Electrophysiological working memory (WM) research has shown that distinct brain areas communicate through macroscopic oscillatory activities across multiple frequency bands. Such cross-frequency interactions generate nonlinear amplitude modulations (AM) in the observed signal. Traditionally, the AM of a signal is expressed as coupling strength between the signal and a pre-specified modulator at a lower frequency. Therefore, the idea of AM and coupling cannot be separately studied. This EEG study shows that the AM of parieto-occipital alpha/beta power and the coupling between frontal theta phase and parieto-occipital alpha/beta AM provide different information on WM processing. Thirty-three participants completed a color recall task with simultaneous EEG recording. The results showed that individual differences in WM precision are associated with frontal theta power enhancement and parieto-occipital alpha/beta power suppression. Furthermore, the AM of parieto-occipital alpha/beta power predicted WM precision after presenting a target-defining probe array. The phase-amplitude coupling (PAC) between frontal theta phase and parieto-occipital alpha/beta AM increased with WM load during the processing of incoming stimuli, but they did not predict the subsequent recall performance. These results indicate that the frontoparietal PAC reflects the executive control for selecting relevant WM representations, but whether the memorized information can be retrieved depends on the subsequent amplitude variation of parieto-occipital alpha/beta power. In conclusion, individuals with higher working memory precision are associated with enhanced frontal theta power and parieto-occipital alpha/beta power suppression.

Working memory (WM) research in electrophysiology reveals that brain areas communicate through macroscopic oscillatory activities across multiple frequency bands. Interactions across different frequency components generate nonlinear amplitude modulation (AM) in the observed signal. Traditionally, AM is expressed as the coupling strength between the signal and a prespecified modulator at a lower frequency. Therefore, the idea of AM and coupling cannot be studied separately. In this study, 33 participants completed a color recall task while their brain activity was recorded through EEG. The AM of the EEG data was extracted using the Holo-Hilbert spectral analysis (HHSA), an adaptive method based on the Hilbert-Huang transforms. The results showed that frontal theta power enhancement and parieto-occipital alpha/beta power suppression predicted individual differences in WM precision. Furthermore, the AM of parieto-occipital alpha/beta power predicted WM precision after presenting a target-defining probe array. The phase-amplitude coupling (PAC) between the frontal theta phase and parieto-occipital alpha/beta AM increased with WM load while processing incoming stimuli, but the PAC itself did not predict the subsequent recall performance. These results suggest frontal and parieto-occipital regions communicate through theta-alpha/beta PAC. However, the overall recall precision depends on the alpha/beta AM following the onset of the retro cue.