Schematic representation of the n-back task.

The n-back working memory task was conducted in two distinct modalities: visuospatial and verbal. In the 2-back condition, participants had to identify whether the current stimulus matched the one presented two steps previously. In the visuospatial modality, the target was a spatial location, whereas in the verbal modality, the target was a letter. In the 0-back condition participants’ task was to respond to a predefined target, with the type of target corresponding to the task modality. Target stimuli are highlighted in orange.

Schematic of the analysis.

The time domain data were first transformed into the time-frequency domain by convolution with superlets (Moca et al., 2021). Next, the periodic and aperiodic components of the power spectrum density were estimated for each time point using FOOOF (Fitting Oscillations and One-Over-F) algorithm (Donoghue et al., 2020b). The aperiodic component was further separated into the slope (exponent) and offset components.

Effect of baseline correction and FOOOF decomposition on time-frequency decomposition in the n-back task.

Illustration of the effect of baseline correction and FOOOF decomposition on EEG time-frequency analysis. (A) “Raw” total power from time-frequency decomposition is difficult to interpret due to the 1/f power scaling of EEG power spectra (here representing average data across subjects, channels and conditions). In panel B we applied baseline correction (decibel conversion) using a pre-stimulus interval of -0.5 to -0.2 seconds for comparison. The baseline correction showed a significant decrease in alpha and beta power from 0 to 1 second and a concomitant increase in theta power lasting up to 2 seconds. The observed changes were very similar across different choices of baseline correction (Figure S1). However, it’s unclear from the baseline-corrected data whether the changes in theta power are oscillatory or aperiodic. Separation into periodic (C) and aperiodic (D) components shows that changes in theta power are due to changes in aperiodic activity.

Changes in periodic (oscillatory) activity as a function of time.

(A) The inspection of periodic activity revealed strong activity in the alpha and beta frequency bands, with a sharp decrease at around 0.5 seconds post-stimulus. The power was stronger in the 0-back condition, compared to the 2-back condition (see also Figure 5). (B) Early beta activity was most prominent at occipital and frontal channels.

Results of the linear mixed model analysis of periodic activity for comparison between conditions.

Significant values are highlighted on the heatmap and marked with yellow circles on the topographies. Only factors with significant differences are shown. A p-value was interpreted as significant if it was significant in at least 3 channels or 3 time points (see Methods for details). Note that there are small discrepancies between significant values on heatmaps and topographies due to averaging across time or channels. A significant difference was observed between the 2-back and 0-back conditions, with a reduction in activity in the 2-back condition, particularly in the alpha and beta frequency bands, starting at 0.5 seconds post-stimulus. The differences between stimulus types were most evident from 0.3 to 1 second post-stimulus, with decreased activity for targets compared to non-targets across the whole scalp. Additionally, a smaller effect of the modality × stimulus type interaction was observed from 0.6 to 1 second post-stimulus (see also Figure S3 for detailed visualisation of the interaction). Associations with reaction times were significant in the beta band across central channels. These associations exhibited a positive correlation in the early phase (0 to 0.5 seconds) and a negative correlation in the later phase (1 to 1.5 seconds).

Changes in aperiodic activity (slope or exponent) as a function of time.

(A) We averaged the time course of the aperiodic activity (exponent) over all channels and observed two components (peaks) of the aperiodic activity. The first component peaked around 0.3 s post-stimulus in the frontal channels. The second component peaked at 0.7 s, was stronger in parietal channels and differed between non-target and target conditions (see Figure 7). The time course of the offset parameter was comparable, although the separation between the frontal and parietal components was more pronounced (see Figure S4).

Results of the linear mixed model analysis of aperiodic (exponent) activity for comparison between conditions.

Significant values are shown in blue on line plots and marked with yellow circles on topographies. Only factors with significant differences are shown. The only significant differences between conditions were observed between target and non-target conditions, where the exponent was higher for targets in the early phase (0 to 0.5 s post-stimulus) and for non-targets in the middle phase (0.5 to 1 s post-stimulus). There was also a small association with reaction times in occipital channels around 1.5 seconds post-stimulus. Results were similar for the offset parameter, with an additional effect of load and n-back × stimulus type interaction (Figure S5).

The effect of baseline correction and FOOOF decomposition on time-frequency decomposition in the item-recognition task.

Similar to the n-back task (Figure 3, Figure S11), a decrease in alpha power is observed following instruction and stimuli presentation (up to 0.8 seconds) and continues throughout the retention period, up to 2.8 seconds post-stimulus (B). This is followed by a decrease in alpha and beta power during probe presentation, which is likely indicative of a motor response. Simultaneously, there is an increase in theta power, which is most pronounced during stimulus presentation (up to 0.8 seconds) and again after probe presentation (after 2.8 seconds). The FOOOF decomposition indicates that theta activity is predominantly aperiodic (C, D). The data shown represent the group average over all conditions at electrode Fz, where theta activity was most pronounced (see also Figure S24).

The comparison of different baseline corrections on time-frequency decomposition results in the n-back task.

We compared four types of baseline correction (in columns): (a) decibel conversion (10 * log10(data/baseline)), (b) relative change ((data – baseline)/baseline), (c) normalised change ((data – baseline)/(data+baseline)), (d) absolute change (data – baseline). We also considered three baseline periods (in rows): (a) from -500 to -200 ms, (b) from -300 to 0 ms, (b) from -500 to 0 ms. The selection of baseline correction type and period had minimal impact on the outcomes. Among the methods, absolute change correction exhibited a slight reduction in deviations from baseline. Nevertheless, the overall results were qualitatively similar for all baseline corrections. The data presented represent the average across participants, conditions and electrodes.

Power spectra of periodic activity on the Fz channel for each participant, averaged over time (between 0.5 and 2 seconds).

Inspection of the individual periodic power spectra showed that only two participants, SKP-1001 and SKP-2022, exhibited periodic activity in the theta band. Both axes of the spectra are displayed on a logarithmic scale.

Power of periodic activity 0.5 to 1 second post-stimulus, averaged across all channels and participants.

This visualisation facilitates the interpretation of the interaction between stimulus type and modality. It illustrates that the difference in power between target and non-target stimuli is more pronounced in the verbal task than in the visuospatial task. Error bars represent 95% Cousineau-Morey (within-subject) confidence intervals (Cousineau, 2017).

Temporal changes in aperiodic activity (offset).

A) (Figure 6).

Results of the linear mixed model on aperiodic activity (offset parameter) of the comparison between conditions.

Significant values are shown in blue on line plots and marked with yellow circles on topographies. Results were similar to the exponent parameter (Figure 7), but there was also an effect of load at about 0.6 seconds post-stimulus.

Grand average event-related potentials (ERPs) on midline electrodes.

ERPs have been low-pass filtered at a 40 Hz cut-off for visualisation.

Correlations between ERPs and aperiodic activity (exponent).

The line represents the mean correlation across subjects and the shaded area represents the interquartile range.

Changes in periodic (oscillatory) activity as a function of time.

Similar to Figure 4, but without subtracted ERPs. The periodic activity remains very similar to that observed with the subtracted ERPs, with a notable increase in power at occipital and frontal electrodes from 0 to 0.5 seconds post-stimulus.

Changes in aperiodic activity (slope or exponent) as a function of time.

Similar to Figure 6, but without the subtraction of ERPs. Aperiodic activity remains broadly similar to that observed with subtracted ERPs, with an increase in power at occipital electrodes and greater differentiation between frontal and parietal/occipital components.

Changes in aperiodic activity (offset) as a function of time.

Similar to Figure S4, a frontal and parietal aperiodic component can also be observed when the ERPs are not subtracted.

Effect of baseline correction and FOOOF decomposition on time-frequency decomposition (control dataset).

As in the main analysis (Figure 3), baseline correction (B) revealed changes in alpha, beta and theta power after stimulus presentation. FOOOF decomposition showed that changes in alpha and beta were periodic (C), whereas changes in theta power reflected task-related changes in aperiodic activity (D) (see also Figure S15, Figure S17).

Power spectra of periodic activity on the E15 channel for each participant, averaged over time (control dataset).

Similar to our main analysis (Figure S2), theta periodic activity was reliably present in only one participant (1115), with another participant (1104) showing a peak at 7-8 Hz. Both axes are displayed on a logarithmic scale.

Changes in periodic (oscillatory) activity as a function of time (control data).

(A) Time course of periodic activity averaged accross all channels. The periodic activity was most pronounced in the alpha band, with a strong decrease after stimulus onset. (B) Alpha and beta activity was most prominent in frontal channels.

Results of linear mixed model on periodic activity for comparison between conditions (control data).

There was a significant effect of load with reduced alpha and beta power at higher loads.

Changes in aperiodic activity (exponent) as a function of time (control data).

Similar to the main analysis (Figure 6), we observed two peaks, early frontal and late parietal components.

Results of linear mixed model on aperiodic activity (exponent) for comparison between conditions (control data).

There was a significant effect of load, most evident immediately after stimulus presentation. There was also an association with reaction times, particularly after 1 second post-stimulus.

Changes in aperiodic activity (offset) as a function of time (control data).

Results of linear mixed model on aperiodic activity (slope) for comparison between conditions (control data).

Schematic representation of the item-recognition task.

In each trial, two or four target stimuli of a given condition were presented sequentially in each half of the visual field while the participant focused on one half. Participants held the relevant visual properties of the stimuli in working memory. This was followed by a probe in which two stimuli were presented, one in each half of the visual field, and participants responded whether the stimulus in the attended half matched one of the previous targets. Each trial began with a fixation cross in the centre of the screen on which participants maintained their gaze. Each trial began with instructions indicating the area of focus and the number of stimuli that would follow. After a blank interval, participants viewed the target stimuli, followed by a maintenance interval and a probe. During this interval, participants indicated whether the probe corresponded to one of the previous targets. Trials were counterbalanced to ensure that an equal number of presentations occurred in both visual fields for each of the conditions and working memory loads. The number of stimuli presented sequentially (i.e., working memory load) was randomised to prevent participants from making predictions.

Periodic activity on item-recognition task.

Vertical lines represent: instruction presentation (0 s), stimuli presentation (0.4 s), end of encoding (0.8 s in load 2; 1.6 s in load 4), end of retention period (2.8 s). Topographies of theta activity were omitted due to the very weak activity and to save space. Task-related modulations were observed in alpha and beta activity, with a decrease during stimulus presentation, a relative increase during the retention phase and a significant decrease, probably motor-related, during probe presentation.

Aperiodic activity (exponent) in the item-recognition task.

As with periodic activity, we also observed task-related changes in aperiodic activity. Specifically, the aperiodic slope increased during instruction, stimulus presentation and probe presentation compared to the retention period. The topographies were similar to those of the n-back task, with activity spanning frontal and parietal channels. However, the two components could not be distinguished (see Figure S22).

Aperiodic activity (offset) in the item-recognition task.

The outcomes were comparable to those of exponent (Figure S21), however, the frontal and parietal/occipital components were discernible.

The power spectra of periodic activity on the Fz channel for each participant in the item-recognition task, averaged over time.

While the group average did not show pronounced theta activity (Figure S20, Figure S24), several participants showed periodic activity in the theta range (Seq2-s18, Seq2-s34, Seq2-s52, Seq3-s22, Seq3-s38, Seq3-s41, Seq3-s44, Seq3-s50). Furthermore, several peaks were observed between 7 and 8 Hz (Seq1-s14, Seq1-s23, Seq2-s37, Seq2-s40, Seq2-s55, Seq3-s28, Seq3-s59). Both axes are displayed on a logarithmic scale.

Group average power spectra of periodic activity in the item-recognition task, averaged over time.

As in Figure S20, only alpha and beta peaks were observed in the group average.