Human study provides evidence that theta phase precession supports memory formation and retrieval

Past neuroscience research has pinpointed many of the neural processes through which the human brain forms, stores and retrieves important information, such as domain-specific knowledge and memories. One dimension of human memory is the ability to link various aspects of experience to specific life events.

Past studies have suggested that this memory-related process is supported by phase precession, which is a shift in the timing at which specific neurons are fired. Up until now, however, this hypothesis had not been confirmed experimentally.

Researchers at the University of California, Davis, Harvard Medical School, Toronto Western Hospital and Cedars-Sinai Medical Center recently carried out a study aimed at probing the relationship between phase precession and memory.

Their findings, published in Nature Human Behaviour, suggest that theta phase precession, the shift in the time at which specific neurons fire relative to the theta rhythm in brains, contributes to memory formation and retrieval.

“Our study was inspired by previous work showing that the relative timing of spikes with respect to population activity in the form of field potentials dynamically changes as animals navigate through an environment,” Jie Zheng, first author of the paper, told Medical Xpress.

“Extending this idea to human memory, the primary objective here was to test the functional role of such dynamic spike timing signals during episodic memory formation and retrieval of complex event sequences in a natural and non-spatial task.”

Zheng and her colleagues recruited 22 study participants and recorded single-neuron activity and local field potentials in their medial temporal lobe as they encoded and retrieved memories from movie clips. The team embedded the movie clips with event boundaries (i.e., contextual shifts), as this allowed them to mimic the process through which humans form memories in their everyday lives.

The researchers subsequently asked participants to recall events they observed in the movie clips and the order in which they occurred. They then analyzed the single-neuron activity they recorded as the participants were both viewing the movie clips and recalling the memory of the watched clips.

“The study was conducted in patients with pharmacologically resistant epilepsy where we could concurrently record the activity of individual neurons as well as population field potential signals,” said Zheng. “We observed that the precise time at which neurons fired was related to the ongoing theta rhythms inside brains and that this relative timing evolved dynamically (called phase precession).”

The experiments carried out by Zheng and her colleagues yielded very interesting results. The team observed dynamic phase precession in the brains of participants throughout the experiment, with specific neurons shifting the timing with which they fired at different stages of the experimental task.

“We found strong theta phase precession at event boundaries when participants watched the clips and also when the participants answered memory questions,” said Zheng. “The strength of theta phase precession was also predictive of participants’ memory accuracy, especially for their order memory.”

Overall, the findings of this recent study confirm the role of theta phase procession in the formation and retrieval of episodic memories. In the future, they could pave the way for further studies exploring the link between phase precession and specific memory processes.

“We would like to investigate the mechanisms underlying memory formation and retrieval of dynamic experiences with different sensory inputs (e.g., both visual and auditory), how the order of events is encoded, and how theta phase precession supports these consolidation and retrieval processes,” added Zheng.

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