14th
What are memories made of?
“There appears to be no single memory store, but instead a diverse taxonomy of memory systems, each with its own special circuitry evolved to package and retrieve that type of memory. Memories are not static entities; over time they shift and migrate between different territories of the brain.
At the top of the taxonomical tree, a split occurs between declarative and non-declarative memories. Declarative memories are those you can state as true or false, such as remembering whether you rode a bicycle to work. Non-declarative memories are those that cannot be described as true or false, such as knowing how to ride a bicycle. A central hub in the declarative memory system is a brain region called the hippocampus. This undulating, twisted structure gets its name from its resemblance to a sea horse. Destruction of the hippocampus, through injury, neurosurgery or the ravages of Alzheimer’s disease, can result in an amnesia so severe that no events experienced after the damage can be remembered. (…)
A popular view is that during sleep your hippocampus “broadcasts” its recently captured memories to the neocortex, which updates your long-term store of past experience and knowledge. Eventually the neocortex is sufficient to support recall without relying on the hippocampus. However, there is evidence that if you need to vividly picture a scene in your mind, this appears to require the hippocampus, no matter how old the memory. We have recently discovered that the hippocampus is not only needed to reimagine the past, but also to imagine the future.
Pattern completion
Studying patients has taught us where memories might be stored, but not what physically constitutes a memory. The answer lies in the multitude of tiny modifiable connections between neuronal cells, the information-processing units of the brain. These cells, with their wispy tree-like protrusions, hang like stars in miniature galaxies and pulse with electrical charge. Thus, your memories are patterns inscribed in the connections between the millions of neurons in your brain. Each memory has its unique pattern of activity, logged in the vast cellular network every time a memory is formed.
It is thought that during recall of past events the original activity pattern in the hippocampus is re-established via a process that is known as “pattern completion”. During this process, the initial activity of the cells is incoherent, but via repeated reactivation the activity pattern is pieced together until the original pattern is complete. Memory retention is helped by the presence of two important molecules in our brain: dopamine and acetylcholine. Both help the neurons improve their ability to lay down memories in their connections. Sometimes, however, the system fails, leaving us unable to bring elements of the past to mind.
Of all the things we need to remember, one of the most essential is where we are. Becoming lost is debilitating and potentially terrifying. Within the hippocampus, and neighbouring brain structures, neurons exist that allow us to map space and find our way through it. “Place cells” provide an internal map of space; “head-direction cell” signal the direction we are facing, similar to an internal compass; and “grid cells” chart out space in a manner akin to latitude and longitude.
For licensed London taxi drivers, it appears that navigating the labyrinth of London’s streets on a daily basis causes the density of grey matter in their posterior hippocampus to increase. Thus, the physical structure of your brain is malleable, depending on what you learn.
With impressive technical advances such as optogenetics, in which light beams excite or silence targeted groups of neurons, scientists are beginning to control memories at an unprecedented level.”
— Hugo Spiers is a neuroscientist and lecturer at the institute of behavioural neuroscience at University College London, What are memories made of?, The Guardian, Jan 14, 2012 (Illustration: Polly Becker)
See also:
☞ Memory tag on Lapidarium notes
