Temporal Context and Emotional Memory

A fundamental goal of neuroscience is to understand how information is encoded and stored in the brain. The physical or functional representation of a memory (the memory trace or “engram”) is thought to be sparsely encoded over a distributed memory network. However, identifying the precise neurons which make up a memory trace has challenged for scientists since Karl Lashley’s famous “search for the engram” in the 1950’s. Moreover, it was not known why one neuron (rather than its neighbour) was recruited or allocated to a given memory trace. Using a variety of different methods [from overexpressing single genes to chemogenetics (using specially synthesized chemical compounds to control the activity of a defined population of cells) to optogenetics (using light to control the activity of a defined population of neurons)], we showed that eligible neurons compete for allocation to an engram in mice. Those neurons that are more excitable at the time of the event are preferentially recruited to the engram representing that event. Moreover, we showed that if two related events occur in a small time window (6 hrs) then the same neurons win the competition for allocation to both engrams. This links the two related memories. If, on the other hand, two events occur > 6 hr apart, non-overlapping populations of neurons are recruited and the memories are kept separate. Our results suggest that this neuronal competition during memory formation is a mechanism that links (or disambiguates) related emotional memories. Memory impairments are a hallmark of aging, major mental illnesses (e.g., schizophrenia and depression) as well as neurological disorders (e.g., Alzheimer's and Parkinson's diseases). Therefore, understanding how the brain encodes and stores information is highly relevant to both mental health and mental illness.