Friday 4 January 2013

A review of meditation studies

A recent review of meditation studies (Fell et al, 2010) raises issues that are worth considering.

The authors reflect on the inconclusive results from previous studies (Cahn, B.R., Polich, J., 2006).  Meditation takes diverse forms, but the authors feel that during the development of meditation practice some common characteristics are shared and passed through.  They offer two hypotheses.  The first is that every meditative training involves a similar scheme of development.  The second is that the steps are similar, beginning with similar physical demands,  With increasing experience the student focuses on an object with consequent slowing of internal dialogue accompanied by greater relaxation.  With advanced practice, the student undergoes a change in the relationship between thoughts and feelings, and the student starts experiencing mental processes as temporary and transient.  The most advanced step in meditation practice occurs when the meditator achieves certain peak experiences and undergoes permanent changes and alterations of consciousness that last outside of the mediation experiences.

In terms of EEG correlates, the authors maintain that there is a state-related slowing of the alpha rhythm in combination with an increase in alpha power which is localized in frontal regions.  This pattern occurs early on in the development of meditation practice.  The alpha oscillations are thought to reflect an increase in internal attention.

A general increase in theta activity occurs with mediation practice and is associated with more advanced levels.  Sharp bursts or theta trains, proceeded and followed by alpha, have been observed in studies and are thought to distinguish the theta that occurs in meditation from the irregular theta that reflects drowsiness.  This type of theta activity has also been observed to occur after meditation when the meditators opened their eyes and were alert.

Synchronized gamma oscillations have been associated with meditation.  Studies with long-term meditators doing different forms of focused and compassion meditation found high levels of gamma activity.  The authors speculate that gamma activity may provide ideal conditions for cortical plasticity and the formation of neural circuits.

While these generalizations provide a context to look at meditation, the diversity of meditation practices and their neurophysiological correlates warrants examination.  And there is abundant evidence that the differences matter.

Cahn, B. Rael, and John Polich. "Meditation states and traits: EEG, ERP, and neuroimaging studies." Psychological bulletin 132.2 (2006): 180.

Fell, Juergen, Nikolai Axmacher, and Sven Haupt. "From alpha to gamma: Electrophysiological correlates of meditation-related states of consciousness."Medical hypotheses 75.2 (2010): 218-224.

Spatial distribution of alpha power during meditation

Two engineers from the Taiwan group associated with the process study previously reported (Liu & Lo, 2006) investigated the spatial distribution of alpha power during meditation.  They studied Zen-meditation practitioners (the experimental group) and compared them with non-practitioners (the control group).  The technical details of the method are complex.   Here is their summary of the method, which you might want to skip if you are averse to complexity:

"We firstly adopted wavelet transform to decompose EEG signals and reconstruct waves in each frequency band using wavelet coefficients.  From the power ratio, we selected the candidates (normalized alpha-power vectors) for further spatial analysis.  Fuzzy C-means based algorithm was applied to the normalized vectors to explore various brain spatial characteristics during meditation (or, at rest).  Here we evaluated correlation coeffiicents to decide the number of clusters."

They found that (1) the alpha power in the control group decreased dramatically but not in the experimental group, (2) after meditation alpha power in the frontal area of meditators increased more than that of the control subjects (after resting-EEG recording).  The authors speculated that activating the medial prefrontal cortex and the anterior cingulate cortex during meditation may be the reason for increasing frontal alpha power.  

Liu, Chuan-Yi, and Pei-Chen Lo. "Investigation of spatial characteristics of meditation EEG using wavelet analysis and fuzzy classifier." Proceedings of the fifth IASTED International Conference: biomedical engineering. ACTA Press, 2007.

Representing the meditation process

Most studies of people meditating look at what brainwaves and brain regions are most prominent over the course of an entire meditation session taken as a whole.  Only a few studies have looked at the meditation process itself and what emerges over time, what is referred to as the "meditation scenario." Anyone who has meditated knows that over the course of a session, alertness may vary, topics and stories arise and pass away, diverse sensations are experienced and, if we are lucky, moments of serenity and mindfulness occur.

One of the most interesting EEG studies I have come across comes from engineers in Taiwan (Chang & Lo, 2005; Lui & Lo, 2006).  They propose methods of studying meditation that show great promise, although the technicalities of the methods will likely be intimidating to non-engineers.

In one study (Chang & Lo, 2005) 8-channel recordings (F3, F4, C3, C4, P3, P4, O1 AND O2) were made of Zen meditators at various levels of experience who meditated over a 40 minute period.  (The number of meditators in the experimental group and the number of non-meditators in the control group are not clearly stated.  The description of the type of meditation does not appear to be insight meditation but something similar to concentration meditation in the Theravada tradition.)

Their analysis involved "novel fuzzy-merging strategies and wavelet features," the explanation of which can be obtained by those brave enough to read the article.  In essence, the "cluster-managing strategies" employed in the automated analysis achieved an interpretation close to the result of naked-eye examination of the EEG.

They distinguished five different meditation states:  delta, delta plus theta, theta plus slow alpha, high amplitude alpha, and an amplitude suppressed wave state, which they call phi.  Five different "meditation scenarios" were observed in the meditators studied.

A)  Persistent alpha activity dominates the entire meditation session.  Phi activities of different duration appear intermittently.  The meditators reported flights of thought throughout the meditation session with abrupt shifts to peaceful states accompanied by feelings of light.
B)  Four EEG patterns--alpha, delta, theta and phi--appear rotationally.  Meditators reported switches among sensations of interference of mental activities, alertness and tenseness, subliminal consciousness and feelings of sacred, peaceful light.  
C)  EEG signals of background phi predominate with very few alpha activities and scattered delta.  This occurred with a very experienced meditator.  She reported that she felt energy or light penetrating her head several times, with feelings of serenity and egolessness and liberation from her body and mind.  Thoughts and drowsiness occurred intermittently.
D)  Background phi is sprinkled with alpha rather than delta.  The meditator reported a bright light and feelings of being fully relaxed during the session.
E)  The phi patterns dominates from the beginning of meditation and no other activity is observed to be significant.

In contrast, the scenarios of the control group were characterized by dominant alpha rhythms with delta plus theta or theta appearing occasionally.  In the control group, the delta and theta emerged because the subjects fell asleep, whereas this was not true for the experimental group.  The authors hypothesized that the delta and theta that emerged in the experimental group had to do with subliminal consciousness rather than drowsiness and sleep.  The most striking result was the emergence of the phi pattern in the experimental group, which was correlated with the experience of "blessings" according to the reports of the meditators.

Fig.5 Five meditation scenarios based on evolution
of meditation EEG illustrated by the running grayscale
chart. The gray-scales from the darkest to the
brightest colors indicate, respectively, the alpha plus, delta, delta plus theta, theta plus alpha and phi patterns.

Chang, Kang-Ming, and L. O. PEI-CHEN. "Meditation EEG interpretation based on novel fuzzy-merging strategies and wavelet features." Biomedical Engineering: Applications, Basis and Communications 17.04 (2005): 167-175.