It is a remarkable fact that the brain, made of neurons and their connections to one another named synapses, is able to remember.
After all, if there is one thing we can say about our bodies and about nature, it's that change is everywhere. To remember is to go the other way, as we somehow try to defy the passage of time and its dispersive effects, even if imperfectly. We go back to some kind of archive in our heads where we can retrieve information. Short-term memories are ephemeral, while long-term memories may fade and change, and sometimes they disappear altogether. But some persist for a lifetime, even if sometimes only at the level of uncertain contours. How does the brain do that?
The research field of how we remember has a vast history. (The interested reader can check the book by Kurt Danziger, a well-known historian of psychology.) It is intuitive that if memories are somehow encoded in the brain, the mechanism must involve neurons and their synapses. When a memory is retrieved, such pathways connecting neurons are reactivated and brought back to consciousness: A certain group of neurons and specific pathways between them fires and we remember.
The encoding is, in a sense, like sculpting: Sensorial inputs demark certain neurons and chisel specific connections between them. With around 100 billion neurons (perhaps a bit less, around 85 billion) and an average of 7,000 synaptic connections to other neurons, the brain of a 3-year-old child has a staggering quadrillion (1015) synapses. The combinatorial number of pathways, even in subregions of the brain dedicated to short-term memories (frontal, prefrontal and parietal lobes), is enormous. Long-term memories seem to be distributed in neuronal networks across the brain and have even larger capabilities.
Even if the general workings seem fairly clear, the details of memory encoding have been elusive. In a recent study, a group of neuroscientists led by Matias Ison from the University of Leicester in England set out to measure how episodic memories are encoded in the medial temporal lobe.
Neurosurgical patients at UCLA Medical Center were first shown pictures of family members and famous people, like Clint Eastwood or Jennifer Aniston. Then they were shown pictures of the same people in front of well-known places (the Tower of Pisa or the Eiffel Tower). The patients already had electrodes implanted in their medial temporal lobe for medical reasons, allowing the scientists to map which neurons flashed when the subjects saw this or that picture.
The same group of neurons that fired when the subjects saw the pictures of the celebrities fired again when the celebrities were in front of the sights. In other words, the scientists were able to observe how single neurons changed their firing patterns to encode the new associations, creating a persistent pathway related to a specific memory.
How, exactly, a repetitive sensorial input awakens the same group of neurons remains unclear. It is also unclear what determines which neurons and pathways relate to a specific input — that is, the choices of specific pathways associated with a specific memory. But the study demonstrates that memories are indeed sculpted in the brain through selective pathways between neurons.
As with sculptures, we should expect time to have a deleterious effect, which is more or less impactful depending on the strength of the synaptic connections that define the pathway. A deeper chiseling carves longer-lasting sculptures.
Marcelo Gleiser is a theoretical physicist and cosmologist — and professor of natural philosophy, physics and astronomy at Dartmouth College. He is the co-founder of 13.7, a prolific author of papers and essays, and active promoter of science to the general public. His latest book is The Island of Knowledge: The Limits of Science and the Search for Meaning. You can keep up with Marcelo on Facebook and Twitter: @mgleiser.