Tuesday, 25 February 2014

Gamma and self-reference

The concept of anatta or not-self is central to Buddhism. Meditation is a project designed to loosen attachment to the self.  A number of neuroimaging studies have focused on regions of the brain associated with self-reference (Brewer et al, 2011; Farb et al, 2007; Heatherington, 2006; Kelley et al, 2002; 2009; Qi & Northoff, 2011). Most of these studies used fMRI, but a study by Berkowitz-Ohana and associates (Berkovich-Ohana et al, 2011) used EEG to investigate the relationship of mindfulness meditation to activity related to self-reference.  The authors focused specifically on gamma power over frontal and midline areas that reflect default mode network (DMN) activity.

This study is interesting for a number of reasons. First, the authors differentiated between two types of self-reference (narrative self-reference and experiential self-reference) and the areas of the brain in which they occur. Second, they suggested that gamma power has an entirely different significance for self- reference in different parts of the brain.  Third, because they studied mindfulness meditation practitioners at three different levels of experience, they were able to illuminate the point at which their practices loosened their attachment to self.

The EEG changes associated with the transition from a resting state to a simple time production task were lower gamma power over frontal and midline regions.  This effect was similar in practitioners with three levels of experience and controls who had not previously meditated.  When the three groups of practitioners were compared to matched controls during a resting state, the practitioners exhibited lower frontal gamma activity, mainly right lateralized, thought to be related to narrative self-reference and default mode activity, and higher gamma in the right parietal-occipital region, possibly related to a more experiential focus with increased awareness of internal and external stimuli.  This suggests the practitioners had made a transition from one form of self-reference to another.  Curiously, these changes were found irrespective of expertise level suggesting that these changes occurred in the earlier stages of practice, ranging from 180 to 1430 hours of practice.
 
This study also highlights the risks of associating any given EEG band with meditation.  For years, alpha was the go-to band for meditation states.  More recently, mid-line frontal theta has enjoyed a hay day.  Studies of meditators doing compassion meditation (Lutz et al, 2004) and vipassana meditation (Cahn et al, 2010) indicated increased gamma activation.  But meditation is clearly complex and different bands at different locations have different significance.

Berkovich-Ohana, A., Glicksohn, J., & Goldstein, A. (2012). Mindfulness-induced changes in gamma band activity–implications for the default mode network, self-reference and attention. Clinical Neurophysiology, 123(4), 700-710.
Brewer, J. A., Worhunsky, P. D., Gray, J. R., Tang, Y. Y., Weber, J., & Kober, H. (2011). Meditation experience is associated with differences in default mode network activity and connectivity. Proceedings of the National Academy of Sciences, 108(50), 20254-20259.
Cahn, B. R., Delorme, A., & Polich, J. (2010). Occipital gamma activation during Vipassana meditation. Cognitive Processing, 11(1), 39-56.
Farb, N. A., Segal, Z. V., Mayberg, H., Bean, J., McKeon, D., Fatima, Z., & Anderson, A. K. (2007). Attending to the present: mindfulness meditation reveals distinct neural modes of self-reference. Social cognitive and affective neuroscience, 2(4), 313-322.
Heatherton, T. F., Wyland, C. L., Macrae, C. N., Demos, K. E., Denny, B. T., & Kelley, W. M. (2006). Medial prefrontal activity differentiates self from close others. Social Cognitive and Affective Neuroscience, 1(1), 18-25.
Kelley, W. M., Macrae, C. N., Wyland, C. L., Caglar, S., Inati, S., & Heatherton, T. F. (2002). Finding the self? An event-related fMRI study. Journal of cognitive neuroscience, 14(5), 785-794.
Lutz, A., Greischar, L. L., Rawlings, N. B., Ricard, M., & Davidson, R. J. (2004). Long-term meditators self-induce high-amplitude gamma synchrony during mental practice. Proceedings of the National Academy of Sciences of the United States of America, 101(46), 16369-16373.
Trapnell, P. D., & Campbell, J. D. (1999). Private self-consciousness and the five-factor model of personality: distinguishing rumination from reflection. Journal of personality and social psychology, 76(2), 284.
Qin, P., & Northoff, G. (2011). How is our self related to midline regions and the default-mode network?. NeuroImage, 57, 1221-1233.

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