Their findings may someday even help scientists understand why dreams are so difficult to remember. Scientists have long known that memories are formed in the brain’s hippocampus, but are stored elsewhere–most likely in the neocortex, the outer layer of the brain. Transferring memories from one part of the brain to the other requires changing the strength of the connections between neurons and is thought to depend on the precise timing of the firing of brain cells.
“We know that if neuron A in the hippocampus fires consistently right before neuron B in the neocortex, and if there is a connection from A to B, then that connection will be strengthened,” explains Casimir Wierzynski, a Caltech graduate student in computation and neural systems, and first author on the Neuron paper. “And so we wanted to understand the timing relationships between neurons in the hippocampus and the prefrontal cortex, which is the front portion of the neocortex.”
The research team–led by Athanassios Siapas, a Bren Scholar in the Caltech Division of Biology and an associate professor of computation and neural systems–used high-tech recording and computational techniques to listen in on the firing of neurons in the brains of rats. These techniques helped them pinpoint a number of neuron pairs that had precisely the kind of synchronous relationship they were looking for–one in which a hippocampal neuron’s firing was followed within milliseconds by the firing of a neuron in the prefrontal cortex.
“This is exactly the kind of relationship that would be needed for the hippocampus to effect changes in the neocortex–such as the consolidation, or laying down, of memories,” adds Wierzynski.
Once these spike-timing relationships between the hippocampal and prefrontal cortex neurons had been established, the team used their high-tech eavesdropping techniques to hear what goes on in the brains of sleeping rats–since sleep, as Siapas points out, has long been thought to be the optimal time for the memory consolidation.
As it turns out, those thoughts were right–but only part of the time. The team did indeed hear “bursts” of neuronal chatter during sleep–but only during a phase of sleep known as slow-wave sleep (SWS), the deep, dreamless periods of sleep. “It turns out that during slow-wave sleep there are these episodes where a lot of the cells in the hippocampus will all fire very close to the same time,” says Wierzynski. In response, some cells in the prefrontal cortex will fire in near unison as well, just milliseconds later. “What’s interesting is that the bulk of the precise spike timing happens during these bursts, and not outside of these bursts,” he adds.