The generators of slow potentials obtained during verbal, pictorial and spatial tasks

The purpose of this study was to test whether slow cortical electrical activity is specific to performance on verbal, pictorial and spatial tasks. Twenty-nine healthy subjects were required to compare pairs of visual stimuli separated by a delay of 2.5 s in a S1-S2 contingent negative variation-type paradigm. Slow potentials (SPs) were recorded by high-resolution EEG (123 channels) and their generators modeled by current density reconstruction using individual MRIs as source space models. Activity in each architectonic area of Brodmann was scored with respect to individual maximum current by a percentile method. Results showed a multifocal pattern of current density foci comprising the SP generators, including frontal and posterior cortices in all subjects, with the most active areas being common to the three tasks. In spite of the intersubject variability in the sets of active areas for each given task, a few cortical areas were observed to discriminate between tasks in a statistically significant way: the verbal task corresponded to stronger electrical activity in right area 45 than the other tasks; the spatial to weaker activity in right area 38 and left area 5 than the other tasks; the pictorial, compared to the spatial task, to stronger activity in left area 39; the verbal, compared to the spatial task, to stronger activity in left area 10, and compared to the pictorial, to weaker activity in right area 20. The present method of SP analysis may aid in the functional mapping of human association cortices in individual cases. We discuss our results emphasizing intersubject variability in cortical activity patterns and the possibility of finding more universal patterns.