Research articlePhase synchrony in slow cortical potentials is decreased in both expert and trained novice meditators
Introduction
Slow cortical potentials (SCPs) are embedded in the spontaneous cortical activity and appear as slowly varying potential shifts of the EEG, ranging from 0.5 to several tens of seconds [1]. Due to their very low-frequency fluctuations, it has recently been postulated that SCPs can integrate information across different cortical regions, and thus may constitute a fundamental neural feature in the functioning of consciousness [[2], [3], [4]]. Following this reasoning, the strength of SCP phase synchrony is of interest in the context of mental training that can promote enhanced awareness and self-regulation of attention [5].
Meditation techniques were shown to induce changes in functional connectivity between brain regions, which are coupled by phase synchronizations between pairs of oscillatory EEG signals. For instance, enhanced functional connectivity in alpha- and gamma-frequency were found during meditations compared to resting states [6,7], but opposite effects were also reported for all frequency oscillations from delta- through gamma-frequencies [8]. These inconsistent results might be due to different types of meditative states, individual differences in mental strategies during meditation, or different levels of meditation experience. While most research concerning neural synchronization has focused on oscillations above delta-frequency [9], very low-frequency fluctuations (below 0.5 Hz) remain unaddressed.
The present study aimed at examining the strength of SCP phase synchrony during a resting state and during mindfulness meditation. Phase-locked SCPs measured from 9 long-term expert meditators were compared to the data obtained from 11 novices (cross-sectional design). Novices participated in an 8-week mindfulness-based stress reduction (MBSR) program [10], after which phase-locked SCPs were measured again, which sheds further light on the findings of the cross-sectional design. MBSR is a well-operationalized training program that teaches several practices, which foster a mindful way of paying attention to the present moment with an accepting attitude [10,11].
Section snippets
Materials and methods
9 long-term expert meditators (mean age = 49.7 years, SD = 7.5; 5 females) were recruited: four monks/nuns from Theravada Buddhist monasteries, three teachers of Vipassana meditation, and two additional practitioners with long-standing meditation experience. On average, they had 22 years of meditation experience (SD = 95.5, range from 12 to 33 years). 11 novices (mean age = 40.2 years, SD = 11.4; 9 females) with no prior experience in meditation or mindfulness practice were recruited for the
Results
During meditation, novices pressed the button 9.64 (SE = 2.58) times before participating in the MBSR program and 12.55 (SE = 3.53) times after participation (p = .214, d = 0.22). In contrast to the novices, expert meditators did not press the button, with the exception of one meditator who pressed twice during meditation.
In general, we observed strong SCP phase synchronies in both groups and blocks, as per high mean nPLFs (values of nPLF > 2.97 equal to p < .001; Fig. 1a). To quantify the
Discussion
Although many studies have characterized functional connectivity in terms of neural oscillations, phase synchrony below delta-frequency and its link to mental states have so far been neglected. The present study extended the functional brain connectivity to very low-frequency fluctuations by demonstrating different amounts of SCP phase synchrony in two different mental states; a resting state vs. a mindfulness meditation state. This was shown in long-term expert meditators as well as in novices
Conclusions
Despite the limitations described above, we were able to demonstrate the differences in functional connectivity between a resting state and a meditation state. While during the resting state expert meditators and novices exhibited the same amount of SCP phase synchrony, decreased synchronization was found during meditation among expert meditators and novices after undergoing MBSR. These findings suggest that SCP dynamics are context-dependent, whereby large-scale phase synchronization may
Acknowledgements
H.-G.J. was supported by the International Research Training Group (IRTG 2150) of the German Research Foundation (DFG).
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