Figure 1.
Illustration of experimental design.
Sets of either 2 or 5 consonants were presented for 2.5 s (stimulus period) and had to be retained in memory for 3.5 s (retention period). After the retention period a probe letter was shown for 2 s (retrieval period). Subjects indicated by button press whether the probe was part of the stimulus. The red line indicates the last 2.5 s of the retention period that was used for EEG spectral analyses and for EEG-BOLD signal correlation analysis.
Table 1.
Summary of significant (p<0.005, FDR corrected) BOLD activations for the contrast ‘ss5–ss2’.
Figure 2.
Illustration of EEG results during the retention interval.
(A) Load independent topographical EEG effects. Results are reported for the following frequency bands: theta (5–7 Hz), alpha1 (8–10 Hz), alpha2 (10–13 Hz), beta1 (13–20 Hz), beta2 (20–30 Hz) and gamma (30–40 Hz). (B) Load dependent topographical EEG effects (‘(ss5–ss2)/ss2’). Significance values (t-tests, short arrow: p<0.05 and long arrow: p<0.01, uncorrected) for both analysis are indicated by a white-to-red (positive effects) and white-to-dark blue (negative effects) colour scale. (C) Results for single electrodes Afz and O2. For Afz, load 5 (dark-blue curve) exhibits stronger theta (yellow rectangle) power than load 2 (grey curve). For O2, alpha2 (light-green rectangle) and beta1 (light-blue rectangle) power is more enhanced for load 5 than for load 2. The black bars indicate significant effects at p<0.05 (corrected for multiple comparisons using the FDR). No significant load independent- or load dependent EEG effects occur at eye electrodes or at temporal (muscle) electrodes.
Figure 3.
EEG source localization (sLORETA) activation maps for the contrast ‘ss5–ss2’.
(A) Theta (5–7 Hz) band power increased (p<0.1) with load at the border of the MPFC and the ACC. (B) Alpha1 (8–10 Hz) showed decreased (blue) activity (p<0.05, corrected for multiple comparisons) at the border of the precuneus and the PCC with an increase of load. (C) Alpha2 (10–13 Hz) showed positive load modulations (p<0.05, corrected for multiple comparisons) in the right middle occipital gyrus (cuneus, BA 18). The t-values are plotted onto a MRI template.
Figure 4.
BOLD activation evoked by each task phase (encoding (A), retention (B), and retrieval (C)) contrasted against baseline (family-wise error corrected, p<0.05, red-yellow colour code).
Activity for each phase is averaged across set size 2 and 5. Negative BOLD activations (i.e., ‘baseline - encoding’) are shown in a blue-green colour code. (D) BOLD activations for the contrast ‘ss5–ss2’ (red-yellow colour code) and ‘ss2–ss5’ (blue-green colour code). Activations are presented at p<0.005 (corrected for multiple comparisons using the FDR). Numbers indicate z-coordinates in MNI space.
Figure 5.
Illustrations of EEG-BOLD signal correlations during the retention interval.
Positive (red) and negative (blue) correlations are shown for the following frequency bands: theta/θ: 5–7 Hz; alpha1/α1: 8–10 Hz; alpha2/α2: 10–13 Hz; beta1/β1: 13–20 Hz; beta2/β2: 20–30 Hz and gamma/γ: 30–40 Hz. All EEG-BOLD signal correlations are shown at an uncorrected voxel threshold of p<0.001 (t = 3.1) and a corrected cluster threshold of p<0.01. Numbers on the bottom indicate z-coordinates in MNI space. Capital letters in the different panels indicate the following brain regions: anterior cingulate cortex/ACC (A), posterior cingulate cortex/PCC (B), medial frontal gyrus/MPFC (C), posterior parietal cortex/PPC (D), inferior frontal gyrus (E), dorsolateral prefrontal cortex/DLPFC (F), precuneus (G), lingual gyrus (H), precentral gyrus (I), and border superior temporal gyrus and inferior parietal lobule (J). Correlations are shown for grey and white matter regions.
Table 2.
Illustration of grey matter high frequency BOLD signal correlations.
Table 3.
Summary of load specific EEG frequency band effects in regions activated by the BOLD contrast ‘ss5–ss2’ (p<0.005, FDR corrected).