The study, led by Michał Kucewicz and published in Nature Communications found that recalling even simple words involves widespread synchronization across multiple brain regions rather than activity limited to language centres.

This, say the researchers, provides new insight into how memories and concepts are reconstructed in the human brain.

Kucewicz, head of the Brain and Mind Electrophysiology Laboratory at the Gdańsk University of Technology BioTechMed Centre, conducted the research with scientists from Medical University of Wrocław and St. Anne's University Hospital.

The team studied patients with drug-resistant epilepsy who had electrodes implanted in their brains to identify the sources of epileptic seizures. The electrodes also enabled researchers to record detailed brain activity while patients performed memory tasks.

Patients from Poland, Czechia and the United States were shown simple nouns on a computer screen and later asked to recall them aloud from memory.

“We used simple nouns that are translated similarly in different languages, for example ‘house’ or ‘tree’”, Kucewicz said.

Researchers expected to observe activity mainly in brain regions linked to speech, but instead found widespread activation across all five lobes of the cerebral cortex.

“Before saying a word, there is synchronization of brain activity in high wave frequencies. Before a word even appears in the mind or is spoken, almost half of the areas in the brain are activated for several dozen milliseconds”, Kucewicz said.

According to the researchers, recalling a word requires the brain to reconstruct a broader concept built from sensory experiences, memories and associations distributed across multiple neural systems.

“For example, the word ‘fish’ can evoke many more associations than just a visual image. Many people's taste cortex will be activated when they think about a fish dish. In the case of anglers - the motor cortex, because they will remember how to use a fishing rod. The fish is also a symbol of Christianity, so if someone thinks about that, the association area of the brain related to symbol processing will be activated”, Kucewicz explained.

The scientists observed synchronized brain activity both when participants first memorized the words and when they later recalled them without any external cues.

“We saw this synchronous activity as words appeared on the screen and patients tried to remember them by imagining the concepts in their mind. The activity appeared even more strongly when they recalled the words at any time without any external stimulus. That is, the word appeared only in their minds, and the visual cortex reproduced or projected what they associated with it”, Kucewicz said.

The study also identified unexpectedly fast synchronized brain waves, known as High-Frequency Oscillations, reaching frequencies of 250–300 Hz during memory encoding and recall.

“We were surprised that fast brain waves (High-Frequency Oscillations, HFOs) were not limited to those we have known for a long time, i.e. with a frequency of approximately 100 Hz (approx. 100 cycles per second). While the patients were encoding and recalling memories, we found globally synchronized waves up to a frequency of 250-300 Hz”, the researcher said.

The synchronization appeared up to 300 milliseconds before participants began speaking, allowing researchers to observe how neural networks activated in sequence as the brain prepared to produce a word.

“We could therefore observe how the brain was preparing to pronounce a word, and how, under the influence of recalling concepts or memories, one neural network after another was activated in a cascading manner”, Kucewicz said.

The researchers said the findings could help scientists better understand how concepts and memory traces, known as engrams, are represented in the brain.

“We already know in what frequencies, parts of the brain and time to look for the activity underlying concepts and engrams, i.e. memory traces. We can now check, for example, at what point a concept appears consciously in the patient's mind and when it is in the subconscious sphere, or check what the activity looks like in the case of more complicated, abstract concepts, such as ‘love’ or ‘justice’. We also want to track electrical discharges in the brain not only during a half-hour test in the hospital, but also when patients later think about these words or sleep”, Kucewicz said.

PAP - Science in Poland, Anna Bugajska (PAP)

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