EEG Responses to Shamanic Drumming . Does the Suggestion of Trance State Moderate The Strength of Frequency Components ?

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. DOI : 10.14302/issn.2574-4518.jsdr-17-1794 Corresponding Author: Mateusz Konopacki, Umea University, Sweden. Email:mateusz.s.konopacki@gmail.com

The key concept in shamanism is the ceremonial use of the state of trance [7]. Altered consciousness may be achieved in various ways, either ascetic -like meditation or sensory deprivation, or ecstatic such as dancing, chanting or use of entheogenic substances. A very popular instrument used in ceremonies is the drum, and monotonous patterns of drumming are often used to induce a state of trance. During ceremonies, unusual perceptions and hallucinations were reported, as well as twitching of the body and generalized convulsions [10].

Subjective response to shamanic 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.
[8] compared the trance state induced by shamanic drumming with that induced by hypnosis.
After listening to a 15 min recording of monotonous drumming the participants (N=206) rated their subjective experience. Forty participants reported experiencing shamanic trance state while listening to drumming. Those participants for whom the trance induction was successful came largely from the groups of medium or high hypnotic susceptibility scores [8], which indicates a link between hypnotic susceptibility and the ability to experience trance state while listening to shamanic drumming. [9] examined the subjective experience of participants (N=12) who listened to different patterns of monotonous drumming. Written descriptions of participants' subjective experiences were categorized. according to common themes and topics. Among the experienced described were shamanic imagery, feeling of becoming energized, mental or physical discomfort, sensations of movement, flying or distorted body image and loosing track of time. All participants experienced visionary experiences during at least one condition. The experiment supports the hypothesis that rhythmic drumming can induce state of trance [9].

The use of EEG in trance research
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 [15]: • 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 [9].
Especially, the presence of alpha and theta frequency has been associated with states of meditation [5].
Although research on EEG activity in shamanic trance is limited, higher alpha and theta activity were registered in participants who claimed to have experienced trance state in experiments by [9,10]. Basing on this research, EEG may be useful in detecting trance experience during shamanic drumming.

Photic driving
Since 1934 [11,12] it was known that rhythmic visual stimulation in the form of flashing light with a frequency close to the alpha rhythm of the brain can produce two effects in the EEG: (1)

Procedure
Prior to the session, participants were informed that they will take part in a non-invasive experiment that After the end of the recording, the electrodes and headphones were removed, and the participants completed the PCI questionnaire.

Experimental groups
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

Data analysis
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 [9], hence from each control condition a number of five 1 min epochs were chosen: • 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 [9] the optimal physiological response to shamanic drumming occurs between 8 and 15 min of listening.

Discussion
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 [8,9,10].    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.

Limitations
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 [5].