The forgotten part of memory


IMAGE/Sam Falconer

Memories make us who we are. They shape our understanding of the world and help us to predict what’s coming. For more than a century, researchers have been working to understand how memories are formed and then fixed for recall in the days, weeks or even years that follow. But those scientists might have been looking at only half the picture. To understand how we remember, we must also understand how, and why, we forget.

Until about ten years ago, most researchers thought that forgetting was a passive process in which memories, unused, decay over time like a photograph left in the sunlight. But then a handful of researchers who were investigating memory began to bump up against findings that seemed to contradict that decades-old assumption. They began to put forward the radical idea that the brain is built to forget.

A growing body of work, cultivated in the past decade, suggests that the loss of memories is not a passive process. Rather, forgetting seems to be an active mechanism that is constantly at work in the brain. In some — perhaps even all — animals, the brain’s standard state is not to remember, but to forget. And a better understanding of that state could lead to breakthroughs in treatments for conditions such as anxiety, post-traumatic stress disorder (PTSD), and even Alzheimer’s disease.

“What is memory without forgetting?” asks Oliver Hardt, a cognitive psychologist studying the neurobiology of memory at McGill University in Montreal, Canada. “It’s impossible,” he says. “To have proper memory function, you have to have forgetting.”

Biology of forgetting

Different types of memory are created and stored in varying ways, and in various areas of the brain. Researchers are still pinpointing the details, but they know that autobiographical memories — those of events experienced personally — begin to take lasting form in a part of the brain called the hippocampus, in the hours and days that follow the event. Neurons communicate with each other through synapses — junctions between these cells that include a tiny gap across which chemical messengers can be sent. Each neuron can be connected to thousands of others in this way. Through a process known as synaptic plasticity, neurons constantly produce new proteins to remodel parts of the synapse, such as the receptors for these chemicals, which enables the neurons to selectively strengthen their connections with one another. This creates a network of cells that, together, encode a memory. The more often a memory is recalled, the stronger its neural network becomes. Over time, and through consistent recall, the memory becomes encoded in both the hippocampus and the cortex. Eventually, it exists independently in the cortex, where it is put away for long-term storage.

Neuroscientists often refer to this physical representation of a memory as an engram. They think that each engram has a number of synaptic connections, sometimes even in several areas of the brain, and that each neuron and synapse can be involved in multiple engrams.

Nature for more

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