Physiological Psychology1976. Vol. 4 (2).137-144

Expectancy effectsin a psychophysiological experiment

ALEX M. CLARKE, PATRICIA T. MICHIE, ALLAN G. ANDREASEN, and LINDA L. VINEYMacquarie University, North Ryde, Australia

and

ROBERT ROSENTHALHarvard University, Cambridge, Massachusetts

The effect of the experimenter's expectations about experimental outcomes on two physiologicalvariables (EMG and EEG) were investigated in an experiment involving a tendon tap stimulus used toelicit a phasic stretch reflex . Experimenter's and subject's perceived locus of control scores wereincorporated into the design to test their relevance as a mediating variable. Immediately after thepresentation of the tendon tap stimulus, subject's EEG alpha measures were found to be more biased inthe direction of the experimenter's expectations when experimenters and subjects were similar in theirscores on the perceived locus of control scale . Biasing effects on EEG alpha measures in the direction ofexperimenter's expectations were found early in the experimental session where subjects or theirexperimenters scored as internally controlled. The EMG measures increased or decreased in the directionof the experimenter's expectations when subjects were internally controlled but there was a reversal ofthe direction of the experimenter's expectations with subjects who scored as externally controlled so thatno significant experimenter expectancy effect was observed.

The study had two aims. The first was to investigatethe effects of experimenter expectancy (Rosenthal,1966, 1969. 1973) on two physiological variables,namely, the gross electrical activity of the cortex asrecorded by an electroencephalogram (EEG) and thestate of the spinal segmental mechanisms reflected inthe amplitude of a phasic stretch reflex measured byan electromyogram (EMG) : The second aim was toinvestigate the relationship between observedexperimenter expectancy effect (EEE) and both thesubject's and the experimenter's generalizedexpectancies of perceived locus of control as measuredby Rotter's 1-E Scale (1966).

The background to the first objective is as follows.Although the pathways mediating the phasic stretchreflex are monosynaptic , the amplitude of the reflexresponse is subject to both facilitation and inhibitionfrom supraspinal centers, in particular the reticularformation and the sensorimotor cortex (Granit, 1970).Both the variance and mean amplitude of the muscleaction potentials (MAPs) from the muscle respondingin the reflex increase by way of facilitation of thegamma and alpha motor fibers when arousing stimuliare present (Clarke, 1967a ,b). Changes in corticalarousal are related to gamma efferent activity(Buchwald & Eldred, 1961a) and the frequency and

Preparation of this paper was supported in part by a researchgrant (GS-39242) from the Division of Social Sciences of theNational Science Foundation and in part by a Fellowship from theJohn Simon Guggenheim Memorial Foundation to RobertRosenthal. We thank M. Robin DiMatteo for performing theprogramming required and Paul Holland . Department of Statistics.Harvard University. for statistical consultation. Address requestsfor reprints to: Alex M. Clarke . Department of Psychology. TheUniversity of Wollongong. New South Wales. Australia 2500.

amplitude of the EEG changes with the level ofarousal. The EEG pattern arising from the occipitaland parietal areas of an awake but relaxed subjecttypically exhibit fairly regular activity in the 8-13-Hzrange, the alpha rhythm . The amplitude of the alpharhythm is either considerably reduced or replaced bylower voltage faster activity by an arousing stimulus,the degree of the change depending to some extent onthe novelty and intensity of the stimulus (Cobb, 1963).Since both the EMG of the phasic stretch reflex andthe EEG pattern are affected by either the arousal orrelaxation of the subject , the effect on these responsesof researcher manipulated experimenter expectanciesabout the activation level of the stimulus conditionsprovides a favorable experimental situation for theinvestigation of EEE.

In this experiment, the stretch reflex was elicited bytendon taps of uniform force over all trials withinsubjects. Experimenters were given the expectationthat one set of tendon taps would be heavy, resultingin large contractions of the subject's respondingmuscle. whereas another set of taps would be light ,resulting in smaller contractions of the subject'smuscle. If an EEE occurred, it was predicted that inthe former condition. the MAPs would be of highamplitude and the concurrent EEG would show alphaamplitude attenuation and perhaps alpha frequencyblocking. In the latter condition, the MAPs would beoflower amplitude and the EEG would show a smallerdegree of alpha amplitude attenuation and alphafrequency blocking.

The background to the second objective is asfollows: The I-E Scale is a measure of the generalizedexpectancies which people have about the

137

138 CLARKE, MICHIE, ANDREASEN, VINEY, AND ROSENTHAL

consequences of their actions being controlled bypersonal (internal) influences or environmental(external) influences (Rotter, 1966). Those whoseperceived locus of control is internal have been foundto be more alert to environmental cues but moreresistant to overt attempts to influence them (Crowne& Liverant, 1963; Rotter & Mulry, 1965). Sinceinternal subjects are more sensitive to environmentalcues. they might, however. be more influenced thanexternals by subtle cues covertly conveying theexperimenter's expectations. They are also expectedto be more controlling as experimenters (Doctor.1971; Phares, 1965). The studies stimulated by Rotterhave dealt with intentional influence processes (forexample, Biondo & McDonald, 1971), whereas thebiasing effects of the experimenter's expectationsmost . probably are mediated by unintentionalinfluence processes (Rosenthal. 1%9). This study.then. was designed to examine unintentional socialinfluences on behavior.

There is some evidence supporting the view thatsubjects who perceive their lives as internally'controlled may emit physiological responses whichdiffer from those of subjects who see them asexternally controlled. For example, Ray (1974) foundthat internal subjects were able to increase their heartrate better than external subjects and that externalsubjects were better able to decrease heart rate thaninternals. It was also apparent that the two groups ofsubjects had adopted different strategies to controlheart rate. Of more relevance to this experiment isthat of Pawlik and Cattell (1%5) relating personalitymeasures to measures of cortical arousal. They foundthat their Factor 1. labeled Assertiveness vs.Submissiveness (but having in common with Rotter'sI-E Scale. correlations with indices of conformity,field-dependence. and verbal fluency), correlatedsignificantly with all of their cortical activity measuresbut most with alpha amount. They also found theirFactor 3. Cortical Alertness vs, Pathemia (having incommon with the I-E Scale aspects such as activeseeking of information from the environment, verbalfluency. parental income, and educational level), tobe related to all the cortical activity measures theyused but especially alpha frequency. Subjectsdiffering in scores on the I-E Scale. then. may welldiffer in their physiological response to theexperimental treatment described above.

An effect of the personal locus of control of theexperimenter was also predicted. Felton (1971) hasshown that the EEE in a photo-rating task varied notonly as a function of the subject's locus, but also of theperceived locus of the experimenter. That the subject'sresponsivity differs according to what personalityattributes he perceives in the experimenter wasdemonstrated by Hicks (1970, 1971). The subject'svasomotor responsivity was shown to be dependent onhis perception of his experimenter's degree ofsociability. An internal experimenter. confident that

he can control events in the environment and be alert tocues from the subject to aid in his control of him hasbeen found to be a more influential experimenter(Phares. 1965) than the external experimenter whohas lower expectations of effectiveness and setshimself lower standards (Joe, 1971). These studiessuggest that any EEE occurring in physiologicalmeasures of arousal should be examined in relationnot only to subject's, but also to experimenter's per­ceived locus of control, to isolate the mediatinginfluence of this generalized expectancy on the processof unintentional social influence.

METHOD

SubjectsThe experimenters (four males and two females) were 3rd year

undergraduate students in a physiological psychology course atMacquarie University. They had acquired a practical knowledge ofEMG and EEG techniques and knew the neurophysiology ofarousal. Each experimenter was paid a fee of $10 for participationin the experiment. Experimenters were told only that theresearchers wanted to conduct an experiment involvingmeasurement of EMG and EEG responses and that they wished toemploy experimenters to appl y the electrodes. give instructions tosubjects. and make ratings of the subjects ' level of arousal duringthe experiment . Each experimenter was assigned four subjects ofthe same sex as the experimenter. Subjects were drawn from 1styear undergraduate classes at Macquarie University . The sequenceof experimenter 's expectancies about the amplitude of the EMGresponses (that is. high-low and low-high) was counterbalancedover the four subjects so that two subjects received the high-lowsequence and two subjects received the low-high sequence.

Physiological MeasuresThe experiments were conducted with the subjects supine on a

couch in a sound-attenuated and air-conditioned room . The controland recording equipment was located in. and operated from . anadjacent room .

The EEG was recorded by silver disc scalp electrodes attachedwith collodion. Records were taken continuously from standardlocations (Pz - 01 . Pz - 02. and 01 - 02) in the parietal-occipitaland occipital areas (Jasper. 1958) using a Both electro­encephalograph with a time const ant of .03 sec and paper speed of1.5 ern/sec. An event marker indi cated the temporal location of thetap on the tendon in relation to the record . so that measurements ofany changes in EEG frequency and amplitude before and after thestimulus could be made.

The tendon tap used to elicit the reflex was delivered to theAchilles tendon by a special hammer driven by an electricallypowered solenoid. Foil strain gauges mounted on the hammer headdetected the force-time relations of the tap so that it could bedetermined that tendon taps of constant force were delivered to thesubject. The muscle response was recorded isometrically (Clarke .Michie . & Glue . 1973) and MAPs were detected by surfaceelectrodes placed 2 in . apart over the belly of the medialgastrocnemius muscle . MAPs were amplified by a balanced system(response 3 dB down at 15 Hz and 10 kHz; gain 80 dB) . The EMGand force of the tendon tap were displayed on the screen of a storageoscilloscope (Tektronix Type 549) as well as being recordedpermanently on ultraviolet light-sensitive paper using a Visigraphrecorder (Type FR-102).

Perceived Locus of ControlAll subjects and experimenters were administered the I-E Scale

(Rotter. 1%6) of perceived locus of control (PLC) within 2 weeks ofthe physiological testing. Mean scores for subjects andexperimenters were 11.00 and 9.17. respective ly. with standarddeviations of 4.94 and 3 .93. respectively . These mean I-E scores fall

between those reported for U.S. students by Rotter (1966) whichvaried from 7.71 to 9.62 and those reported for Australian studentsby Viney (1974) which were 12.45 for males (SO = 3.75) and 11.56for females (SO = 3.80>.

Awareness of the Purpose of the ExperimentAfter the experiment. all experimenters and subjects were asked

to give brief written answers to two questions: (I) Describe theactual purpose of the experiment completed in the laboratory; and(2) Is there anything we did not tell you about the experimentcompleted in the laboratory? The responses showed that noexperimenters and no subjects suspected that experimenterexpectancy variables were being investigated.

ProcedureOne of the researchers (A.G.A.) assisted the experimenter to

apply some of the electrodes in order to shorten the time spent onattaching them. Every effort was made. however. to establish thecredibility of the experimenter in the eyes of the subject. A numberof pretrials were conducted to establish the best position and forceof tap to the tendon to elicit a brisk but moderate reflex and toreduce the novelty of the stimuli. After the subject had been madeready for the experiment. another researcher (A.M .e.) took theexperimenter outside the room and induced an expectancy for aparticular outcome for the experiment. This was done by giving theexperimenter the following information : "As part of theexperimental design. we are balancing the order of presentation ofheavy and light force of tap on the tendon. For half of the Ss thefirst 10 trials (taps on the tendon) will be light in force of tapwhere we will expect a small response or contraction of the muscle.and the second set of trials will be heavy ones where we will expect alarge response. For the other half of the group of Ss, the heavy tapswill be given in the first set of 10 trials. and the light ones in thesecond set of 10 trials . This S will be given the light taps first andheavy taps later. This information will enable you to ensure that theexperiment runs smoothly; you should not tell your S about thiscounter-balancing procedure." For the other presentation.condition. the experimenter was shown the same informationexcept for the inversion to "This S will be given the heavy taps firstand the light taps later ,"

The experimenter was then given two data sheets to rate thesubjects on scales of arousal -relaxation. One sheet was forrecording data from the first set of 10 taps on the tendon and theother for entering data from the second set of 10 taps. These sheetswere used for two reasons . First. to provide a reason for theexperimenter to remain with the subject during the experiment andthus make an EEE possible and. second. to provide a means ofreinforcing the experimenter's expectancy of the amplitude of theresponse by having him mark on the data sheet the order ofpresentation of the sets of light and heavy taps. The experimenterthen went into the experimental room and sat alone with thesubject.

A third researcher (P .T .M .), who was blind to the order ofpresentation, controlled the experiment when necessary by one-wayvoice communication to the experimenter from an adjacent roomand by operating the solenoid to tap the tendon. as well as recordingthe responses . The level of aural intensity at the experimenter'sheadphones was too low for the subject to hear what wascommunicated by this researcher . Although the experimenter hadbeen given information to lead him to expect a set of 10 light tapsand a set of heavy taps resulting in small and large muscle responses,respectively. each of the 20 taps was of equal force. Because of thelow level of illumination of the room during the experiment. theexperimenter had no means of knowing from his position beside thesubject that the force of the taps was uniform across the 20 trials.

The experimenter then read these instructions to the subject froma printed sheet. "There will be a preliminary period during whichsome trial taps will be given to determine the best position to tap thetendon to obtain a good response. The main experiment will thenfollow. This is an experiment investigating the relationship betweenarousal and relaxation. on the one hand. and reflexive response of

EXPECTANCY EFFECTS IN AN EXPERIMENT 139

the calf muscle. on the other hand. We will be tapping your Achillestendon at irregular intervals of between 15 and 30 seconds . I wouldlike you to try to relax but to keep your eyes open. I will reduce theillum ination level in the laboratory to a low level to help you torelax. I will remain with you in the room to observe your state ofrelaxation and to make any necessary adjustments of the positionwhere the small hammer will tap your tendon."

"There will be two parts to the experiment. I shall tell you whenthe first part is over. and we will have a short break. Try not to alterthe position of your leg during the experiment and during the breakin the middle. Are you comfortable? Right. we will begin now."

The following note for the experimenter was included in theinstructions sheet: " Sit on your chair. put on your earphone. andfill in your questionnaire about the subject's state of relaxation .There will be ten trials (taps on the S's tendon) . Make your rating ofthe S's relaxation when you are asked to do so over the intercom.You wili also be informed from the control room via the earphonecommunication when the first set of 10 trials is over. At that pointread the following instructions to the S: 'We will now begin thesecond part of the experiment. Remember that you are to keep youreyes open and to relax. I will again remain with you in the room toobserve your state of relaxation. Are you comfortable? Right, wewill begin again now,' ..

The experimenter sat on a chair about 1 m away from. andwithin view of, the subject. If the subject closed his eyes during theexperiment. the experimenter reminded him to keep them open butto relax .

Quantification of DataScoring of the EMG and EEG data was done by paid personnel

who were blind to the purpose of the experiment.Electroencephalogram. The amplitude and frequency character­

istics ofthe EEG were measured over the time intervals of 4 sec andI sec preceding and following the tendon tap.

Frequency characteristics were measured by counting thenumber of occasions that activity in the alpha range (8-13 Hz)occurred in the eight 'l,-sec intervals immediately preceding andimmediately following the tendon tap. The frequency counts weresummed over the first five trials and last five trials of eachexpectancy condition . This index. called alpha amount. therefore.is a measure of percent time alpha (Pawlik & Cattell. 1%5) . Theamplitude of alpha activity was measured on an 8-point scaleranging from 0 microV to 120 microV in 15-microV intervals. foreach of the eight 'Il -sec intervals. These amplitude measures weresummed over the pre- and poststimulus intervals and separatetotals were computed for the first five trials and last five trials.

The measures of EEG frequency and amplitude characteristics.then. were (a) alpha amount: 4 sec pre and 4 sec post. (b) alphaamount: 1 sec pre and 1 sec post, (c) alpha amplitude: 4 sec preand 4 sec post, and (d) alpha amplitude: I sec pre and 1 sec post.High alpha amount and alpha amplitude scores indicated thatalpha activity was present for a large percentage of themeasurement period. a pattern typical of a relaxed subject in a lowlevel of arousal. Low alpha amount scores (alpha blocking) and lowalpha amplitude scores (alpha attenuation) indicated a higher levelof arousal. Although the EEGs were recorded from subjects withtheir eyes open, all subjects produced records with substantialamounts of alpha activity because of the low level of illumination inthe experimental room. and the relaxed state of the subjects.

Electromyogram. The amplitude of the diphasic MAPs of thestretch reflex response was measured in units of 100 microV. Theamplitudes were summed over the first five trials and the last fivetrials of each expectancy condition. yielding four scores for eachsubject.

RESULTS

ElectroencephalogramMethod of analysis. For each of the EEG measures.

140 CLARKE, MICHIE, ANDREASEN, VINEY, AND ROSENTHAL

log transformations were used to achieve greaterhomogeneity of variance, and constants of 1 and 4were added to the 1-sec and 4-sec measures,respectively, to avoid negative log values.

The basic analysis of the EEG measures was bymeans of a fully crossed five-factor ANOVA in whichlevels of experimenters' PLC and subjects' PLC scoreswere between-subject sources of variation. Experi­menter's experimental expectancy, first half vs. lasthalf of trials, and pre- vs. poststimulus time intervalwere within-subject sources of variation. Thoseexperimenters scoring above the grand median PLCscore of 10.51 were designated as externals, whilethose experimenters scoring below the grand medianwere designated as internals . This cutoff point isappropriate in terms ofthe Australian normative dataprovided by Viney (974). Levels of experimenter'sPLC were crossed by levels of subject's PLC, againemploying the grand median as the point dividingsubjects into those considered externals vs. internals.These complex tables of variance were simplified by amethod of aggregation in which no mean squares werecombined when F was as large as 2. This proceduregives stable and robust estimates of the error terms(Green & Tukey, 1960). Because this method wasused, the degrees of freedom of aggregated errorterms differed for alpha amount and alpha amplitudemeasures .

Main effects. The ANOVAs indicated a significantmain effect attributable to the experimenter'sexperimental expectancy in the l -sec intervals bothpreceding and following the stimulus. The effect wasevident in alpha amplitude [F = 7.110,114) , P < .01]but was only a trend in alpha amount [F = 3.310 ,63),P < .08]. The means showed that for !!Ie l-secintervals, the high expectancy condition (X = ,334)was associated with less alpha amplitude than the lowexpectancy condition (X = .419). A similar patternappeared in the 4-sec interval but did not reachsignificance. The effect of the experimenter'sexperimental expectancy was greatest, then, on theshort interval preceding and following the stimulus.

The EEG results also indicated , as expected, thatalpha amplitude attenuation indicating alertingoccurred on presentation of the tendon tap stimulus,as there was a significant main effect of pre- vs.poststimulus time interval [F = 13.22(1,114), P <.001] measured 1 sec before and 1 sec after thestimulus. The prestimulus EEG alpha amplitude(X = .455) was significantly greater than thepoststimulus amplitude (X = .298) . The drop inamplitude did not persist over the 4-sec interval sincethe corresponding effect for the 4-sec interval was notsignificant. Further analysis on the data revealed thatthe significant alpha amplitude attenuation persistedover an interval of 2 sec but not for 3 sec. There wasno effect on alpha amount scores for either the l-secor 4-sec intervals. These results indicate that activityin the alpha range was present both before and afterthe tendon tap, but was of lower amplitude in thel-sec interval after the stimulus than in the l-secinterval beforehand.

There was no main effect for sequence in thecomparison ofthe alpha amount and alpha ampJitudemean scores between the first half trials and thesecond half trials in each expectancy condition, norwere there significant main effects for either thesubject's or experimenter's PLC, although there was astrong trend [F = 3.42(1,22), p < .08] for thesubject's PLC to be an important variable in l-sec pre­and poststimulus intervals for alpha amount andalpha amplitude.

Interaction of Experimenters' Expectancy withOther Variables

There was a significant three-way interaction foralpha amplitude measures between experimenter'sExperimental Expectancy by First Half vs, Last HalfTrials by Experimenter's PLC which was evidentacross both the 4-sec [F = 5.250,139), p < .03] andI-sec [F = 4.51(1 ,114), P < .04] pre- and poststimu­Ius intervals. Table 1 indicates that the magnitude ofthe EEE decreased over trials for internalexperimenters but increased over trials for external

Table 1Mean EEG Alpha Amplitude (Log Transformed) for Two Levels of Experimenter's PLC Under Two Conditions of Experimenter's

Expectancy Measured During Subject's First IWf and Last IWf Trials

Length of Measurement Interval

Experimenter'sLast Half First Half Last HalfPLC Expectancy First Half

High 1.003 1.012 .249 .274Internal Low 1.094 1.013 .322 .291

EEE -.091 -.001 - .073 -.017

High 1.191 1.146 .447 .367External Low 1.167 1.221 .464 .598

EEE .024 - .075 -.017 -.231

4-sec 4-sec l-sec l-secPrestimulus through Poststimulus Prestimulus through Poststimulus

Note-A negative EEE score indicates that an experimenter expectancy effect occurred; a positive EEE score indicates that areversal of the experimenter expectancy effect occurred.

EXPECTANCY EFFECTS IN AN EXPERIMENT 141

Table 2Mean EEG Alpha Amount (Log Transformed) for Two Levels of Subject's PLC Under Two Cond itions of Experimenter's

Expectancy Measured During Subject's First Half and Last Half Trials

Length of Measurement Interval

4-secthrough

4-~ec l-sec th h l-sec

Subject 'sPrestimulus Poststirnulus Prestimulus roug Poststimulus

PLC Expectancy First Half Last Half First Half Last Half

High 1.268 1.318 .613 .640Internal Low 1.351 1.325 .717 .660

EEE -.083 -.007 - .104 -.020

High 1.218 1.178 .519 .456External Low 1.197 1.208 .504 .538

EEE .021 - .030 .015 - .082

Note-A negative EEE score indicates that an experimenter expectancy effect occurred: a positive EEE score indicates that areversalof the experimenter expectancy effect occurred,

experimenters, with the externals showing a reversalof the EEE in the first half over the 8-sec (4 sec pre­through 4 sec poststimulus) interval.

A similar · interaction was observed betweenExperimenter's Experimental Expectancy by FirstHalf vs. Last Half Trials by Subject's PLC for alphaamount measures for the 4-sec pre- through 4-secpoststimulus intervals [F = 5.33(1,139), P < .03]with only a trend in the pre and post l-sec intervals[F = 2.97(1.163), p < .09]. Table 2 shows that theEEE occurred in both the first half and last half trials(with a slight decrease over trials) for internalsubjects. For external subjects, however, there was areversal of the direction of the EEE in the first halftrials, but there was an EEE in the predicted directionin the second half trials.

Finally, a significant four-way interaction ofExperimenter's Experimental Expectancy by Pre- vs.Poststimulus 1ntervals by Experimenter's PLC bySubject's PLC was observed for both alpha activitymeasures in the 4-sec interval for amount[F = 11.330 ,139), P < .001] and amplitude [F =7.700, 139), P < .0 1] and in the l-sec interval,[F = 6.030,163). P < .02] (alpha amount), but onlyas a trend for alpha amplitude [F = 3.380,114) ,P < .07]. Table 3 gives the differences between thepre- and poststimulus alpha scores for l -sec an d 4-secintervals and for the four combinations of the levels ofthe experimenter's and subject's PLC. Subjects andexperimenters with similar levels of PLC producedmore alpha blocking and alpha attenuation in thehigh expectancy condition than with the low, that is,the predicted expectancy effect. For subjects andexperimenters with discordant levels ofPLC, however,more alpha blocking and alpha attenuation wasproduced in the low expectancy condition than in thehigh; that is, the reverse of the predicted expectancyeffect. There was only one exception to this generalpattern where the combination of externalexperimenter with the external subject produced lessattenuation on the l-sec alpha amplitude measurewith high expectation compared with low (Table 3).

Electromyogra mMethod of analysis. An analysis of variance

(ANOVA) also was computed on the EMG scores. Apreliminary test of the ANOV A model was made todetermine whether individual differences amongexperimenters in the MAP scores they obtained fromtheir subjects were greater than individual differencesin subjects' MAP scores as nested withinexperimenters. The result (F < 1) showed that theexperimenter factor need not be considered further.Experimenters were, however, divided into two groupson the basis of their PLC scores. Homogeneity ofvariances was achieved by a log transformation, andan unweighted means ANOV A carrie d out on the dataconsidered four factors and their interactions,namely, experimenter's PLC (between subjects),subject's PLC (between subjects), experimenter'sexperimental expectancy (wit hin subjects), and first

Table 3Alpha Blocking and Att enuation Scores (Pre- vs, PoststimulusInterval Differences) for Mean EEG Measures (Log Transform ed)for Four Combinations of Experimenter's and Subject 's PLC

Under Two Conditio ns of Experimenter'sExperimental Expectancy

4-sec Pre-Post l-sec Pre-PostDifferences Differences

Experi- Subject's Expec- Alpha Alpha Alpha Alphamenter's PLC tancy Amount Ampli- Amount Ampli-

PLC tude tude

Internal Internal High -.055 - .118 -.023 -.249Low .026 .119 .022 - .188

External External High - .099 - .078 -. 182 - .182Low .077 .027 -.089 -.216

Internal External High .014 -.016 .057 - .098Low - .020 -.036 - .142 - .247

External Internal High .036 -.006 .140 .108Low - .048 - .105 - .037 -.182

Note-A negative score indicates that the poststimulus alphaactivity was less than the prestimulus activity. That is. alphablocking and attenuation occurred in the poststimulus interval.A positive score indicates the opposite effect, that is, greateralpha activity occurred in the poststimulus interval than inthe prestimulus interval.

Table 4Mean Muscle Action Potentials (Log Transformed) for FourCombinations of Experimenter's and Subject's Level of

Perceived Locus of Control Under Two Conditionsof Experimenter Expectancy

PLC

Internal Internal External ExternalInternal External Internal External

5 7 7 5

.70 .33 .37 .50

.65 .28 .45 .48

.68 .30 .41 .49

.05 -.03

HighLow

SubjectExperimentern

EEE

ExperimenterExpectancyMean MAP

DISCUSSION

142 CLARKE, MICHIE, ANDREASEN, VINEY, AND ROSENTHAL

five trials vs. last five trials (within subjects).Interaction effect. Only one effect was significant,

the interaction of experimenter's experimentalexpectancy with level of subject's PLC [F =4.380,20) , P < .05]. Table 4 shows that while therewas effect on internal subject's MAP scores in thesam e direction as the ir experimenter's expectation(EEE = .05), the external subjects showed MAPeffects in the reverse direction to that predicted fortheir experimenter's experimental expectations(EEE = - .03).

This research has shown that the EEG reflectsconcurrent changes in arousal , associated with EEEs,that occurred over short time intervals immediatelypreceding and following the presentation of thestimulus. The EMG results were less conclusive. Thegeneralized expectancy of perceived locus of controlfor subjects and experimenters was found to be amediator of EEG responses when it was considered asan independent variable, and the effect of subject'slocus of control on the EMG measure was readilyapparent.

ElectroencephalogramThe analysis of the EEG responses indicated that

they are a very sensitive measure of nonverbalcommunication in terms of the development ofexpectancy effects over trials . The first importantfinding was that the EEE was most clearly reflected inalpha amplitude and as a strong trend in alphaamount scores over the l-sec intervals preceding andfollowing the tendon tap. A possible explanation isthat experimenters and subjects may have been moreattentive to one another's facial expressions and bodymovements just before and just after the presentationof the stimulus, because, just prior to the tendon tap,the experimenter was requested by one of theresearchers viaearphone communication to make hisrating of the subject's state ofrelaxation at that time.Thi s greater attentiveness to the subjects ma y have ledto larger and more significant biasing effects of theexperimenters during this l-sec period prior to andfolIowing the tap.

The significant three-way interaction betweenexperimenter's experimental expectancy, first halfvs .lasthalf trials, and experimenter's perceived locus ofcontrol for alpha amplitude showed that themagnitude of the EEE decreased over trials forinternal experimenters but increased over trials forexternal experimenters. A similar interaction wasobserved between experimenter's experimentalexpectancy, first half vs. last half trials, and subject'sPLC. Internal subjects and experimenters, perhapsbeing more sensitive to subtle cues, may have beenable quickly to monitor each other's nonverbalcommunications and thi s resulted in an EEE early in

Note -A negative EEE score indicates that an experimenterexpectancy effect occurred; a positive EEE score indicates thata reversal of the experimenter expectancy effect occurred.

the sequence of trials. As the trials progressed,however, the internalIy controlled experimenters andsubjects exhibited a lesser EEE indicating that:(a) the response had habituated, or (b) later in theseries of trials , experimenters may have become"aware" of the effect they might have been having onsubjects' responses and reduced the intensity of thecues they emitted, that is, a subtle negative feedbackmay have operated, or, (c) as the series of trialsprogressed, the internal subjects became "aware" ofthe covert influences being attempted by theexperimenter and then resisted these influences(Brehm. 1966). Externally controlled experimentersand subjects , however , failed to demonstrate an EEEin the early trials but as trials progressed they mayhave learned to monitor each other's nonverbalcommunications so that by the last five trials an EEEwas observable.

In summary, the ma in effect of experimenterexpectancy was significant. Experimenters andsubjects were successful in producing an EEE in thehigh expectation condition associated with low alphaactivity, that is, relatively high level of arousal. in theI-sec interval preceding and following the stimulus.However, when experimenters and subjects werecongruent for PLC, they were more successful in theirnonverbal communication in that they produced morecortical arousal with the high expectation conditionthan experimenters and subjects whose perceivedlocus of control scores were incongruent.

ElectromyogramThere was no significant main effect representing

EEE in the EMG. However, a significant interactioneffect was obtained. Internal subjects showed EMGresults, the change in the magnitude of which was inthe same direction as their experimenter'sexpectation, but external subjects showed a reversal.Two other studies have examined the moderatingeffects on the experimenter's experimental expectancyvariable of subjects' PLC scores. Daniels (1969) foundno effects of subjects' perceived locus of control scores

on the operation of the expectancy effect. McFall(1967) found, as in the present study, that internalsubjects showed somewhat greater effects in thepredicted direction of experimenter's expectations.Both of the earlier studies employed a photo-ratingtask as the dependent variable rather than a reflexresponse, so that any comparisons must be tentative.McFall also found that the experimenter's PLC mightserve to moderate the effects of the experimenter'sexpectancy, with his more external experimentersobtaining greater biasing effects in the direction oftheir expectation. Further research is needed toelucidate the interaction of experimenter expectancyeffect and the generalized expectancies of perceivedlocus of control.

CONCLUSIONS

An examination of the interrelationship betweenthe EMG and EEG measures reveals that the failureto obtain a main effect for experimental expectancy inthe EMG may be explained in terms of the two maineffects obtained in the EEG. These latter alphaamount and amplitude results showed an EEE whichwas confined to the l-sec pre- vs, poststimulusintervals. Descending neural activity from EEGarousal presumably activated alpha and gammamotor fibers during the l-sec interval preceding thestimulus . However, for natural stimuli to facilitate theEMG of a stretch reflex of normal human subjectswithin 1 sec prior to a tendon tap, a sudden and overtpresentation of a stimulus is required, for example,Beale (1971) observed that a tone of high intensity(1,800 Hz, 1 sec duration, and 97 dB) was necessary.Subtle, covert influences such as those attempting toarouse or to relax the subjects in the presentexperiment apparently failed to significantly modifythe magnitude reflex response, as measured by theEMG, by way of either the direct alpha route affectingthe spinal cells or the indirect gamma route affectingthe sensitivity level of the muscle spindles. Thisstatement must be qualified by the finding in thisexperiment that internal subjects showed a significantexperimenter effect in the predicted direction whileexternal subjects showed a reversed effect.Apparently, internal subjects, being more sensitive toenvironmental cues, responded to subtle cues(whether arousing or relaxing) emitted by theexperimenters, while the externally controlledsubjects were continually aroused because theexperimenter was present in the . room and, hence ,required a particularly strong cue to further affecttheir EMG response.

The observation of a significant influence on theEEG, without a concomitant main effect in the .EMGof the responding muscle, is analogous to the findingof Buchwald and Eldred (1961a,b). They were able todetect conditioning of activity in gamma efferentfibers, using a classical conditioning paradigm, in the

EXPECTANCY EFFECTS IN AN EXPERIMENT 143

absence of a learned response in the alpha motorfibers supplying the target muscles and without anovert conditioned response occurring. The latterconditioning was not observed until many furthertrials had been conducted. The EEG during thattraining was the fast low-amplitude pattern of an alertanimal. Presumably, in the present experiment, theinfluence of cortical activity was not strong enoughafter only 10 trials in each of the two expectancyconditions to affect the alpha motor response .

The significant interactions of the experimenterexpectancy effect with the generalized expectancymeasures for each of the two physiological measuresthrow some further light on the results. They show,quite clearly, that the perceived locus of controlvariable is a powerful mediating influence on theexperimenter expectancy effect. The interaction ofExperimenter's Experimental Expectancy by Sub­ject's PLC in the EMG measures revealed thatinternal subjects showed the predicted EEE whileexternal subjects showed a reversal (Table 4). Theperceived locus of control data for the EEG measuresreveals consistent results (Table 3), namely, subjectswith generalized expectancies of internal control,irrespective of experimenter's perceived locus ofcontrol .: had EEG alpha activity changes in thepredicted direction over the 4-sec pre- andpoststimulus intervals (mean EEG alpha amplitudedifference scores: high expectancy = - .062; lowexpectancy = +.007) . The parallel data for thesubjects with generalized expectancies of externalcontrol show that the mean EEG alpha amplitudedifference scores were in the predicted direction forthe high experimenter expectancy condition(X = - .047). The reason for the overall reversal isapparently that external subjects reversed the EEE inthe low expectancy condition (mean difference score= -.004). Hence , when the subject's perceived locusof control scores are taken into account, thedifferential impacts of the EEE are similar in both theEEG and EMG results. Probably external subjectsreversed the EEE in the low expectancy conditionsbecause, as defined, they were prone to the influenceof powerful others (in this case the experimenters)and, under the conditions of this experiment with theexperimenter present in the same room to observereactions, external subjects, at all times during theexperiment, were more aroused than internals on bothalpha measures (Table 3). This result is consistentwith the finding of Pawlik and Cattell (1965), whodemonstrated the reliable positive relationshipbetween two personality factors (assertiveness vs.submissiveness and cortical alertness vs. pathemia,which have factor loadings high on variables known tocorrelate with I-E locus of control) and arousalmeasured by alpha amount (their alpha indexmeasure) and alpha amplitude. .

Alternative interpretations of these data are, ofcourse , possible. For example, transfer of information

144 CLARKE , MICHIE, ANDREASEN, VINEY, AND ROSENTHAL

from experimenter to subject may not have occurred.Such alternative interpretations do not, however.negate the findings of the study but suggest that anevaluation of them is desirable in further research.The observation of an experimenter expectancy effectin a response which was not under the voluntarycontrol of the subject is an indication of the powerfuleffect that unintentional social influences may have onexperimental results. The findings of the study haveshown that at least one mechanism of theexperimenter expectancy effect is cortical activation .The levels of EEG arousal and relaxation exhibited inthis experiment presumably would have been followedby parallel facilitation and inhibition of the EMGresponses had many additional trials been COn­ducted (Buchwald & Eldred, 1961a, b). The processeswhereby covert information is exchanged betweensubject and experimenter. however. remain to beelucidated.

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(Received for publication June 17. 1975:revision accepted September 12. 1975.)