The authors have declared that no competing interests exist.
One of the techniques used to induce trance state in shamanic ceremonies is monotonous drumming. According to previous EEG research, certain patterns of drumming cause an increase in power of alpha and theta frequencies of the listener. Present study aimed to determine, if suggestion to experience trance state could increase the relative alpha and theta amplitude and the intensity of experienced state. A group of twenty-four subjects participated in the study. Suggestion to experience trance state decreased alpha frequency power during the drumming condition. No other significant effects of suggestion or drumming were found.
The key concept in shamanism is the ceremonial use of the state of trance
Although the topic of shamanic trance states was extensively covered in the field of anthropology, only a few researchers set out to examine its behavioral aspects in an experimental setting.
A more direct way to assess trance or altered states of consciousness is to measure patterns in the electrical activity of the brain. The electroencephalograph (EEG) provides information about neural oscillations. The frequency components of the neural oscillations spectrum allows to detect for example states of alertness, relaxation or drowsiness. According to frequency brainwaves are categorized into following main types
beta 13-40 Hz – active, strongly engaged mind
alpha 8-12 Hz – relaxed wakefulness, sometimes also alertness
theta 5-7 Hz - occurs in a range of different mind states: drowsiness, threshold between wakefulness and sleep but also meditation or solving mathematical tasks
delta 1-4 Hz, associated with sleep and drowsiness
EEG can be used to discern between various altered states of consciousness, such as hypnosis, meditation or states resulting from yoga practices
Since 1934
It reasonable to believe that certain patterns of drumming may induce states of trance due to the brainwave entrainment (photic driving) mechanism. However, a shamanic ceremony is a complex phenomenon. An important factor is the ritual structure itself. In Deep Listeners
Therefore, the individual’s response to shamanic drumming may be moderated by expecting the trance state to happen. Expectancies can shape the ability to resist suggestion and can influence hypnotic suggestions, such as arm catalepsy, or spontaneous amnesia
Out of the very limited scientific literature about shamanic drumming, only two studies used EEG to examine the trance state induced by monotonous drumming
Hence, the aim of the present experiment was to contribute to the existing scientific knowledge about shamanic drumming. Specifically, the first aim of the experiment was to determine if the results obtained by Neher and Maxfield by could be replicated. The second aim was to examine the possible role of expectation in experiencing trance state during listening to monotonous drumming. Verbal suggestions can mediate the effect of meditation on executive brain functions
Two research questions were derived:
1) Does shamanic drumming induce a change in EEG frequency components?
2) Does the suggestion to experience trance state increase the relative amplitude of EEG frequency components?
A group of 24 people participated in the experiment consisting of 13 males and 11 females (mean age=28.8 SD=8.05). The group was recruited in Sweden mainly among Umeå university students and members of Yogasällskapet yoga center. All participants lived in Umeå, twenty of them were of Swedish origin and all of them were fluent in English. The inclusion criteria were normal hearing and no prior experience with shamanic drumming. Exclusion criteria was epilepsy and psychosis. To ensure a normal state for the EEG procedure, all participants were asked not to drink alcohol or use drugs the day of the session and the day before. They were also advised not to come on empty stomach and not after a big meal.
To produce a recording for the experiment an accurate replica of shamanic frame drum was constructed, with wooden frame and deer hide membrane. Samples of drumming with the pattern of approximately 4 beats/s were recorded. According to previous research this pattern was likely to induce the strongest physiological and behavioral response due to the mechanism of photic driving
Using Ableton Live 9 software, the chosen sample of 1.5 min was looped to produce a recording of steady 15 min of drumming. Beginning and end of the each sample repetition were adjusted to make the transition unnoticeable. A small reverb effect was added to the the recording to create the feeling of listening to the drum played in a room. The samples were not corrected to achieve a metronomically perfect 4 beats/second speed, to simulate a live performance during a shamanic ceremony. The beginning and end of the recording were faded to create a smooth transition between the drumming and the control conditions: in the beginning the sound level increased up to the basic level for 10 s, and in the end the sound decreased for the last 10 s. Intensity of the sound was the same for all the participants. Recordings similar to the one used in present experiment can be found on Youtube, as seen in a video by
An AD Instruments Powerlab 8/35 device with a Dual BioAmp amplifier was used to collect the EEG data, together with five Ag electrodes (impedance < 5kΏ)
Channel 1 gathered signal from the occipital lobe. The first electrode in this channel was placed on the occipital lobe (OZ), and the reference electrode was placed on the parietal lobe (PZ). Alpha rhythm is distributed maximally in the occipital regions
Channel 2 gathered signal from the temporal lobe, with the first electrode placed on the left temporal lobe (T3), and the reference electrode placed on the right temporal lobe (T4). This setting of the channel 2 was chosen in order to detect theta signal if it would occur.
An additional grounding electrode was placed on the forehead. The sampling frequency was 1000 Hz and the time resolution 1 cm/s. A mains filter was applied to exclude 50 Hz interference and a 50 HZ low pass notch to cut off high frequency noise. No means of correction for eye movement artifacts were needed because Chanel 1 and Chanel 2 electrodes were placed away from the forehead.
The Phenomenology of Consciousness Inventory (PCI) is a self-rating instrument developed to measure subjective experience during altered states of consciousness, such as hypnosis or various types of meditation. PCI questionnaire is a reliable and valid instrument and testing of its dimensions and subdimensions proved them to be stable construct
Prior to the session, participants were informed that they will take part in a non-invasive experiment that involves listening to drumming. All experimental sessions were conducted in the afternoon to control for the time of the day. During two weeks, two experimental sessions were conducted at 16.00 and 17.30, one participant at a time. All sessions took place in a calm, quiet laboratory, and the time of the day was chosen to limit the amount of possible outside noises.
Upon arrival in the laboratory, participants were advised to use the toilet if needed before the session. Every session started with signing the consent form and reading the informational text – neutral for Group 1, and a text containing suggestion for Group 2. After signing consent and reading the text, participants were informed about the procedure of the experiment. They were asked to keep their eyes closed throughout the recording, and to move as little as possible, but also to adjust their position whenever they feel uncomfortable. They were also instructed to continue lying still and relaxing after the drumming track will end, until the experimenter informs them that the recording is over. Subsequently, EEG electrodes were glued onto the chosen places on their scalp (
First, 10 min of baseline control condition was recorded. An EEG procedure requires an adaptation period of approx. 5 min for the participants to settle in and calm down. Data from the first five min of the control recording were treated as an adaptation period and excluded from the analysis.
After the first control condition, the 15 min drumming track was played through the headphones.
Subsequent to the drumming condition, second control recording was taken. Because of the structure of the experiment which involved lying in supine position for a prolonged period of time, second baseline recording was needed to determine if any observed response in EEG signal could be due to the time passing.
After the end of the recording, the electrodes and headphones were removed, and the participants completed the PCI questionnaire.
Participants were randomly assigned to two groups. Group 2 received a suggestion in form of a written text before the experiment, whereas Group 1 received no suggestion (they were given a neutral text).
Group 1 (N=12) received a neutral text. It referred to the drumming, but no trance state effects were mentioned.
“Drumming if often used in native ceremonies. It is sometimes combined with singing or playing other instruments, like rattles or flutes. During the experiment, you will hear drumming on an exact replica of the drum typically used during such ceremonies, with a deer rawhide membrane mounted on a wooden frame using traditional methods. The drum is usually beaten with a stick with a steady, monotonous pattern. No other instruments were used in the recording.”
Group 2 (N=12) received a text containing a suggestion that they may experience a trance state during the drumming.
“Rhythmic drumming is used in shamanic ceremonies to induce a state of trance. During the altered state of consciousness caused by drumming you may feel either energized or relaxed. Your body experience often becomes distorted, and you may feel pressure, expansion or vibration in various parts of your body. It is also common to experience sensations of movement: flying, falling, or spiraling. Usually, your imagination gets very vivid, and you may experience visions or a dreamlike state of mind. Participants of shamanic drumming ceremonies often describe their experience as a “journey” emphasizing the sensation of distorted time and space, and a feeling of leaving the body and traveling.”
Statistical analyses were conducted using AD Instruments LabChart 8 and LibreOffice Calc software. The structure of the present experiment was derived from the research by
From the first control condition, the first 5 min were ignored as adaptation time, and the epochs starting 5, 6, 7, 8, and 9 min into the recording were chosen for further analysis.
From the drumming condition, epochs starting 8, 10, 12, 13, and 14 min after the drumming begun were chosen. The choice of epochs was determined by the observations from the previous research. According to
From the second control, epochs starting 2, 4, 6, 8 and 9 min into the recording were chosen.
No movement artifacts were present in the chosen epochs. For each epoch, LabChart software was used to derive the power of each frequency component from the raw EEG signal. Alpha peak was the highest amplitude density in the 8-12 Hz frequency range, theta peak was the highest amplitude density in the 5-7 Hz frequency range and delta peak was the highest amplitude density in the 0-4 Hz frequency range. For each chosen epoch, the mean peak values of alfa and delta amplitude density in μV/√Hz were derived for the further analysis.
The experiment used one between subjects factor: suggestion and neutral text groups and two within-subjects factors: three conditions and five epochs in each condition. Therefore, to determine if suggestion increases the relative amplitude of frequency components a two groups x 3 conditions x 5 epochs repeated measures ANOVA test was used.
Preliminary observations of the EEG spectrum revealed that there were almost no theta components in the EEG signals. Two peaks corresponding to alpha and delta bandwidth were clearly noticeable for all participants, and the subsequent analyses were therefore conducted on these.
Examining the mean amplitude values from both channels led to the observation that the power of the alpha component was stronger in Channel 1 (occipital lobe), whereas the power of the delta component was stronger in Channel 2 (temporal lobes). Therefore, Channel 1 was chosen as the source of alpha data and Channel 2 was chosen as the main source of delta data.
Firstly, the question of whether the suggestion to experience trance state increases the relative amplitude of EEG frequency components was addressed. Two 2 group x 3 condition x 5 epochs repeated measures ANOVA was performed to reveal any significant differences in mean alpha and delta amplitudes as a function of suggestion and condition. There was a significant main effect of epoch (F4. 24 = 11.67, p < .01), but there was no significant main effect of suggestion (F1, 24 = 0.81, p > .05), and no significant main effect of condition (F2. 24 = 2.71, p > .05). There were no significant two-way interactions. However, there was a significant three way interaction between condition, suggestion, and epoch (F8, 24) = 2.19, p = .031). For delta, no significant main or interaction effects were found (suggestion F1. 24 = .01, p > .05, condition (F2, 24 = .74, p > .05), epoch (F4. 24 = 1.72, p > .05).
Secondly, the question of whether the suggestion to experience trance state during drumming increases the intensity of subjective experience (measured by the PCI instrument) was addressed. An independent t-test found no statistically significant difference between suggestion and neutral group in the Altered Experience (AE) scores (p > .5). Mean value on AE scale for the whole group was 2.45 (on a 0-6 scale).
Treating PCI questionnaire Altered Experience score as an indicator of experienced trance state it can be doubted whether the drumming stimulus and suggestion succeeded in inducing the altered experience reported by previous research
It is also possible that the cultural differences could be accounted for different scores in PCI questionnaire. Many participants expressed difficulties with understanding some of the questions connected for example with expanding of the body or visionary states and imagery. It may mean that they are not used to describing their experience in such terms. Cultural differences may also limit the effectiveness of suggestion. In other words, Swedish people may be unlikely to believe that they will experience a state of trance. For future research on shamanic drumming it would be reasonable to adjust the used instruments to the cultural background of the participants.
A surprising finding of the present experiment was registering the decreased alpha frequency during the drumming condition in the suggestion group. This observation is in variance with the results of
Another possible reason for the differences in alpha activity between the present and the Maxfield study may be different placement of the electrodes. In the present study, the occipital location for Channel 1 electrode was chosen due to the fact that the occipital lobe is the main source of alpha activity
One other unexpected result was that theta rhythm was not registered neither during baseline nor during stimulus conditions. Together with decreased alpha and increased delta frequencies, the absence of theta may indicate deep relaxation and drowsiness of participants.
One of the goals of present experiment was to replicate the results of the previous EEG research on shamanic drumming, at the same time avoiding their methodological problems. The number o participants (N=24) was twice bigger than Maxfield’s (N=12) or Neher’s (N=10). One of the biggest drawbacks of Neher (1962) experiment was very short stimulus condition (T= 4 min). In
This situation of the use of similar stimuli yielding different physiological and behavioral responses indicates that shamanic drumming is a complex phenomenon requiring further scientific investigation. It is possible that there are other variables than merely the amount of beats per second that determine the trance–inducing character of the drumming. Typically, different combinations of strong and weak drum strokes are used during drumming. Future research would benefit from comparing patterns of drumming with the same speed of beats per second, but different combinations of accents.
Another aspect of the present experiment was the horizontal position of the participants. Although some shamanic ceremonies require the participants to lye down, most of the indigenous rites involve movement and dance. The original ritual context provides a richer and much more stimulating atmosphere than a sterile and rigid laboratory setting. Unfortunately, due to the problem of movement artifacts replicating such conditions is not possible in EEG study. Remaining in horizontal position throughout the session allowed obtaining steady EEG signal form the participants, but might have increased their drowsiness.
The differences between participants in terms of mean values of alpha and delta were very large. This may stem from a highly heterogenous group of participants. The age span was very wide, ranging from 20 years for the youngest participant to 43 for the oldest one, which may have increased the individual differences in EEG signal between the participants. EEG signal is very sensitive to specific state of the participant, and older people may respond differently to remaining in horizontal position for a prolonged time. The increased variance in EEG signals may be the cause of reduced statistical power, and therefore insignificant results.
Moreover, nine of the participants were recruited from the Yoga studio were the present author used to teach. Some of the participants recruited from this source had experience with meditation practice, which tends to be associated with higher baseline alpha activity
In conclusion, suggestion about experiencing a state of trance might decrease alpha frequency brainwave activity when listening to shamanic drumming, which indicates a deeper state of relaxation, rather than the predicted trance state. The present experiment did not replicate the results of the previous studies, which demonstrates that the phenomenon of trance induced by shamanic drumming required careful scientific examination.