Publication: Oscillatory delta and theta frequencies differentially support multiple items encoding to optimize memory performance during the digit span task
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Date
2022-09-24
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Abstract
The human brain has limited storage capacity often challenging the encoding and recall of a long series of multiple
items. Different encoding strategies are therefore employed to optimize performance in memory processes such
as chunking where particular items are ‘grouped’ to reduce the number of items to store artificially. Additionally,
related to the position of an item within a series, there is a tendency to remember the first and last items on the list
better than the middle ones, which calls the “serial position effect”. Although relatively well-established in behavioral research, the neuronal mechanisms underlying such encoding strategies and memory effects remain poorly
understood. Here, we used event-related EEG oscillation analyses to unravel the neuronal substrates of serial encoding strategies and effects during the behaviorally controlled execution of the digit span task. We recorded EEG
in forty-four healthy young-adult participants during a backward digit span (ds) task with two difficulty levels
(i.e., 3-ds and 5-ds). Participants were asked to recall the digits in reverse order after the presentation of each set.
We analyzed the pattern of event-related delta and theta oscillatory power in the time-frequency domain over
fronto-central and parieto-occipital areas during the item (digit) list encoding, focusing on how these oscillatory
responses changed with each subsequent digit being encoded in the series. Results showed that the development
of event-related delta power evoked by digits in each series matched the ‘serial position curve’, with higher delta
power being present during the first, and especially last, digits as compared to digits presented in the middle
of a set, for both difficulty levels. Event-related theta power, in contrast, rather resembled a neural correlate of
a chunking pattern where, during the 5-ds encoding, a clear change in event-related theta occurred around the
third/fourth positions, with decreasing power values for later digits. This suggests that different oscillatory mechanisms linked to different frequency bands may code for the different encoding strategies and effects in serial
item presentation. Furthermore, recall-EEG correlations suggested that participants with higher fronto-central
delta responses during digit encoding showed also higher recall scores. The here presented findings contribute to
our understanding of the neural oscillatory mechanisms underlying multiple item encoding, directly informing
recent efforts towards memory enhancement through targeted oscillation-based neuromodulation.
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Keywords
Brain oscillation, Working memory, Serial position effect, Chunking, Encoding, Number perception
Citation
Aktürk T., de Graaf T. A. , Erdal F., Sack A. T. , Güntekin B., "Oscillatory delta and theta frequencies differentially support multiple items encoding to optimize memory performance during the digit span task.", NeuroImage, cilt.263, ss.119650, 2022