Academic journal article International Journal of Yoga

Brainstem Auditory-Evoked Potentials in Two Meditative Mental States

Academic journal article International Journal of Yoga

Brainstem Auditory-Evoked Potentials in Two Meditative Mental States

Article excerpt

Byline: Sanjay. Kumar, H. Nagendra, K. Naveen, N. Manjunath, Shirley. Telles

Context: Practicing mental repetition of "OM" has been shown to cause significant changes in the middle latency auditory-evoked potentials, which suggests that it facilitates the neural activity at the mesencephalic or diencephalic levels. Aims: The aim of the study was to study the brainstem auditory-evoked potentials (BAEP) in two meditation states based on consciousness, viz. dharana, and dhyana. Materials and Methods: Thirty subjects were selected, with ages ranging from 20 to 55 years (M=29.1; [+ or -]SD=6.5 years) who had a minimum of 6 months experience in meditating "OM". Each subject was assessed in four sessions, i.e. two meditation and two control sessions. The two control sessions were: (i) ekagrata, i.e. single-topic lecture on meditation and (ii) cancalata, i.e. non-targeted thinking. The two meditation sessions were: (i) dharana, i.e. focusing on the symbol "OM" and (ii) dhyana, i.e. effortless single-thought state "OM". All four sessions were recorded on four different days and consisted of three states, i.e. pre, during and post. Results: The present results showed that the wave V peak latency significantly increased in cancalata, ekagrata and dharana, but no change occurred during the dhyana session. Conclusions: These results suggested that information transmission along the auditory pathway is delayed during cancalata, ekagrata and dharana, but there is no change during dhyana. It may be said that auditory information transmission was delayed at the inferior collicular level as the wave V corresponds to the tectum.

Introduction

The functions of the brain in meditation have been studied using different techniques. These include the electroencephalogram (EEG), [sup][1] evoked potentials, [sup][2] regional cerebral glucose utilization as well as, more recently, functional magnetic resonance imaging. [sup][3] Among these methods, a specific technique is selected for each experiment as each of them have different spatial and temporal resolutions. [sup][4]

Evoked potentials are used in meditation studies because a correlation between different evoked potential components and underlying neural generators is reasonably well worked out. [sup][5] Apart from this, it appears that the cerebral cortex is actively involved in meditation. [sup][6] Hence, one may expect corticoefferent gating with changes occurring at the subcortical relay centers. [sup][7] For these reasons, there have been studies of short and midlatency auditory-evoked potentials during meditation. The studies on midlatency auditory-evoked potentials have most often shown changes in a component called the Na-wave, a negative wave occurring between 14 and 19 msec. The changes have been in the form of an increase in amplitude, [sup][8] suggesting the requirement of more neurons. A decrease in latency has also been reported, [sup][9] suggesting a decrease in time taken to transmit sensory information.

Studies on short latency auditory-evoked potentials have not shown such clear changes. [sup][2] In that study, brainstem auditory evoked potentials (BAEP) were measured in five advanced practitioners of transcendental meditation (TM) to determine whether such responses would reflect an increase in perceptual acuity to auditory stimuli following meditation. The BAEP provide an objective physiological index of auditory function at a subcortical level. Repeated measures of the BAEP of TM practitioners were taken before and after a period of meditation and were compared with those of age-matched controls. Peak latencies as well as interwave latencies between major BAEP components were evaluated. No pre-post meditation differences for experimental subjects were observed at low-stimulus intensities (0-35 dB). At moderate intensities (40-50 dB), the latency of the inferior collicular wave (wave V) increased following meditation. However, at higher stimulus intensities (55-70 dB), the latency of this wave was slightly decreased. …

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