International Journal of Psychophysiology 61 (2006) 167 – 178
www.elsevier.com/locate/ijpsycho
EEG correlates of multimodal ganzfeld induced hallucinatory imagery
Peter Pütz *, Matthias Braeunig, Jiřı́ Wackermann
Department of Empirical and Analytical Psychophysics, Institute for Frontier Areas of Psychology, Wilhemstrasse 3a, D-79098 Freiburg i. Br., Germany
Received 14 December 2004; accepted 15 September 2005
Available online 7 November 2005
Abstract
Multimodal ganzfeld (MMGF) frequently induces dreamlike, pseudo-hallucinatory imagery. The aim of the study was to explore EEG
correlates of MMGF-induced imagery. In a screening phase, seven Fhigh-responders_ were selected by frequency and quality of their reported
hallucinatory experience in MMGF. Each of these subjects then participated in three MMGF sessions (45 min) with simultaneous 19 channel EEG
recordings and indicated occurrences of imagery by pressing a button. Relative spectral power changes during percept formation (30 s preceding
subjects’ reports) with respect to intra-individual baselines (no-imagery EEG) were analysed. At the beginning of the 30-s Fimage formation_
period alpha was slightly reduced than in the Fno-imagery_ periods. This was followed by increased power in the higher alpha frequency band
(10 – 12 Hz) which then declined in a monotonic fashion. This decline in higher alpha power was accompanied by increased power in the beta
frequency bands. Throughout the image formation period there was a steady decline in power of low frequency alpha (8 – 10 Hz).
Correlations between descriptors of subjective experience and EEG power changes were evaluated in terms of their global average magnitude
and variability in time. Results indicate that the acceleration of alpha activity is a nonspecific effect of MMGF. In contrast, the tri-phasic profile of
faster alpha activity seems to be a specific correlate of the retrieval and transformation of memory content in ganzfeld imagery.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Altered states of consciousness; Multimodal ganzfeld; Imagery; Alpha activity; EEG; Inhibition theory
1. Introduction
Stimulation with homogenous light and sound, also known
as multimodal ganzfeld (MMGF), induces a state of perceptual
deprivation. Sustained exposure to MMGF can lead to an
altered state of consciousness (ASC) that is characterised by
dreamlike imagery. This imagery is subjectively comparable in
quality and intensity to hypnagogic imagery occurring during
sleep onset (Witkin and Lewis, 1963).
In an earlier study, comparing electrophysiological
signatures of sleep onset, waking mentation and ganzfeld,
it was established that ganzfeld imagery occurs in a brain
functional state which is distinctly different from that
occurring at sleep onset (Wackermann et al., 2002).
Frequency spectra of ganzfeld EEG data indicated an
activated waking state; thus, contrary to the common belief,
ganzfeld imagery is related to a different brain functional
state than that associated with hypnagogic imagery. This
finding has been confirmed independently by source
* Corresponding author. Tel.: +49 761 207 2176; fax: +49 761 207 2179.
E-mail address: [email protected] (P. Pütz).
0167-8760/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ijpsycho.2005.09.002
localisation methods (Faber et al., 2002). Despite these
new insights, much of the physiological and psychological
effects of MMGF has yet to be examined. In particular,
little is known about the specific electrophysiological
signatures of ganzfeld-induced imagery.
Ganzfeld usually does not induce a continuous stream of
imagery. Subjective experience in MMGF is a dynamically
changing state, characterised primarily by adaptive processes
in the respective sensory systems with variation of colour
saturation, brightness, loudness, etc., intermingled with
episodes of typical ganzfeld imagery. Spontaneous thoughts,
ideations, fantasies, associations, sometimes even distortions
of body scheme and orientation in space and time are reported
(Holt, 1964; Schacter, 1976). Reports of ganzfeld experience
thus reveal a true potpourri of mentation of various origins.
The dynamic nature of ganzfeld experience cannot be
adequately assessed by the method of Freports on demand_
used in our earlier study. Frequency of imagery episodes
considerably vary between individuals and some participants
do not experience any imagery. Therefore, for the purpose of
the present study, we opted for the method of Fself-initiated
reports_. Subjects were instructed to report the moments of
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P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
maximally developed imagery and this enabled us to separate
episodes of ganzfeld-induced hallucinations from the noimagery background.
A similar approach was successfully used to study the
relationship between subjective experiences and objective
brain electrical activity. In an experiment studying the
relationship of long-term sensory deprivation and hallucinatory experiences (Hayashi and Hiroshima, 1992), the participants signaled occurrences of hallucinatory experiences by
button presses. In a study by Line et al. (1998), that attempted
to localise regions of brain activity associated with the onset
of schizophrenic auditory hallucinations, subjects also signaled
the onset of auditory hallucinations by button pressing.
Germain and Nielson (2001) studied EEG power changes
associated with sleep onset images. In this case, subjects
marked the onset of hypnagogic imagery by pressing a microswitch. These studies have shown that Fself-initiated reporting_
using a button press is a viable and reliable method applicable
with subjects who are about to fall asleep, or even in the case
of those who experience genuine psychotic hallucinations
(Line et al., 1998).
Our previous study indicated that ganzfeld induced an
activated waking state characterised by the acceleration of
alpha activity. These results, and the results of other authors
(Klimesch, 1997; Ray and Cole, 1985; Line et al., 1998;
Hayashi and Hiroshima, 1992; Cooper et al., 2003), indicate
the importance of the alpha frequency band with respect to
research on voluntary, induced, or spontaneously occurring
mental imagery. The aim of the present study was to investigate
the electrophysiological signatures of episodes of ganzfeldinduced imagery, and to study correlations between objective
electrophysiological measures and dimensions of subjective
experience.
2. Methods
2.1. Subjects
Forty paid volunteers, 28 female and 12 male, in the age
range 20 – 59 years (mean 39.0 years), recruited by local
newspaper ads, participated in the experiment. Thirteen
subjects had already participated in another study and were
thus familiar with the laboratory facilities. One subject was
excluded because he was using anti-depressant medication. The
remaining 39 participants were in good health with no
neurological or psychiatric history.
2.2. Experimental design
The study consisted of two phases. The purpose of the
first, screening phase was to select subjects producing
frequent and well-structured imagery in MMGF (Fhigh
responders_). In the second phase, repeated ganzfeld sessions, including electrophysiological recordings, were arranged with the selected subjects. The aim of this two-phase
design was to enhance the efficiency of the experimental
procedure.
In both phases, the same MMGF technique was applied:
Participants were sitting relaxed in a reclining chair, in a soundattenuated recording room, with eyes open. The subjects’ eyes
were covered by translucent, anatomically shaped goggles,
which were illuminated by bright red light. Simultaneously they
were listening to the monotonous sound of a waterfall through
headphones. Sound intensity was individually adjusted to a
subjectively comfortable level. This procedure has proven to
accomplish an almost perfect homogenisation of visual and
acoustic sensory field.
In the screening phase, each subject participated in one
session, consisting of two periods of 20-min and 30-min
duration, respectively, with an interview between the two
periods. During the first period, subjects were exposed to
ganzfeld for 20 min to familiarise themselves with the new
situation and possibly to experience ganzfeld-induced imagery
without prior suggestion from the experimenter. The subjects
were instructed to sit in a relaxed manner, with their eyes open,
and to watch closely but passively their perceptions, avoiding
guided imagery, directed daydreaming, etc. Afterwards they
were interviewed about their experiences during the period.
As expected, some subjects reported spontaneously that they
had experienced vivid imagery during the ganzfeld session.
Those who did not mention such experiences were asked about
the occurrence of any inner perceptions. If such perceptions
were reported, they were told this was the intended effect of the
ganzfeld procedure. Those participants who had not experienced any imagery were informed that such experiences may
occur later, during the subsequent session, and typical
examples were described. The pseudo-hallucinatory nature of
the phenomenon of interest was also explained and contrasted
with other perceptual phenomena that might occur in the
ganzfeld, such as retinal adaptation processes, Fmouches
volantes_, etc. Thus, after the interview, all participants were
informed about the effects of the ganzfeld and the specific
features of the imagery to be reported. Before the second
period of the experiment, the subjects provided information on
their mental and somatic status (Table 1).
In the second period, participants were exposed to MMGF
for 30 min. They were instructed to signal the moment when
imagery was most intense, or just about to vanish, via a small
micro-switch held in their right hand. The button press elicited
an acoustic signal to the experimenter, who then interrupted the
Table 1
Overview of items and associated codings used for the pre-experiment status
questionnaire
No.
Item
Description
Type
Coding
1
2
3
Alert
Relax
Mood
Alertness
Relaxation
Emotional condition
Ordinal
Ordinal
Ordinal
4
Cond
Physical condition
Ordinal
5
6
7
Alcohol
Events
Sleep
Alcohol consumption
Life events
Sleep quality
Binary
Binary
Ordinal
1 = tired—5 = fresh
1 = strained—5 = relaxed
1 = very good mood—5 =
very bad
1 = bodily very good—5 =
very bad
0 = absent, 1 = present
0 = absent, 1 = present
1 = very bad—5 =
very good
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
169
Table 2
Overview of items and associated codings used for the ganzfeld inquiry
No.
Item
Description
Type
Coding
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Visual
Acoust
Kinest
Tactile
Olfactory
Imagecleara
Imagereala
Imagegen
Variation
Imagectrla
Imagedur
Timest
Bodysens
Relax
Sleepwakea
Memory
Visual percepts
Acoustic percepts
Kinaesthetic percepts
Tactile percepts
Olfactory percepts
Distinctness of imagery
Reality character of imagery
Genesis of imagery
Steadiness of imagery
Control of imagery
Duration of imagery
Estimated elapsed time
Altered bodily sensations
Relaxation
Sleepiness
Remembered content
Binary
Binary
Binary
Binary
Binary
Ordinal
Ordinal
Binary
Nominal
Ordinal
Numeric
Numeric
Binary
Ordinal
Ordinal
Ordinal
0 = absent, 1 = present
0 = absent, 1 = present
0 = absent, 1 = present
0 = absent, 1 = present
0 = absent, 1 = present
1 = unclear—5 = clear
1 = unreal—5 = real
0 = gradual, 1 = sudden
1 = steady, 2 = rising, 3 = sinking, 4 = varyingb
1 = guided—5 = free floating
. . . [s]
. . . [min]
0 = as usual, 1 = changed
1 = stressed—5 = relaxed
1 = wide awake—5 = deep asleep
1 = few—5 = everything
a
b
Item utilised to select the Fhigh-responding_ subjects.
After the screening-phase category 4 = varying was introduced and used during the EEG sessions.
acoustic stimulation and communicated with the subject via
intercom. After a free verbal account of her/his imagery, the
subject answered a structured inquiry consisting of 16 ordinal
or binary items assessing various aspects of the experience
(Table 2). All communication was tape-recorded. This procedure (stimulation –report – inquiry) was repeated until a total
time of 30 min of ganzfeld exposure was reached. The duration
of the entire experimental session including both ganzfeld
exposures was about 70 min. Based on the results of the first,
screening phase, seven subjects were selected for the second
phase of the study, as described in the next subsection.
Each of these selected subjects attended three ganzfeld
sessions with simultaneous EEG recordings (Fig. 1). These
sessions comprised 45 min of ganzfeld each, again with the
instruction to signal the moments of the most expressed
imagery via button press, and to give a verbal report
followed by the structured inquiry. The button press now
also triggered an event marker in a separate channel recorded
in parallel with the EEG data (marked as SR Fsubject
response_ in Fig. 1a).
This procedure was repeated until a total of 45 min of
ganzfeld was reached (net time, i.e., not including interruptions
for the report/inquiry). The typical duration of one ganzfeld
session was thus about 60 –75 min. The total time spent for one
experimental session, including time needed for electrode
attachment, impedance checks, etc., was about 2 h. The
average time to accomplish all three sessions with one subject
was about 2 weeks (range 3 –22 days).
(a)
(b)
(c)
Fig. 1. Time charts of an experimental session and data extraction (example). (a) Whole exp. session comprising three mentation reports (SR) out of four stimulation
intervals (starting with RE). The typical session duration was 60 – 75 min. (b) Stimulation interval (RE – SR). Period between 120 and 30 was excluded. (c) The
last 30 s preceding subject response were analysed by 29 overlapping 2-s segments starting at 2, 3, 4, . . ., 30 s before SR.
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P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
2.3. Selection procedure
In the screening phase, a total of 93 reports were obtained
from 39 subjects (see Fig. 2). The aim of the selection
procedure was to identify subjects who were willing to
participate in the second phase of the study and who had reported well-structured imagery frequently (Fhigh responders_).
The selection was thus based on a combination of qualitative
and quantitative criteria, i.e., the number of reports delivered
by the subject as well as their ratings on the following four
scales: clearness of imagery (imageclear F4_), reality-like
character, or vividness, of imagery (imagereal F4_), control
of imagery (imagectrl F3_), and sleepiness (sleepwake F3_)
(cf. Table 2). The fourth item in the compound selection
criterion, sleepwake F3_, was used to exclude hypnagogic
imagery occurring close to the sleep onset. Since no
physiological recordings were available in the screening phase,
we had to rely on subjective estimates of vigilance. Seven
subjects who met the above-listed criteria for at least one
report, and gave no less than three reports were selected. All
selected subjects were female, in the age range 38– 53 years
(mean = 49.2). These Fhigh-responders_ delivered 30 reports
(mean = 4.29) while the remaining 32 participants yielded 63
reports (mean = 1.97, i.e., less than half of the average yield in
the selected subjects).
A post hoc review of the transcribed reports delivered by
those selected subjects confirmed well-structured percepts,
usually visual and/or acoustical, sometimes reaching pseudohallucinatory quality (Appendix A, example 1).
2.4. Electrophysiology data recording and preprocessing
The experimental sessions were conducted in an electrically
and acoustically shielded, air-conditioned, video-monitored
recording room; the experimenter was in an adjacent control
room, communicating with the participant via intercom. A
Fig. 2. Barchart of report frequency in the screening phase. Selected Fhighresponders_ meeting all selection criteria and being available for additional
sessions with EEG-recording are marked with dark bars.
BrainScope 220 (M&I Ltd., Prague, Czechia) amplification and
A/D conversion system was used for electrophysiological
recordings. EEG was recorded from 19 Grass 8 mm Gold cup
electrodes placed according to the 10/20 system (Jasper, 1958).
A common reference electrode was located between Fz and Cz.
The low- and high-pass EEG filters were to 70 Hz and 0.15 Hz,
respectively. Vertical electrooculogram (right eye) and electromyogram (m. masseter) were also recorded for later artefact
elimination. The electrocardiogram was recorded using Ag/
AgCl disposable electrodes placed on the right upper and the left
lower thorax. Respiration was recorded using a flexible rubber
tube placed around the abdomen and connected to a tensoelectric
transducer. Data was digitised at a sampling frequency of 256/s
in all channels. EEG recordings were visually inspected and
artefact-free 2-s epochs were marked for subsequent analyses.
EEG data was then digitally FIR-filtered to 1 –30 Hz frequency
range, transformed to average reference and detrended using a
polynomial fit of third order. The tape-recorded mentation
reports were transcribed using a simplified version of the
guidelines for dream report transcription by Gass et al. (1987).
3. Data analyses
3.1. EEG data
The subsequent analyses relied on intra-individual differences of imagery-related EEG against no-imagery EEG
(baseline). For each session, all artefact-free 2-s EEG epochs
from all stimulation periods from the start of stimulation
(marked as RE in Fig. 1b) up to 120 s preceding the subject’s
report (SR) were collected and combined to a single dataset
which was used for computation of the individual baseline (Fig.
1b). EEG data from the 30-s period preceding SR was processed
separately, using a 2-s window shifted forward by 1 s, thus
resulting in 29 overlapping segments of 2-s duration (Fig. 1c).
Data from the time interval from 120 to 30 s preceding SR were
omitted from further analyses to clearly separate baseline from
imagery-related data. In 51% of reports, subjective estimates of
imagery duration were below or equal to 60 s, with only 13% of
duration estimates exceeding 2 min.
After artefact rating, a total of 1616, or 67%, of 2-s EEG
epochs remained for the 30-s interval preceding SRs. Only
reports with at least 10 out of 29 2-s epochs were included in
further analyses, thus reducing the amount of usable EEG
epochs to 1571 associated with 75 reports. Furthermore, all
reports with self estimated imagery durations of longer than 2
min were excluded from the analyses, to avoid every possibility
that baseline EEG was contaminated with imagery-related EEG
data. This reduced the number of usable EEG epochs to 1364
from 64 reports. All artefact-free baseline and imagery EEG
segments were FFT-transformed to frequency spectra ranging
from 1.5 to 30 Hz at 0.5 Hz resolution. These were normalised to
unity integral across the full frequency range. These normalised
FFT-spectra were integrated over seven frequency bands,
Z fup;x
Ix ¼
S ð f Þdf ; where xafd; h; a1; a2; b1; b2; b3g:
ð1Þ
flo;x
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
171
The frequency boundaries f lo and f up were as follows: 1.5 –6
Hz (d), 6– 8 Hz (h), 8– 10 Hz (a1), 10 –12 Hz (a2), 12 – 18 Hz
(b1), 18 –21 Hz (b2) and 21– 30 Hz (b3). Of interest are
imagery-related changes of integral spectral power relative to
the no-imagery EEG power. We thus define the effect measure
as follows:
and six selected area-transformed variables of the ganzfeld
inquiry were calculated. As the chosen effect measures were
log-power differences from individual baselines, the productmoment correlations were based on products of the deviates
from the baselines without subtracting their mean values,1
Ex ¼ log I x log Ix;base :
1
rxy ¼ I
n
ð2Þ
That is to say, the effect measure is the difference between
the log-transformed integral power in the analysis epoch and
the mean log-transformed integral power averaged across all
non-imagery epochs to provide a baseline. The effect measures
were calculated for each frequency band in each channel and
across all subjects.
The ensembles of effect measures (E) obtained for each
frequency band, each channel, and each 2-s time segment, were
further processed as follows:
1) Arithmetic means were calculated across reports;
2) Deviations of the ensemble distributions from zero were
assessed by Wilcoxon one-sample test. The Wilcoxon
statistics were transformed to Z values:
W nðn þ 1Þ=4
Z ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
nðn þ 1Þð2n þ 1Þ=24
n
X
sx Isy
ð4Þ
where Y Z {imagclear, imagreal, memory, imagctrl, memory,
sleepwake} and s x = S.D. of E x, s y = S.D. of Z-transformed
variable y. (Note that mean Z y = 0 and s y = 1 by definition, see
Section 3.2.) This resulted in 19 3 6 = 342 correlations for
each frequency band. To obtain a comprehensive picture out
of this material, the correlations matrices were condensed as
follows: For each channel and each pair of variables we had
three correlations, calculated for three above-mentioned time
intervals, 30 to 21, 20 to 11, and 10 to 2 s. These Fcorrelation
triplets_ were reduced to two descriptors2 : (a) global
correlation (arithmetic mean of all three correlations across
all 19 channels),
r̄¯ ¼
ð3Þ
where W denotes the rank sum yielded by the Wilcoxon’s test,
and n is the sample size. Under the null hypothesis, the Z
values obtained by formula (3) should have a normal Gaussian
distribution.
For each frequency band, and each report, the effect
measures (E) and the corresponding Z-values were further
averaged over three intervals: (a) 30 to 21 s, (b) 20 to 11 s and
(c) 10 to 2 s before subject responses.
In this way, we obtained a quantitative characterisation of
report-related EEG changes in time, expressed as average
differences of log-powers (effect measure E, in dB), and
transformed to quantities of known statistical properties (Zscores, normally distributed).
Ei;x IZi;y
i¼1
3
1 X
r¯ j ;
3 j¼1
ð5Þ
where j = 1, 2, 3 is an interval index and r j is calculated:
r¯ j ¼
19
1 X
rjk ; where k ¼ 1; N 19; is a channel index;
19 k¼1
ð6Þ
and (b) correlation lability, expressed by the maximal
difference between any two out of three of the global
correlation triplets:
s ¼ maxðjr¯ 1 r¯ 2 j; jr¯ 2 r¯ 3j; jr¯ 1 r¯ 3jÞ:
ð7Þ
4. Results
4.1. Report frequency
3.2. Experiential data
3.3. Correlations between EEG and experiential data
A total of 82 reports with simultaneous EEG recordings
were obtained during the second phase of the study. The
number of reports per subject and session varied from zero to
nine (mean = 3.90). Report frequency was thus comparable to
the screening phase, although one subject showed a very
reduced response rate (cf. Table 3).
To assess increasing or decreasing frequency of reports
during sessions, time intervals between subjects’ reports (preresponse intervals, PRI) were normalised (divided by individual means), their rank-correlations with serial positions of
reports were calculated, and the signs of the correlations
To assess the relationship between brain electrical activity
and subjective experience for each of the three intervals
(30 to 21 s, 20 to 11 s and 10 to 2 s) product-moment
correlations between the seven effect measures (see Eq. (2))
1
Due to this modification, the critical values tabulated in statistical tables for
the Pearson’s correlation coefficient are not applicable.
2
Due to the generally low correlation coefficients, these were not Fisher Z
transformed before the calculations.
All variables were area-transformed to Z-values (mean = 0,
S.D. = 1), assuming a normal Gaussian distribution of the
underlying latent dimension (Chase, 1976), similarly as in
our previous study (Wackermann et al., 2002). Out of 16
variables collected during the post-report inquiries, 6 were
ordinal scales: imageclear, imagereal, imagectrl, memory,
relax, sleepwake (see Table 2). These variables were used for
the correlation analysis described in the next subsection.
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P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
Table 3
Report frequencies in screening and experimental phase
Subject
Screening
1
2
3
4
5
6
7
Mean
3
7
3
4
4
6
3
4.29
Experiment
Mean
E1
E2
E3
5
6
3
0
3
4
4
3.57
6
5
4
1
2
5
5
4.00
4
9
2
1
3
7
3
4.14
5.00
6.66
3.00
0.66
2.66
5.33
4.00
3.90
inverted for convenient interpretation. Positive values indicate
an acceleration tendency (progressively shortened PRI, i.e.,
increasing imagery potential), while negative values reflect
deceleration tendency (progressively lengthened PRI indicating
a decreasing imagery potential). This procedure was applied to
data from 12 sessions with more than three reports. Subjects
with accelerating tendency produced on the average more
reports (mean = 6.33) than subjects with decelerating tendency
(mean = 4.33). The development of PRI averaged across
repeated sessions reveals a slightly increasing tendency
(acceleration).
4.2. Subjective experience
Most reports mentioned visual imagery, with acoustical
imagery ranking as the second most common modality. Other
sensory modalities were only rarely reported (cf. Table 4).
Modality frequencies differed only marginally between the
screening and experimental phase, and the relative order of
reported modalities (visual > acoustic > tactile > olfactory) was
in agreement with our earlier study (Wackermann et al., 2002).
Compared to our earlier study occurrences of olfactory and
tactile percepts are noticeably lower in both data sets, this
difference being probably due to different methods used in the
earlier and the present study.
Imagery was predominantly described as Fgradually_ evolving (65.9%) vs. Fsudden_ (34.1%). FGradually_ evolving
percepts tended to be associated with longer PRI (r = 0.17).
Analysis of perceived imagery steadiness revealed that less
than a third of all imagery was reported as Fsteady_ (30.5%),
while most imagery percepts were described as Frising_
(42.6%) or Fvarying_ (22%) and only very few (4.9%) as
Fsinking_. Contents of the hallucinatory percepts were often
described as being familiar to the participants. Participants
reported that they were surprised by the occurrence of these
fragmentary elements from earlier experience but emerging in
an alien context. As in our previous study, water and its
derivatives (lakes, streams, etc.) occurred frequently as scenic
or dynamic elements of the reports. Most percepts were rated
by subjects as Frather clear_ or Fclear_ (imageclear F4_:
80.5%). About two thirds of experiences were rated as ‘‘rather
real’’ or Freality-like_ (imagereal F4_: 64.6%). These two
experiential dimensions, clearness and vividness of the percept,
were positively correlated (r = +0.602).
During the ganzfeld sessions, participants were awake and
relaxed, as revealed by their self-ratings on the corresponding
scales (sleepwake F2_: 67.1%, relax F4_: 61.0%). This
indicates that they really attained the state of passive
wakefulness, as intended by the instruction.
The relationships between the status variables recorded
before the experimental session (alertness, relaxation, etc.) and
the experiential variables were assessed by means of Spearman
rank-correlation coefficients (Table 5). While the quantity of
produced reports was related to alertness of the participants, no
association was found between alertness and any of the
selected experiential variables, except for the variable Frelax_
and this was trivial. There was a significant correlation between
the number of reports per subject and self-rated alertness before
the experiment started (r = + 0.475, p < 0.04).
Clearness of imagery was related to the degree of
experienced relaxation before the beginning of the session
(r = 0.607, p < 0.004) and to vividness of imagery with life
events (r = 0.630, p < 0.002). The latter was the highest
correlation of experiential and status variables observed.
Neither clearness of imagery nor vividness of imagery was
significantly correlated with report frequency. Two other
observations are worth mentioning. First, there was no
significant correlation between the self-rated degree of relaxation at the beginning of the experimental session and
experienced relaxation during imagery. Secondly, there was a
significant correlation between negative mood reported before
the experimental session and sleepiness during imagery
(r = 0.601, p < 0.005).
Representative examples of hallucinatory ganzfeld imagery
are given in Appendix A. The content of some of the reported
imagery episodes consisted of short Fflashes_ of dynamic
changing actions (Example 1). Acoustic imagery mostly did
not simply emerge by itself but was embedded in a meaningful
context, as illustrated by Example 2. Sometimes the halluci-
Table 5
Correlations of status variables and selected variables from ganzfeld inquiry
Table 4
Percentage of reported sensory modalities by experiments
Modality
GFS
GFE
SOGF
Visual
Acoustic
Olfactory
Tactile
Kinesthetic
94.3
16.1
3.2
9.7
5.4
97.6
23.2
3.7
8.5
2.4
90.4
28.8
16.4
26.0
N/A
Shown are data from the screening phase 2 (GFS) and experimental phase
(GFE). Data from the earlier study (SOGF) are shown for comparison.
Alert
Relax
Imageclear
0.358
0.607**
Imagereal 0.070
0.065
Imagectrl
0.048
0.235
Relax
0.527T
0.066
Sleepwake 0.239 0.299
Memory
0.422
0.556**
* p < 0.05.
** p < 0.01.
Mood
Cond
0.412
0.150
0.219
0.121
0.601**
0.143
0.407
0.353
0.200
0.145
0.630**
0.146
0.149
0.186
0.121
0.418
0.185
0.247
0.431
0.216
0.164
0.376 0.183
0.068
Events
Sleep
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
natory percepts attained a dreamlike and almost surrealist
quality, e.g. the alien manikin approaching the subject and then
amazingly disappearing (Example 3). Example 4 demonstrates
a typical incorporation of water as a scenic element (waterfall)
into the recurring imagery of a mountain landscape; the
reported scenery is of a dreamlike character.
4.3. EEG signatures of imagery
Most pertinent effects occur in the a frequency range,
especially a2 (Fig. 4a,b). Inspection of the time courses of
effect measures (E) for both frequency bands a1 and a2 reveals
deviations from zero, i.e., deviations of EEG power prior to
subjects’ report from the baseline-EEG power; significance of
these changes is verified by corresponding Z scores. This is
illustrated in Fig. 3a –d for the frequency bands a1, a2 and the
ratio a2/a1 at location Pz. a activity at other scalp locations
shows very similar time course, as illustrated by Z-values
averaged across all 19 EEG recording sites (Fig. 3e; cf. Fig.
4a,b and f).
Noteworthy are different time courses of a1 and a2-power
changes during the analysis interval. a1 power shows a minor
but continuous decrease during the entire 30-s analysis interval,
while a2 displays a distinct tri-phasic time course: initially (30
to 21 s) reduced power, relative to baseline level; followed by a
Fburst_ up to the baseline level in the second sub-interval (20 to
11 s), and finally a monotonic and strong decrease during the
last sub-interval (10 to 2 s). These time courses are reflected by
the corresponding Z values (Fig. 3c– d).
These differences in the time course of the two a sub-bands
are particularly well expressed by the a2/a1 ratio, which is very
sensitive to minute shifts of the alpha peak frequency, and
attains a maximum for the second analysis interval. This
pattern is again globally distributed over the scalp.
Power in bands b2 and b3 is constant or raised. Overall, no
larger changes during the analysis interval are evident, except
(a)
(b)
(c)
173
for a minor increase of b power during the interval
immediately prior to SRs (Fig. 4c,d). Within the last 10-s
interval oscillations of b power occur which are not reflected
by the coarser time-resolution chosen for the present analysis.
During the entire 30-s interval centro-parietal d power is raised
with respect to the baseline level. The increase of d power
during the last analysis epoch (10 –2 s) probably reflects
preparation of the motor response (FBereitschaftspotential_).
4.4. Correlation of experiential data and EEG
The following section describes the results of the
correlation analysis of seven EEG effect measures (Section
3.1) and six experiential variables (Section 3.2), evaluated in
terms of global correlation and correlation lability (Section
3.3). Note that for a given report, there is only one value of
each experiential variable per subject. Thus, changes of
correlation coefficients reflect the dynamics of EEG changes
during the analysis epochs but the resulting correlation
profiles need not necessarily be similar to the profiles of
EEG power changes.
Fig. 5a and b both display global correlation versus
correlation lability and are based on the same data. Fig. 5b
shows the appropriate profiles of global correlation triplets for
the time intervals 30 –21, 20– 11 and 10 – 2 s prior to SRs.
Profiles are centered at the appropriate global mean over all
analysis intervals and shown without axes for the sake of
legibility. To facilitate identification of the correlations under
discussion these are marked by an open circle in Fig. 5a. The
most instable correlation were found for variables memory and
h with s = 0.20 (see also Fig. 5b). Global correlation was 0.05
and the correlation changed sign during the three analysis
intervals, starting with 0.05 (30 – 21 s) and Fincreasing_ to
0.15 (10 – 2 s) in the last interval. A slightly lower correlation
lability (s = 0.16) was found for the variables memory and a1,
but here there is higher negative global correlation. Vividness
(d)
(e)
Fig. 3. Average effect measure E and Z statistics for frequency band a1, a2 and ratio a2/a1. Sections a – d show values for scalp site Pz, section e shows averaged Z
across all channels T one standard error. Abscissae (all sections a – e): time from 30 to 2 s prior to subject response. Ordinates (sections a, b): effect measure in dB,
(sections c – d) corresponding Z values. Sections a and c show appropriate effect measure E and Z statistics for each 2-s segment, sections b, d and e averages for
three time intervals 30 to 21, 20 to 11 and 10 to 2 s prior to report signal (0 s).
174
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4. Topographic synopses of average frequency bands. Data are normalised, log-transformed difference values with respect to baseline condition. Shown are
averages across subjects’ for three time intervals: 30 – 21, 20 – 11 and 10 – 2 s preceding report-signal (0 s).
of imagery and a2 were also globally negatively correlated but
showed a more stable correlation profile. No comparable
correlation profile was found between a1 and vividness of
imagery where there is a stable but very weak global
correlation (Fig. 5a). There was a positive global correlation
for variables b3 power and vividness of imagery (Fig. 6f) with
correlations diminishing through the analysis intervals and
reaching a minimum immediately prior to SRs.
The highest global correlations were found for a2 power,
which showed a pattern of positive correlation with self-rated
sleepiness during imagery (Fig. 6d). Other correlations showed
less clear or conflicting topographical patterns and the
corresponding global correlations were generally globally
small and relatively stable in time.
5. Discussion
Our results can be grouped into three categories
& nonspecific effects of the multimodal ganzfeld (MMGF)
stimulation (i.e., not directly related to the hallucinatory
imagery);
& effects related to the active task of the participants (button
pressing);
& specific effects related to emergence or formation of
hallucinatory imagery.
The multimodal ganzfeld induced an activated waking state
with pronounced and accelerated a activity. This supports our
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
(a)
175
(b)
Fig. 5. Scatterplots of global mean correlation (ordinate) vs. lability in time (abscissa). Charts are based on correlations between seven log-transformed EEG
frequency band power values and six experiential variables. Correlations were computed for three intervals: 30 – 21, 20 – 11 and 10 – 2 s backwards in time with
respect to SR. Shown are (a) global means over all three intervals vs. maximum difference between two correlations and profiles for each variable combination (b).
Absolute values are used for maximum difference values. Variables are coded according to the following rule: c = imageclear, r = imagereal, e = imagectrl, = relax,
s = sleepwake and m = memory. Frequency bands are coded with Greek letters, e.g. ra2 correlation between imagereal and a2.
earlier finding that the multimodal ganzfeld does not necessarily
induce a hypnagogic state (Wackermann et al., 2002), and is in
agreement with earlier reports of researchers who studied the
effects of a homogenised visual field on the human EEG. Adrian
and Matthews (1934) reported that when subjects open their eyes
in a uniform visual field (illuminated or completely dark) a short
time after the usual a blocking effect synchronisation reappears.
Lehtonen and Lehtinen (1972) examined the effects of a uniform
visual field on a rhythm. The authors found that eyes open in a
uniform visual field (sheet of paper placed in front of eyes) or
ganzfeld (translucent hemisphere) led, after an initial delay, to
reappearance of a activity that is comparable to the eyes closed
level. This effect of a homogenous visual field was also observed
with higher vertebrates: Rougeul-Buser and Buser (1997) used
visual ganzfeld to induce a synchronisation in cats. We may thus
interpret the observable a synchronisation as a nonspecific effect
of eyes open under conditions of unstructured visual input.
Our participants had a double task: (i) to observe passively
their inner percepts, and (ii) to signal by a button press the
moment hallucinatory percepts were maximally experienced.
This latter, active task involved preparation and execution of a
motor response, i.e., the button press. The increase of d power
observed immediately prior to the button press probably
reflects the FBereitschaftspotential_ (Kornhuber and Deecke,
1964) and was therefore not directly related to the imagery.
Results by Germain and Nielson (2001) indicate that the
sensory content of sleep onset imagery in stage SO4 (Hori et
al., 1994) is related to d activity. It is, however, unlikely that
the increase in d power observed in our study was related to the
hallucinatory imagery as well. Firstly, visual inspection as well
as frequency analysis of MMGF EEG showed no indications of
sleep onset episodes during imagery formation. Secondly, the
observed increase of d power was relative to the ganzfeld
baseline condition. Thirdly, our participants never reported that
they had fallen asleep.
It is possible that the ganzfeld baseline condition still
contained hallucinatory experiences which were below the
individual Fthreshold_ for initiating a mentation report. This is
partly reflected by occasional statements of our subjects that,
although they perceived very weak recurrent imagery, it never
was strong enough for them to report it. In our opinion, these
reports suggest that participants were not eager to give
mentation reports when unsure about perceived imagery and
suggests that the actual reports were based on true hallucinatory ganzfeld imagery.
Our findings indicate that a2 activity is related to the
emergence of hallucinatory imagery in the multimodal ganzfeld. Experimental studies showing a relation between faster a
activity and semantic memory performance (Klimesch, 1999),
suggest an interpretation as a sign of activity of thalamocortical feedback loops involved in retrieval, activation and
embedding of memory content in the ganzfeld imagery. This
interpretation is in agreement with subjects’ reports, according
to which hallucinatory percepts often consisted of partly well
known items/scenes in new and often alien contexts.
In contrast to a2 activity, a1 power showed only a decline
throughout the analysis epochs. This slower a band
Fdesynchronisation_ may be related to a shift of attention
(Klimesch, 1999): participants now had to re-focus their
attention on the active component of the experimental task,
i.e., on their assignment to signal occurrence of hallucinatory
imagery. The increase of b2 and b3 activity during the last time
segment preceding SR (10 – 2 s), showing marked oscillations of
power, is possibly reflecting a shift of cognitive processes from
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P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. Topographic synopses of correlations between EEG and area-transformed experiential data. Correlations were computed for three time intervals: 30 – 21, 20 –
11 and 10 – 2 s preceding report-signal (0 s).
the state as a passive observer to judgement of perceived
imagery and initialisation and preparation of motor response.
The characteristic pattern of alpha activity during the
analysed time segments can be interpreted as electrophysiological signatures of ganzfeld-induced hallucinatory percepts in
statu nascendi. This pattern indicates a functional significance
of faster a activity (10 – 12 Hz) for activation, restructuring, and
transformation of memory content into a hallucinatory percept.
Since the early 1930s, a synchronisation has been seen as
indicator of an inactive Fidle state_ (Adrian and Matthews, 1934;
Pfurtscheller et al., 1996). Recently, alternative hypotheses have
been formulated, based on the idea that alpha synchronisation is
due to active inhibition of either cortical areas related to sensory
information processing when attention is internally directed
(mental imagery), or to inhibition of non-task-relevant cortical
areas, regardless of direction of attention. Both hypotheses are
known as Finhibition hypotheses_ (Klimesch et al., 1999; Ray
and Cole, 1985). There is also now experimental evidence
which challenges the traditional concept of a synchronisation as
an indicator of an inactive cortical Fidle state_. Cooper et al.
(2003) studied the functional role of alpha activity during tasks
requiring either externally or internally directed attention, and
observed greater a amplitudes during internally directed
attention and during increased workload. The authors concluded that the observed a synchronisation indicated active
inhibition of cortical areas processing sensory input or areas
not related to the actual task of the subjects. Hayashi and
Hiroshima (1992) studied the effects of prolonged sensory
P. Pütz et al. / International Journal of Psychophysiology 61 (2006) 167 – 178
deprivation on appearance of hallucinatory imagery. They
noticed that a amplitude in the 7.6 –13.4 Hz band increased
from 2 min before participants signaled the occurrence of
hallucinatory experiences. The authors concluded that sensory
deprivation-induced hallucinations were related to an increase
of a activity.
The observed general a inducing effect of ganzfeld is more
in line with the inhibition hypothesis than with Fcortical idle
state_ assumption. Multimodal ganzfeld induces a mental state
favorable to internally directed attention and non-voluntary
mental imagery and is not a typical Fidle state_. Activity of
cortical areas associated with processing of optical and
acoustical sensory input needs to be suppressed to allow for
internally directed attention. Our data thus seem to be in
agreement with the inhibition theory.
Our result show that electrophysiological correlates of
emergence and formation of hallucinatory imagery in ganzfeld
can be identified. They also demonstrate the fundamental
functional difference between slower and faster a activity
under these conditions.
177
suddenly appeared. . . It looked pretty interesting. . . it had
a. . . well, it was all in black, had a black. . . well, had a long
narrow head, fairly broad shoulders, very long arms and
then again a relatively small trunk. It had legs and very long
feet and it came to me, putting forth its hands, well, both
hands, which were very long, very big, like a bowl and so it
stood for a while and then it went back to where it came
from. . . so slowly out, I could follow that. There was a wall,
a rock, with ornaments, and from the wall a white tube
extended, a white tube, and it was like drawn inside. And
then it disappeared almost as if flying, very gently it was
drawn into the white tube.
Example 4. Experimental phase, subject 2, session 1, report 2.
The only thing I saw again and again was a waterfall at the
mountains, well only a small one. Only this waterfall,
occurring again and again, well, not a waterfall, more a rill
through the mountains—which was slowly running to this
island. In between I saw children in water, but only very
short, it disappeared immediately.
Acknowledgments
We would like to thank Dr. Frauke Schmitz-Gropengieher,
Jakub Späti and Daniel Riewe for technical assistance in the
experiments. Dr. Frauke Schmitz Gropengieher also transcribed the mentation reports.
Appendix A. Examples of ganzfeld-induced imagery
The following are English translations of selected fragments of imagery reports from original German transcripts.
For the sake of easy reading, the text does not use the original
transcription system (Gass et al., 1987). All dialogs with
experimenters and all paraverbal or nonverbal communication
is excluded. Interruptions and omissions are symbolised by. . .
Example 1. Experimental phase, subject 7, session 1, report 1.
I still see its face, well its, its head, it’s strongly expressed. It
jumps exactly toward me, well, it gallops. I have. . . it
jumped across me. . .. Only this horse, that comes like out of
clouds, yes. Its mask clearly visible, and eyes and. . . all in
all, a white horse which jumped across me.
Example 2. Screening phase II, subject 36, report 2.
Yes, again such a picture, but again fairly short. . . fairly
short and fairly minimal acoustic. . . well, it was a children’s
party, a children’s birthday or something in a garden and I
have heard such a children’s laughter very short and very
hushed.
Example 3. Experimental phase, subject 6, session 3, report 4.
Well today, now again in the area of the right eye, in fact
is. . . yes the right field of view, of the right eye a manikin
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