Long-term associations between two items or people can be predicted by the individual firing pattern of neurons in the human brain, shows a study published in Nature Communications this week. The findings shed light on how humans store long term, remembered associations.

When you perform an internet search for Hillary Clinton, how many of the results also include a mention of Bill Clinton- Counting these joint “hits” is a way of analysing how strongly these two people are associated, and Rodrigo Quian Quiroga and colleagues find that measuring human neural activity in response to pairs of images predicts this degree of association. They measured the firing patterns of 49 individuals who had been implanted with electrodes (for epilepsy treatment). The patients looked at a number of pictures, and researchers found that the patterns with which individual neurons fired was able to predict how related two pictures were. This means that when somebody saw a picture of Hillary Clinton, the neurons’ firing pattern was very similar to when they saw a picture of Bill Clinton, but not similar to the firing pattern in response to a picture of Stevie Wonder.

The neurons observed in this study are located in the medial temporal lobe region of the brain. Neurons in this region have long been known to have a role in learning to associate people, places, and things. However, it was not clear if this region had only a temporary role in coding associations during learning, which then consolidated in other areas, or whether this coding of associations remained in this area after initial learning. The new findings show that the medial temporal lobe provides a long-term coding associations, which plays a key role in human memory.