Investigating the generators of the scalp recorded visuo-verbal P300 using cortically constrained source localization

Abstract

Considerable ambiguity exists about the generators of the scalp recorded P300, despite a vast body of research employing a diverse range of methodologies. Previous investigations employing source localization techniques have been limited largely to equivalent current dipole models, with most studies identifying medial temporal and/or hippocampal sources, but providing little information about the contribution of other cortical regions to the generation of the scalp recorded P3. Event-related potentials (ERPs) were recorded from 5 subjects using a 124-channel sensor array during the performance of a visuo-verbal Oddball task. Cortically constrained, MRI-guided boundary element modeling was used to identify the cortical generators of this target P3 in individual subjects. Cortical generators of the P3 were localized principally to the intraparietal sulcus (IPS) and surrounding superior parietal lobes (SPL) bilaterally in all subjects, though with some variability across subjects. Two subjects also showed activity in the lingual/inferior occipital gyrus and mid-fusiform gyrus. A group cortical surface was calculated by non-linear warping of each subject's segmented cortex followed by averaging and creation of a group mesh. Source activity identified across the group reflected the individual subject activations in the IPS and SPL bilaterally and in the lingual/inferior occipital gyrus primarily on the left. Activation of IPS and SPL is interpreted to reflect the role of this region in working memory and related attention processes and visuo-motor integration. The activity in left lingual/inferior occipital gyrus is taken to reflect activation of regions associated with modality-specific analysis of visual word forms.

Figure 1

Averaged ERP activity (μV) to target words for all five subjects at 70 representative scalp sites, depicted from 200 msec pre‐stimulus to 800 msec post‐stimulus. The check marks on the x‐axis represent 200‐msec intervals. The data shown have been corrected for EOG artifact.

Figure 2

Grand average ERP activity (μV) to target words (thick line) and non‐target words (thin line) at 70 representative scalp sites, depicted from 200 msec pre‐stimulus to 800 msec post‐stimulus. The check marks on the x axis represent 200‐msec intervals. The data shown have been corrected for EOG artifact.

Figure 3

Scalp topographic activity (μV) to target words at the peak of the P3 component for (A) individual subjects and (B) the grand mean ERP activity (group). Topographic maps were scaled equally around zero according to the maximum value (microvolts) at the peak of the P3 component with amplitudes and latencies as follows: S1 ± 26.5 μV, 340 msec; S2 ± 29 μV, 356 msec; S3 ± 17 μV, 366 msec; S4 ± 10.9 μV, 360 msec; S5 ± 25.1 μV, 360 msec; Group ± 19.8 μV, 358 msec. Isocontours were derived by partitioning scalp activity into 20 equal bands, depicting 5% steps in microvolts across the range for each subject's topographic maps. C: Cortical current density maps for individual subjects and (D) group maps shown on the modeled inner surface of cortical gray matter to target words at the peak of the scalp P3 component after spherical spline Laplacian transformation. Source activity reflects radial current density (A/m²) estimated by cortical constraint of the source solution to radial dipoles distributed equally throughout the cortex. The scalar bar indicates positive (yellow to red) and negative (blue to aqua) current source density, with CCD maps scaled equally around zero with amplitudes and latencies as follows: S1 ± 0.138 A/m², 336 msec; S2 ± 0.203 A/m², 336 msec; S3 ± 0. 0513 A/m², 364 msec; S4 ± 0.231 A/m², 332 msec; S5 ± 0. 0732 A/m², 348 msec; Group ± 0. 0623 A/m², 348 msec. Cortical activity for the P3 component is depicted from superior and posterior, and left and right lateral viewpoints.

Figure 4

Group cortical current density maps to target words showing the temporal evolution of the P3 component between 250 and 470 msec post‐target. The CCD maps are averages across consecutive 20‐msec time windows. They are shown from inferior, posterior, and superior viewpoints. The scale bar indicates positive (yellow to red) and negative (blue to aqua) current source density, with each CCD map scaled to the peak of the P3 component (±0. 0623 A/m²).