Reconstruction of EEG alpha band sources during a working memory task

Abstract

Working memory denotes the temporary retention of a small amount of information in a readily accessible form for subsequent action. It facilitates planning, comprehension, reasoning, and problem-solving. Although there is still some debate, working memory has been specifically associated with the prefrontal cortex based on animal intracranial recordings. On the other hand, recent scientific evidence indicates that the alpha frequency range of the electroencephalogram (EEG) is related with working memory storage, but the anatomical sources of these signals are still unknown. The goal of this project is to determine the brain regions involved in generating stimulus-selective alpha-band EEG signals during a working memory task, using a source reconstruction approach. To this end, I compared activity currents estimated in three regions of interest (ROIs), the prefrontal cortex, the posterior parietal, and the visual cortex, during a visuospatial working memory task in 5 subjects. Structural Magnetic Resonance Imaging (MRI) was obtained for each subject and EEG was acquired during the performance of the task. I applied previously defined source reconstruction algorithms to the EEG filtered signal – in alpha waves – to data extracted during the task for each of these 5 subjects. The method used to perform the source reconstruction – to solve the inverse problem - was LORETA. To assess stimulus selectivity, I compared sources in the two brain hemispheres in two different conditions during the task, when memorized stimuli were presented either in the left or the right hemifields and considering the three ROIs of interest. We found that prefrontal cortex and posterior parietal regions showed more selectivity for the stimulus and had more activity during the delay of the working memory task than the visual cortex. This result validates my approach to estimate the anatomical sources of stimulus-selective alpha-band EEG signals in cognitive tasks.