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Special issue: Research report
Ganzfeld-induced hallucinatory experience, its
phenomenology and cerebral electrophysiology
Jirˇı
´Wackermann*, Peter Pu
¨tz and Carsten Allefeld
Department of Empirical and Analytical Psychophysics, Institute for Frontier Areas of Psychology and Mental Health, Freiburg i. Br., Germany
article info
Article history:
Received 30 October 2006
Revised 3 March 2007
Revised 22 May 2007
Accepted 27 May 2007
Published online 5 June 2008
Keywords:
Altered states of consciousness
Electroencephalogram
Ganzfeld
Imagery
Subjective perceptual phenomena
Telepathy
abstract
Ganzfeld, i.e., exposure to an unstructured, uniform stimulation field, elicits in most ob-
servers pseudo-hallucinatory percepts, and may even induce global functional state
changes (‘altered states of consciousness’). The present paper gives a comprehensive over-
view of the phenomenology of subjective experience in the ganzfeld and its electrophysi-
ological correlates. Laboratory techniques for visual or multi-modal ganzfeld induction are
explained. The spectrum of ganzfeld-induced phenomena, ranging from elementary per-
cepts to complex, vivid, dream-like imagery is described, and the latter illustrated by tran-
scripts of subjects’ reports. Similarities and differences to related sensory/perceptual
phenomena are also discussed. Earlier findings on electrophysiological correlates of the
ganzfeld are reviewed. Our own studies of electroencephalographic (EEG) activity in the
ganzfeld are presented in some detail, and a re-analysis of data on EEG correlates of hallu-
cinatory percepts in statu nascendi is reported. The results do not support the hypothesis of
the hypnagogic origin of the percepts; the ganzfeld-induced steady-state is an activated
state, and the spectral EEG dynamics in the alpha frequency range reveals processes of
attention shifts and percept formation. The final section is devoted to the controversial
topic of allegedly anomalous communication between human subjects (‘ganzfeld telepa-
thy’). It is shown that the use of ganzfeld in this research field relies partly on unsupported
hypotheses concerning ganzfeld-induced states, partly on a weak conceptual background
of the experimental procedure. The roˆ le of a particular belief system shared by the partic-
ipants and experimenters is critically discussed.
ª2008 Elsevier Srl. All rights reserved.
1. Introduction
Sensory systems, i.e., structures specialised to inform the
organism of certain physical properties of the environment,
play an essential roˆ le in the behavioural integration of the
organism into its ‘world’ (von Uexku
¨ll, 1926). A necessary
condition for proper functioning of a sensory system is an
adequate stimulus, where ‘adequate’ refers to three important
aspects of the sensory input: (1) its physical nature must corre-
spond to the specialised function of terminal receptors; (2)
intensity has to fit the dynamical range of the receptors; and,
in case of sensory systems mediating the ‘epicritic’ sensitivity
(Head, 1920), (3) variation range and structure of the stimulus has
to meet the feature-extracting and representation-building
*Corresponding author.
E-mail address: jw@igpp.de (J. Wackermann).
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cortex
0010-9452/$ – see front matter ª2008 Elsevier Srl. All rights reserved.
doi:10.1016/j.cortex.2007.05.003
cortex 44 (2008) 1364–1378
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capability of the central neural component of the respective
system. The importance of the structural aspect of perception
has been revealed by discoveries of visual contrast enhancing
mechanisms, e.g., lateral interactions operating on the retinal
(von Be
´ke
´sy, 1967; Ratliff, 1974) or cortical level (Stemmler
et al., 1995). Briefly, the stimulusdor, in case of spatially
distributed analysers such as the visual system, the stimulation
fielddmust fall within a relatively wide but necessarily limited
range in terms of its energetic and informational characteris-
tics. Perception is the perceiving of a structure; e.g., vision is
perception of the structured ‘ambient optical array’ (Gibson,
1979).
Of interest here are phenomena occurring when one or
several sensory systems are exposed to adequate physical
stimulation of inadequate structure ordas a limiting
casedtotally unstructured. Such situations are rather rare in
our natural physical environments: for example, viewing
a uniformly blue sky on a clear summer day, or vision obfus-
cated by a dense fog cloud illuminated by the sun from
outside. Under such circumstances, perceivable patterns or
even complex and structured illusory percepts may appear
in the homogeneous visual field.
An earlydperhaps the first?dscientific account on vision
of an unstructured bright field can be found in Purkyneˇ’s
(1819) pioneering study of subjective visual phenomena. Pur-
kyneˇ described light and dark spots occurring spontaneously
‘‘[w]hen gazing at a large field of almost blending luminance,
e.g., at the sky uniformly covered by clouds, or watching a can-
dle flame from a short distance’’ (Purkinje, 1819, pp. 67–68),
and also similar phenomena appearing in a completely dark
visual field. He compared their appearance with meteors,
and speculated on their possibly electrical nature, by analogy
with luminous phenomena of atmospheric electricity. Impor-
tantly, these subjective phenomena must not be confounded
with images of fine anatomical structures (e.g., the ‘blood
vessel figure’ also described by Purkyneˇ ) or microscopic bodies
floating in the intra-ocular media.
A century later, perceptual phenomena occurring in homo-
geneous visual fields became the subject of a more systematic
research, mainly due to the focus of gestalt psychology on the
principles of perceptual organisation (Wertheimer, 1938). In
opposition to the stimulus–response paradigmdborrowed
from 19th century’s sensory physiology and adopted by early
psychologydgestalt psychology emphasised the active roˆle
of the perceiving subject in the genesis of a structured percept;
hence the interest in formation of subjective percepts in
absence of an objective structure imposed by the physical
sensory input. The term ganzfeld, derived from German
ganz ¼‘whole, entire’ and Feld ¼‘field, area’, was coined as
a generic term for the unstructured visual field (Metzger,
1930). (Note that the noun Ganzfeld does not make much sense
unless the quality fulfilling the visual field is specified; e.g.,
farbiges Ganzfeld ¼‘visual field entirely filled with colour’.)
Further research focused mainly on the conditions of fig-
ure-ground differentiation in the ganzfeld and colour percep-
tion (see Avant, 1965, for a review, cf. also Tsuji et al., 2004).
From the middle of the 20th century on, the ganzfeld is used
in diverse research contexts. In addition to the studies of
perceptual organisation, the ganzfeld is used as a technique
to manipulate the subjects’ global mental state; specifically,
to induce an artifical ‘hypnagogic’ state, similar to states
occurring spontaneously at sleep onset (Witkin and Lewis,
1963). Following this turn towards so-called ‘altered states of
consciousness’ (ASC), the ganzfeld has been repeatedly
applied in experimental parapsychology to induce a state
presumably facilitating ‘telepathic communication’ (Braud
et al., 1975; Parker, 1975;Honorton et al., 1990). Due to this
diversity of contexts and purposesdranging from vision
research to the far frontiers of psychologydthe term ‘ganz-
feld’ has lost its topically precise meaning and acquired, unde-
servedly, a somewhat mystical flavour.
The aim of the present paper is to provide a comprehensive
overview of the phenomenology of subjective experience in
the ganzfeld, as well as of its objectively measurable electro-
physiological correlates, based mostly on our experimental
studies of ganzfeld-induced phenomena. According to the
focus of this special issue, particular attention is given to the
roˆ le played by the ganzfeld in studies of ‘telepathic communi-
cation’ (or ‘anomalous information transfer’: Bem and
Honorton, 1994), and to a critical discussion of the underlying
concepts and results of that research.
2. Experimental techniques for ganzfeld
induction
The term ganzfeld originally denoted a homogeneous visual
field. By analogy, unstructured or de-structured stimulation
can be applied to other sensory systems, e.g., auditory or
tactile. Studies aiming at induction of ASC have been using
‘multi-modal ganzfeld’ (MMGF), i.e., simultaneous exposure
to unstructured visual and auditory input.
A number of techniques have been developed to create the
visual ganzfeld. The simplest method is to let the subject gaze
at a uniformly illuminated surface, e.g., a large sheet of paper
(Goldstein and Rosenthal, 1930) or a perfectly smooth wall
(Metzger, 1930)(Fig. 1a). To ascertain the homogeneity of the
visual field under minor changes of the subject’s direction of
view (eye movements, postural changes), smoothly curved
wings can be used. In another variant, the subject watches
the interior of a spherically shaped, uniformly illuminated
cavity (Fig. 1b) (Gibson and Dibble, 1952).
Generally, ganzfeld stimulation rooms with planar or
spherical surfaces are rather space-demanding options, and
require much technical sophistication to achieve a perfectly
homogeneous visual field. An inexpensive and convenient
alternative is to use light diffusors mounted on the subject’s
head, e.g., special goggles or semi-translucent eye-shields,
and illuminated by a light source from outside (Gibson and
Waddell, 1952)(Fig. 1c). A simple and wide-spread technique,
introduced by Hochberg et al. (1951) and routinely employed
also in our laboratory, makes use of anatomically shaped
halves of a ping-pong ball applied directly on the subject’s
eye orbits (Fig. 2), while the subjects’ eyes remain open. This
experimental setup yields a smooth, almost perfectly
homogeneous visual field, and avoids disturbing perception
of contours of the visual field (nose, cheekbones) present
under the natural conditions.
In our earlier experimental studies (Wackermann et al.,
2002; Pu
¨tz et al., 2006) a red-coloured incandescent 60-W
cortex 44 (2008) 1364–1378 1365
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lamp, placed at a distance w120 cm from the eye-shields, was
used as the light source; in recent studies, where a precise
control of the ganzfeld colour is important, a computer-
driven, xenon lamp based D-ILA projector has been used
(Pu
¨tz and Wackermann, 2007). The choice of red colour
reportedly (Cohen, 1958) facilitates the observers’ ‘immersion’
in the ganzfeld.
A wide range of acoustic stimuli has been used to homog-
enise sensory input in the auditory modality. The notion of
perfectly unstructured sensory field is ideally met by using
a broad-band, flat-spectrum noise (‘white noise’), but a longer
exposure to pure white noise may be annoying or irritating for
many subjects; spectrally shaped ‘pink noise’ is a suitable al-
ternative. Arguably equivalent results may be obtained using
monotonous natural noises or sounds. In our laboratory
a CD record of the sound of a waterfall, played-back to the sub-
jects via headphones, is routinely used as the auditory compo-
nent of the MMGF stimulation. A comfortable sound intensity
is adjusted individually to the subject’s preference prior to the
experiment, and kept constant during the session.
3. Subjective experience in the ganzfeld
Three principal methods to access the subjects’ experience in
psychophysiological research are (a) ‘post hoc’reports, based on
the subject’s retrospective memory recall; (b) ‘on demand’
reports initiated by the experimenter; (c) ‘self-initiated’reports
by the subject her/himself. Retrospective reporting is a method
of choice in exploratory studies, suitable for subjects trained
in introspection (often experimenters themselves, such as
Purkyneˇ , quoted in Section 1). Its use is limited or questionable
in case of mid-term to long-term alterations of the state of
consciousness, including ganzfeld-induced states. The latter
two methods are preferable in studies where the stream of
subjective experience has to be correlated with objective
physiological measurements. The ‘on demand’ method is
easy to use and well suited to collect random samples of the
subjects’ momentary ‘mentation’ (e.g., Lehmann et al., 1995).
Where a circumscribed class of phenomena is of interest,
the method of ‘self-initiated’ reports is more economical, but
requires prior training of the subjects. Observations in this
section are mostly based on the ‘on demand’ or ‘self-initiated’
reports from our studies.
3.1. Phenomenological characteristics
Elementary changes of sensory qualities are usually observed
already after a relatively short exposure to the visual or MMGF
(a few minutes). The visual field’s luminance diminishes and
the field shows diffuse inhomogeneities, often described as
a ‘cloudy fog’. In case of a colour ganzfeld, the field’s colour
gradually bleaches, up to the point of a loss of the sensation
of colour: the field is of indefinite grey, sometimes with an un-
dertone of the complementary colour, e.g., greyish-green if
red light is used. In addition, more distinct structures may ap-
pear against the diffuse ‘foggy’ background: dots, zig-zag
lines, or more complex patterns. Generally, these elementary
perceptual phenomena can be accounted for by adaptive
retinal processes: saturation of the receptive elements and
their mutually inhibitory interactions (Helson and Judd,
1932; Hochberg et al., 1951).
After a prolonged exposure (a few minutes up to tens of
minutes) to the ganzfeld, some subjects report complex
percepts, apparently unrelated to the above-described sen-
sory phenomena and thus of presumably central nervous
origin. Clarity and distinctness of these percepts vary inter-
as well as intra-individually; they may achieve a hallucinatory
quality, that is, vividness comparable to that of dreams or hyp-
nagogic percepts (see below). [More precisely, we should say
‘pseudo-hallucinatory’ (Jaspers, 1953), since the subjects’
awareness of the experimental situation and unreality of the
perceived remains intact.] The occurrence of hallucinatory
percepts justifies the classification of the ganzfeld-induced
state as an ASC (Vaitl et al., 2005).
In most cases, visual phenomena largely prevail in subjects’
reports, so that the term ‘ganzfeld imagery’ is often used as
a synonym for the totality of the ganzfeld-induced subjective
experience. However, almost any sensory modality may be
involved (see Table 1). The next most frequently reported
sensory modality is auditory, occurring either alone or
accompanying visual percepts. The acoustic hallucinations
a
S
LL
r
b
S
L
LL
t
c
S
L
t
Fig. 1 – Spatial arrangements used to create a uniform visual field. S[subject, L[light source, r[reflecting surface,
t[translucent material. (a) Planar stimulation field. (b) Spherical stimulation field. (c) Head-mounted light diffusors.
cortex 44 (2008) 1364–13781366
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may range from relatively simple (e.g., ringing bells, bursts of
laugh) to complex percepts (speaking voices, musical melo-
dies). Tactile or kinesthetic sensations, or remarkable subjec-
tive changes of the body scheme may also occur. Olfactory
and gustatory sensations are only rarely reported. As it is
difficult to control stimuli to these senses under normal labo-
ratory conditions, the differentiation of truly hallucinatory
sensations from technical artefacts is often uncertain (cf.
Wackermann et al., 2002, p. 134), unless the reported sensa-
tions are of evidently bizzare or impossible quality.
Ganzfeld imagery is not experienced continuously but
rather in transient episodes, sometimes developing in time to
reach full clarity, often occurring abruptly. In the screening
part of a study by Pu
¨tz et al. (2006), 40 subjects were trained
in self-initiated imagery reporting. Seven selected ‘high-
responders’ were then exposed to MMGF for 45 min; the
numbers of imagery episodes reported per session were in
the range 0–9, thus leaving (on the average) a time period of
w5 min for the ‘preparatory’ phases between the episodes.
Subjective estimates of the duration of the hallucinatory
episodes were in the range from 3 sec to 7 min.
There seem to be inter-individual differences in the
subjects’ ‘responsiveness’ to the MMGF: distributions of report
frequencies per session of fixed duration are usually U-
shaped, with a local minimum separating ‘high-responders’
from the majority of average/low-responders (Pu
¨tz et al.,
2006, 2007). Little is known by now about the psychological
conditions or personality correlates of the responsiveness to
ganzfeld, and virtually nothing is known about its neurophys-
iological substrate. Unpublished data from the study by Pu
¨tz
et al. (2006) shows that the personality profiles of the seven
selected high-responders, assessed by the NEO Five Factor
Inventory (Costa and McCrae, 1992) differed from the rest of
the pool only in personality factor ‘Conscientiousness’ (C). A
negative correlation between individual Cscores and the
numbers of reports per session, (r¼.31, pz.05) suggests
that a laisser passer attitude is favourable for the production
of ganzfeld imagery. Interestingly, all seven high-responders
were women, while the pool of unselected subjects consisted
of 28 women versus 12 men.
Another phenomenon occasionally reported from the
ganzfeld are episodes of ‘‘complete disappearance of the
sense of vision for short periods of time’’, also called ‘blank-
outs’ (Cohen, 1960), occurring after prolonged exposure (10–
20 min) to the ganzfeld. Subjects also report that during these
periods they were uncertain whether their eyes were open or
closed, or even unable to control their eye movements. In the
‘luminous fog’ of the ganzfeld the subjects do not see any-
thing; in the ‘blank-out’ periods, they may experience pres-
ence of ‘nothingness’ (Gibson, 1979).
Ganzfeld-like conditions may be met in exceptional natural
environments, e.g., during high-altitude flights, exploratory
journeys in mountain or desert landscapes, or practice of ex-
treme sports. Individuals prone to hallucinatory experience in
the ganzfeld may under such ‘favourable’ circumstances per-
ceive unreal things and beings (cf. Brugger et al., 1999; Arzy
et al., 2005). While in experimental situations the subjects are
aware of the illusory character of their percepts, conditions
for apprehension of percepts spontaneously occurring in the
Fig. 2 – Construction of a head-set of light diffusors for
ganzfeld experiments. (a) Ping-pong ball cut into two
halves along anatomically shaped contours. (b) Subject
exposed to multi-modal ganzfeld. (Photo A. Fischer).
Table 1 – Description of three studies of ganzfeld-induced
imagery and occurrence of different sensory modalities
in the subjects’ reports
Study I II III
Nof subjects 12 40 7
Sessions/subject 2 1 3
Session duration 30 min 30 min 45 min
Reporting method On demand Self-initiated Self-initiated
Sensory modalities reported (percent)
a
Visual 90 94 98
Auditory 29 16 23
Tactile 26 10 9
Kinaesthetic d52
Olfactory 16 3 4
Wackermann et al. (2002) and Pu
¨tz et al. (2006).
a Column sums exceed 100% because of percepts involving more
than one sensory modality.
cortex 44 (2008) 1364–1378 1367
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‘natural ganzfeld’ may be different. Factors such as social isola-
tion, impossibility of a direct (physical) reality check, exhaus-
tive states or high arousal, etc. may contribute to deficient
cognitive processing of the percept, thus resulting in claims
of real experience of the unreal, or even the ‘super-natural’.
Depending on the subject’s disposition and pathoplastic social
or cultural factors, experience of ‘blank-out’ episodes may elicit
even mystical or religious interpretations.
3.2. Examples of ganzfeld-induced imagery
Typical characteristics of visual imagery in the ganzfeld are
sudden appearance and disappearance; mostly static charac-
ter, but sometimes also abrupt, dynamical changes and occur-
rence of new elements; clarity and distinctness equaling or
exceeding that of most vivid dreams. Yet, the subjects’ aware-
ness of the percepts being not ‘objectively real’ is preserved.
One of the authors reported on his very first experience
with the ganzfeld:
‘‘For quite a long time, there was nothing except a green-
greyish fog. It was really boring, I thought, ‘ah, what
a non-sense experiment!’ Then, for an indefinite period
of time, I was ‘off’, like completely absent-minded. Then,
all of sudden, I saw a hand holding a piece of chalk and
writing on a black-board something like a mathematical
formula. The vision was very clear, but it stayed only for
few seconds and disappeared again. The image did not
fill up the entire visual field, it was just like a ‘window’
into that foggy stuff.’’
Shortly later, the subject had a vision of
‘‘an urban scenery, like an empty avenue after a rain, large
areas covered with water, and the city sky-line reflected in
the water surface like in a mirror.’’
Still during the same session, the subject saw an image of
‘‘a clearing in a forest [Lichtung], a place bathed in bright
sun-shine, and the trunks of trees around. A feeling of
a tranquile summer afternoon in a forest, so quiet, so
peaceful. And then, suddenly, a young woman passed by
on a bicycle, very fast, she crossed the visual field from
the right to the left, with her blond long hair waving in
the air. The image of the entire scene was very clear,
with many details, and yes, the colours were very vivid.’’
These three reports demonstrate the most salient charac-
teristics of ganzfeld-induced imagery, as listed above. Unlike
many dreams, the examples show coherence of content and
proximity to real-world situations, or could even be recollec-
tions of sensory percepts from the past (the hand writing on
a black-board). Similarly to dreams, minor deviations from
the material logic (how could the bicyclist ride at such
a breath-taking speed in a dense forest?) are accepted without
notice. Interestingly, the very first hallucinatory percept of the
three reported above emerged after a ‘blank-out’ period.
While the introductory examples describe purely visual
imagery, percepts combining more sensory modalities are
no exception, as shown in the following. The texts are
excerpts from subjects’ reports collected in three large exper-
imental studies carried out in our laboratory. The transcripts
were translated from German to English, abridged and edited
for the sake of legibility.
Some percepts are very impressive for their visual clarity
and dynamism. A 54-year-old woman had a vision of a horse,
as if seen face-to-face:
‘‘I can see his face, still, it’s very expressive.[I could see]
only the horse that comes as if out of clouds. A white horse
that jumped over me.’’
As mentioned above (and documented by Table 1), visual
and auditory sensations may combine to a coherent, mean-
ingful percept, such as in the following report (woman, 36
years):
‘‘A friend of mine and I, we were inside a cave. We made
a fire. There was a creek flowing under our feet, and we
were on a stone. She had fallen into the creek, and she
had to wait to have her things dried. Then she said to
me: ‘Hey, move on, we should go now’.’’
In the following example, given by a 37-year-old woman,
the visual and auditory components are accompanied by a kin-
aesthetic sensation:
‘‘It was like running a bob sleigh on an uneven runway
right down.[There] was snow or maybe water running
down.I could hear music, there was music coming from
the left side below.’’
The contents of the hallucinatory percepts are often famil-
iar to the subjects, taken from their past experience but
combined in an unusual way or framed into a novel context
(Pu
¨tz et al., 2006). However, the reported percepts may attain
a really bizzare quality, as in the following fragment (woman,
37 years):
‘‘In the right side of the visual field, a manikin suddenly
appeared. He was all in black, had a long narrow head,
fairly broad shoulders, very long arms and a relatively
small trunk.. He approached me, stretching out his
hands, very long, very big, like a bowl, and he stayed so
for a while, and then he went back to where he came
from, slowly.’’
Not only the appearance of the alien, but also his way of
leaving the stage was spectacular:
‘‘There was a rock wall [decorated] with ornaments, and
a white tube stretched out of the wall, and it was as if he
was sucked into the tube interior. He disappeared as if
flying, he was sucked smoothly into the tube.’’
Interestingly, a water element appeared very often in the
visual imagery, e.g., lakes, creeks, streaming fluids, which is
possibly related to the nature of the acoustic MMGF compo-
nent, viz. the monotonous sound of a waterfall (see Section 2).
cortex 44 (2008) 1364–13781368
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It is known from experimental studies of dreams that a simple
sensory stimulus may be transformed and meaningfully inte-
grated in the dream plot (de Becker, 1968). Thus, similar mech-
anisms may be at work in production of the oneiric and
ganzfeld imagery. Acoustic percepts co-occurring with the
visual imagery may have the form of articulated speech or
music. Purely acoustic percepts are often simpler, e.g.,
elementary sensations of environmental sounds, unclear
voices, laughter, and the like.
Finally, we should mention rather rare reports of non-
visual, purely kinaesthetic or proprioceptive experience in
the ganzfeld-induced state. Some subjects reported changes
in their body scheme, such as extensions of their extremities,
changes of perceived body weight, or even a feeling of ‘levita-
tion’; for example, a 44-year-old woman described a feeling
of her body moving continuously upwards, an impression
of ‘ascension’. Such experiential modes do not fit the
category of ‘imagery’, but they seem to have been really
perceived, not ‘just imagined’, and their character clearly
indicates an ASC.
4. Related sensory/perceptual phenomena
The following is an overview of experimental or natural
situations in which percepts similar to those observed in the
ganzfeld may occur. The aim of this section is to provide
a larger context into which the phenomenology of ganzfeld
can be embedded.
4.1. Flickering ganzfeld
Purkyneˇ described geometrical patterns and colours occurring
in a flickering visual field (Purkinje, 1819). The emergence of
colours may be related to ‘subjective colours’ observed by
Fechner (1838) on rotating disks with black/white sectors.
These ‘stroboscopic patterns’ were later studied by Smythies
(1959) who provided phenomenological classification of
perceived forms: straight lines, honeycomb patterns, complex
mosaics. Herrmann (2001) in an electroencephalographic
(EEG) study of the visual cortex’s response to a flickering visual
field observed the appearance of subjective colours and forms.
Herrmann and Elliott (2001) described the variety of these
perceptual phenomena as a function of flicker frequency (1–
40 Hz). Recently, Becker and Elliott (2006) reported co-
occurrences of forms and colours in a flickering ganzfeld being
dependent on flicker frequency, and phase relationship
between the subject’s response and the flicker period.
Analysis of these phenomena may provide a deeper insight
into spatio-temporal dynamics of the retinal and/or cortical
processes (Spekreijse et al., 1971; Kelly et al., 1976; Billock
and Tsou, 2007) that may be related to the formation of visual
structures in the static ganzfeld. In contrast to the latter, the
flickering ganzfeld seems to induce a complex, spatio-
temporal resonance response. Also, it is well-known that the
brain’s response to periodic photostimulation (PPS) may be
not limited to visual sub-systems but may propagate further
in the brain (e.g., photosensitive epilepsy). The global dynam-
ics of the brain’s response to PPS is thus a subject of study of
its own (Wackermann, 2006).
4.2. Dark field vision
Luminous phenomena (phosphenes) appearing in a com-
pletely dark visual field were also described by Purkyneˇ (cf.
Section 1). A few years later, Mu
¨ller (1826) described complex
‘visual phantasms’ appearing in the dark visual field
(Augenschwarz), in a relaxed state with eyes closed. Elemen-
tary luminous phenomena are considered a manifestation of
spontaneous activity of the visual system (cf. Hurvich and
Jameson, 1966); complex visual phenomena sensu Mu
¨ller are
more likely of hypnagogic origin (see below).
4.3. Immobilised retinal images
The image created by the eye’s optical system can be fixed on
the retina by special techniques (Heckenmueller, 1965). The
structure of the visual field thus remains preserved but the
scanning motion due to eye movements is inhibited. Under
these conditions, partial or total ‘fade-outs’ of the visual field
may occur (Yarbus, 1967), indicating that a regular refreshing
is necessary for maintaining the visual structure. We may
hypothesise a relationship between these ‘fade-outs’ and the
‘blank-out’ periods in ganzfeld, where eye movements are
reportedly reduced.
4.4. Sensory deprivation
Ganzfeld is sometimes incorrectly denoted as sensory depri-
vation. In sensory deprivation, the physical intensity of ex-
ternal stimuli is minimised, ideally in most sensory
modalities (Zubek, 1969). In the ganzfeld, the sensory field
is unstructured but the physical intensity of sensory input
is kept at the average or even above-average level; a proper
term for the ganzfeld would thus be perceptual deprivation.
But the opposition sensory versus perceptual deprivation is
merely schematic: mixed experimental designs are possible,
in which the visual field is only partially blurred (Kubzansky
and Leiderman, 1965). Hallucinatory percepts under
prolonged sensory deprivation show features similar to ele-
mentary percepts in the luminous ganzfeld or in the dark
field, e.g., transient sensations of light flashes or colours
(Zuckerman et al., 1969), and, like ganzfeld-induced percepts,
they may also develop to ‘‘full-blown scenes’’. Heron (1965)
observed that with the use of opaque goggles hallucinatory
percepts were initially intensified then abolished, but with
translucent goggles (i.e., ganzfeld condition) the hallucina-
tions re-appeared.
4.5. Hypnagogic imagery
Hypnagogic states are episodes of dream-like hallucinatory
experience, occurring at sleep onset (Mavromatis, 1987),
described first by Mu
¨ller (1826), later given name by Maury
(1848). In contrast to ‘true’ night dreams, which are mostly
of a narrative character and develop continuously, hypnago-
gic hallucinations are usually rather static and occur abruptly.
It has been hypothesised that hypnagogic states may be
a source of accounts of nocturnal ‘paranormal’ experiences
(Cheyne et al., 1999).
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Ganzfeld-induced imagery shows a remarkable similarity
to hypnagogic hallucinations. This similarity plus reported
global changes of consciousness in the ganzfeld (reduced vig-
ilance, ‘blank-outs’) were the basis for the hypothesis of the
hypnagogic origin of ganzfeld imagery (‘hypnagogic-like’
states: Schacter, 1976). The hypnagogic hypothesis attained
a fact-like status for decades, and only recently was submitted
to serious experimental scrutiny (see Section 5).
5. Brain electrical correlates of
ganzfeld-induced phenomena
Analysis of the brain’s spontaneous electrical activity (electro-
encephalogram, EEG: Berger, 1969; Niedermayer and Lopes da
Silva, 1993) is an important method in studies of states of
consciousness, perceptual and cognitive processes, etc., for
several reasons: (i) the brain’s electrical field, reflecting the
summary post-synaptic activation of neuronal populations,
is a ‘direct expression’ of the brain’s functions; (ii) there is
a large empirical database on correlations between brain
functional states (states of consciousness, sleep stages, etc.)
and EEG characteristics; (iii) spectral analysis of EEG signals
into different frequency components allows a differentiated
functional interpretation; and (iv) the method permits to
study the brain’s functioning on a sub-second scale, or to trace
the brain’s state changes by means of aggregated data on
a seconds scale. In spite of the contemporary trend towards
‘brain imaging’ via functional magnetic resonance, EEG is still
the method of choice, providing superior temporal yet moder-
ate spatial resolution.
However, EEG studies of ganzfeld-induced states have been
relatively rare. Reported effects where mostly related to alpha
activity, which is a term for a regular rhythmical activity at
frequency w8–12 Hz, occurring usually in a no-task no-
stimulation relaxed state, e.g., with eyes closed (Berger,
1969). Those early studies usually referred to the alpha
rhythm as a unitary phenomenon. However, later studies
revealed functional differences between sub-bands within
the alpha frequency range: low-frequency alpha, reflecting
rather attentional processes, and high-frequency alpha
reflecting cognitive processes (Klimesch, 1997, 1999; cf. also
Shaw, 2003). This functional differentiation may be also
relevant for the interpretation of EEG-based findings on the
ganzfeld.
Cohen and Cadwallader (1958) reported correlation be-
tween higher alpha activity in the resting EEG and individual
susceptibility to ‘blank-outs’. Cohen (1960) interpreted occur-
rence of alpha activity during the ‘blank-outs’ as alpha
rebounddthis is a well-known phenomenon where, after
a transitory suppression e.g., due to an external stimulus,
eyes opening, etc., alpha activity attains the original level, or
even increases. Tepas (1962) found an increase of alpha ampli-
tude during the ‘blank-outs’, which was intermediate to ‘eyes
closed’ and ‘eyes open’ conditions, but could not confirm the
hypothesised relation between high alpha activity and
blank-out susceptibility. These findings are in line with early
observations by Adrian and Matthews (1934), who had previ-
ously reported alpha rebound after eyes opening in a uniform
visual field. Later, Lehtonen and Lehtinen (1972) also reported
re-occurrence of alpha activity in the ganzfeld, comparable to
the ‘eyes closed’ condition. Increase of alpha activity was also
observed during the ‘fade-out’ periods in perception of stabi-
lised retinal images (Lehmann et al., 1967); this supports the
relation to ganzfeld ‘blank-outs’ hypothesised above.
As shown in the preceding sections, the variety of ganz-
feld-induced phenomena is fairly rich and suggests relations
to several different classes of perceptual phenomena and/or
states of consciousness. Objective characterisation of the
brain’s functional states under ganzfeld stimulation by means
of EEG measures may help to elucidate these relations. This
was the objective of our two major ganzfeld studies, results
of which are summarised below.
5.1. EEG spectral signatures of the ganzfeld-induced
steady-state
A study by Wackermann et al. (2002) aimed at a comparison of
the ganzfeld-induced state with the hypnagogic state at sleep
onset (i.e., transition waking–sleep stage 1–sleep stage 2). EEG
data recorded in different states of consciousness were com-
pared: day-time relaxed waking, ganzfeld exposure, waking
before sleep onset, and sleep stages 1 and 2. Subject’s eyes
were closed in all conditions except ganzfeld exposure. [Ad-
mittedly, this fact imports an inhomogeneity in the experi-
mental conditions. There is recent experimental evidence
that eyes-open and eyes-closed conditions are not equivalent
even in the absence of any visual input (Marx et al., 2003). This
topic deserves more attention in studies on visual imagery.]
As expected, sleep states on the one hand and waking
states on the other hand showed clearly different spectral
profiles (Fig. 3). The spectrum of the ganzfeld EEG is very close
to that of the relaxed waking state EEG, and it is clearly differ-
ent from that of sleep onset, where the alpha peak is absent.
Evidently, the ganzfeld-induced brain functional state differs
from the brain state at sleep onset; therefore, it is unlikely
that ganzfeld-induced hallucinations were of hypnagogic
nature. Within the waking states, the ganzfeld and ‘normal’
waking states were best distinguished by the band power ratio
a
2
/a
1
(frequency ranges 10–12 Hz and 8–10 Hz, respectively),
which was increased in the ganzfeld EEG, indicating an accel-
eration of the alpha activity. Visual inspection of the spectra
reveals a power drop along the lower flank of the alpha peak
in the ganzfeld EEG, leading to an increase of the peak
frequency (Fig. 3).
5.2. EEG spectral signatures of ganzfeld-induced
imagery
A study by Pu
¨tz et al. (2006) specifically addressed correlates of
ganzfeld-induced imagery, using the method of self-initiated
reports. EEG was recorded in 19 channels during continuous
ganzfeld stimulation, and subjects were asked to signal hallu-
cinatory episodes by a button press, followed by a verbal
report. Spectral characteristics of EEG recorded during ganz-
feld imagery (GFI), i.e., epochs up to 30 sec before a subject’s
report, were compared to EEG during the ganzfeld baseline
(GFB) condition, i.e., epochs more than 120 sec before a report.
Using the a
2
/a
1
band power ratio as the variable of interest, the
authors found a tri-phasic pattern of spectral changes related
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to ganzfeld imagery, with the maximum alpha acceleration in
the time segment 20–10 sec before the report. A time–fre-
quency analysis of the data gave additional evidence for alpha
acceleration (Wackermann et al., 2003). A frequency domain
principal component analysis (PCA) revealed a component
reflecting the shift from the lower to the higher alpha band ac-
counting for w6% of the variance in single-epoch spectra of
imagery-related EEG (relative to the baseline condition
(Fig. 4, curve #3)). Original, unrotated PCA solution is shown.
Interestingly, earlier factor-analytical studies of EEG spectra
by Ro
¨sler (1975) and Mecklinger and Bo
¨sel (1989) revealed
two independent components within the alpha band.
5.3. Time–frequency dynamic of imagery-related EEG
To further elucidate the EEG spectral changes corresponding
to ganzfeld imagery, for the purposes of the present paper
the same data set was re-analysed, investigating in detail
the time-course of changes preceding subjects’ reports. The
data consisted of three recording sessions for each of seven
subjects. The analysis was performed on artefact-free 2-sec
epochs (512 samples) with 1-sec overlap, collected into two
conditions, GFB and GFI, defined by the time relative to the
subject’s next report (see above). EEG spectra were computed
via Fourier transform, using a Hamming taper. Baseline
0 5 10 15 20 25 30 [Hz]
frequenc
y
spectral density (normalised)
.00
.01
.02
.03
.04
.05
Channel Pz / Segment T–2
wm
gf
sw
s1
s2
Fig. 3 – Fourier spectra of 2-sec EEG epochs recorded from the parietal region prior to ‘on demand’ reports of subjective
experience in five different states: wm [relaxed waking state; gf [multi-modal ganzfeld; sw [waking state before sleep
onset; s1, s2 [sleep stages 1 and 2. Source: Wackermann et al. (2002, p. 134); reprinted with permission from Elsevier
Science.
0 10 20 30 [Hz]
frequenc
y
−.3
−.2
−.1
0
+.1
+.2
+.3
p.c. coef
1
2
3
4
1 55.9%
2 15.6%
36.1%
44.8%
Fig. 4 – Principal components of imagery versus baseline differences between log-transformed normalised EEG spectra; data
merged from 19 scalp locations according to the 10/20 system. Plotted are coefficients of four eigenvectors associated with
greatest eigenvalues, i.e., contributions to the total variance (see the insertion), as functions of frequency.
cortex 44 (2008) 1364–1378 1371
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spectra revealed very similar individual alpha peak frequen-
cies (range 10.5–11 Hz) for six of seven subjects; one subject
showing a deviant alpha peak frequency (8 Hz) was excluded
from further analyses. Statistical evaluation of the between-
conditions difference over sessions showed a significant de-
crease in power for GFI versus GFB mainly in the lower alpha
range (w7–10 Hz). For a detailed examination, single session
data were selected into two smaller data sets, (a) and (b); the
selection was based on the similarity of their spectral profiles
with the overall average according to visual inspection. The
typical spectral forms are shown in Fig. 5 (scalp location P
z
).
In both cases, in the imagery-related EEG the power in the
lower alpha range (w7–10 Hz) is reduced, leading to a shift of
the alpha peak to a higher frequency; in addition, EEG spec-
trum (a) shows a local power increase at the upper bound of
the alpha range, w13 Hz.
For each data set, the average spectra of EEG epochs at the
same time coordinate (i.e., time remaining to the next report)
were computed, resulting in a spectrogram representation
(Fig. 6, upper panels). Local maxima of the spectra were deter-
mined, and the estimates of the peak frequencies were refined
using a three-point quadratic interpolation (Fig. 6, lower
panels).
In data set (a), along with a relatively stable primary local
maximum at w11 Hz (the alpha peak), frequent secondary
and tertiary maxima in the range 12–14 Hz are observed.
The time period immediately preceding the subjects’ reports
(w20 sec) is characterised by a power decrease in the range
below 10 Hz as well as a complementary increase in the
range of secondary local maxima at higher frequencies, but
not a peak shift. The redistribution of local maxima accounts
for the secondary local maximum visible in the GFI average
spectrum in Fig. 5a. In data set (b), alpha peak frequencies
are scattered over a broader range. The time period before
the subjects’ reports is characterised by an overall power de-
crease and by a general shift towards higher frequencies.
This process starts at about 60 sec before the subject’s re-
port, possibly followed by an additional shift in the last
10 sec. In summary, the generally observed alpha accelera-
tion effect can be individually realised in at least two differ-
ent forms: (a) as a slight dislocation of the barycenter of the
spectral distribution which preserves the location of peaks,
or (b) as an increase of the local alpha peak frequency,
a real shift.
5.4. Correlations with experiential data
Subjects’ reports were followed by a short structured enquiry,
in which the participants rated qualitative properties of the
reported percept on several ordinal scales (for details see
Pu
¨tz et al., 2006, p. 169). These experiential data were corre-
lated with GFI–EEG spectra and revealed various forms of ‘cor-
relation profiles’ over the analysed 30 sec time window. The
most stable correlation over this time window was a global
(i.e., involving all 19 channels) negative correlation between
a
2
power, measured relative to individual GFB baselines, and
subject-reported vividness of imagery.
The relation between fast a
2
activity and imagery forma-
tion was interpreted by Pu
¨tz et al. (2006) as an indicator of
activation of thalamo-cortical feedback loops involved in re-
trieval, activation and embedding of memory content in the
ganzfeld-induced imagery. The observed a
1
attenuation dur-
ing the analysis epoch may reflect a shift of attention towards
the visual percept and, later, preparation of the required mo-
tor action (button press signalling occurrence of imagery). The
unspecific alpha-inducing effect of the ganzfeld-induced
steady-state (no imagery) is in line with the inhibition hypoth-
esis (i.e., alpha synchronisation due to inhibition of cortical
areas related to external sensory information processing),
and with earlier findings of other authors mentioned above.
5.5. Global properties of brain functional
states under ganzfeld stimulation
Global descriptors (Wackermann, 1999; Wackermann and
Allefeld, 2007) of the 19-channel EEG data from the study by
Pu
¨tz et al. (2006) were evaluated for the conditions GFB and
GFI (baseline versus imagery, see above), and for the resting
state with eyes closed. No significant difference in global field
strength (S) was found; global generalised frequency (F) was
increased in both ganzfeld conditions, reflecting the above-
described alpha acceleration. Global spatial complexity (U)
was higher in both ganzfeld conditions than in the idling state,
and inter-hemispheric complexity deficit (Wackermann, 2003)
5 10 15 20
9
10
11
12
13
14
f / Hz
5 10 15 20
9
10
11
12
13
14
f / Hz
ab
Fig. 5 – Fourier spectra of two selected subsets of EEG data recorded from P
z
in the baseline condition (GFB, solid line) and
within 30 sec before imagery report (GFI, dashed line). (a) Subject 1, session 2. (b) Subject 6, sessions 1–3. The vertical axis
shows the logarithmic variance contributions from frequency bins of .5-Hz width.
cortex 44 (2008) 1364–13781372
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was reduced. These findings indicate higher diversity of brain
activation processes and reduced inter-hemispheric coopera-
tion as conditions that are probably favourable for emergence
of internally generated imagery (Pu
¨tz and Wackermann, 2004).
5.6. Intracerebral sources of ganzfeld-related
EEG activity
Faber et al. (2002) re-analysed data from the study by Wacker-
mann et al. (2002) to localise intra-cranial model sources of
EEG activity (frequency band 2–30 Hz). The source locations
during sleep onset differed significantly from those during
waking state and in the ganzfeld; there were no significant
differences between the ganzfeld and waking state.
Results of reported original studies and post hoc re-analyses
thus do not support the hypothesis of the hypnagogic origin of
ganzfeld imagery. The subtle but objectively demonstrable
differences between frequency spectra of the brain’s electrical
activity during the MMGF exposure and the relaxed waking
state indicate rather an activated state, in which the subject’s
attention is directed towards the emerging percepts. There-
fore, we have proposed a term hypnagoid states to cover a broad
class of ASCs that are characterised by spontaneously dream-
like imagery, and may or may not be associated with reduced
vigilance (Wackermann et al., 2002; Vaitl et al., 2005).
Given the increasing body of findings on neural correlates of
hallucinatory percepts of psychotic origin (e.g., Behrendt and
Young, 2004; Collerton et al., 2005; Weiss and Hecker, 1999),
a comparison with ganzfeld-induced pseudo-hallucinations
would be of interest. We are, however, not aware of any study
comparing the ganzfeld-induced states to other hallucinatory
states, pathologically caused or experimentally provoked by
different methods.
6. Anomalous communication
in the ganzfeld?
The ganzfeld’s potential to induce an ASC producing vivid im-
agery has been utilised by experimental parapsychology in
a so-called ‘ganzfeld telepathy’ (GFTP) paradigm (Honorton
and Harper, 1974; Braud et al., 1975; Parker, 1975). Historically,
this experimental paradigm resulted from two convergent yet
distinctly different developmental lines: studies on ‘spontane-
ous psychic phenomena’, reportedly occurring often in ASCs,
and experiments with ‘dream telepathy’ in the early 1960s
(Ullman et al., 1989). Following the hypothesis of the hypnago-
gic origin of ganzfeld imagery (Witkin and Lewis, 1963; Bertini
et al., 1969), the ganzfeld was considered ‘‘a simpler and
cheaper technique’’ compared to the dream studies (Parker,
2005).
6.1. Experimental procedure
In a typical GFTP experiment (Honorton et al., 1990), there are
two subjects, a ‘sender’ (S) and a ‘receiver’ (R), located in two
different rooms and thus spatially and sensorily separated
from each other. Subject Ris exposed to the MMGF and is
reporting her/his subjective experience; simultaneously,
subject Sis focusing her/his attention on a ‘target’ stimulus,
usually of visual content: a static picture (photograph or draw-
ing) or a short video sequence. The aim of the experimenters is
to establish a ‘communication’ between Rand S: subject Sis
assumed to ‘transmit’ her/his mental content to subject R,
where it should appear (in a manifest or a disguised form) in
subject R’s reported imagery.
In the standard experimental setting, subject Ris allowed
or even encouraged to verbalise continuously her/his ‘menta-
tion’ (percepts, thoughts, emotions, etc.), and the stream of
verbal reports is recorded for later evaluation. This makes
a substantial difference to the more formalised reporting
methods in psychophysiological experiments, as described
in the preceding sections. Also, it is unclear if and how
the genuine ganzfeld-induced imagery is differentiated
from merely cognitive ingredients (free associations of
thoughts, reflections of daily concerns, etc.). One or more
f / Hz
6
8
10
12
14
16
18 8 10 12 14
−300 −250 −200 −150 −100 −50 0
6
8
10
12
14
16
18
time / s
f / Hz
f / Hz
10
8
6
12
14
16
18
6
8
10
12
14
16
18
f / Hz
9 10 11 12 13
−300 −250 −200 −150 −100 −50 0
time / s
a
b
Fig. 6 – EEG spectral changes in the alpha frequency range,
over time relative to the next subject report, for two
selected data subsets as in Fig. 5. Upper panels:
spectrogram representation, logarithmic variance
contributions from frequency bins of .5-Hz width. Lower
panels: peak frequencies of the 1st, 2nd, and 3rd highest
local maxima, indicated by dots of large, medium or small
size, respectively.
cortex 44 (2008) 1364–1378 1373
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communication trials may be carried out during one experi-
mental session.
The evaluation of a communication trial is based on (a)
direct target identification by subject S; or (b) assessment of simi-
larity between the stimulus and subject S’ own experience;
or (c) assessment of similarity between the stimulus and subject
S’ verbal reports recorded during the MMGF session, done by
an external ‘judge’. Whatever the evaluation method, the
results are usually presented in the form of ‘hit-rates’, i.e.,
relative frequency with which the target could be correctly
identified in an array of malternatives (m¼4 used as a stan-
dard). Correct identification rates significantly exceeding the
mean chance expectancy ¼1/m(¼25% in standard designs)
are considered as suggestive or indicative of the ‘telepathic
communication’ in the ganzfeld.
The claims of positive results of GFTP studies are thus
based on statistical evaluation of series of experiments. This
poses a serious problem for the interpretation of results of sin-
gle trials, and their possible linking with objective, e.g.,
psychophysiological measurements. Even if the ‘hit-rate’
from a certain study is ‘significantly’ higher than the mean
chance expectancy, it is virtually impossible to indicate which
correct identifications were due to the alleged dyadic commu-
nication and which were ‘successful’ merely by chance.
Facing this difficulty, some authors (e.g., Parker, 2000) pointed
out the importance of so-called ‘qualitative hits’, i.e., real-time
coincidences between the contents of the target stimulus at
subject Rand the stream of verbal report given by subject S
(cf. Goulding et al., 2004). However, evaluation of such coinci-
dences is necessarily matter of subjective judgment, and thus
may involve some arbitrariness.
Bem and Honorton (1994) summarised results of 10 GFTP
studies carried out by Honorton and his colleagues during
the period 1983–1989 that yielded an overall ‘hit-rate’ of 32%,
significantly higher than the 25% expectancy. The authors
considered these results as a ‘‘replicable evidence for an
anomalous process of information transfer’’. However, their
findings were questioned by a later meta-analysis of a larger
GFTP database (Milton and Wiseman, 1999), which elicited
further discussion between the advocates and the critics of
GFTP research. It is not the aim of the present paper to go
into details of the debate; an interested reader is referred to
the cited papers (see also Bem et al., 2001 and Storm and
Ertel, 2001). Briefly, no agreement has been reached yet: at
present as well as a decade ago, GFTP is far from being ac-
knowledged by the scientific community as an experimentally
established fact.
6.2. Theoretical background
Experimental parapsychologists assume that ganzfeld in-
duces in the ‘receiver’ a ‘psi-enhancing’ (Honorton, 1977)or
‘psi-conducive’ state (Bem and Honorton, 1994; Parker, 2005)
favourable for the alleged telepathic communication. The
term ‘psi’ was proposed by Thouless and Wiesner (1948) to de-
note the totality of ‘psychic’ phenomena, under the assump-
tion that ‘‘telepathy, clairvoyance, and precognition [.]
might be the same capacity working under different circum-
stances’’; it was originally purely descriptive as ‘‘it implied
no theory about the psychological nature of the process’’
(Thouless, 1972). According to Stanford (1977), the term ‘psi-
conducive’ was originally coined by W. G. Braud. Stanford
himself distinguished between research ‘‘in naturalistic con-
texts’’ and laboratory research focusing on ‘‘the search for ex-
perimental procedures which will optimize the function of the
organism for the purpose of deliberate extra-sensory perfor-
mance of the perceptual cognitive sort. [.] Research in this
area seems to be guided by the concept of a psi-conducive syn-
drome’’ (Stanford, 1977, p. 826; emphasis by author).
However, the term ‘psi-conducive state’ is too vague and
useless for any theoretical reasoning, as long as it is tautolog-
ically defined by a success of telepathic communication, or oc-
currence of a ‘psi’ phenomenon. The terminological confusion
is aggravated by some authors’ using the adjective ‘psi-
conducive’ not only for the ‘receiver’s’ psychophysiological
state, but with virtually any component of the experimental
setup. For example, Parker (2005) speaks not only about ‘‘psi-
conducive techniques’’, but also ‘‘psi-conducive subjects’’,
and even ‘‘psi-conducive experimenters’’ (sic!). Another,
more specific concept is thus needed to provide a rationale
for the GFTP paradigm.
Honorton (1977) coined the term internal attention states to
designate ‘‘any condition in which conscious awareness is
maintained in the absence of patterned exteroceptive and
proprioceptive information’’ (Honorton, 1977, p. 435). Later
and elsewhere, the attenuation of external sensory stimula-
tion is also referred to as ‘noise reduction’ (Honorton, 1977;
Parker, 2005). The choice of this expression suggests an
implicit signal-detection model: the ‘anomalous information
transfer’ from Sto Ris considered as a weak ‘signal’ obscured
by sensory input of higher magnitude, considered as a disturb-
ing ‘noise’. How this signal-like transfer should be realised,
and what is the mechanism of its conversion into reportable
subjective experience, remains unclear and open to specula-
tions. For example, Parker (2001, p. 28) pointed out that ‘‘mis-
perceptions [in psi-mediated communication] occur like those
in normal perception’’ and hypothesised that ‘‘psi shows the
same form of top–down processes as occur in normal percep-
tion during non-optimal conditions’’.
But even if one accepts the ‘noise reduction’ hypotheses,
this begs a question: why should inundation of two main sen-
sory systems, the visual and the auditory, effectuate anything
like ‘noise reduction’? In fact, it would be more rational to cre-
ate for subject Rconditions of complete sensory deprivation.
Reasoning in terms of sensory physiology is obviously not
very helpful in the domain of presumably extra-sensory
communication. The construction of the GFTP paradigm
becomes understandable rather out of the ganzfeld’s ability
to induce an imagery-productive ASC. Honorton (1977, p.
459) emphasised that ‘‘ganzfeld stimulation is associated
with increased attention to internal mentation’’, and accepted
the (unverified) assumption of the hypnagogic nature of
ganzfeld imagery; but he simultaneously pointed out (with
Naranjo and Ornstein, 1971) similarities between ganzfeld
and concentrative meditation, and suggested a parallel
between the ganzfeld-induced ‘blank-outs’ and meditation-
induced ‘periods of void’ (ibid.). The notion of ‘internal
attention states’ thus seems to be broad enough to embrace
productive states of vivid dream-like imagery as well as med-
itative experience of nothingness. Too broad, indeed; we are
cortex 44 (2008) 1364–13781374
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facing a rather confusing multitude of references to a wide
spectrum of ASCs, as illustrated by the following quotation:
‘‘The ganzfeld protocol’s combination of mild sensory depri-
vation, the provision of an undifferentiated visual and audi-
tory field, and relaxation provides the same combination of
ideal conditions [as] described for hypnosis and dreaming.’’
(Carpenter, 2005, p. 85).
In sum, the experimental technique developed for GFTP
studies is neither sufficiently justified by the ‘internal atten-
tion state’ or ‘noise reduction’ hypothesis, nor well grounded
in the established knowledge on the ganzfeld and its psycho-
physiological effects. The experimental findings contradicting
the hypnagogic hypothesis of the ganzfeld-induced phenom-
ena have been either ignored by the parapsychology commu-
nity, or dismissed as untrustworthy. Parker (2005) pointed out
several ‘‘shortcomings’’ of the study by Wackermann et al.
(2002): discomfort of the EEG electrodes (sic!), no prior experi-
ence, no special relaxation sessions before experiments, and
shorter MMGF exposure. However, in the later study by Pu
¨tz
et al. (2006) the subjects participated repeatedly in MMGF ses-
sions, had enough opportunity to accommodate to the exper-
imental situation, and the sessions duration was extended to
45 min (the ‘discomfort’ issue does not deserve discussion),
and the appearance of EEG spectra under MMGF stimulation
confirmed the findings from the earlier study.
6.3. Working alliance: a shared belief system?
As seen above, a GFTP experiment is of quite a complex
design, and little is known of the importance of its particular
components; e.g., the choice of the target material, the phys-
ical characteristics of the ganzfeld stimulation, the duration
of the ganzfeld exposure, physiological conditions and psy-
chological characteristics of the participants, etc. Systematic
variation of experimental conditions would be necessary to
elucidate their relative contribution to the alleged anomalous
communication. There are, however, factors that are gener-
ally thought of as being critically important: one of them is
the participants’ acceptance of, or just belief in, the reality of
‘psi phenomena’; the other is the concept of the experimental
situation as a ‘social ritual’.
There is no doubt that the subjects must admit at least
a possibility of the telepathic communication between Sand
Rin order to actively participate in a GFTP experiment. This
implies an attitude of positive expectation, which is further
reinforced by subject R’s experience of imagery in the ganz-
feld. As far as we know, novice participants in the GFTP exper-
iments have no prior experience with ganzfeld in a non-
dyadic settingdin other words, they do not know that their
imaginary percepts would occur even without the S’s presence
and her/his efforts. The occurrence of the hallucinatory
percepts plus the expectation of a ‘transmission from out
there’ creates conditions for a self-reinforcing belief, which
is welcomed and shared by the experimenters themselves.
This cognitive and emotional closure seems to be the
essential component of the working alliance between the par-
ticipants and the experimenters. Honorton et al. (1990) and
Bem and Honorton (1994) emphasise the importance of
a ‘warm social ambiance’ and conclude, ‘‘[w]e believe that
the social climate created in psi experiments is a critical
determinant of their success and failure’’ (Bem and Honorton,
1994, italics ours). It seems that all parties involved in the ex-
periment need a sort of belief: the participants, the experi-
menters, even the (meta)analysts of the data. Other leading
researchers in this area go even farther and plea for ‘‘return-
ing the magic to the laboratory’’ (Parker, 2005). This is really
a non-standard notion of the experimental situation, at least
for those among us whose understanding of experiment has
been shaped by physical sciences.
But is the belief of importance of ‘acceptance of psi’, ‘warm
ambiance’, and ‘atmosphere of magic’ really substantiated by
empirical data? In a recent study by Pu
¨tz et al. (2007), the
experiment was in principle designed by the GFTP model,
but presented to the participants as two parallel, unrelated ex-
perimental tasks. The subjects were thus not aware of the
‘telepathic communication’ possibly involved and not intend-
ing any ‘transfer’. Accordingly, the subjects were not
instructed to ‘identify’ the target stimulus (short video
sequence) but they were just asked to evaluate similarity
between their subjective experience in a MMGF session
(20 min/trial) and four different video sequences shown to
them thereafter. Trials in which the subjects assigned the
highest similarity score to the stimulus which was really
presented to the other member of the pair were counted as
‘correct identifications’. The rate of ‘correct identifications’
of the target stimuli was 32.5%, that is, significantly above
25% as expected by chance (pz.04).
This result, if it were obtained in a ‘standard’ GFTP experi-
ment, could be interpreted as indicative of anomalous infor-
mation transfer; but is such an interpretation feasible in
a situation where there is no intent of communication at all?
Or does the ganzfeld-induced ASC increase the subject S’s
unspecific ‘extra-sensory sensibility’ to distant stimuli in
her/his environment, even without participation of the other
subject (R)? The importance of the subjects’ active involve-
ment in telepathic communication, and of their belief in the
paranormal, ‘psi’, and the like, has been certainly questioned;
the problem deserves further experimental investigation.
7. Concluding notes
The discovery of the ganzfelddan experimentally created vi-
sual ‘nothingness’dopened views to a plenitude of interesting
perceptual phenomena that have been studied from various
research perspectives: sensory physiology, psychology and
psychophysics, psychology of consciousness, and even para-
psychology. The multitude of approaches reflects the multi-
levelled organisation of the neural substrate of the ganzfeld
phenomena. Conceptually, we should distinguish (a) ganz-
feld-evoked sensory phenomena from (b) subjective experi-
ence characterising ganzfeld-induced global state change
(deep relaxation, possibly diminished vigilance), which may
range up to (c) genuinely hallucinatory imagery. Practically,
the boundaries between these groups of phenomena are
rather blurred and often left to the observer’s interpretation.
While phenomena sub (a) belong to the domain of sensory
physiology, phenomena named sub (b and c) are of broad in-
terest to neuro- and psychophysiology as well as to psychol-
ogy of ASC.
cortex 44 (2008) 1364–1378 1375
Author's personal copy
Theuseoftheganzfeldinstudiesof‘anomalouscommuni-
cation’ is a rather special chapter. Due to the proof-oriented, em-
piricist tradition of experimental parapsychology on the one
hand, and its rather weak conceptual background on the other
hand, this line of research has been situated in relative isolation
from other scientific disciplines. Integration of the existing
knowledge on neurophysiology of ganzfeld-induced phenom-
ena is desirable if a progress in this still controversial domain
of study is to be achieved. In the context of this special issue,
the insistence of parapsychologists on a shared belief system
as a background of the ‘working alliance’ between the partici-
pants and the experimenters is especially interesting. More re-
search is needed to explore the roˆle played by particular
components of the complex experimental setup. For the time
being, we remain reservedly open to the possibility of yet unex-
plored ways of inter-individual communication; but we stay
equally open to the possibility that future research may not val-
idate the GFTP hypothesis; alternative ways of explanation of
the reported results may be sought.
Finally, we should point out that the ganzfeld provides an
inexpensive, non-invasive, and (as to our knowledge) risk-
free method to induce hallucinatory experience in normal
subjects. As such it may serve as a suitable model for experi-
mental research on neural correlates of hallucinations, with
possibly relevant output for e.g., clinical neuropsychiatry
and related fields. Very little is known still about the psycho-
logical and neurophysiological basis of responsiveness to
ganzfeld. We can only speculate that the latter may be related
to a proneness to spontaneous occurrence of hallucinatory
states or other forms of psychopathology: another potentially
promising area of research.
Acknowledgements
The authors wish to thank all earlier collaborators and assis-
tants in the experimental studies reported in the present pa-
per: Simone Bu
¨chi, Daria Kaluza, Nicole Rechsteiner, Frauke
Schmitz-Gropengießer, and Jakub Spa
¨ti. We are also thankful
to Andreas Fischer for taking photographs of the ganzfeld
setup (Fig. 2b), to Matthias Ga
¨ßler for technical assistance,
and to two anonymous reviewers for their critical comments
and helpful suggestions.
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