviors develop in complex sociobiology “is really the great challenge of the 21st century.”

He later elaborated on that challenge in response to an audience question, alluding to the daunting work still to be done at neuroscience’s latest frontier: unraveling organisms’ “connectomes,” the complete diagrams of neural circuitry.

“There are a lot of cells up there,” he said. “Each one of them connects to 1,000 other cells, so you’ve got more synapses than there are stars in the universe. When you finish counting those stars in the universe, I will be ready for the connectome.”