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c Mind and Matter Vol. 2(1), pp. 105122

Dyadic Correlations

between Brain Functional States:Present Facts and Future Perspectives

Jir WackermannDepartment of Empirical and Analytical Psychophysics

Institute for Frontier Areas of Psychology and Mental HealthFreiburg, Germany

Abstract

For about four decades data suggestive of correlations betweenfunctional states of two separated brains, not mediated by sensoryor other known mechanisms, were reported, but the experimentalevidence is still scarce and controversial. In this paper we brieyreview studies in which one member of a pair of human subjectswas physically stimulated and synchronous correlates were searchedfor in the brain electrical activity of the other, non-stimulated sub- ject. We give a comprehensive account of our study of dyadic EEGcorrelations, discussing pros and contras of its design, and we re-view parallel and follow-up studies on the same topic carried outelsewhere. Possible directions of future research are discussed andnovel experimental paradigms are proposed.

1. Introduction

Functional states of living organisms and, particularly, of their brains,are continuously varying; these variations are partly autonomic, partly

driven by enviromental input. The autonomic state variations take placeon time scales ranging from 10 1 sec (functional micro-states of thebrains activity) over 10 0 102 sec (e. g. spontaneous uctuations of at-tention), up to the order of magnitude of 105 sec (circadian rhythms,sleep/wake cycle). Time scales of externally induced state variations aredependent on time scales of the environmental processes as well as on thecharacteristic times of their neural processing.

Of our interest here are co-variations between states of two selectedorganisms, or, briey, dyadic correlations 1 (Greek : pair, couple) be-

1 By a correlation between X and Y we understand any departure of the jointdistribution of the pair ( X, Y ) from the product of marginal distributions of X and Y ,i. e. any violation of statistical independence of X and Y . A product-moment cross-correlation between two time series is obviously only a special case of this generalnotion. Experimental studies reviewed in section 2 used various measures of correlat-ions between electrophysiological data.

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tween brain functional states as expressed by correlations between particu-lar electrophysiological observables pertaining to those states. 2 Observeddyadic correlations can usually be explained by (1) parallel processingof identical environmental input, or (2) parallel autonomic developmentfrom a similar or identical initial state. For example, two human sub- jects may be synchronously yawning, if (1) they both attend a boringlyuninteresting lecture (reaction to identical input), or (2) they have beenentrained to same diurnal cycle, due to which they are feeling sleepy atthe same time at different geographical locations. 3

It is a legitimate question whether all cases of dyadic correlations arereducible to mechanisms as just mentioned, or whether they can also occur

if communication/interaction via physical environment is precluded. Thisproblem has a long history, which only partially intersects with the historyof natural science. More frequently the quest has led into para-scienticor pseudo-scientic discourse that is not of interest here. Nevertheless, oc-casionally the problem emerges within the very eld of empirical researchconducted by established experimental methods. In the following we willexplore a trace of this problem in experimental neuroscience.

2. Historical Predecessors 4

About fourty years ago, Duane and Behrendt (1965) reported remark-able correlations between electroencephalograms (EEG) recorded frompairs of spatially separated monozygotic twins. If one of the subjectsclosed, on the experimenters demand, her/his eyes, then regular -rhythmoccurred not only in the instructed subjects EEG but sometimes alsoin the EEG of the other, remote twin. The authors observed this phe-nomenon in two out of fteen pairs tested, and called it extrasensoryelectroencephalographic induction. The term is purely descriptive, notexplanatory; the adjective extrasensory says nothing except that syn-

chronous occurrences of the -rhythm could not be explained by sensori-ally mediated information. This short comunication was an early prologueto the history of the topic of our interest.

About one decade later, Targ and Puthoff (1974) modulated one sub- jects EEG by stroboscopic stimulation at varied frequencies (so-calledphotic driving) and searched for correlative changes in the frequency

2 For the purpose of the present paper we adopt the assumption that universal ruleslink a function and an observable manifestation (in our case: electrophysiological data)of that function as an epistemic principle, without necessarily adopting the essentialidentity of the two.

3 An non-trivial example of inter-subject correlations in response to shared environ-mental input of two subjects can be found in a recent paper by Hasson et al. (2004).

4 Here we are dealing only with the line of research establishing the experimentalparadigm used in our study, and its parallels or follow-ups. We do not intend an exhaus-tive review of all related research done in the past, since conceptual and operationalramications would lead us too far from our central topic.

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Dyadic Correlations between Brain Functional States 107

spectrum of the other subjects EEG. They reported signicant reductionof power in the frequency range (911 Hz). In the same vein, Orme-Johnson et al. (1982) studied the inuence of meditation on inter-subjectcorrelations of EEG coherence patterns. These studies, combining correl-ations of selected EEG parameters and a kind of mental exercise, paved away to experiments of Grinberg-Zylberbaum and his collaborators, whichestablished a new and inuential experimental paradigm.

Grinberg-Zylberbaum and Ramos (1987) recorded EEGs from pairsof subjects located in the same room, with a distance of 50 cm be-tween them under so-called direct communication (DC) conditions, de-ned as becoming aware of each others presence; subjects reported

images and thoughts of the other person or even physical sensations(Grinberg-Zylberbaum and Ramos 1987, p. 43). The authors studiedinter-hemispheric correlations (within subjects) of EEG signals and foundthe interhemispheric correlation patterns for each subject [ . . .] to becomesimilar during the communication sessions as compared to the control sit-uations (p. 41). They also calculated cross-correlations between EEGsignals derived from both subjects what they called intersubject con-cordance patterns and reported higher concordances in the DC condi-tion (p. 52) than in the control no-DC condition.

In a later study, Grinberg-Zylberbaum et al. (1994) investigated brainelectrical responses to sensory stimuli (evoked potentials, EP) in pairsof subjects. The two participants were meditating together for about20 minutes to establish so-called direct communication, and were thenseated in separated Faraday chambers, to avoid possible electromagneticinterferences. During experimental sessions, one member of the pair wasexposed to visual stimuli light ashes generated by a Grass photostim-ulator while the other, non-stimulated subject was relaxing, and EEGswere recorded simultaneously from both subjects.

A standard method, averaging post-stimulus EEG signals aligned bythe stimulus onset, was used to extract stimulus-related activity of thebrain from the spontaneous EEG. The averaged data recorded from thestimulated subjects showed characteristic waveforms of visual evoked po-tentials (VEP), as expected. However, the authors observed also VEP-likewaveforms in the averaged EEG data of some non-stimulated subjects ,suggesting that their brains were, in some way, reacting to the physi-cal stimulus presented to the stimulated subject. The authors claimedthat these VEP-like signals which they called transferred potentials(TP) occurred only if the DC interaction was deemed successful ( sic! )and the stimulated subjects data showed a distinct evoked potential(Grinberg-Zylberbaum et al. 1994, p. 424).

Fenwick et al. (1998) attempted a replication of Grinberg-Zylberbaumet al. s ndings, also establishing an empathic relation between the sub- jects as the allegedly necessary condition for the occurrence of TPs. They

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applied acoustic stimuli short auditory clicks instead of visual stim-uli to elicit evoked potentials in the stimulated subject. They reported[n]o transferred potential [ . . .] between non-empathically relating controlsubjects but some perturbation in the EEG response [ . . .] in the non-stimulated brains of 6 pairs of empathically-relating subjects (Fenwick et al. 1998, p. 2). The reported results were obtained with pairs of subjectslocated in the same room, with only elementary precautions against sen-sory leakage. In a replication study conducted by the same group (Sabellet al. 2001), subjects were located in separate rooms, and no TPs werefound.

3. The Freiburg StudyA conceptual replication of the experiments of Grinberg-Zylberbaum

et al. (1994) and their followers was conducted at Freiburg as part of adiploma thesis (Seiter 2001). Preliminary results were reported by Walachet al. (2001) and Seiter et al. (2002). The nal results, together with tech-nical details, were published by Wackermann et al. (2003). The followingis a condensed summary of the experimental design and the results.

Figure 1: Experimental rooms and main components of the equipment.EEG ampliers (headboxes) h A,B in experimental chambers A,B are con-nected to data acquisition stations pc A,B in the control room C . The thirdcomputer pc S generates visual stimuli displayed on the CRT monitor d ,and delivers synchronization signals ( sync ) to computers pc A,B . Dottedlines: digital data transfer. Dashed line: connection betwen the pushbut-ton b and the electric bell b in the experimenters resting room D .

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Thirty-eight subjects participated in the study, distributed into twoexperimental groups, E 1 , 2 , with 14 participants in each group, and onecontrol group, K . Seven pairs of related persons reporting emotional re-lationship were recruited for the group E 1 . Before the experiment, thepairs were instructed to establish an empathic bond, dened as intensefeeling of the presence of the other person, in a 20 minute session, andto maintain the bond during the experimental session. Group E 2 con-sisted of seven pairs randomly combined from 14 subjects recruited on anindividual basis; instead of the empathization session, they were indi-vidually relaxing for 20 minutes. The control group K was composed of three related pairs, and four individual subjects.

The experiments were carried out in two separated, acoustically andelectromagnetically shielded rooms (Fig. 1), with the stimulated subjectin room A and the non-stimulated subject in room B. Six-channel EEGswere recorded simultaneously from both subjects, sampled at a frequencyof 256 data vectors per second, and stored on disks of two data-recordingstations ( pcA , pcB ).

Figure 2: Visual stimulus used to elicit brain electrical response. Acheckerboard pattern composed of alternating black and white squares

appears from the neutral background (stimulus onset) and is reversedthree times in 250 ms intervals.

Visual stimuli, generated by the computer pcS and displayed on a CRTmonitor in room A, were used to elicit VEPs in the stimulated subject;each stimulus consisted of a checkerboard pattern and three reversals of it,occurring in 250 ms intervals (Fig. 2). Markers of stimulus onset/outsetwere sent by the computer pcS to the computers pcA , pcB , and stored withthe EEG data. 72 stimuli were delivered in each experimental session,with inter-stimulus intervals (ISI) randomly varied in the range 3.54.5 sec. Subjects in room B were neither informed about the kind of stimuli presented to the subject in room A, nor about their intermittentnature, timing, etc.

Control recordings with pairs of subjects were done in exactly the sameway as in the experimental group E 2 (i. e., without the empathization

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procedure), but the front of the CRT monitor in room A was coveredby an opaque shield, so that the subject could not perceive any stimuli.Control recordings with individual subjects in room B were carried outwhile the stimuli were displayed in the empty room A. Subjects in roomB were not informed about manipulations carried out in room A.

Off-line, 1-second epochs of artefact-free EEG data, synchronized tostimulus onset, were averaged separately for the stimulated subject A andthe non-stimulated subject B in each pair. Visual inspection of the aver-aged EEG data revealed, as expected, VEPs in the EEG data of the stim-ulated subjects. No VEP-like waveforms were seen in the averaged EEGdata of the non-stimulated subjects, contrary to the claims of Grinberg-

Zylberbaum et al. (1994). However, we could not a priori exclude that astimulus-related brain response was hidden in the residual EEG noiseresulting from the averaging. The averaged data were further analyzed asfollows:

The effective voltage of the residual signal, i. e. the root of meansquared data values, was calculated within a window of 35 sam-ples (137 ms) width, and the window was shifted on a sample-by-sample basis. This resulted in series of 222 effective voltage values

V eff (t), where t refers to the time coordinate of the windows cen-ter. 5 These effective voltages were normalized, V eff (t) = V eff (t)/V ref ,where V ref was dened as the effective voltage of a reference dataepoch, obtained by averaging the same number of 1-second artefact-free EEG epochs chosen randomly from the inter-stimulus periods.The reference values were calculated individually for each subjectand electrode site.

The time series V Aeff (t) for stimulated subjects A showed distinctlocal maxima corresponding to the maximum response of the stim-ulated brain. For each stimulated subject A in a pair and for eachelectrode site, the latency

t relative to the stimulus onset was de-termined. The normalized effective voltages of the non-stimulatedsubject B, obtained for the same electrode position at time

t , Q =V Beff (

t), were taken as the response correlate of the non-stimulatedbrain. 6

Given the null-hypothesis, i. e., the brain of the non-stimulated sub- ject does not respond to a stimulus presented to the stimulated

5 The data epoch length was 256 data points, and the window length was 35 datapoints, so 256 35 + 1 = 222.

6 This strategy implied that the response in subject B occured synchronously withthe electrical brain response in subject A. This was, in fact, a deliberate decision, bywhich we restricted the analysis to a uniquely determined time instant, accepting therisk of possibly overlooking a stronger response occuring earlier or later than

t .

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member of the pair, the mean expectancy of the Q-ratio is Q = 1;but the exact distribution of Q is unknown. Therefore, we useda simple randomization statistics, constructing reference distribu-tions of V Beff (t) calculated from averaged EEG data epochs as above,but now the EEG epochs were drawn randomly from the pool of available artefact-free EEGs from the inter-stimulus periods. Theobserved Q values were compared with the min-max ranges of thesereference distributions. In some cases the observed Q value was outof the range, indicating a suspectly decreased or increased effectivevoltage of the EEG data; these cases were labeled as outliers. Therandomization and min-max range construction procedure was re-

peated 20 times, and the total sum of outliers, C , was calculatedindependently for each subject and electrode site.

An occurrence of an outlier per se does not mean that a responseof the non-stimulated brain was detected. If the null-hypothesisapplies, outliers can still occur with small although not a priori known probability, so their counts C should be approximatelyPoisson distributed (statistics of rare events). For the nal statis-tics, we (1) tested the goodness-of-t of the empirical distributionsof outlier counts C against the Poisson distribution, and (2) wecompared the three empirical distributions of C values obtained forgroups E 1 , E 2 , and K .7

The results of the analyses, summarized for the seven subjects in eachgroup and the six electrode sites, are shown in Fig. 3. There was no signif-icant deviation from the Poisson distribution in the control group K , butthe Poissonian ts for the experimental groups E 1 and E 2 failed due tounexpectedly high outlier counts C > 10 (Fig. 3A,B). A between-groupscomparison of the three outlier count distributions (Fig. 3C) shows no

difference between the experimental groups E 1 and E 2 , but a strong dif-ference between the experimental groups on the one hand and the controlgroup on the other ( P < 0.01).

These results suggest that, at times of the maximal electrical brainresponse to the visual stimulus in the stimulated subjects, the brain elec-trical activity of non-stimulated subjects showed small but detectable uc-tuations of the EEG power, compared to the EEG during inter-stimulusperiods. In other words, the results seem to substantiate the hypothesis of state correlations between two separated brains, although these have notthe form of transferred potentials as suggested by Grinberg-Zylberbaumet al. (1994).

7 No VEPs were available in the control group K as the subject in room A was notstimulated, or was absent. The statistics was in principle the same except that thetimes

t , at which the ratios Q were determined, were randomly drawn from a pool of

t s constructed from the data obtained for the groups E 1 , 2 .

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out. Although the two studies were conceived independently from eachother, their designs were very similar: checkerboard stimuli were used toelicit VEPs, squared averaged voltages were the values of interest, andthe latency of the assumed effect from the stimulus onset was determinedby the time range of maximum response in the stimulated subject (80180 msec). Ten pairs of subjects participated in the study by Standish et al. (2001), acting alternately as the stimulated or non-stimulated subject(sender or receiver in the authors nomenclature). Relaxation sessionsof eight minutes duration preceded the experimental sessions. Statisticalevaluation was done on the pair basis, using non-parametric statistics tocompare stimulation versus non-stimulation EEG epochs. Signicantly

(P < 0.01) increased brain activation was reported in two out of tenreceivers. In an extension of the study, ve subjects out of 30 reportedlyshowed signicantly higher brain activation (Standish et al. 2003a).

The same group (Standish et al. 2003b) recently published results ob-tained using functional magnetic resonance imaging (fMRI), claiming asignicant increase of the blood oxygenation level in the visual cortex of one subject while the other subject was visually stimulated by an alternat-ing checkerboard pattern. The statistical merit of this case report basedon one pair only is unclear.

Radin (2003, 2004) carried out a study using a real-time video imageof the non-stimulated person as the visual stimulus, and recording EEGresponses on stimulus onsets and outsets from both subjects in parallel.His method of analysis differed from all other procedures ever used inearlier studies: EEG epochs were aligned at stimulus onsets/outsets, asusual in EP studies, means and variances across repeated stimulus pre-sentations were calculated; the values of interest were cross-correlationsbetween time series of EEG variances obtained for the stimulated and thenon-stimulated subjects (sender and receiver in his wording). No ra-

tionale was given for this choice of the variable of interest. Data combinedacross all 13 pairs of participants reportedly yielded a highly signicantcorrelation ( P < 5 10 4 ).

At present, a replication of the Freiburg study is being carried out inour laboratory. It is based on essentially the same paradigm as the originalstudy, with several improvements of the experimental design (discussedin section 6). Results will be published soon.

5. Commentaries and Criticisms

After about four decades of research, the status of dyadic brain statecorrelations not mediated by physical environment is still surprisingly un-clear. Also, in spite of its fundamental importance for our concepts of communication/interaction between living systems, the problem is virtu-ally ignored by the mainstream of neuroscience.

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Several factors may be responsible for this state of affairs. The re-ported ndings of dyadic correlations may let a majority of neuroscientiststhink of methodical or technical artefacts rather than of a phenomenonworth their attention and further exploration. Indeed, a closer look atwork published until recently makes such an attitude toward this sensi-tive subject understandable and justied. Early publications on the topicwere frequently of sub-standard quality, suffering from an absence of for-mal statistical hypotheses testing, supporting their arguments mostly byselected visual material, etc.

Another factor that may affect the reputation of this research couldbe its apparent association with areas stigmatized as pseudo-sciences:

most of the recent ndings are scattered in conference proceedings or journals dedicated to parapsychology, alternative medicine, etc. Under-standable prejudices may be further amplied by occasional referencesto unproven, speculative theories (Goswamis theory of consciousness,Grinberg-Zylberbaums syntergic theory of experience), and an incli-nation to supercial analogies with quantum physics. 8 Also, many re-searchers working in this eld are obviously biased towards positive nd-ings of brain state correlations, since these seemingly provide objec-tive support for their preconceived notions (telepathy in parapsychol-

ogy, connectedness in transpersonal psychology, or the diffuse notion of non-locality used in particular theories of consciousness; cf. Walach et al. 2002, Radin 2003, Thaheld 2003).

The notion of a transferred potential, introduced by Grinberg-Zyl-berbaum et al. (1994), requires particular consideration. The term sug-gests that the brain of the non-stimulated subject B produces a responsesimilar to the physiological response of the brain of the stimulated sub- ject A, the so-called evoked potential (EP). This, in fact, has never been observed: data published by Grinberg-Zylberbaum et al. (1994) andother auhors show oscillatory patterns rather than well-shaped EP-likewaveforms (cf. Fenwick et al. 1998, perturbations shown in Fig. 1). 9 Inour study, we did not observe EP-like waveforms in the averaged EEGdata of non-stimulated subjects (Wackermann et al. 2003, p. 63).

8 A brief quotation illustrates this point: [in] the earlier work [ . . . ] it was found thatif two people meditate together, their brains EEG display phase coherence with respectto each other. Phase coherence is a well-known signature of quantum nonlocality.Accordingly, subjects of this study were correlated by meditating together for about20 minutes. (Grinberg-Zylberbaum et al. 1994, p. 422).

9 The averaging procedures are based on the assumption that the brains response tothe stimulus is a constant function of time (EP-waveform) while the background EEGis random relative to stimulus onset, and thus averages out to zero. However, in anyreal experimental study only a nite number of EEG epochs is being averaged, so theresidual EEG activity in the averaged data is never exactly zero. Authors of earliercommunications never seriously discussed the possibility that the observed waveformsmay be random bursts of the residual EEG, which appear as evoked activity onlyprima facie .

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In addition, the notion of transfer suggests a kind of causal process,a transfer from a cause X to an effect Y , for which there is no evidenceat present. Noteworthy, the notion of a causal signal transfer between thetwo brains contradicts the leading metaphor of Grinberg-Zylberbaumswork, referring to quantum entanglement as expressed by non-causal cor-relations between the states of two particles. After all, the term trans-ferred potential should be omitted in future discussions of the topic, andthe discourse should adher to operationally dened characteristics of thebrain electrical activity under study. 10

It may still be conceivable, however, that the brain electrical activ-ity of the non-stimulated subject may have been (in some unknown way)

altered at times when the stimulated subject has been presented a sen-sory stimulus eliciting an electrical response in his brain. Technically, ourproblem reduces to the detection of minor changes of a suitably chosenparameter of the EEG signal. This was exactly the concept of the dataanalysis employed in the Freiburg study described in section 3. The ob-served variations of EEG power in non-stimulated subjects thus seem tosubstantiate the (widely conceived) notion of dyadic EEG correlations.Nonetheless, our own ndings also deserve critical comments:

The evidence is very indirect. The data analysis comprised severalsteps of data aggregation; at the very outcome, the averaged EEGdata for each subject and electrode location were reduced to binaryindicators, 0 (no outlier) or 1 (outlier) which entered the nalcounting statistics.

A neurophysiological interpretation of the effect is difficult if notimpossible because of the lack of a directional effect and no well-dened locus of maximal effect. The effect could be obtained onlycounting outliers in both directions, i. e. reduced or increased EEGpower. The EEG was recorded from only six electrodes, and thusthe data do not provide precise topographical information about thealleged effect.

Different subjects were participating in experimental sessions and inthe control group, so inter-individual differences beyond our controlmay have played a role. A design varying stimulated versus non-stimulated conditions within each pair of subjects would be moreappropriate. 11

10 Some authors still adhere to the notion of transferred potentials and falsely relatetheir ndings to our work, cf. Thaheld (2004).

11 The original design comprised only two groups, E 1 labelled as the experimentaland E 2 as the control group, since the existence of the TP phenomenon was taken forgranted and the study aimed mainly at the effect of connectedness between subjects(Seiter 2001). Note that preliminary communications by Walach et al. (2001) and

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The last of these points was recently brought up in a commentary byKalitzin and Suffczynski (2003). Their criticism is justied, and for thepurpose of the follow-up study we modied the experimental protocol ac-cordingly (see section 6). In their other comments, they touched minorpoints which, in their own words, could not have produced any method-ological artefact (Kalitzin and Suffczynski 2003, p. 193), or even pleadedfor a less conservative approach.

Summarizing: in our own publication we argued that we are facinga phenomenon which is neither easy to dismiss as a methodical failureor a technical artefact nor understood as to its nature (Wackermann et al. 2003, p. 63). Given the limitations of the design and of the data ma-

terial of the rst study, it would certainly be unwise to conclude moreconsequences (or speculate about hypotheses) than those that it impliesuncontrovertibly. The situation may change when the results of the repli-cation study become available.

6. Directions of Future Research

Obviously, the main problems of the research concerning dyadic EEGcorrelations are the absence of a solid theoretical background and still in-sufficient empirical knowledge about the phenomenon under question. Insuch a situation, we have to rely mostly on the intuition of experimental-ists, guided by few general principles of empirical research. First, we needto nd a minimal set of conditions rendering the investigated effect stable,reproducible, and quantiable. Then we have to vary conditions system-atically to study the dependence of the magnitude of the effect on controlvariables. The following notes are intended to contribute to a cartographyof the landscape of possible choices and decisions of experimenters.

6.1 Classication of Experimental Designs

In asymmetrical experimental designs , the functional state of one sub- jects brain is manipulated, by a specic stimulation, and a correlativechange of the brain state of the other subject is searched for. Most ex-perimental paradigms reviewed above belong to this group (Duane andBehrendt 1965, Targ and Puthoff 1974, Grinberg-Zylberbaum et al. 1994,

Seiter et al. (2002) were still referring to the two-groups design. Only later, withthe advancement of the data-analysis method, a third, independent control group wascreated from data material which was originally recorded merely for safety measuresagainst possible technical artefacts. Variation of the stimulation conditions duringexperimental sessions was excluded anyway, since the empathic relation between theE 1 subjects was believed to be the essential component of the design, and experimentersthus avoided any interactions with the subjects after the empathic bond had beenestablished. This sub-optimal design illustrates how uncritical preconceptions inthis case, the a priori assumption of the principal role of empathic relation maycompromise experimental protocols and/or strategies of data analysis.

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Fenwick et al. 1998, Sabell et al. 2001, Standish 2001, 2003a,b, Wacker-mann et al. 2003, Radin 2003, in press). The advantage of such asym-metrical designs is that the experimenter is in control of the conditionsconcerning stimulation versus non-stimulation periods. Evidence for aneffect consists in a detection of a measurable response in the brain of thenon-stimulated subject at selected instances of maximal response, or overextended time periods of stimulation. 12

In symmetrical experimental designs , spontaneous correlative changesof functional states of two subjects are searched for. Experiments withinter-subject EEG correlations or coherences under varied conditions of mental exercise belong to this group (Orme-Johnson et al. 1982, Grinberg-

Zylberbaum & Ramos 1987). Symmetrical designs t the notion of dyadiccorrelations ideally and do not suggest directed relations of inuence ortransfer between the two brains. On the other hand, they may posedifficult problems with the denition of the baseline conditions againstwhich effects are to be evaluated.

We may also distinguish asymmetrical and symmetrical measures of the functional states of the brains involved. All parameters derived fromcross-correlation or cross-spectral functions of pair-wise recorded signalsare, by their nature, symmetrical. The value of interest used in our study,

i. e., effective voltage V B

eff (

t) of the electrical activity of brain B at time

tof the maximum V Aeff (

t) of the electrical activity of brain A, is an exampleof an asymmetrical measure, as it is not invariant with respect to anexchange of A and B. Obviously, symmetrical measures can be used withboth symmetrical and asymmetrical designs, while using asymmetricalmeasures is conned to asymmetrical experimental designs.

6.2 Renement of Established Designs

Since the results of our study did not conrm the principal role of connectedness between the participants for the occurrence of the effect,we eliminated (at least temporarily) this component from the design of the replication study. This enables us to vary the stimulation versus non-stimulation condition within single sessions with each pair of subjects: thestimulation display d (see Fig. 1) was uncovered during one half of thesession and covered for the other half, in a counter-balanced order. Dueto this modication we can use non-stimulation data from each pair as

12 Another factor contributing to the preference for asymmetrical experimental de-signs is probably the implicit assumption of the signal propagation model, i. e. trans-mission of a perturbation from the brain of the stimulated subject to the brain of thenon-stimulated subject, which acts as a signal detector. In this sense, Standish et al.(2001, 2003a,b) conceptualize their experiments as so-called neural energy transfer,although they do not say anything specic about the hypothetical neural energy. Theuse of the sender-receiver nomenclature among many researchers in the eld suggeststhe same signal propagation model.

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its own control data (cf. footnote 11). This control condition, in whichthe stimulation of subject A is attenuated, is necessary for a crucial testbetween two interpretations of the results: is the observed effect in subjectB conditioned by the presence of a physiological response to the stimulusin subject A, or does the experimental procedure contain a leakagestimulating subject B directly? Further improvements in the design of the replication study are an increased number of 16 pairs participatingin the study, increased number of 168 stimuli per session, and full scalpcoverage with 19 EEG electrodes conforming to the international 10/20system.

Later, conceptual replications of our dyadic EEG correlations studies

should involve variations of physical stimulus parameters , e. g. contrastand angular size of elementary squares of the checkerboard pattern, tocontrol the amplitude of the evoked potential. This will allow us to studythe dependence of the effects magnitude in the non-stimulated brain onthe magnitude of the electrical event in the stimulated brain: is it amonotonically increasing function, or rather an all-or-none response? Theanswer to this question may play an important role for the theoreticalconceptualization of the alleged EEG correlations. 13

Another important and closely related question is whether there is a

relationship between spatial distributions of activations in the stimulatedand non-stimulated brain. To address this problem, we intend to uselateral hemi-eld visual stimulation, which is known to induce VEPs of asymmetrical eld topography (Skrandies and Lehmann 1982), and exam-ine topographies of the brain electrical elds of the non-stimulated brainfor their (dis)similarity with topographies of the VEP elds.

6.3 Novel Experimental Designs

In studies based on an asymmetrical experimental design, the roles of the members of the pair (stimulated versus non-stimulated) are assignedin advance and immutable. We suggest to introduce pseudo-symmetrical designs as extensions of asymmetrical designs, in which stimuli are de-livered to one or the other member of the pair (on a strictly alternatingor randomized basis). EEG correlates are then searched for in brain ac-tivities of both subjects, at times when the stimulus was presented to theother member of the pair.

13 Until now little attention has been paid to this question, as most authors weresatised with merely detecting a correlate. Grinberg-Zylberbaum et al. (1994) stateda dependence in qualitative terms, writing that the presence of a distinct evokedpotential was a necessary condition for the alleged TP to occur. Radin recently founda positive relationship between the stimulated brains response and the magnitude of the reported effect (2003, pp. 194195). However, in both these reports the variationsof the stimulated brains response were spontaneous and their causes were not clearlyidentied. Our modication of the experiment intends a controllably gradual response.

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While all recent research concerning dyadic brain state correlations re-lies on asymmetrical experimental designs, symmetrical designs may ap-pear more promising. We are developing experimental paradigms basedon dyadic games played in a virtual world. Essential components of thesegame paradigms are (1) incomplete information , i. e., the players are pro-vided only partial, complementary projections of the game as a whole, and(2) cooperation rewarding , controlled by an adjustable parameter in thegame conguration. Experiments with dyadic cooperative games shouldinvolve parallel recordings of electrophysiological and behavioral data fromboth subjects, and investigate dyadic correlations on both observationallevels, i. e., in the manifest choice behavior as well as in the accompanying

brain activities, with varying reward for cooperative actions. Correlativeeffects in the subjects behavior would manifest themselves in a signi-cant increase of cooperative choices; inter-subject correlations in neuralprocesses could be detected by methods sketched in preceding sections.

7. Concluding Remarks

Where are we right now? Is there any considerable progress since thetopic was introduced about fourty years ago? Our answer, based on theaccount of the present state of research given in the preceding sections,can only be prosaically demure.

There are undoubtedly particular indications of progress. Comparedto early studies, the level of statistical data treatment and presentationof results reached current standards. Hence, academic communities tendto accept the ndings for publication and unprejudiced discussion less re-luctantly (Kalitzin and Suffczynski 2003, Wackermann 2003). There arenow several researchers or research groups all over the world that haveput the topic on their agenda; new, usually signicant results are beingreported. But none of these indications justies premature satisfaction.

The absence of a theoretical background as well as the divergence of ex-perimental designs and methods are rather frustrating. As long as we haveno denitely working experiment i. e., an experiment that, built-up indifferent laboratories, would yield comparable, if not perfectly identical,results we cannot speak of substantial progress.

We may, however, hope that such progress will be achieved by con-certed efforts of experimenters and theoreticians. Experimentally, thedependence of the alleged effects on systematically varied experimentalconditions should be explored. Theoretically, a framework into which the

present results could be embedded and which gives rise to further exper-imental proposals would be highly desirable.

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Note Added in Proofs

Meanwhile, data from the replication study addressed above were ana-lyzed. Comparisons between the experimental condition (subject A phys-ically stimulated) and the control condition (subject A not stimulated)reveal a signicant suppression of EEG power at ve out of 19 electrodelocations in the experimental condition. However, the results indicatethat this difference is due to deviations of observed Qs from expectedvalues in both conditions, experimental and control, going in opposite di-rections. This result presents an additional difficulty for interpretationsince, according to the logic of the experiment, deviations from expected

values (determined from the EEG recorded during inter-stimulus periods)in the control condition were unforeseen.

Acknowledgements

The author wishes to thank two referees for their critical reading andhelpful comments.

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Received: 30 December 2003 Revised: 23 April 2004Accepted: 27 April 2004

Reviewed by Christoph M. Michel and another, anonymous, referee.