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 with electroencephalograms (EEGs) sampled from 115 channels and spati ally sharpened with magnetic resonance imaging (MRI)-based finite elem ent deblurring. Eight subjects performed tasks requiring comparison of each stimulus to a preceding one on verbal or spatial attributes. A f rontal midline theta rhythm increased in magnitude with increased memo ry load. Dipole models localized this signal to the region of the ante rior cingulate cortex. A slow (low-frequency), parietocentral, alpha s ignal decreased with increased working memory load. These signals were insensitive to the type of stimulus attribute being processed. A fast er (higher-frequency), occipitoparietal, alpha signal was relatively a ttenuated in the spatial version of the task, especially over the post erior right hemisphere. Theta and alpha signals increased, and overt p erformance improved, after practice on the tasks. increases in theta w ith both increased task difficulty and with practice suggests that foc using attention required more effort after an extended test session. D ecreased alpha in the difficult tasks indicates that this signal is in versely related to the amount of cortical resources allocated to task performance. Practice-related increases ire alpha suggest that fewer c ortical resources are required after skill development. These results serve: (i) to dissociate the effects of task difficulty and practice; (ii) to differentiate the involvement of posterior cortex in spatial v ersus verbal tasks; (iii) to localize frontal midline theta to the ant eromedial cortex; and (iv) to demonstrate the feasibility of using ana tomical MRIs to remove the blurring effect of the skull and scalp from the ongoing EEG. The results are discussed with respect to those obta ined in a prior study of transient evoked potentials during worsting m emory.