Changes in aperiodic (1/ f slope) activity during a picture-word interference task: Effects of congruency and sequence manipulations

Abstract

Aperiodic neural activity (1/f EEG) has been proposed to reflect the balance between excitatory and inhibitory (E:I) inputs, with steeper spectral slopes reflecting increased inhibition and flatter slopes indicating excitation. This activity also reflects the temporal coordination of neural firing, offering insights into fundamental brain dynamics. Recent studies have shown that the 1/f slope is sensitive to stimulus onset, characterized by initial inhibitory shifts followed by excitatory rebounds, which may reflect cognitive control mechanisms involved in suppressing distractions and preparing goal-directed responses. However, previous works have relied on fixed temporal windows and insufficient control of ERP contamination, limiting our understanding of rapid control dynamics. Here we used newly developed time-resolved analyses to study 1/f spectral slope modulation during a Picture-Word Interference task, focusing on two canonical cognitive control markers: the Congruency Effect (CE) and Congruency Sequence Effect (CSE). Forty-nine participants categorized pictures while ignoring congruent or incongruent words. Behaviorally, we replicated robust CE and CSE patterns. Spectral slope analyses showed that incongruent trials elicited steeper slopes - consistent with increased inhibition - particularly in frontal and central regions, reflecting conflict-related control engagement. Moreover, CSE analyses revealed dynamic slope modulations across frontal, central, and occipital components over time, suggesting control adjustments influenced by previous trial congruency. These results provide the first fine-grained evidence that aperiodic 1/f EEG activity can track both immediate conflict resolution and cognitive adjustments, offering a temporally sensitive neural marker of cognitive control through modulation of E:I balance.

Keywords: aperiodic 1/f EEG; cognitive control; congruency effect; congruency sequential effect; neural noise; spatial distribution.

Figure 1.

Overview of experimental design. Examples of two consecutive trials in the changed response category condition (Panel A) and in the repeated response category condition (Panel B), shown separately for the Novel Picture Condition (left panels) and the Frequent Picture Condition (right panels). The distractor word could be congruent with the picture (e.g., the word animale — animal in English — on an image of an animal) or incongruent (e.g., the word veicolo — vehicle in English — on an image of an animal).

Figure 2.

Flowchart of the spectral analysis procedure. The diagram shows the main processing steps.

Figure 3.

Current Congruency as a function of Previous Congruency for Reaction Times. Mean reaction times for current congruency, broken down by previous congruency. Error bars represent ±1 within-subject standard errors of the mean (Cousineau, 2005).

Figure 4.

Results of the Spatial Principal Component Analysis. Distribution of electrode-wise standardized loadings after varimax rotation (Panel A), and percentage of variance explained by each corresponding component (Panel B). Components are ordered from highest to lowest explained variance. Those outlined in green in Panel B accounted for more than one-fifth of the variance and were retained for further analysis.

Figure 5.

Results of the Temporal Principal Component Analysis. Distribution of time-wise standardized loadings after varimax rotation (Panel A) and percentage of variance explained by each component (Panel B). Components in Panel B are ordered from highest to lowest explained variance. Those outlined in green accounted for more than 1/13 of the variance and were retained for further analysis. FWHM, Full Width at Half Maximum.

Figure 6.

Changes in Aperiodic Activity (Spectral Slope) as a Function of Time. Stimulus-locked spectral slopes for Current Congruency (blue line for congruent, red line for incongruent) by Previous Congruency (solid line for previous congruent, dashed line for previous incongruent), shown for raw data from midline electrodes (Panel A) and for key components extracted from spatial PCA (Panel B). The dashed vertical line indicates stimulus onset (time zero). Panel A displays slope values from selected electrodes for illustrative purposes, whereas Panel B presents factor scores estimated from all electrodes using PCA weights; therefore, the panels are not directly comparable.

Figure 7.

Global Stimulus-Induced Changes in Spectral Slope. The time course of condition-averaged spatial scores for the frontal (top), central (middle), and occipital (bottom) components derived from spatial PCA. The dashed vertical line indicates stimulus onset (time zero). Shaded areas represent the time windows defined by temporal PCA. Lines at the bottom of each subplot denote statistically significant effects from paired t-tests comparing post-stimulus values to the pre-stimulus interval (values within the time windows were averaged for t-tests; black horizontal line for p < .05, green horizontal line for p < .01).

Figure 8.

Effects of Experimental Manipulation on the Spectral Slope. The time course of differences in spatial scores for Current Congruency (Panel A), Previous Congruency (Panel B), and their interaction (Panel C) for the frontal (top), central (middle), and occipital (bottom) components derived from spatial PCA. The dashed vertical line indicates stimulus onset (time zero). Shaded areas represent the time windows defined by temporal PCA. Lines at the bottom of each subplot denote statistically significant effects from repeated-measures ANOVA (values within the time windows were averaged for testing); black horizontal line for p < .05, green horizontal line for p < .01; Con, congruent; Inc, incongruent.

Figure 9.

Current Congruency as a Function of Previous Congruency for Spectral Slope. Mean spectral slope values for Current Congruency as a function of Previous Congruency in the time windows and spatial components where significant interactions between these factors were observed: 160–320 ms and 960–1120 ms in the frontal component (top left and top right panels, respectively), 160–320 ms in the central component (bottom left panel), and 480–640 ms in the occipital component (bottom right panel).

Figure 10.

From neuronal inhibition to cognitive control level. Schematic overview of the proposed mechanisms linking neuronal-level changes in excitation and inhibition to cognitive control. (Top left) A shift in the local excitation–inhibition balance, captured by local field potentials (LFPs), reflects increased GABAergic inhibition relative to AMPA-mediated excitation. (Top right) This shift leads to spectral changes characterized by a steeper 1/f slope in the power spectrum. (Bottom right) At the brain-network level, increased inhibition manifests as enhanced activation in regions involved in resolving competing responses. (Bottom left) At the cognitive level, stronger cognitive control supports performance in interference tasks, reflected by increased reaction times (RTs) for incongruent compared to congruent trials.