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 characterised by the aperiodic slope (the negative counterpart of the exponent parameter) and the offset, which together describe the underlying broadband spectral shape.

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 low-frequency power lasting up to 2 seconds. The observed changes were very similar across different choices of baseline correction (Figure S2). However, it’s unclear from the baseline-corrected data whether the observed changes in the low-frequency range reflect oscillatory or aperiodic contributions. To disentangle these components, we decomposed the time-frequency signal into periodic and aperiodic contributions using spectral parameterisation. (C) The periodic component includes only the parameterised oscillatory peaks, reconstructed from Gaussian fits to the power spectrum (see Methods for details). (D) The aperiodic component reflects the 1/f-like background activity. This decomposition suggests that changes in power in the low-frequency (delta and theta) range may largely reflect changes in aperiodic activity. See Figure S1 for the corresponding figure with a logarithmic y-axis.

Correlations between baseline-corrected time-frequency and FOOOF-decomposed EEG activity

For each FOOOF parameter, we estimated its similarity to baseline-corrected time-frequency EEG activity at each channel-frequency-timepoint using a linear mixed model. We observed strong correlations between periodic activity and baseline-corrected time-frequency activity in the alpha and beta ranges. In contrast, the exponent and offset showed strong correlations in the lowfrequency range, with the exponent also showing negative correlations in the gamma range. These results suggest that changes in aperiodic activity appear as low-frequency oscillations in baseline-corrected time-frequency plots. Note that while R2 is strictly non-negative, we assigned its sign based on the beta co-efficient from the fixed effect in the model to facilitate interpretation. The values presented are averaged across all channels; see Figure S3 for topographical distributions.

Phase autocorrelation function and its correlations with FOOOF parameters.

Phase autocorrelation function (pACF) (Myrov et al., 2024) is an alternative measure of oscillations that is sensitive to rhythmicity rather than the amplitude. Patterns of pACF (A) were largely consistent with periodic activity (B), particularly in the alpha range, supporting the notion that pACF and periodic activity reflect the same underlying processes. In contrast, the correlations with exponent and offset were low (only results for exponent are shown as results for exponent and offset were essentially the same). See Figure S4 and Figure S5 for detailed figures.

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 7). (B) Early beta activity was most prominent at occipital and frontal channels. Note that colour scale ranges in panel B differ between frequency ranges.

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 S7 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 (exponent, interpreted as aperiodic slope) 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 9). The time course of the offset parameter was comparable, although the separation between the frontal and parietal components was more pronounced (see Figure S8). The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals, adjusted so that non-overlapping intervals correspond to statistically significant differences (Cousineau, 2017).

Results of the linear mixed model analysis of aperiodic exponent (interpreted as slope) 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 S11).

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 S18), 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 low-frequency 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 a substantial portion of low-frequency activity could be attributed to the aperiodic component (C, D) (see also Figure S33). The data shown represent the group average over all conditions at electrode Fz, where low-frequency activity was most pronounced (see also Figure S40). Horizontal lines indicate the boundaries of the frequency ranges. See Figure S32 for the corresponding figure with a logarithmic y-axis.

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

Same as Figure 3 but with a logarithmic y-axis.

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.

Topographies of correlations between baseline-corrected time-frequency and FOOOF-decomposed EEG activity.

See Figure 4 for heatmap of average across all channels.

Changes in phase autocorrelation function (index of rhytmicity) as a function of time.

Correlations between phase-autocorrelation function and FOOOF parameters.

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).

Similar as Figure 8, but for the offset parameter. The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals.

FOOOF goodness-of-fit measures.

Averaged across channels, subjects and conditions. Goodness-of-fit was lowest around 0.5 s after stimulus onset, corresponding to the decrease in aperiodic and periodic activity.

Number of identified peaks in FOOOF models.

(A) Average number of peaks across all subjects, channels, and conditions. Light lines represent individual channels. (B) Histogram of the number of peaks in all models. On average, 1.9–2 peaks were identified per model, showing high consistency across models and indicating that the models did not overfit by detecting an excessive number of peaks.

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 9), 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.

Topographies of stimulus- and response-locked event-related potentials (ERPs).

Correlations between ERPs and aperiodic activity.

Associations between stimulus-locked ERPs and aperiodic activity were estimated using a linear mixed model for each channel-time point. Correlations between ERPs and aperiodic activity peak right after stimulus onset but remain generally low. Notably, moderate correlations are observed between 0.5 and 1 second after stimulus onset on one channel, which we suspected were due to noise. To address this, we recalculated the model by capping values below the 0.1th percentile and above the 99.9th percentile (panels B, D). The results confirm that the associations between stimulus-locked ERPs and aperiodic parameters are very weak beyond 0.5 seconds.

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

Similar to Figure 6, 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 8, 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. The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals.

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

Similar to Figure S8, a frontal and parietal aperiodic component can also be observed when the ERPs are not subtracted. The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals.

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 S27, Figure S29).

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

Same as Figure S18 but with a logarithmic y-axis.

Correlations between baseline-corrected time-frequency and FOOOF-decomposed EEG activity (control dataset).

The results were very similar to those obtained with the other two datasets (Figure 4, Figure S3, Figure S33).

Changes in phase autocorrelation function (index of rhytmicity) as a function of time (control dataset).

As with the other two datasets, the patterns of pACF were broadly consistent with the periodic activity estimated by FOOOF (see also Figure S22).

Correlations between phase-autocorrelation function and FOOOF parameters (control dataset).

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

Similar to our main analysis (Figure S6), 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 dataset).

(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.

FOOOF goodness-of-fit measures (control dataset).

Averaged across channels, subjects and conditions. Similar to our primary dataset (Figure S9), the decrease in goodness-of-fit around 0.5 s after stimulus onset, coincided with the decrease in periodic activity.

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 8), we observed two peaks, early frontal and late parietal components. The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals.

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).

The vertical lines in panel A represent mean reaction time modes. The shaded areas represent 95% Cousineau-Morey within-subjects confidence intervals.

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.

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

Same as Figure 10 but with a logarithmic y-axis.

Correlations between baseline-corrected time-frequency and FOOOF-decomposed EEG activity (item recognition task).

The results were very similar to those obtained with the other two datasets (Figure 4, Figure S3, Figure S20), but in the item recognition task we also observed smaller correlations with aperiodic parameters in the alpha range.

Changes in phase autocorrelation function (index of rhytmicity) as a function of time (item recognition task).

As with the other two datasets, the patterns of pACF were broadly consistent with the periodic activity estimated by FOOOF (see also Figure S35).

Correlations between phase-autocorrelation function and FOOOF parameters (item recognition task).

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). Prominent task-related modulations were observed in alpha and beta range, with a decrease during stimulus presentation, a relative increase during the retention phase and a decrease 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 S38).

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

The outcomes were comparable to those of exponent (Figure S37), 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 S36, Figure S40), 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 S36, only alpha and beta peaks were observed in the group average.

FOOOF goodness-of-fit measures for the item recognition task, averaged across channels.

R2 was above 0.97 and model error was below 0.045 throughout the task. There was a small decrease in goodness-of-fit around 3 seconds after stimulus onset, coinciding with a decrease in periodic activity, similar to the other two datasets (Figure S9, Figure S25).

Simulated power spectra in log-log space.

We added a single periodic component to each power spectrum, with the exponent fixed at 1.5 and the offset at 1.

Exponent estimates.

Red lines indicate ground truth. Panel A covers 1–30 Hz fits, while panel B covers 3–30 Hz fits. Exponent estimates were inflated at larger bandwidths and higher periodic power, especially when the periodic component’s central frequency was low. This suggests a mixing of periodic and aperiodic components.

Offset estimates.

As in Figure S43, estimates tended to be inflated when the periodic component was difficult to detect.

Bandwidth estimates.

Red lines show ground truth, while red numbers mark how often the periodic component was undetectable (out of 100 cases). Low frequency, low power, and the restricted 3–30 Hz fitting range often made periodic features hard to detect.

Central frequency estimates.

Central frequency estimates were inflated for large bandwidths and high power, particularly when fitting was restricted to ≥ 3 Hz (panel B). Variability of estimates also increased at low simulated power. The red numbers indicate how many times out of 100 the periodic component was undetectable.

Power estimates.

In contrast to bandwidth and central frequency estimates, power estimates remained largely accurate at low simulated power, regardless of bandwidth. However, at high simulated power and large bandwidths power estimates were deflated. The red numbers indicate how many times out of 100 the periodic component was undetectable.

R2, index of model fit.

R2 values were very high in most cases, even when parameter estimates were largely inaccurate (see previous figures). The lowest R2 values were observed for low bandwidths, low central frequencies and high power.

Model errors.

Similar to R2, higher model errors were associated with low central frequencies and large powers, and the effect was more pronounced when only frequencies ≥ 3 Hz were fitted.