Working memory is more complex than we thought

Rhythmic interaction of brain waves in the hippocampus are key to our memories. Two brain states must vibrate rhythmically in order for information to be retained.

In order to retain a piece of information — even for a short period of time — working memory is required. Now research finds the underlying processes are considerably more complex than had been assumed.

Researchers from the Ruhr-Universität Bochum and Bonn University, Germany, report in the journal Cell Reports that two brain states must alternate rhythmically in order for a piece of information to be successfully maintained.

When we want to remember a new piece of information for a short time, a phone number for example, working memory is called up. Different brain regions become involved in this process including the hippocampus, known for its crucial role in long-term memory.

A team headed by Prof 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 study subjects memorised sequences of numbers, or faces.

The scientists worked with epilepsy patients with electrodes implanted into their hippocampus for the purpose of surgical planning. Those electrodes enabled the researchers to measure hippocampal activity embedded deeply in the brain.

While patients memorised sequences of faces or numbers, researchers observed two activity states in a subject's hippocampus. These two states alternated twice per second: 1) an excited state and 2) a less excited state.

If this rhythmic pattern did not occur in the hippocampus, patients tended to have mistakes in memory. Based on these patterns, researchers were also able to estimate how many numbers or faces test subjects could reliably memorise.

"The results show that the brain performs highly complex processes even during seemingly simple tasks. Our subjective feeling if something is simple or complex is not a reliable marker for how the brain actually solves a task."

Nikolai Axmacher PhD, Professor Department of Epileptology, University of Bonn, German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany; Department of Neuropsychology, Institute of Cognitive Neuroscience, Ruhr University Bochum, Germany, and Corresponding author

Abstract 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.

This is an open access article under the CC BY-NC-ND license DOI: http://dx.doi.org/10.1016/j.celrep.2015.09.081
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Image Credit: Bonn University