Washington: Similar to members of an orchestra, neurons in the brain require a well-timed conductor (waves) to stay on tempo.
In the hippocampus,the brain’s memory center, temporal ordering of the neural code is important for building a mental map of where you’ve been, where you are, and where you are going.
As a mouse navigates its environment, the central hippocampal area called CA1, relies on rhythmic waves of neural input from nearby brain regions to produce an updated map of space. When researchers turned off the input from nearby hippocampal area CA3, the refreshed maps became jumbled.
While mice could still do a simple navigation task, and signals from single neurons appeared to represent space accurately, the population level code, or ‘orchestra’ was out of time and contained errors.
“The neural music didn’t change, but by silencing CA3 input to CA1 in the hippocampus we got rid of the conductor,” said senior author Thomas McHugh,
The discovery of the mental map of space in the hippocampus was awarded the 2014 Nobel Prize in Physiology or Medicine, but the circuitry connecting ensembles of place cells, which are also used for memory processing, and how they update in realtime was not known.
Without CA3 input, accurate prediction of the spatial location from the ensemble neural code is impaired.
The mouse still knows where it is, but small errors in representing space from individual neurons become compounded without CA3 directing the CA1 ensemble.
If shutdown of CA3 was possible in humans, McHugh suggests, memories would likely become useless and jumbled. Earlier work with these same mice pointed to a similar role for the CA3 neurons in organizing information during sleep, a process required for long-term memory storage.
While individual hippocampal neurons continue to tick, this study shows that the neural orchestra needs CA3 input to serve as a conductor. This means that different neural coding strategies are dissociated but interdependent in the brain.
This research was published in Nature Neuroscience.