Electroencephalograph (EEG) analysis of oscillatory networks supporting human short-term memory

Lisa Payne

doi:10.17918/00010140

Functional imaging studies have demonstrated that short-term memory (STM), the ability to store information that is no longer available in the environment, involves prefrontal, parietal, and temporal areas. In order to form a complete understanding of this important phenomenon, functional models must combine neuroanatomical substrates with oscillatory network dynamics. Although electroencephalograph (EEG) oscillations in the theta band (4-7 Hz) have been associated with STM, there are inconsistencies in its response to increasing memory demand. In addition, recent reports of alpha frequency activity during STM tasks challenge the historical view that alpha oscillations represent cortical-idling and demands that we interpret its functional role. The first aim was to utilize EEG wavelet coherence to analyze how memory load-related increases in alpha power correspond to changes in STM network coherence and if coherence would be a more sensitive measure of load-related changes in network theta activity. Results revealed that frontal-midline theta and posterior alpha both exhibit demand-induced correlations with left hemisphere temporal and parietal regions associated with verbal STM retention. The finding that frontal-midline theta and parietal-midline alpha exhibit a nexus of coherence with left temporal/parietal sites is indirect evidence, consistent with functional neuroimaging studies, that this region supports retention. These results support the notion that these interactions represent complimentary mechanisms supporting retention through top-down selective strengthening of memory representations and gating of potentially interfering bottom-up inputs. The next goal was to test the hypothesis that posterior alpha activity serves to inhibit the flow of potentially interfering bottom-up sensory processing during STM retention with an experiment that required participants to attend to a fixation point on display while retaining consonants in memory. In accordance with inhibition theory, it was found that the requirement to visually attend during the retention interval did significantly decrease alpha power. The requirement to attend also altered the coherent relationships of the retention network. The interactions between frontal and parietal sites depicted in the control condition did not occur when the retention interval was varied, indicating that these long-distance coherent alpha relationships during retention are more likely to occur when external stimuli can be ignored.

Electroencephalograph (EEG) analysis of oscillatory networks supporting human short-term memory

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Electroencephalograph (EEG) analysis of oscillatory networks supporting human short-term memory

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