The brain emits varying waves according to the state of consciousness of the individual. These can be roughly divided into two groups:
The barrier between waking and sleeping seems to exist in the zone between alpha and theta, somewhere around 7½ cycles/second. Most people fall into unconsciousness below this level, however, yogis claim to be able to maintain conscious awareness even during sleep, in the state of samadhi. Their consciousness is so animated by the spark of life that it is not extinguished by sleep and some claim not even by death.
Ananda, Chhina and Singh of the Physiology Department, All-India Institute of Medical Sciences, New Delhi, have investigated deep meditative states, supposedly of samadhi, and they claim that the raja yogis in samadhi "are oblivious to 'external' environmental stimuli while their higher nervous activity remains in a state of 'ecstasy', mahananda".*1
The researchers used an EEG to study:
Group 1 - Before meditation alpha waves, indicative of relaxation and introversion, were prominent. These were blocked by 'external' stimulation so that beta waves emerged, indicative of extroversion or tension. This blocking pattern did not go away after a period of time in which the disturbances were repeated.
During meditation the alpha waves increased their intensity (amplitude or height). One yogi showed theta wave activity but reported that he was awake throughout the meditation. He maintained conscious awareness even when his brainwaves showed he had entered the state of dreaming. No 'external' stimuli blocked alpha activity during meditation, and though they appeared quite relaxed and in a sleep-like state, the EEG showed mainly alpha, not delta.
These results have confirmed the work of Bagchi and Wenger (1957)*2 and other researchers who showed that meditators increased their alpha wave activity during meditation, even with their eyes open.
Anand et al. reported that this awake meditative state, with lack of external awareness, is probably due to spontaneous autonomous discharge of the reticular activating system. This occurs in meditation because when the body is kept absolutely still, nervous activity decreases and may eventually stop to register on the conscious areas of our brain, as in sleep. The awareness moves inside. It seems that the brain is capable of stimulating itself without the need for stimuli from our external environment and in this way may be registering events from our inner world, at a completely different level and dimension of experience. Garoutte et al. (1958)*3 reported that alpha and beta rhythms are probably caused by sub-cortical pacemakers (centres of independent electrical activity beneath the external cortex of the brain) independent of external and internal input. This seems to imply that the brain is capable of generating its own reality.
Group 2 - The two yogis with their hands immersed in water showed alpha waves before and during the experiment. The same explanation of brain patterns applies.
The state reported by the researchers is that of a withdrawn consciousness, or pratyahara, in which the yogi consciously withdraws his mind and the major part of his pranic energy into his brain, shutting off the sense activity and thereby all external input into the brain.
It appears that through yogic training it is possible to consciously withdraw our consciousness from the external world and to voyage into the realm of inner dimensions. In sleep the same thing happens but we are not aware of it. We gradually lose our consciousness of the external world. The reticular activating system prevents all but the strongest of stimuli from reaching the highest part of the brain and our conscious awareness. Of course, our unconscious mind is still registering stimuli from the external environment but awareness is missing and we do not appreciate our inner experience. This is the aspect developed by yogic techniques so that a fundamental change takes place in our consciousness and within the brain. The reticular activating system may shut off external awareness but we can retain our conscious awareness of internal planes that are usually not accessible to us. In this state we are free to explore the inner world. From here the consciousness goes deeper, and becomes more one-pointed. The individual awareness becomes more and more aware of its link to the cosmos until it finally merges in samadhi.
*1. B. K. Anand, G. S. Chhina, B. Singh, "Some Aspects of Electroencephalographic Studies in Yogis", 'Electroenceph. Clin. Neurophys.', 13: 452-456, 1961.
*2. B.K. Bagchi, M.A. Wenger, "Electrophysiological Correlates of Some Yogic Exercises", 'Electroenceph. Clin. Neurophys, Suppl.' 7: 132-149, 1957.
*3. B. Garoutte, R.B. Aird, "Studies on the Cortical Pacemakers, Synchrony and Asynchrony of Bilaterally Recorded Alpha and Beta Activity", 'Electroenceph. Clin. Neurophys.', 10: 259-268, 1968.