Dissociation between phase‐locked and nonphase‐locked alpha oscillations in a working memory task

Abstract

The functions of human alpha oscillations ( approximately 10 Hz) were related to cognitive processes such as memory and top-down control. Recent models suggest that alpha phase serves as a mechanism especially relevant for the timing of neural activity, whereas alpha amplitude is important for the inhibition of task-irrelevant brain areas. This study investigates directly the influence of top-down modulation on phase-locked and nonphase-locked alpha rhythms. We conducted an EEG experiment where subjects performed a working memory task. In the encoding phase of the task subjects had to learn presented pictures of nonliving objects that could later be asked to be retrieved. We varied the top-down modulation by including cues indicating either to remember or to forget (not to remember) the next following item. Spectral analyses showed that nonremember cues elicited pronounced alpha amplitude increase compared to remember cues. Furthermore, phase-locking in low frequencies, especially in the alpha range (7-12 Hz), was stronger for remember as opposed to not-to-remember items. In conclusion, we propose that alpha amplitude reflects top-down modulated inhibition and that alpha phase is important for the exact timing of neural activity and can be related to binding processes.

Inter-individual performance differences in younger and older adults differentially relate to amplitude modulations and phase stability of oscillations controlling working memory contents

“…More importantly, the application of rTMS (at alpha frequencies) over cortical sites processing irrelevant information led to increments in WM performance. Relatedly, when participants successfully ignore taskirrelevant stimuli, alpha amplitudes increase already prior to stimulus-onset (Freunberger et al, 2009) (Fig. 1).…”

“…One important factor contributing to interindividual differences in the WM performance range is the efficient control of WM contents (Freunberger et al, 2009;Gazzaley et al, , 2008Sauseng et al, 2009;Vogel et al, 2005), in particular, when both relevant and irrelevant information is present in the environment.…”

“…Recent evidence suggests that the inhibition of irrelevant information is closely related to the power of oscillatory brain activity in the alpha-frequency range (i.e., 7-14 Hz) (Freunberger et al, 2009; for a recent review, see Klimesch et al, 2007;Palva and Palva, 2007;Romei et al, 2008;Thut et al, 2006;van Dijk et al, 2010). For example, by combining EEG and repetitive transcranial magnetic stimulation (rTMS), Sauseng et al (2009) recently found that the amplitude of lateralized alpha oscillations co-varies with the amount of irrelevant information and predicts individual WM capacity.…”

“…Towards this end, younger adults and older adults performed a cued version of the Sternberg WM paradigm with variable set sizes (Freunberger et al, 2009;Sternberg, 1966). In this task, participants were informed prior to stimulus presentation whether they should memorize or ignore the upcoming stimulus.…”

Prefrontal cortex and sensory cortices during working memory: quantity and quality

“…Besides the above studies linking alpha oscillations with WM quantity, two new studies have indicated their role in WM precision [70,71] , so it cannot be ruled out that alpha oscillations are also correlated with WM quality. Future studies to disentangle the roles of alpha power and phase would help to answer this question, since it has been shown that the phase-locked and non-phaselocked parts of alpha oscillations are related to different processes during WM [72] .…”