Today, there is a paradigm shift around the concepts of health, illness, and treatment options. We are living in a time where medical physicians largely rely on technology and drugs to treat illness and disease. Yet in the midst of the best health care system that western medicine has to offer, millions of people are seeking alternative health care (Barnes,2004 ). In the recent past, efficacy and therapeutic effects of yoga have been reported in various medical journals using latest technology, suggesting that yoga has scientific basis. Moreover, millions of people are exploring and paying for such complementary treatment primarily out of their own pocket, which again emphasises and acknowledges the positive and healing effect of yoga.

In the continuing endeavour to unravel the neurobiological mysteries of yoga, latest imaging technologies are constantly being used, like functional magnetic resonance imaging (fMRI) (Baerentsen et al, 2001), positron emission tomography (PET) of regional cerebral metabolic rate & regional cerebral blood flow, radio-ligand binding to receptors of neurotransmitters (Kjaer,2002, Roggia,2001), diffusion tensor imaging (DTI), magneto encephalography, conventional electroencephalography (EEG), quantitative electroencephalography (qEEG) and event-related potentials (ERP).

Especially, qEEG is a non-invasive tool that is capable of assessing quantitatively the resting and evoked activity of the brain, having a high sensitivity and a temporal resolution superior to those of any other imaging method.

In the past 20 years, research in EEG has made significant contributions to the understanding of brain-electrophysiology. Recently, digital electroencephalography has come into widespread use and has become an established alternative to conventional EEG (Nuwer, 1997). EEG records the action potentials of electrical energy generated by cortical neurons, by a non-invasive method through electrodes placed on the scalp. An extension to this is the electrocorticograms, an invasive procedure that records electrical potentials over the human cortex.

The EEG rhythms recorded on the scalp are the result of the summation effect of many excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) produced in the pyramidal layer of the cerebral cortex. In humans, the thalamus is thought to be the main site of origin of EEG activities (alpha and beta bands). Thalamic oscillations activate the firing of the cortical neurons. The depolarisation (mainly in layer IV) creates a dipole with negativity at layer IV and positivity at more superficial layers. The scalp electrodes will detect a small but perceptible far-field potential that represents the summed potential fluctuations. Scalp electrodes cannot detect charges outside six square centimetres of the cortical surface area, and the effective recording depth is several millimetres (Thakor and Shanbao, 2004).

High-resolution EEG investigation of meditation

According to recent investigations, theta and alpha oscillations are defined as narrow frequency bands reflecting the activity of multifunctional neuronal networks.These are differentially associated with orientation, attention, memory, affective, and cognitive processing. 128-channel ESI System (ESI-128, NeuroScan Labs.) and 64-channel QuikCap with imbedded Ag/AgCl electrodes (NeuroSoft, Inc.) were used inorder to record these EEG from 62 active scalp sites referenced to the tip of the nose along with both vertical and horizontal electrooculograms (EOGs). EEG spectral power and coherence was estimated in the individually defined delta, theta, alpha-1, alpha-2, and alpha-3 bands and were used to identify and characterize brain regions involved in the meditative states, in which focussed internalized attention gave rise to emotionally positive 'blissful' experience.

Blissful state was accompanied by an increase in anterior frontal and midline theta synchronization as well as an enhanced theta long-distant connectivity between prefrontal and posterior cortex with distinct 'center of gravity' in the left prefrontal region (AF3 site). Therefore, subjective scores of emotional experience significantly correlated with theta waveforms whereas scores of internalized attention were correlated with both theta and alpha lower synchronization.

Conclusion

These results suggest selective associations of theta and alpha oscillating networks activity with states of internalized attention and positive emotional experience.

Spectral power changes between eyes closed and meditation conditions in the short-term (STM) and long-term (LTM) meditators in the theta, alpha-1, and alpha-2

Coherence changes between eyes closed and meditation conditions in the STM and LTM in the theta band. Solid lines indicate coherence increase whereas dashed lines point to coherence decrease (the thicker lines relate to error probability of P < 0.001, the thinner lines relate to P < 0.01

EEG WAVEFORMS

Traditional time domain EEG spectra are separated into fundamental bands qualitatively based on shape and range of frequency for clinical and research applications. These generally occur within the limits of 0.1 to 35 Hz for clinical and include alpha, beta, delta, and theta waves. When many of the individual bands occur repeatedly in a specific area of the brain, they produce a complex EEG waveform observed in traditional EEG recording methods.

Alpha wave

Normal alpha rhythms are characterized by sinusoidal waveforms occurring between 8 to 13 Hz. Although the specific amplitude varies from one individual to another, it typically ranges from 20 to 60 mV and rarely exceeds 100 mV. They are believed to originate in the posterior region of the brain and are generally observed in the parietal, occipital, and posterior temporal areas. Alpha rhythms are best detected when an individual is mentally inactive, and they are often seen when the subject is awake, relaxed, and in an environment relatively free of stimuli. These rhythms are inhibited by the ascending reticular activating system at the onset of an unanticipated stimulus or when an individual exhibits increased mental and visual activity. The rhythms disappear completely when a person becomes drowsy. This "alpha dropout" is characterized by the eventual replacement of the alpha waves by a low voltage, mixed frequency pattern. Once asleep, patterns known as sleep spindles may appear which resemble alpha rhythms but periodically produce clusters of extremely large spikes in 1 to 2 second interval (Niedermeyer, 1993). These spindle formations are referred to as spindle coma patterns when observed in comatose patients who have preserved their normal sleep patterns (Synek, 1988). Despite the somewhat similar appearance to alpha waves, spindle waves are clearly different and originate in the thalamus where they inhibit the synaptic transmission of that structure (Steriade, 1993).

Beta wave

Beta rhythms include all frequencies above 13 Hz with low amplitudes rarely exceeding that of 30 mV. They can exist simultaneously throughout the cortex at various frequencies but are most common to the frontal and central head regions in nearly all healthy adults. Beta rhythms can be extremely fast with an upper limit between 50 and 100 Hz. Enhanced or fast beta activity occurs over isolated bone defects and is also an effect of minor tranquillisers, barbiturates, and some nonbarbiturate sedatives. Remarkably accentuated beta rhythms are usually classified as only slightly abnormal unless they occur in unresponsive individuals, which may be an indication of a severe abnormality (Niedermeyer, 1993). Frontal beta activity may be one of the fastest EEG frequencies and is common in normal sleeping individuals. Posterior beta activity also may be present in some individuals where it mimics the alpha rhythms blocking and enhancement reactivity to eye opening. In addition, localized bursts of 40 Hz oscillations are characteristic prior to voluntary movement, such as wrist or finger extensions, and beta synchronization appears at approximately 20 Hz after movement (Pfurtscheller, 1992; Pfurtscheller 1996).

Delta wave

Delta rhythms consist of low frequency, high-amplitude waveforms recorded between 1 to 4 Hz with amplitude ranges commonly from 20 to 30 mV. Delta waves can be seen in the posterior regions of the head, and/or they can occur on either side of the temporal region. However, they are most often recorded over the left cerebral cortex. These rhythms are produced by thalamocortical neurons and are virtually absent in the EEGs of normal alert individuals. Delta waves are associated with periods of unconsciousness typically appearing in cerebral monitoring during sleep, coma, or after convulsive seizure. They also are common following traumatic brain injury (TBI) and can occur in conjunction with elevations in intracerebral pressure (ICP) due to an obstruction of the cerebral spinal fluid system or an expanding lesion (Rumpl, 1979). In such cases, waveforms of 0.5 to 5 Hz are recorded diffusely over the cranium. Customarily, waveforms below 1 Hz have been classified as delta waves. However, intracellular recordings indicate that these waveforms are derived from different mechanisms than those waves ranging from 1 and 4 Hz. The slower oscillations are generated by corticothalamic and reticular thalamic neurons, and they are significant abnormalities in severe coma patients (Steriade, 1993). 'Psychomotor poverty' is positively correlated with both delta and beta power and 'reality distortion' was significantly positively correlated with alpha-2 power (Harris, 1999).

Theta and Gamma wave

Theta waves measure from 4 to 7.5 Hz and have low to moderate amplitudes. They are presumed to originate in the thalamus and are associated with the hippocampus and limbic system. Theta rhythms can be recorded in the frontal, temporal, central, and posterior head regions and are rarely the predominant waveform, frequently mixed with alpha and beta waves. In fact, theta waves are most often seen in conjunction with alpha waves despite their different production mechanisms. Theta rhythms appear in various capacities at different stages of development and maturation. These waveforms also play a vital role in conditions of drowsiness and sleep in all ages and may be linked to the emotional processes in children (Niedermeyer, 1993). Frontal midline theta rhythm is a distinct theta activity of EEG in the frontal midline area that appears during concentrated performance of mental tasks in normal subjects and reflects focused attentional processing. Analysis showed bilateral medial prefrontal cortices, including anterior cingulate cortex, as the source of frontal theta, suggesting suggests that focused attention is mainly related to medial prefrontal cortex (Ishii et al, 1999). It has been suggested that immediate memory in humans may be mediated in the theta band (Towle et al., 1999).

Arousal may be a necessary condition for Gamma activity. In states of extremely low arousal (anaesthesia and non-REM sleep), there is minimal Gamma activity and evidence points to a positive linear relationship between arousal and level of Gamma. Sheer (1984) captured the essential role of arousal in the modulation of Gamma in his interpretation of Gamma activity as a `focused state of cortical arousal'. It has been hypothesised that in patients with schizophrenia, the integration, associating, timing, coupling or binding of spatially diffuse cerebral activity related to a specific cognitive task may be a key feature of the pathophysiology.

Neuroimaging studies of hypnosis have identified many of the same cerebral responses posited in the model of meditation proposed by Newberg and Iversen. In both meditation and hypnosis, attention drives the prefrontal and cingulate cortices which interact with other structures including nuclei of the thalamus and brainstem as well as parietal cortices, resulting in states of decreased vigilance and increased attention. Furthermore, hypnosis studies have demonstrated distinctive associations between certain brain networks and mental relaxation and absorption. Specifically, hypnotic relaxation involves brain areas known to regulate arousal and vigilance while mental absorption involves a brain network underlying attention mechanisms. Additional increases in occipital rCBF during guided meditation and hypnosis may reflect a decrease in vigilance and in cross-modality suppression, associated with decreases in the cortical release of norepinephrine, and leading to a facilitation of experiential changes. Meditative techniques form a dichotomy roughly akin to the extremes of the allegorical spotlight of attention. Concentrative techniques involve sustained focal attention (e.g. on the breath) whereas receptive techniques involve unfocused sustained attention (e.g. mindfulness meditation). Further, meditative techniques may be self guided or externally guided via an instructor or recording. Similarly, hypnosis can be self induced or induced by a hypnotist. Considering the striking similarities in their experiential and brain correlates, meditation and hypnosis appear to be closely related phenomena and hypnosis may be conceived as a western form of guided meditation.