How memories are stored in the living brain.

Imagine you had a tiny device inside your brain that could ‘see’ the moment a new memory was formed.

What would it record?

Since the brain is made up of neurons linked together in a vast network, it seems reasonable to suppose that some part of that network would activate when you, say, saw your first child being born.

But how big a network?

Is that memory contained in a handful of neurons or is it distributed widely across the network?

Fascinating clues come from a new study of nine patients who had had electrodes implanted in their brains to help monitor their seizures.

These electrodes can monitor the activity of single neurons in the brain.

With their permission, neuroscientists took advantage of this window into the electrical activity of the brain to examine how memories are laid down and recalled.

A small network

For the test, participants simply learned a series of words, then were presented with another list which contained some words they’d learned along with some new ones.

They were asked to say which ones they’d seen before.

The results amazed the study’s first author says:

“Intuitively, one might expect to find that any neuron that responds to one item from the list would also respond to the other items from the list, but our results did not look anything like that.

The amazing thing about these counterintuitive findings is that they could not be more in line with what influential neurocomputational theorists long ago predicted must be true.”

As expected, the memory was encoded in the hippocampus, an area of the brain that is vital for memory.

What they saw was that the memory for a single word was encoded across hundreds of thousands of neurons in a distributed network.

While this may sound like a lot, it’s actually only a small fraction: the number of neurons firing in response to a single word was about 2% of those in the hippocampus.

Inside your brain

So, if you could look inside your own brain when a new memory is formed, what you’d see is that a relatively small network of neurons in the hippocampus jumps into action.

When you recall a memory, that same network leaps into action again.

The study’s authors explain:

“[This is]…a sparse distributed coding scheme in which each memory is coded by the activity of a small proportion of hippocampal neurons, and each neuron contributes to the representation of only a few memories.”.

Some have theorised that this is the most efficient way for the brain to work because:

If memories were localised to individual neurons then they could easily be lost if those particular neurons died.

If memories were too widely spread across the network, memories would be too easily overwritten or confused.

One of the study’s authors:

“To really understand how the brain represents memory, we must understand how memory is represented by the fundamental computational units of the brain — single neurons — and their networks.

“Knowing the mechanism of memory storage and retrieval is a critical step in understanding how to better treat the dementing illnesses affecting our growing elderly population.”