1 Introduction

In 1969 Swāmī Rāma of the Himalayas arrived in the United States and was introduced to Elmer and Alyce Green of the Menninger Foundation in Topeka, Kansas (Green and Green, 1977). They had a well-equipped laboratory designed to investigate voluntary control of psychophysiological processes. In their initial discussions, as they described their work, Swāmī Rāma decided that this was a place to realize his master's ambition that the abilities manifested by a masterful practice of yoga be documented scientifically. He demonstrated an array of abilities including simultaneously raising and lowering the temperature in adjacent muscles by 10°F, placing his heart in such rapid atrial fibrillation that his heart stopped pumping blood for several minutes, manifesting telekinesis in an environment carefully controlled for the motion of air and for electromagnetic forces, spontaneously creating cysts under his skin and also voluntary entry into alpha, theta, and delta brain wave states. One of these involved a demonstration of a yogic practice called yoga-nidrā, conscious entry into deep, non-REM, delta wave sleep. He was connected to an electroencephalogram and Swāmī Rāma then entered into a delta wave state where he was creating delta waves almost exclusively. During the period of the experiment, two technicians were quietly talking in the next room. When he resumed ordinary waking awareness after the experiment, Swāmī Rāma recounted that conversation verbatim. This illustrated the criterion he articulated that in addition to being focused on his inward state, he also remained aware of his external environment. There is no current explanation in neuroscience for his ability to do this.

Before this time, yoga-nidrā was not well known in the West. Since that time, a number of other yoga traditions have pursued teaching the practice to students including the Bihar School of Yoga (Saraswati, 1998), Richard Miller (2005), Amrit Desai and his daughter, Kamini (Desai, 2017) and others (e.g., Panda, 2003). Swāmī Rāma's close disciple Swāmī Veda Bhāratī set out to replicate his master's experiments at the Institute for Noetic Sciences in California in 2006 under the experimental leadership of Dean Radin Ph.D. During these trials, in addition to duplicating Swami Rama's experiment, Radin accidentally discovered that Swāmī Veda remained almost perpetually in a state of yoga-nidrā, his brain producing theta and delta waves even with his eyes open, talking and moving around (Bhāratī, 2006, p. 69). Again, there is no neurological explanation for the possibility of this. Swāmī Veda subsequently published a small book about his personal experiences and experiments with yoga-nidrā, which actually began from the age of three, even though he did not recognize it as such at the time (Bhāratī, 2014).

Unfortunately none of these demonstrations were published in the peer reviewed scientific literature. There were several explanations of these demonstrations in popular literature (Green and Green, 1974, 1977; Moffat, 1973), but peer reviewed investigations into the state of yoga-nidrā still await. There are a number of studies of the preliminary practices leading up to yoga-nidrā, but none of these really investigate the state itself.

There are several reasons why these experiments were not published in the formal scientific literature. The most important reason is that to the true masters of yoga, science needs yoga; yoga doesn't need science. The masters of yoga know what they can do and they are never interested in demonstrating these things as a matter of self-promotion. Swāmī Rāma was ordered by his own master, Bengali Bāba, to do these demonstrations as a way to help the West understand the degree to which a state of health and wellness can be within the control of an individual practitioner. Another reason for the failure to publish in peer reviewed circles was the fact that these were individual cases which lacked statistical generalizability and usually also lacked matched control subjects. As such they were not favored by editors. Finally, there was the skepticism of many scientists, and their fear of their colleagues' skepticism, which often extended to not believing what they had seen with their own eyes, much less found acceptable in a peer review. Their unwillingness to believe the very senses on which they relied for their evidence eventually persuaded Swāmī Rāma that many, perhaps most, scientists were not ready to see and he stopped demonstrating. The willingness of neuroscience to even entertain the possibility of such skills is a relatively recent development.

This willingness has been greatly aided by the more recent investigations of meditation and mindfulness in collaboration with Buddhist practitioners, the most obvious example of which is the Mind and Life Institute collaboration begun by several neuroscientists in collaboration with the Dalai Lama and the community of Tibetan Buddhist monks. From this collaborative beginning research on meditation and the nature of consciousness has mushroomed. To get a sense of the sweep of this work over a somewhat longer and more general historical perspective, see Walsh and Shapiro (2006). It greatly added to our understanding of a number of systems within the brain and nervous system. It has also contributed enormously to the understanding of psychotherapeutic healing processes, particularly regarding the healing of depression, anxiety and trauma and so they have immediate practical applications in psychotherapeutics. Mindfulness based stress reduction (Kabat-Zinn, 2003), Benson's relaxation response (Benson, 2000), mindfulness based cognitive therapies (Segal et al., 2018) and coping skills training (Linehan, 1993), mind/body-based trauma therapies (Levine, 2010; van der Kolk, 2014), and interpersonal neurobiology (Siegel, 2007, 2012) are just a few examples.

This author will argue for the inclusion of yoga practices in these neuroscientific investigations of consciousness. The practices of the yoga system parallel in many ways the practices of Buddhism, evidence for their universality in human experience. The examples of Swāmī Rāma and Swāmī Veda indicate that collaboration with accomplished yoga practitioners offers an opportunity to further extend the frontiers of neuroscience. And the relationship between yoga-nidrā and the deepest states of meditation (samādhi) provides one measurable way to investigate them.

2 Methodological considerations

Here the potential for collaboration may go beyond simply supplying accomplished “subjects” or “informants,” especially from the phenomenological perspective. Purely quantitative experimentation had for a long time no interest in the subjective experience of subjects and in Swāmī Veda's frequent conversations with scientists who examined him this was, to him, a consistent limitation to the usefulness of their research. The tendency of experimental science to omit such data impoverishes our ability to interpret quantitative findings. Any hint of subjectivity has long been suspect in the scientific enterprise as a compromise to “objective” truth, facts ascertained solely through our cognitive senses.

The limitations of objective measurement have persisted in social science and in neuroscience research, even though the existence of objectivity itself came into question more than a century ago in physics. The reliance on quantitative data alone may provide some precision of measurement but it often lacks both ultimate precision and a rich interpretive context. As a result, much interpretation is a matter of trial and error and successive approximation, which is neither very efficient nor very edifying. In physics, upon which psychology and neuroscience base their experimental models, the experiments that established the exclusion principle of Wolgang Pauli demonstrated that how an experiment is structured and observed largely determines its outcome. As a result, he realized that one may know separately a particle's position or its velocity fairly accurately but never both simultaneously with that much accuracy. This fundamentally limits the precision of results. There are limits to what can be objectively observed and one's choice of observational method substantially affects experimental outcomes.

The advent of qualitative research methodologies (e.g., Langs, 1999; Ravitch and Carl, 2015) in the 1980s and 1990s began to provide a disciplined way to investigate qualitative experience, which led to an increase in interpretive richness. One example, which the author stumbled upon in preparing his doctoral paper, involved the use of stochastic mathematics to derive a stochastic regression equation that would predict who would speak in the next second of a psychotherapeutic interview. The authors then searched for violations of the pattern predicted by the equation and analyzed the recorded content of the interview at those points. What they discovered was that these violations of the predicted pattern pointed to episodes of empathic failure on the part of the therapist. This provided an efficient way to identify specific examples of empathic failure for the sake of learning and supervision (Badalamenti and Langs, 1992).

More recently neuroscience has seen the advent of neurophenomenological methodologies, first proposed by Francisco Varela, and carried forward since Varela's passing by his colleagues and students (see, for example, Lutz et al., 2002, 2004). This form of qualitative and quantitative collaboration grows out of the phenomenology of Husserl, Heidegger and others. It seeks to incorporate the experience of the subject into the experimental design as a complement to whatever quantitative measurements are taken. The result is a much richer context for the interpretation of data.

The method is framed in response to the “hard problem of consciousness,” which Petitmengin (2017) describes, quoting Chalmers (1995):

The argument is that a careful and disciplined study of the experience of subjects serves as a complement to the objective data obtained through physiological measurements and greatly enriches the meaning of that objective data for the understanding of consciousness.

Petitmengin (2017) argues that there are two types of responses to the “hard problem” of why consciousness should exist as a product of physiological processes. (That consciousness exists as a product of physiology is an important assumption here.) In what she describes as “light” or “mild” neurophenomenology, correlations are explored between qualitative experiential (i.e., subjective) and quantitative physiological (i.e., objective) data to try in various ways to create bridges between the subjective and objective domains. As observed by Berkovich-Ohana in her commentary on this article, this generally describes the entire body of neurophenomenological studies to date (Berkovich-Ohana, 2017, p. 118). The problem is that no matter how much bridging occurs, the bridge is never entirely crossed and the distinction between the subjective and objective domains remains intact and remains assumed. As such, there is still no way to explain why consciousness should arise from physiology (this has never been more than an assumption), even though such a method does help to refine the interpretation of quantitative data.

In “deep” or “radical” neurophenomenology, rather than simply correlate subjective experience with the measurement of objects, there is a shift in introspective attention from the contents of observation, i.e., objects, to the experience of the process of observation itself. Petitmengin (2017) explains:

It accomplishes this by refocusing introspective attention not on the contents of the mind that arise but on the process of content arising itself:

In other words, our habitual, conditioned tendency to focus on objectifying what is arising in the first moment of perception blocks the observation of the process of arising of a perception itself. As we pursue these “microgeneses” in emerging perception, the subject/object distinction (and the “hard problem” itself) begins to dissolve. Petitmengin goes on to detail how this works in terms of each sensory modality. She then concludes,

Citing examples of how so-called “mild” neurophenomenology has already assisted the understanding of a number of neurodynamic processes, Evan Thompson, a contemporary philosopher of mind and cognition, concludes that,

A fine example of what this kind of adept, co-equal, phenomenological research collaboration can accomplish is provided in Ataria et al. (2015) and Dor-Ziderman et al. (2013). Here an adept 40-year practitioner of mindfulness meditation demonstrated states of selflessness and lack of body boundaries that yielded much “mild” neurophenomenological insight into the nature of that state from both introspection of the subjective experience and the correlative measurement of the neurophysiology involved.

This kind of collaboration between neuroscience and Buddhist practitioners has already begun, most conspicuously in the work of the Mind and Life Institute, and with others as illustrated above. A parallel system of introspective self-observation has always existed in the yoga tradition as well, but is yet to be accessed by neuroscientific investigators. Textual evidence of its existence goes all the way back to the tradition of the later Upaniṣads (as manasā mana ālokya, “observation of the mind by the mind,” Yoga-śikha-upaniṣad 62–64), tantra texts and the Yoga-sūtras and its commentary tradition (as early as the 2nd century B.C.E.). (For a detailed discussion of the textual descriptions of this kind of self-observation, see Bhāratī, 2001, 661ff.)

This mode of introspection is called uha or tarka. In ordinary Sanskrit tarka denotes logic or rational inquiry. In the meditative traditions of yoga, tarka is a technical term indicating a kind of introspection that goes well beyond ordinary logic and conscious thinking. It can occur only when the mind-field (citta) has been cleansed of any possibility of subjective (emotional) distortion and it is a principal method of moving into the deepest states of meditation, samādhi. As a formal component, a “limb” or aṅga, of systems of yoga alternative to Patañjali's “eight limbed” or aṣṭāṅga yoga, it facilitates the transition between the final two limbs of yoga, dhyāna or “meditation” and samādhi, the deepest superconscious meditation where the subject-object dichotomy collapses into a singularity and mind apprehends its object directly without any intermediary concepts, ideas or (objective) categories. The so-called ṣaḍ-aṅga-yoga, the yoga of six limbs, of which tarka is a part, is shared among both Hindu and Buddhist meditation practitioners (Bhāratī, 2001, p. 659).

If we compare Bhāratī's description of the process of introspection in tarka to the description of deep neurophenomenology above the similarity is striking:

Bhāratī describes the penetration of meditative introspection into the “initial instant, occurring very fast and usually unrecognized, lacking differentiation, where inner world and outer world, subject, and object are still indistinct,” in a 2011 lecture:

In another section of this description he makes reference to the disappearance of a fellow disciple's sense of body awareness in a meditative experience of 3 days' duration in addition to direct observation of vibrations of purest consciousness of the sort described by Petitmengin (2017) above.

The implication of “deep neurophenomenology” that the hard problem of the origins of consciousness in physiology is side stepped by the direct observation of the co-constitution of subject and object in the meditative direct observation of consciousness may be a large and difficult pill for conventional science to swallow. There is a theoretical explanation for this in the school of quantum mechanics that holds consciousness as a first principle of the universe (for a non-technical explanation, see Goswami, 2000). But as Varela warns (Varela, 1996, p. 331), many may choose not to swallow it and remain in the gulf between subject and object. Why pay the price of the long and rigorous training and the personal and professional risks of this course, Petitmengin asks?

The pursuit of scientific understanding aside, this experiential experimental method, the method of meditative self-observation, eventually holds the key to the core of human suffering.

If neurophenomenology can harness this level of disciplined introspective observation from practitioners of yoga-nidrā, there is considerable promise to learn a great deal about how our ordinary dualistic awareness can transition into the highest states of non-dual meditation in samādhi. And there are measurable correlates in “mild” neurophenomenology that can take us part way there, for example, the detection of a time interval between two stimuli that is shorter than the known threshold.

The methodological insight of “deep neurophenomenology” also opens a way toward understanding of the quantum physical idea that consciousness itself is the first principle of the universe and that consciousness precedes the emergence of objects. A detailed discussion of this idea is well beyond the scope of this paper and must be left for the reader to contemplate. As previously stated, a popular rendition of the idea is described in Goswami's A Visionary Window: A Quantum Physicist's Guide to Enlightenment (Goswami, 2000).

The history of empirical studies of yoga-nidrā has been dominated by experiments on what Bhāratī (below) describes as relaxation practices preparatory to entry into the state of yoga-nidra. This literature was reviewed in Parker et al. (2013) and Parker (2017a,b). The principal findings of those reviews were:

1. 1. Across the board, studies lacked a sufficient definition of the goal state of yoga-nidrā, grounded either in the traditional definitions of the state in the yoga literature or the descriptions given by Swāmī Rāma in his original demonstrations.
2. 2. Only a handful of studies utilized electroencephalography to measure the subjects' brain waves. When they did utilize EEG, most of these were measuring states of simple alpha wave relaxation or alpha/theta activity, preparatory to yoga-nidrā itself.
3. 3. No studies measured whether subjects maintained an awareness of their external environment, a criterion for the practice articulated and demonstrated by Swāmī Rāma and replicated by Swāmī Veda Bhāratī.
4. 4. The experiments all utilized only quantitative measures or relatively non-specific questionnaires. Neither method allowed for a detailed and precise phenomenological account of the subjects' subjective experience.
5. 5. One group of studies of the secularized iRest method (Miller, 2005) did only questionnaire based outcome studies that measured subjective relaxation and anxiety reduction and made no effort to ascertain whether a state of yoga-nidrā actually occurred.

3 The state of yoga-nidrā

The aforementioned articles by Parker et al. (2013) and Parker (2017a,b) detail the textual sources for a precise definition of the state of yoga-nidrā. At the risk of repetition, we can summarize these as coming primarily from three sources: Haṭha-yoga-pradīpikā IV. 43–50 (Digambarji and Kokaje, 1998), Śāndilya Upaniṣad I. 35 (Aiyar, 2000), and the Yoga-tarāvalī 17–26 attributed to Śaṅkarācārya (Deshikachar and Deshikachar, 2003). These sources describe first the awakening of the suṣumnā-nāḍī, the cardinal channel of subtle energy (prāṇa) in the subtle body, corresponding to the physiologically observed point in the nasal cycle where both nostrils flow exactly evenly (Hasegawa and Kern, 1977; Kahana-Zweig et al., 2016). Maintaining that state, the mind then makes a transition into khecari-mudrā, often described as a physical practice of folding the tongue back on itself. It is actually a state of meditation where alteration in the shape and orientation of the tongue occurs spontaneously. (For a detailed discussion, see Vijñāna-bhairava-tantra 77 with commentary in Singh, 1999, pp. 72–72, and Śiva Sūtras II. 5, Lakshmanjoo, 2007, pp. 89–91.) The resultant of maintaining that state of mind is a transition into unmani, a state where the mind is withdrawn that soon becomes yoga-nidrā.

This state of unmani is difficult for the reader who is not an accomplished yogi with the direct experience (pratyakṣa) to comprehend. Fortunately Swāmī Rāma elucidates it in his Om, The Eternal Witness: Secrets of the Mandukya Upanishad (Rama, 2007, pp. 196–197). There he states that:

We may conclude, then, that unmani is a state where there is no focused specific content in the mind, the senses being withdrawn from their external objects similar to pratyāhāra [sensory withdrawal in Patañjali's Yoga Sūtras], but they remain internally active in preparation for the dream state. Contrary to pratyāhāra, though withdrawn from external objects the senses are not yet totally dissolved in the mind-field (citta). The mind (as manas) is not yet dissolved or asleep until the state of deep sleep is reached.

Śaṅkara further describes how the yogi moves from a state of unmani where there is a spontaneous cessation of breath with no impulse to breathe again (the yoga term is kevala-kumbhaka) into amānasaka-mudrā where the discursive and sensory mind (manas) ceases to function altogether, becomes “vṛtta-śūnya,” “devoid of activity.” Then the yogi enters the state of yoga-nidrā (Deshikachar and Deshikachar, 2003, pp. 56–63) (Table 1).

From these traditional textual descriptions we can infer that the state of yoga-nidrā is one where one experiences the mind-field devoid of ordinary mental activity (sensory function and discursive thought), although this is not yet nirodha.^a One is in a state of deep, non-REM delta wave sleep, which is observed internally by the buddhi, the subtlest function of citta, the overall mind-field. In Swāmī Rāma's description of the method (Rama, 1988) he describes that state as resting in the heart center (anāhata-cakra) witnessing only the awareness of breathing. Interestingly, in his explanations of the state to Green and Green (1977), cited previously, he also stated that one remains aware of one's external surroundings and he demonstrated that skill. One puzzle is how this is done with the senses externally withdrawn.

There are two possibilities. One is that the senses are not entirely withdrawn in the sense that a sensation reaches the ear and registers in awareness (buddhi), but triggers no interpretive responses (thinking, manas) through the hearing and language centers in the cerebral cortex. Another possibility is that the observation of the sound happens through buddhi alone, without the operation of senses as described below in terms of the mind-field observing itself. Of course, any contributions of cortical function would be measurable.

Thus, in order to ascertain whether the state yoga-nidrā has been reached, it will be necessary to measure:

1. 1. brain waves via electroencephalography to see whether a predominant delta state is attained;
2. 2. some measure of awareness of the external environment despite the non-functioning of senses and manas. This author has utilized a list of words read to the subject through headphones at intervals of approximately 1 min during the duration of the practice. The list should then be repeated by the subject in the correct order upon exiting the practice.
3. 3. It would also be interesting to measure the state of the nasal cycle and whether the breath continues to move given the textual descriptions of the process above. Documentation of the state of kevala-kumbhaka, to the knowledge of this author, has never been conducted.

4 The problem of qualified subjects and training new subjects

One of the chief difficulties in conducting research on yoga-nidrā is identifying capable subjects. If one is very fortunate, one might encounter a subject like Swāmī Rāma or Swāmī Veda who were both very highly skilled and enthusiastic to contribute. Unfortunately, since they left their bodies in 1996 and 2015 respectively, they are now lost to us except in terms of data gathered on Swāmī Veda while he was yet alive. At a recent conference on Indian Psychology, I was presenting a plenary session on this topic when, in the midst of my remarks, a 99 or 100 year old yogi, a 50 year disciple of Swāmī Satyānanda Saraswatī walked into the hall and during the question period humbly volunteered himself. I could hardly believe our good fortune. More often, subjects of this degree of experience are either entirely uninterested in research or concerned that demonstrating what they can do will in some way become captive of their ego and become an obstacle to their sādhana [(spiritual practice].

Aside from such striking moments of good fortune we will have to rely on training to provide a subject pool that can reliably enter the state of yoga-nidrā. Swāmī Veda Bhāratī had a number of suggestions about how this might be done. In the first place he elaborated a model of yoga-nidrā with four plus stages of the depth of practice. In the first stage are preparatory relaxation exercises primarily engaging the physical body, which produce alpha brain wave states. Many teachers of yoga-nidrā confuse these preliminary relaxations with yoga-nidrā itself. In the second stage there are deeper practices in the subtle and mental bodies, which bring one primarily into the theta brain wave range. Here one can practice affirmations for healing, learn languages, and compose literature. This level is quite similar to clinical hypnosis. As practice at this stage deepens, one's brain waves begin to verge on delta. In the third stage one reaches the state of yoga-nidrā as it is described in the texts where one produces delta waves predominantly. At this stage of practice Swāmī Rāma placed a limit of 10 min on the practice. Finally, in the fourth stage, after thorough mastery of the preceding three stages, one remains in yoga-nidrā for up to 3.5 h. Finally, after long practice and further mastery one begins to enter the stage of turīya, the fourth state of consciousness, where the highest samādhi, begins to permeate the other states of consciousness and becomes one's normal state of awareness. At this stage Swāmī Veda's experiential hypothesis is that even with a waking sensorium and bodily activity the EEG would remain flat as the activity of mind will have ceased altogether. Obviously the full scope of this model would require many years of practice. In a manageable period of time many students should be able to move into level three of this model. Progression in the model, he states, may also require the direct induction of the state of yoga-nidrā through initiation by a master of the practice (Bhāratī, 2014; Parker et al., 2013) (Table 2).

The minimal requisite practices to reach this third level of practice are the component subtle body practices described by Swāmī Rāma in his Path of Fire and Light, Volume II (Rama, 1988). These include the progressive muscle relaxation with which many practitioners are familiar (Parker, 2017a,b, pp. 272–274; Rama, 1984, p. 11), the 61 point exercise (Parker, 2017a,b, pp. 276–278; Rama, 1988, pp. 183–186), point-to-point breathing or śithali-karaṇa (Rama, 1988, pp. 186–191; Parker, 2017a,b, pp. 278–280). Unlike many teachers, his rendition of the progressive muscle relaxation begins with the head and relaxes major muscle groups, or muscles and joints, down to the toes and back again with pauses to feel the full body breath at the fingertips, heart center and toes. The 61 point exercise utilizes the visualization of a point of white light in each of 61 marma points. This is an exercise for both concentration and gaining an experience of the prāṇa-maya-koṣa, the layer of embodiment made of intangible subtle energy. Śithali-karaṇa utilizes visualization of exhaling from the crown of the head to the toes and exhaling from toes to crown for 10 breaths, crown to ankle for 10 breaths, crown to knees for 10 breaths, crown to perineum for 5 breaths, crown to navel center for 5 breaths, crown to throat center for 5 breaths, breathing between the eyebrow cakra and nāsāgra, the point where the nose-bridge meets the upper lip, and the crown to the eyebrow center for 5 breaths. Then the process is reversed: crown to throat center, crown to heart center, crown to navel center and crown to perineum for 5 breaths each, then crown to knees, crown to ankles, and crown to toes for 10 breaths each. As the breath moves through the body the visualization in the trunk of the body is moving through a hollow in the spine.

The method of yoga-nidrā from this point is quite simple. Five breaths each through the center between eyebrows, the center at the pit of the throat and the heart center, between the breasts. The mind is then sunk into a silent cave within the heart center with no object of concentration and no other thought, only awareness of breathing, for 10 min. This last process is the actual technique of yoga-nidrā; the rest are simply preliminaries.

Bhāratī, in his book on his personal experiments with yoga-nidrā dating from childhood (Bhāratī, 2014), further recommends a more complete list of practices that lead to the state of yoga-nidrā. These include (Bhāratī, 2014, pp. 36–37, some references added):

1. 1. Makarāsana, the crocodile posture, for learning
2. 2. Abdominal-diaphragmatic breathing (Rama, 1984, pp. 8–9).
3. 3. Śavāsana, the corpse posture, with adjustments for long practices,
4. 4. Exercise Without Movement, a series of exercises taught by Swāmī Rāma in a book of the same title (1984).
5. 5. Tension-relaxation practices (Rama, 1984, pp. 13–32).
6. 6. Progressive muscle relaxation, as described above.
7. 7. Subtler physical relaxations involving one side of the body and then the other, finer tissues, and internal organs.
8. 8. 31–61 point exercises described above with several variations.
9. 9. Whole body breaths, feeling the movement of the prāṇa-vāyu, the current of subtle energy in the prāṇa-maya-koṣa, in several different patterns.
10. 10. Short version of quick conscious rest (no published reference).
11. 11. OM kriyā, described by Swāmī Rāma in the appendices to Rama (2007).
12. 12. Yoga-nidrā via direct entry into the heart cave, guhā-praveśa [cave entry].
13. 13. Technique of exiting the yoga-nidrā state.

One might also include the meditative contemplative exercises of the non-dualist Vedānta practitioners and the increasingly subtle uses of mantra recitation in tāntrika practice as these processes also appear to be meditative correlates of the deep neurophenomenological method of introspection described by Petitmengin (2017). For the sake of brevity we have not made a detailed comparison in this paper.

In yoga-nidrā we have a precisely defined and measurable contemplative practice from the yoga tradition that can be taught to carefully trained participant-observers which produces a state of awareness, a modification of consciousness, that is readily identifiable both objectively and subjectively. As such it creates an opportunity to collaboratively carry the process of neuroscience research into new frontiers of understanding and scientific epistemology. Importantly, it also offers the relationship between scientists and their contemplative collaborators greater mutual respect for their respective gifts. It also holds the promise for contemplative methods that can reliably transform the human experience of suffering, the principal aim of both the yogis and the Buddha.