From alpha to gamma: Electrophysiological correlates of meditation-related states of consciousness
Introduction
In recent years, an increasing number of studies attempted to explore neurophysiological processes occurring during altered states of consciousness resulting from meditative training. These studies abstracted from the cross-cultural differences between meditative practice in different religious contexts and focussed on common mechanisms arising from this voluntary alteration of conscious states, which is characterized by both deep relaxation and increased internalized attention. A number of studies revealed the influence of meditation practice on autonomic parameters such as breathing patterns, heart rate, skin conductance and blood volume pulse [1], [2], [3]. In this article, we will concentrate on the neurophysiological correlates of meditation, in particular effects observed in EEG experiments.
The basic thought behind these studies is the premise that an altered state of consciousness is always accompanied by an analogously altered neurophysiological state (so-called psychophysical isomorphism, e.g. [4]). If meditation is used as a repetitive influence on consciousness, certain measurable qualitative and quantitative effects should develop on the neurophysiological side, which may be either transient or permanent.
A large number of studies aimed at classifying and categorizing observed effects but were only able to assess certain general changes of the EEG which are not directly related to meditation practice (for a review, see [5]): As conscious states can only be accessed introspectively, they depend on subjective descriptions, which are difficult to inquire during meditation practice. Moreover, the enormous bandwidth of not clearly circumscribed meditation styles and the lack of a commonly accepted phenomenal classification of waking states of consciousness do only allow one to conclude about general findings and rough tendencies. Thus, even though meditation research has produced a large number of studies during the past years, there is still a strong need for clear and standardized definitions, in terms of meditative techniques, as well as in terms of the involved states of consciousness.
Hence, we will first have to discuss the use of the term ‘meditation’, explain the difficulties arising from the need of clear definitions, and point out why the large field of meditation research still lacks clear and practicable categorizations. With regard to these problems, we will change our perspective by trying to describe meditation practice from a more general point of view. We will suggest hypotheses which will allow us not only to arrange the electrophysiological findings, but also to predict in which way certain meditation-related states of consciousness may be reached. In the following sections, we will describe EEG findings on meditation-related states of consciousness supporting our hypotheses. We will ask how, according to present knowledge, meditation practice is manifested in oscillatory EEG activity.
Section snippets
What is meditation?
The word ‘meditation’ can be derived from the Latin word ‘meditatio’, which referred to an exercise originally not predefined between intellectual or physical. In both ways it points to the center (lat. ‘medium’ = ‘center’) of either the body or the mind. The word ‘medium’ again is rooted in the Indo-Germanic stem ‘∗me(d)’, meaning ‘to ambulate’ or ‘to measure’. Today, ‘meditation’ is related to various practices aiming to alter the state of consciousness, hence belonging to a more spiritual
A new approach to describe meditation practice
Our main hypothesis is that long-term practice of different forms of meditation is associated with similar developments. By this idea, we do not intend to state that various kinds of meditation exhibit similar mental states and neurophysiological correlates regarding all mental/neurophysiological aspects. We rather suggest that during the development of meditation practice some common characteristics are shared and passed through. This view is supported by the experiences of meditation experts
Alpha activity
The most dominant effect standing out in the majority of studies on meditation is a state-related slowing of the alpha rhythm (8–12 Hz) in combination with an increase in the alpha power [8], [13], [14]. These findings are relatively robust, because they do not depend on either a certain meditation tradition or the experience of the meditator. Subjects engaged in meditation of various styles were reported to demonstrate increased alpha power [15], [16], [17], [18], which is localized mainly over
Synchronized gamma oscillations and cortical plasticity
Neural plasticity comprises the creation of additional neurons and new synaptic connections, as well as the expansion and shift of functional areas. These modifications are most evident in patients with brain lesions or in subjects who have been trained in specialized cognitive functions such as musicians for the control of sensory-motor abilities, taxi drivers for spatial navigation, and so on. Similarly, meditation training may be accompanied by alterations of neural structures. Indeed, it
Are meditation-related brain/mind states unique?
A response to this question requires a clarification of what is actually meant by a “unique” brain/mind state. One may consider a mind state, in other words a state of consciousness, as a point or a small area in a state space describing all possible mind states [4]. The variables defining the axes of the mental state space (i.e. the co-ordinate system) are then different psychological properties. For instance, Vaitl and colleagues [88] have suggested a state space for the classification of
Conflicts of interest statement
None declared.
Acknowledgement
We would like to thank Ulrich Ott for helpful comments and suggestions.
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