HMN 2026: How Episodic memory encoding fluctuates with brain’s theta rhythms

For almost a century, psychologists and neuroscientists have been trying to understand how humans memorize different types of information, ranging from knowledge or facts to the recollection of important events. Past studies consistently showed that humans recall some experiences for longer and in greater detail than others.

Some psychological theories suggest that the encoding and retrieval of past event-related memories is not a continuous process. Instead, these two aspects of memory could be separate and could manifest at different times.

One memory-related theoretical framework, rooted in behavioral science, is the Separate Phases for Encoding and Retrieval (SPEAR) model. This model outlines the idea that the human brain rapidly switches between the encoding of information and the retrieval of stored information.

The switch between encoding and retrieval could be associated with a particular type of brainwaves, known as theta rhythms, which repeat several times per second, typically between 3 and 10 hertz (Hz). These brain waves have been hypothesized to support the coordination of memory processes.

Researchers at University of Toronto, Toronto Western Hospital and other institutes recently carried out a study aimed at testing this theory and the possibility that memory processes change moment-by-moment following this rhythmic pattern. Their findings, published in Nature Human Behavior, are aligned with the SPEAR model’s predictions and suggest that the brain is only disposed to learn new information during brief time windows.

“Why do some experiences endure in memory better than others?” wrote Thomas M. Biba, Alexandra Decker and their colleagues. “We explore the possibility that learning fluctuates rhythmically several times per second, with fortuitously timed experiences being more memorable. Although such fleeting opportunities for encoding would evade our awareness, they are predicted by a prominent model describing how theta rhythms in the brain coordinate memory—the SPEAR model.”

Assessing memory encoding at a millisecond level

To carry out their study, Biba, Decker and their colleagues recruited 125 adult participants and asked them to take part in a memory experiment. The study participants were presented with information that they were meant to recall at very precise times, which varied across different experimental trials.

The researchers later asked participants to recall the stimuli they were previously shown and recorded their responses. By analyzing these responses and the precise timing with which the participants were presented with stimuli earlier, they could determine whether the ability to memorize information rose and fell following a repeating pattern.

“In a preregistered study, we adapted a dense sampling approach to reconstruct the millisecond time course of memory encoding in 125 participants,” wrote Biba, Decker and their colleagues. “We found that memory encoding fluctuated at a theta rhythm (3–10 Hz), that these rhythms were not a by-product of rhythmic attention and that—like theta rhythms in the brain—memory rhythms were modulated by putative markers of acetylcholine. Our findings provide behavioral evidence consistent with the SPEAR model of episodic memory.”

Hints of a specific memory ‘rhythm’

Biba, Decker and their colleagues found that people’s ability to memorize information did not stay constant, but it instead fluctuated rhythmically several times per second. This recorded rhythm was consistent with the frequency of theta brain waves, as predicted by the SPEAR model.

Interestingly, the results gathered by the researchers also suggest that the brain rhythms associated with the encoding of episodic memories are modulated by a chemical known as acetylcholine. This is a neurotransmitter known to play a role in attention, learning and memory processes.

This study offers evidence that supports the SPEAR model, suggesting that the encoding and retrieval of information occurs at alternating phases. Other research teams could soon try to validate the team’s findings, for instance by conducting similar experiments while also monitoring participants’ brain rhythms using electroencephalography (EEG).

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