Figure 3. A: Grand-average ERP waveforms (contralateral
minus ipsilateral activity relative to the memorized display hemifield)
time-locked to the onset of the memory display at parieto-occipital
electrodes (PO3, PO4, PO7, PO8, O1, and O2) for the different object
configurations. For illustration purposes, the presented waveforms were
low-pass filtered at 12 Hz (24 dB/octave). Scalp distribution maps were
comparable across all components (PPC, N1pc, N2pc, and CDA), we
therefore chose to present the point in time at which the respective
difference waves (between grouped and ungrouped configurations) reached
their maximum. B: Mean amplitudes of the PPC, N1pc, N2pc, and CDA
components as a function of Object Configuration. Error bars denote 95%
(within-subject) confidence intervals. Significant differences revealed
by pairwise comparisons are indicated by asterisks; * p< .05. C: Correlations. The scatterplots show the relationship
between individuals’ behavioral performance in grouped (left panel) and
partially grouped (right panel) configurations and their corresponding
PPC amplitudes. Solid lines indicate the best-fitting regressions,
shaded regions illustrate 95% confidence intervals.
Oscillatory Amplitude. Figure 4A shows the
time–frequency profile across trials. As can be seen, variations of
grouping strength modulated activations in the alpha band (8–12 Hz),
with no other frequency ranges showing comparable changes in activity.
Given this, we examined changes in the lateralized alpha amplitude
(contralateral–ipsilateral) as a function of Object Configuration using
a repeated-measures ANOVA with the within-subject factor Object
Configuration (ungrouped, partially grouped, and grouped). The
lateralized alpha during the pre-stimulus period (-200–0 ms)
yielded no effect of configuration (-0.41 µV, -0.49 µV, and -0.37 µV for
ungrouped, partially grouped, and grouped configurations, respectively),F (2, 46) = 0.60, p = .55,ηp2 = .03,BF10 = 0.18, showing that alpha amplitudes were
comparable across conditions before trial onset. However, during thedelay period (350–1300 ms), there was a significant main effect
of Configuration, F (2, 46) = 8.73, p < .001,ηp2 = .28, indicating that
lateralized alpha is suppressed the most for ungrouped (-0.76 µV),
followed by partially grouped (-0.69 µV), and least for grouped (-0.62
µV) configurations (all p’ s < .02,
|dz |s > 0.44, for
the pairwise comparisons between configurations; see Figures 4B
and C ). Note that the observed differences in the lateralized alpha
amplitudes were mainly associated with contralateral variations (which
was strongly modulated by Object Configuration, F (2, 46) = 7.71,p = .001, ηp2 = .25),
thus reflecting processing of the task-relevant stimulus configurations
rather than the inhibition of task-irrelevant placeholders (the latter
being associated primarily with ipsilateral alpha activity, which was
overall comparable for different configurations, F (2, 46) =
0.12, p = .88, ηp2 =
.005, BF10 = 0.13).