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Sample Review of Scientific Literature

MEDITATION AND EVOKED POTENTIALS

One of the most useful techniques in modern clinical neurophysiology is the recording of evoked potentials. Evoked Potentials are the neurophysiological tool that makes it possible for us to make an extensive study of the cerebral cortex, the brainstem and the spinal cord. Evoked potential (EP) is an electrical response of the brain to a given stimuli which occur in a time locked fashion (Chippa, 1997). Evoked potentials are used in meditation research because the association between different components of evoked potentials and underlying neural generators are well understood (Woods & Clayworth, 1985). Apart from this, it appears that cerebral cortex is actively involved in meditation. It is believed that even if the main changes occur in the cortex, corticoefferent connections would result in subcortical changes (Woods & Clayworth, 1986). Hence, short latency and mid-latancy auditory evoked potentials were studied during meditation.

    In evoked potentials, a decrease in peak latency is considered suggestive of facilitated transmission due to increased speed of conduction in the underlying neural generators (Malhotra, 1997). On the other hand, an increase in peak latency can be assumed to suggest inhibited transmission due to slower conduction in the underlying neural generators. Increased amplitudes of evoked potential components are interpreted as activation of the underlying neural generator with recruitment of a greater number of neurons (Woods & Clayworth, 1986).

The use of evoked potentials in yoga research started almost 3 decades ago. In 1976, Slow cortical auditory responses were recorded from the practitioners of transcendental meditation (Wandhöfer, Kobal & Plattig, 1976). Latencies for most of the initial peaks during TM as well as during normal consciousness were significantly shorter than in a control group in a dozing state or during normal consciousness. In a subsequent year, auditory evoked potentials to tone stimuli were recorded from 8 practiced meditators before, during, and after meditation, and also during light sleep (Barwood et al., 1978). No consistent changes were found between baseline and meditating AEPs, or between meditating and sleep AEPs. In a subsequent study, visual evoked potentials (N120, P200, P300) and reaction time were studied during choice reaction time situations in 10 meditators and 10 matched controls (Banquet, Bourzeix & Lesèvre, 1979). After meditation the reaction time significantly increased with less mistakes, and amplitude of P300 increased significantly. After the rest there was a decrease of P300 amplitude and no change in the reaction time of the controls.

In another study, brainstem auditory evoked potentials (BAEPs) were measured in five advanced practitioners of Transcendental Meditation (McEvoy, Frumkin & Harkins, 1980). In this study Peak latencies as well as interwave latencies between major BAEP components were evaluated. Results showed no pre-post meditation differences for experimental subjects were observed at low stimulus intensities (0--35 dB). At moderate intensities (40--50 dB), latency of the wave V increased following meditation, but at higher stimulus intensities (55--70 dB), latency of this wave was slightly decreased.

In the year 1993, middle latency auditory evoked potentials during yogic consciously regulated breathing and attentive state of mind were studied (Telles et al., 1993). Ten healthy male subjects with ages ranging from 21-33 years were studied. Consciously-controlled rhythmic breathing involving timed breath-holding in each cycle of breathing, while the subject holds utmost attention and experiences the touch of inhaled air in the nasal passage were studied. The results showed  an increase in Na-wave amplitude and decrease in latency during pranayamic practice, whereas no significant change in Pa-wave.  

In the same year, changes in middle latency auditory evoked potentials were assessed in 7 experienced meditators during the practice of meditation on the Sanskrit syllable 'OM' (Telles & Desiraju, 1993). The results showed a group significant decrease in the latency of Nb wave. The Nb wave corresponds to the dorso-posterior medial part of the Heschl’s gyrus, i.e., the primary auditory cortex (Liégeois-Chauvel et al., 1994). Hence, in experienced meditators, mental repetition of OM, facilitated the transmission of neural information at the level of the dorso-posterior medial part of the Heschl’s gyrus.

In a subsequent study on experienced and naïve meditators, who were asked to mentally repeat ‘OM’ on one day and ‘one’ on another day, there was a significant change in the Na component (Telles et al., 1994). When both experienced and naïve meditators repeated ‘one’ there was a significant decrease in the peak amplitude of the Na wave. Hence the decreased Na amplitude indicated a possible decrease in neurons recruited at the level of mesencephalic or diencephalic while mentally repeating one.

In contrast, when experienced meditators and naïve persons were asked to mentally repeat ‘OM’ on another day, the results were quite different. The Na wave peak amplitude significantly increased in experienced meditators but significantly decreased in naïve meditators. These results were suggestive of recruitment of increased neurons at the mesencephalic-diencephalic level in experienced meditators repeating OM, whereas naïve practitioners had less neurons recruited at that level. Hence, both mentally repeating ‘OM’ and ‘one’ cause neural changes at the same level but  in opposite directions. In another study an auditory oddball task was used to assess experienced TM meditators at pretest baseline, after 10 min of rest, or after 10 min of TM practice with conditions counterbalanced across subjects (Travis & Miskov, 1994). The P300 latency decreased at Pz after TM practice relative to no change after the rest condition.

Effect of Sahaja Yoga meditation on auditory evoked potentials (AEP) and visual contrast sensitivity (VCS) in 32 epileptics was studied (Panjwani et al., 2000). Sahaja Yoga meditation group showed significant improvement in VCS following meditation and also a significant increase in the Na-Pa amplitude. A study on middle latency auditory evoked potentials during Brahmakumaris Raja Yoga meditation showed decrease in the peak latency of the Na wave during meditation (Telles & Naveen, 2004). Since the neural generator of this wave lies at the midbrain-thalamic level, during meditation conduction time reduced at this level.

Changes in p300 following two yoga-based relaxation techniques were studied in 42 male volunteers (Sarang & Telles, 2006). There was a reduction in the peak latencies of P300 after cyclic meditation. A similar trend of reduction in P300 peak latencies was also observed after supine rest. Although the magnitude of change was less after supine rest compared to after cyclic meditation. The P300 peak amplitudes after CM were higher compared to the "pre" values. In contrast, no significant changes were observed in the P300 peak amplitudes after supine rest. The results support the idea that cyclic meditation enhances cognitive processes underlying the generation of the P300.

The mismatch negativity (MMN) which is an indicator of preattentive processing was studied in Sudarshan Kriya Yoga meditation (combination of breathing exercises and concentrative meditation) (Srinivasan & Baijal, 2007). The results showed larger MMN amplitudes in meditators compared to non-meditators. The meditators also showed significant increase in MMN amplitudes immediately after the practice of meditation suggesting transient state changes due to meditation. The results suggest that concentrative meditation practice enhances preattentive perceptual processes, enabling better change detection.  

Midlatency auditory evoked potentials were studied before and after the practice of cyclic meditation (CM) and supine rest (SR) in 47 male volunteers (Subramanya & Telles, 2009). The results showed a significant increase in peak latency of Pa wave and Nb wave following CM compared to before CM. There was a significant increase in the peak amplitude of the Nb wave compared to before CM. The peak latency of the Na wave significantly increased after SR compared to before SR. Hence, following CM the latencies of neural generators corresponding to cortical areas was prolonged, whereas following SR a similar change occurred at mesencephalic-diencephalic levels. A subsequent study on Vipassana meditation showed reduction in P3a amplitude suggesting decrease in automated reactivity and evaluative processing of task irrelevant attention-demanding stimuli (Cahn & Polich , 2009).

A study was conducted to measure the effect of pranayama & yoga-asana on cognitive brain functions in type 2 diabetes-P3 event related evoked potential (ERP) (Kyizom et al., 2010). There was a significant improvement in the latency and the amplitude of N200, P300 in the yoga group as compared to the control group.

Changes in brainstem auditory evoked potentials were studied following four mental states (Kumar et al., 2010). The results showed a significant increase in the wave V peak latency in caïcalatä, ekägratä and dhäraëä, but not during dhyäna session. These results suggest that information transmission along the auditory pathway is delayed during caïcalatä, ekägratä and dhäraëä, but there was no change during dhyäna.

A recent study assessed the impact of meditation on emotional processing using visual event-related potential (ERP) in long-term meditators (Sobolewski et al., 2011). Results showed differences in the late positive potential (LPP) in meditators and controls. The study concluded that, meditators are less affected by stimuli with adverse emotional load, while processing of positive stimuli remained unaltered.

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