Brain ‘Conductor’ Keeps Time in Memory Orchestra

In mice, memories became useless and jumbled when a region of the brain called CA3 was shut down.

AsianScientist (Jun. 1, 2016) – Just as members of an orchestra need a conductor to stay on tempo, neurons in the brain need well-timed waves of activity to organize memories across time.

Research from the RIKEN Brain Science Institute published in the journal Nature Neuroscience has pinpointed how the neurons that represent space in mice brains stay in time.

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’re going. As a mouse navigates its environment, a central hippocampal area called CA1 relies on rhythmic waves of neural input from nearby brain regions to produce an updated map of space.

By genetically engineering mice to express a nerve toxin in a hippocampal area called CA3, the researchers shut down the synaptic junctions between CA3 and other brain areas, including CA1.

The authors monitored each theta cycle—the time period over which the hippocampus updates its neural map of space—as the mice ran up and down in the track. Comparing the individual and population activity in normal and transgenic mice, they made an apparently paradoxical observation: As the transgenic mice moved in the enclosure, individual neurons continued to update their activity at a regular interval of 8 Hz, known as theta-cycle phase precession. This cyclic organization of information, however, was absent across the population of neurons.

“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 Dr. Thomas McHugh.

In a nutshell, while the mice could still carry out simple navigation tasks and signals from single neurons appeared to represent space accurately, the absence of CA3 input, or the ‘conductor,’ meant that the population level code, or ‘orchestra,’ was out of time and contained errors.

If it were possible to shut down CA3 in humans, McHugh suggests that memories would likely become useless and jumbled. A deeper understanding of how the rhythms of the brain organize information may shed light on the circuit mechanisms of disorders ranging from schizophrenia to Alzheimer’s disease, he says.

The article can be found at: Middleton & McHugh (2016) Silencing CA3 Disrupts Temporal Coding in the CA1 Ensemble.


Source: RIKEN; Photo: Shutterstock.
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