High-resolution EEG mapping of cortical activation related to working memory: effects of task difficulty, type of processing, and practice

Changes in cortical activity during working memory tasks were examined withelectroencephalograms (EEGs) sampled from 115 channels and spatiallysharpened with magnetic resonance imaging (MRI)-based finite elementdeblurring. Eight subjects performed tasks requiring comparison of eachstimulus to a preceding one on verbal or spatial attributes. A frontalmidline theta rhythm increased in magnitude with increased memory load.Dipole models localized this signal to the region of the anterior cingulatecortex. A slow (low-frequency), parietocentral, alpha signal decreased withincreased working memory load. These signals were insensitive to the typeof stimulus attribute being processed. A faster (higher-frequency),occipitoparietal, alpha signal was relatively attenuated in the spatialversion of the task, especially over the posterior right hemisphere. Thetaand alpha signals increased, and overt performance improved, after practiceon the tasks. Increases in theta with both increased task difficulty andwith practice suggests that focusing attention required more effort afteran extended test session. Decreased alpha in the difficult tasks indicatesthat this signal is inversely related to the amount of cortical resourcesallocated to task performance. Practice-related increases in alpha suggestthat fewer cortical resources are required after skill development. Theseresults serve: (i) to dissociate the effects of task difficulty andpractice; (ii) to differentiate the involvement of posterior cortex inspatial versus verbal tasks; (iii) to localize frontal midline theta to theanteromedial cortex; and (iv) to demonstrate the feasibility of usinganatomical MRIs to remove the blurring effect of the skull and scalp fromthe ongoing EEG. The results are discussed with respect to those obtainedin a prior study of transient evoked potentials during working memory.