Memories that are formed within a few hours of one another can be encoded by a shared set of neurons.

Sheena Josselyn and Paul Frankland at the Hospital for Sick Children in Toronto, Canada, and their colleagues exposed mice to two fear-inducing experiences, each consisting of a different musical tone paired with a mild electric shock to the foot. When the events occurred within 6 hours of each other, the associated memories were encoded by some of the same neurons in the lateral amygdala — a brain area involved in fear memory. However, fear memories separated by 24 hours were encoded by separate groups of neurons.

The researchers found that neurons stimulated by the first experience transiently suppressed the excitability of neighbouring neurons, increasing the chances that the same cells will respond to a second experience that follows soon afterwards.

Science 353, 383–387 (2016)