Individual neurons in the brain fire differently the instant new memories are formed, even if a person has only seen a single image, according to newly published research based on human brain studies.

However, if a person has seen an image that combines two concepts, the resulting neuron will fire in response to an image of either concept, without the other being present.

The five-year study, undertaken by scientists at the University of Leicester, in the UK, and Ronald Reagan UCLA Medical Centre in the US, and published today in the journal Neuron, has resulted in findings that provide unprecedented insight into how humans form new memories.

“The discovery that individual neurons in the Medial Temporal Lobe, the brain’s main engine for memory formation, changed their firing to encode new associations even after one single presentation provides a plausible mechanism underlying the creation of new memories,” said study lead author Dr Matias Ison, lecturer in bioengineering at the University of Leicester.

“The study suggests that the experience of learning can be traced back to changes in individual neurons in the brain.”

The study involved showing volunteers images of people and places while monitoring the activity in the brain’s medial temporal lobe (MTL). The people in question, a variety of somewhat dated celebrities, including Jennifer Anniston, Halle Berry and Tiger Woods, were shown in front of famous landmarks, as well as both the places and the people being shown separately to the volunteers.

“The remarkable result was that the neurons changed their firing properties at the exact moment the subjects formed the new memories – the neuron initially firing to Jennifer Aniston started firing to the Eiffel Tower at the time the subject started remembering this association,” said co-author Rodrigo Quian Quiroga, head of the Centre for Systems Neuroscience at the University of Leicester.

“Moreover, we observed these changes after just a single presentation. This is a radical departure from previous experiments in animals where changes have been observed mainly after long training sessions.

“This is critical to understanding the neural processes underlying real-life memory formation, as in real life we are not repeatedly exposed to an event in order to remember it – just one exposure is enough.”

The change in neuron firing was not entirely unexpected – the team did hypothesise such a result in a previous study – but the fact that this change was so dramatic did come as a surprise.

“Given the involvement of MTL neurons in memory formation, we hypothesised that we would be able to see some changes in the firing of the neurons. But the astonishing fact was that these changes were dramatic, in the sense of neurons changed from being very silent to firing a lot, and that these changes occurred at the exact moment of learning, even after one trial,” said Ison.

“The emergence of association of concepts established after single trials, linked to rapid neural activity changes, turned out to be ideal for the creation of new episodic memories.”

The research not only provides greater understanding of how humans form memories, but may help further research into neurological disorders affecting memory.

“A better understanding of how assemblies of neurons represent learning and memory might lead to novel ideas about our memory capacities and how these might deteriorate in patients suffering from certain neurological disorders,” added Ison.

Journal reference: Neuron, Ison et al. “Rapid Encoding of New Memories by Individual Neurons in the Human Brain” http://dx.doi.org/10.1016/j.neuron.2015.06.016