Underlying Processes of Working Memory More Complex Than Thought

Rhythmic brain activity in hippocampus is the key. Successful memory performance is based on alternating activity states.

In order to retain a piece of information for a short time, working memory is required. The underlying processes are considerably more complex than hitherto assumed, as researchers from the Ruhr-Universität Bochum and Bonn University report in the journal “Cell Reports”. Two brain states must alternate rhythmically in order for a piece of information to be successfully maintained.

Working memory: maintaining new information for a short time

When we want to remember a new piece of information for a short time, for example a phone number, working memory is called upon. Different brain regions are involved in this process, including the hippocampus, which is known for its crucial role in long-term memory. The team headed by Dr. Nikolai Axmacher from the Institute of Cognitive Neuroscience in Bochum and Marcin Leszczynski, researcher in Bochum and at the Department of Epileptology at Bonn University, studied rhythmic activity patterns in the hippocampus while the subjects memorised sequences of numbers or faces.

Two activity states at semi-second intervals

To this end, the team worked with epilepsy patients who had electrodes implanted into the hippocampus for the purpose of surgical planning. Those electrodes enabled the researchers to measure the activity of the region embedded deeply in the brain. While the patients memorised sequences of faces or numbers, the researchers observed two activity states in the hippocampus, which alternated twice per second: an excited and a less excited state.

Image shows a DNA double helix.
The results are important because the human genome is filled with proteins that have similar sequences and almost all understanding of these proteins so far has focused on pathological states. Credit: The researchers/Cell Reports.

Seemingly simple tasks require highly complex processes

If the rhythmic pattern did not occur in the hippocampus, the patients tended to make mistakes during the task. Based on the activity patterns, the researchers were also able to estimate how many numbers or faces the test subjects could reliably memorise. “The results show that the brain performs highly complex processes even during seemingly simple tasks,” says Prof Nikolai Axmacher. “Our subjective feeling if something is simple or complex is not a reliable marker for how the brain actually solves a task.”

About this memory research

Source: Nikolai Axmacher – RUB
Image Source: The image is credited to the researchers/Cell Reports and is licensed CC BY-NC-ND 4.0
Original Research: Full open access research for “Rhythmic working memory activation in the human hippocampus” by Marcin Leszczyński, Juergen Fell, and Nikolai Axmacher in Cell Reports. Published online October 29 2015 doi:10.1016/j.celrep.2015.09.081


Abstract

Rhythmic working memory activation in the human hippocampus

Highlights
•Working memory depends on rhythmic fluctuations in the human hippocampus
•Periods of memory activation are interleaved with periods of constant power levels
•Fluctuations between these two modes of processing are organized by delta rhythm
•A hierarchy of oscillations predicts successful performance and individual capacity

Summary
Working memory (WM) maintenance is assumed to rely on a single sustained process throughout the entire maintenance period. This assumption, although fundamental, has never been tested. We used intracranial electroencephalography (EEG) recordings from the human hippocampus in two independent experiments to investigate the neural dynamics underlying WM maintenance. We observed periodic fluctuations between two different oscillatory regimes: Periods of “memory activation” were reflected by load-dependent alpha power reductions and lower levels of cross-frequency coupling (CFC). They occurred interleaved with periods characterized by load-independent high levels of alpha power and CFC. During memory activation periods, a relevant CFC parameter (load-dependent changes of the peak modulated frequency) correlated with individual WM capacity. Fluctuations between these two periods predicted successful performance and were locked to the phase of endogenous delta oscillations. These results show that hippocampal maintenance is a dynamic rather than constant process and depends critically on a hierarchy of oscillations.

“Rhythmic working memory activation in the human hippocampus” by Marcin Leszczyński, Juergen Fell, and Nikolai Axmacher in Cell Reports. Published online October 29 2015 doi:10.1016/j.celrep.2015.09.081

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