The brain simmers with activity. Different groups of neurons (nerve cells), responsible for different thoughts or perceptions, drift in and out of action, ever changing.

Memories are formed from persistent changes in the strength of connections between neurons. These connections, or synapses, can be made stronger or weaker depending on when and how often they have been activated in the past.

So, when a thought enters our head - coffee! - the strengthened connections funnel activity along particular neural pathways, and we free associate the thought with coffee mugs, coffee beans, a coffee date.

A connection between two neurons becomes stronger when neuron A consistently activates neuron B, making it fire an action potential (spike), and the connection gets weaker if neuron A consistently fails to make neuron B fire a spike. Lasting increases and decreases in synaptic strength are called long-term potentiation (LTP) and long-term depression (LTD).

There’s no doubt that these changes - referred to as synaptic plasticity - are a biological memory.

At present, synaptic plasticity is the main candidate for the cellular theory behind memory, but researchers don’t yet understand how - or even if - such synaptic changes translate to the network-level alterations that underlie a memory.

Different 'neuron ensembles' for different memories

Memories occur when specific groups of neurons are reactivated. In the brain, any stimulus results in a particular pattern of neuronal activity—certain neurons become active in more or less a particular sequence. If you think of your cat, or your home, or your fifth birthday cake, different ensembles, or groups, of neurons become active. The theory is that strengthening or weakening synapses makes particular patterns of neuronal activity more or less likely to occur.

As a five-year-old, if given the word 'house', you might have imagined a drawing of a house. As an adult, upon hearing the same word you may well picture your own house—a different response for the same input. 

This is because your experience and memories have changed the connections between neurons, making the old 'house' ensemble less likely to occur than the new 'house' ensemble. 

In other words, recalling a memory involves re-activating a particular group of neurons. The idea is that by previously altering the strengths of particular synaptic connections, synaptic plasticity makes this possible.

Memories are stored by changing the connections between neurons. A five-year-old child will activate a certain group of neurons (Ensemble A); whereas adults will activate a different ensemble (Ensemble A') with the same stimulus. Synaptic plasticity driven by repeated experience can change the connection strengths between neurons. This is how there can be the different neuronal responses to the same input. (Image: Alan Woodruff / QBI).