Research by Dr. Ravi Prakash:
An important reason for the discrepant findings in different electroencephalographic studies of meditation seems to be the fact that meditation is not a single psychological or physiological state. Visual and quantitative analysis of the 192 channel EEG recordings of twelve experienced practitioners of Vihangam yoga who claimed perception of an inner light through out their meditation practice was carried out before, during and after meditation. Visual analysis revealed an increase in the persistence and amplitude of alpha activity and appearance of theta bursts during the meditation state. Quantitative analysis revealed a marked decrease in the delta and theta power and coherence across all the regions, and a consistent increase in the power and coherence of alpha over all the regions. The possible correlation of the inner light perception state with decrease in delta and theta power is discussed.
Meditation is not a single psychological or physiological state. There are different kinds of meditation and equally varying states through which one might pass during meditation practice. The efforts to find the electrophysiological correlates of meditation have increased tremendously in the last 50 years (1). However, the results from different studies have been quite conflicting (1,2,3). An important reason for this seems to be the variable nature of the experiences associated with meditation. Perhaps different studies have tapped the electrophysiological correlates of different states of meditation. It thus seems pertinent to perform state-specific investigations rather than exploring a meditative practice as a whole. Also, there is dearth of studies using dense array EEG to get a better spatial resolution of the electroencephalographic changes during meditation. With this idea in mind, we conducted a 192 channel EEG study of the inner light perception state during meditation.
Subjects: 12 experienced practitioners of Vihangam yoga were selected based on:
- a) claim of an inner light perception during every meditation session that would last through out the session,
- b) a clear differentiation of the perception of the inner light from i) an external source of light, ii) visual imagery, iii) visual hallucinations, and iv) dream like states. EEG Recording: 192 channel EEG was recorded using Neurofax EEG-1100 system (Nihon Kohden) with a time constant 0.1 sec. and a high frequency filter of 120 Hz. EEG was recorded for 15 minutes before and after the meditation and 30-60 minutes during the meditation in the same sitting, with the meditator maintaining the same posture through out the recording (Figs. 1 and 2).
The visual analysis of the 192 channel EEG was carried out in blocks of 64 channels each. After visual analysis 2 minute artifact free EEG samples were selected from the recordings from each of the three phases of the recording (before, during and after meditation). Power spectrum and coherence analysis was carried out on these samples in the frequency bands of delta (0.5-3.5Hz), theta (4.0-7.5), alpha (8.0-12.5), beta (13-30), gamma1 (30.5-50), and gamma2 (50.5-100) using Matlab 6.5 version. To have an understanding of regional changes in power and coherence, averaging of the individual power and coherence scores of channels over the respective regions was done.
- Cahn, B. R. & Polich, J. (2006).Meditation States and Traits: EEG, ERP, and Neuroimaging Studies. Psychological Bulletin, 132, 2, 180–211
- Travis, F., & Wallace, R. K. (1999). Autonomic and EEG patterns during eyes-closed rest and transcendental meditation (TM) practice: The basis for a neural model of TM practice. Consciousness and Cognition, 8,302–318.
- Aftanas, L. I., & Golocheikine, S. A. (2002). Non-linear dynamic complexity of the human EEG during meditation. Neuroscience Letters, 330,143–146.
Dr. Mohammad Zia Ul Haq; Senior Resident; Centre for Cognitive Neurosciences;
Central Institute of Psychiatry, Ranchi, India-834006
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The study is still going on and the analysis of EEG data of only 3 participants is presented here. Socio-demographic Data: Most of the participants were married and were engaged in their individual jobs in the society (N=7), while the rest were living in an ashram. The age of the participants ranged from 32-59 yrs (M=43.4yrs). The duration of meditation practices ranged from 5-14 years (M=8.5 yrs) and the daily time spent in meditation ranged from 2-6 hrs (M= 4 hrs). In a brief interview after the EEG recording, all the participants whose EEG data have been acquired so far reported inner light perception through out the meditation session. Visual Analysis of EEG: Visual analysis of the EEG revealed an increase in the persistence and amplitude of alpha activity during meditation compared to pre- and post-meditation EEG. Appearance of occasional bursts of theta activity lasting more than I sec. was seen only during the meditation phases (Figs. 3, 4 and 5).
Quantitative analysis of EEG
Power spectrum values showed an significant decrease in delta and theta power and an increase in alpha power across all the regions, predominantly over the frontal region during meditation. Intra-hemispheric coherence showed an increase in the alpha band and a reduction in the delta and theta bands during meditation. While the interhemispheric coherence showed a similar trend but the coherence in the alpha band remained less during meditation than pre- and post-meditation (Figs. 6 and 7).
The decrease in delta and theta power and coherence observed during meditation in Vihangam yoga practitioners who participated in our study may be a correlate of the inner-light perception which they had through out the period of meditation. As meditation is not a single psychological or physiological state, the search to find the electrophysiological correlates of mediation should focus on specific states rather than meditation as a whole.