Scientific Americans looks into the human brain, trying to figure out why some events just tend to stick in our memories forever, while the others are gone: “How does a gene “know” when to strengthen a synapse permanently and when to let a fleeting moment fade unrecorded? And how do the proteins encoded by the gene “know” which of thousands of synapses to strengthen?

In the movie thriller Memento, the principal character, Leonard, can remember everything that happened before his head injury on the night his wife was attacked, but anyone he meets or anything he has done since that fateful night simply vanishes. He has lost the ability to convert short-term memory into long-term memory. Leonard is driven to find his wife’s killer and avenge her death, but trapped permanently in the present, he must resort to tattooing the clues of his investigation all over his body.



That disturbing story was inspired by the real case history of a patient known in the medical literature only as “HM.” When HM was nine years old, a head injury in a bicycle accident left him with debilitating epilepsy. To relieve his seizures that could not be controlled in any other way, surgeons removed parts of HM’s hippocampus and adjoining brain regions. The operation succeeded in reducing the brain seizures but inadvertently severed the mysterious link between short-term and long-term memory. Information destined for what is known as declarative memory–people, places, events–must pass through the hippocampus before being recorded in the cerebral cortex. Thus, memories from long ago that were already stored in HM’s brain remained clear, but all his experiences of the present soon faded into nothing. HM saw his doctor on a monthly basis, but at each visit it was as if the two had never met.



This transition from the present mental experience to an enduring memory has long fascinated neuroscientists. A person’s name when you are first introduced is stored in short-term memory and may be gone within a few minutes. But some information, like your best friend’s name, is converted into long-term memory and can persist a lifetime. The mechanism by which the brain preserves certain moments and allows others to fade has recently become clearer, but first neuroscientists had to resolve a central paradox.



Both long- and short-term memories arise from the connections between neurons, at points of contact called synapses, where one neuron’s signal-emitting extension, called an axon, meets any of an adjacent neuron’s dozens of signal-receiving fingers, called dendrites. When a short-term memory is created, stimulation of the synapse is enough to temporarily “strengthen,” or sensitize, it to subsequent signals. For a long-term memory, the synapse strengthening becomes permanent. Scientists have been aware since the 1960s, however, that this requires genes in the neuron’s nucleus to activate, initiating the production of proteins.



Memory researchers have puzzled over how gene activity deep in the cell nucleus could govern activities at faraway synapses. How does a gene “know” when to strengthen a synapse permanently and when to let a fleeting moment fade unrecorded? And how do the proteins encoded by the gene “know” which of thousands of synapses to strengthen? The same questions have implications for understanding fetal brain development, a time when the brain is deciding which synaptic connections to keep and which to discard. In studying that phenomenon, my lab came up with an intriguing solution to one of these mysteries of memory. And just like Dorothy, we realized that the answer was there all the time.



More here.

0