Perceptual and Motor Skillr, 1976,42, 371-378.
@ Perceptual and Motor Skills 1976
HYPNOTIC SUGGESTION OF STIMULUS CHANGE A N D VISUAL CORTICAL EVOKED POTENTIAL1 J. L. ANDREASSI, B. BALWSKY, J. A. GALLICHIO,
J. J. DE SIMONE, B. W. MELLERS Baruch College, C i ~ yUniversity of New York' Summary.-Two experiments were conducted to determine whether the hypnotic suggestion of stimulus intensity change would affect the physical parameters (latencies, amplitudes) o f the visual cortical evoked potential. Stimulus intensity was kept constant as visual cortical evoked potentials were measured under conditions in which it was suggested that stimuli were "bright" or "dim." The results of both experiments led to the conclusion that, under the present condition, hypnotic suggestion of stimulus intensity change did not result in changes in visual cortical evoked potentials.
The problem that concerned us in this study was whether hypnotic suggestion could alter the amplitude of a physiological response, i.e., the visual cortical evoked potential (VEP). Studies of hypnotic suggestion and physiological responses, in general, seem to have produced varied results. For example, stapedial reflex threshold was not changed under hypnotic instruction that sounds were "loud" or "soft" although suggestion altered the experience of the two Ss tested in the expected direction (Schoenfeld, et al., 1973). Barber (1973) mentions several studies in which as many control subjects as hypnotized subjects produced the suggested change in physiological activity. For example, suggestions of heart-rate change were effective with the control group as with the hyponotized group (Klemrner, 1963, as cited in Barber, 1973). Visual acuity was improved in myopic individuals, both in hypnotic and control groups, through suggestion (Kelley, 1958) and allergic responses were altered in both control and hypnotized subjects (Ikemi & Nakagawa, 1962, as cited in Barber, 1973). In an earlier review, Barber (1961) concluded that "Physiological functions vary in the same way during 'hypnosis' as they do during 'waking' behavior" ( p . 411). With regard to brain activity in particular, it has been demonstrated that the hypnotized person shows a similar electroencephalogram (EEG) as when awake (Chertok & Kramarz, 1959). It is well known that amplitude of the visual evoked potential (VEP) is related to stimulus intensity, such that higher intensity stimuli produce greater amplitude responses and shorter latencies (e.g., see Vaughan & Hull, 1965; Vaughan, et a/., 1966). The present two experiments were conducted in an attempt to answer the question, "Will a suggested change of stimulus intensity result in a change in the visual evoked potential?" Three conditions were used 'This research was supported by the Environmental Physiology Branch of the Physiology Program, Office of Naval Research, under O N R Contract N00014-72-A-0406-0006, and ONR Contract Authority No. N R 201-053. '155 East 24th Street, New York, New York 10010.
372
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in Exp. I to explore the effects of suggested "brightness," suggested "dimness," or no suggestion at all, about the intensity of invariant light flashes. Method Subjects.-The subjects were 7 male and 5 female undergraduate students at Baruch College of the City University of New York, ranging in age from 19 to 37 yr. The subjects had no visual system defects other than myopia (corrected to at least 20/30). A p p a ~ a c uand ~ p~ocedu~e.-Subjects were seated in an electrically shielded, soundattenuated room (IAC Chamber) during testing. All experimental sessions were conducted with the lights out in the chamber. In order to obtain the averaged cortical evoked potential, the electroencephalogram (EEG) of each subject was recorded from 0. ("Ten-Twenty" system; Jasper, 1958), with s silver cup electrode referenced to a silver clip electrode on the subject's left ear lobe. The subject was grounded by means of a clip electrode attached to the right ear lcading to "patient ground" of a Beckman RM Dynograph. The 9806A coupler of the Dynograph was used to condition the EEG signal (bandpass set at 0.5 to 32.0 H Z ) . The filtered and amplified signal was then fed into a Mnemotron Computer of Average Transients (CAT 1000). The elecuo-oculograrn (EOG) was measured by a separate channel of the Beckman Dynograph and averaged by the CAT to check on possible distortion of the visual evoked potential (VEP) due to excessive eye movement. A "start" signal from a Photo-stimulator triggered the CAT to take EEG and EOG samples of 500 msec. duration following the presentation of each stimulus to the subject. A Grass Model PS2 Photo-stimulator was used to present flashing light stimuli. The Photo-stimulator was set at Intensity 1 with an interstimulus interval of 1 sec. for all conditions. The PS2 was mounted at the subject's eye level outside the chamber at a distance of 71 cm. (28 in.). A Bausch and Lomb chin rest was used to keep the subject's head in a fixed position. After 100 stimulus presentations the summated VEP responses from CAT memory were plotted by a Hewlett-Packard X-Y Plotter. Selection of subjects was based on a group test of postural sway by the second author in rwo of his classes. Potential subjects were asked to close their eyes, extend their arms and sway while in a standing position. The degree of swaying, readiness to cyose eyes prior to being told, and the immediate closing of the eyes after instruction to do so were used as indicators of hypnotizability. Observance of these indicators was accomplished by the other authors. As each subject arrived for his/her appointment, it was explained that the purpose of the experiment was to measure brain responses to a flashing light while he/she was under hypnosis. Electrode leads were then applied to the appropriate locations and the subject was seated in the IAC Chamber. While the subject was in the chamber, a hypnotic state was induced by one of the experimenters (B.B.). The following is a summarized version of the hypnotic procedure: Initially, subjects were told to relax and that they would be placed in a very special state. They were instructed to listen very carefully to the hypnotist's voice and to disregard all other sounds they might hear. Each subject was asked to breathe deeply throughout the induction procedure. When the subject's eyes appeared to be closing, explicit instruction was given for them to close their eyes. The hypnotist would tell the subject that he/she feels very relaxed, very blissful, free of all worries-like sleeping on a cloud. A test of arm rigidity and sensitivity to pain (hand pinch) was then administered to indicate whether subjea was in a hypnotic state. The subjea was given the instruction once again to relax and listen only to the hypnotist's voice. The individual was then instructed to rest, place his chin and to relax while await-
SUGGESTION AND VISUAL CORTICAL EVOKED POTENTIAL
373
ing further instructions. The person was then read the instructions for Conditions A, B or C as in the following. Condition A : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash of light and tell me how many were presented when I ask you. The light flashes will be very bright, but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. The flashes are very bright. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'the flashes are very bright' three more times for a total of 5 times.)" Condition B : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. The light flashes will be very din,but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. The flashes are very dim. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'the flashes are very dim' three more times for a total of 5 times.)" Condition C : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. Watch the light flashes and count each flash with your eyes closed. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'count each flash' three more times for a total of 5 times.)" After the presentation of each condition, subjects were encouraged to continue relaxing and told they were doing well. Each subject was asked to report the number of flashes they counted. In addition, they were asked to indicate how the light seemed to them. If this did not elicit a comment about the intensity of the flash, they were further asked "what was the intensity like?" This was done to determine the degree to which the hypnotic suggestion was accepted. These verbal reports were recorded. The three conditions were completely counterbalanced across the 12 subjects in a Latin-square design. Each subject was presented with each condition two times in the experimental session, for a total of six trials and six visual cortical evoked potential traces from 0.. The EOG was measured from over the subject's left eye and averaged by the CAT to monitor possible distortion of the visual cortical evoked potential trace due to excessive eye movement or blink. Inspection for eye-movement contamination involved a comparison of the averaged EOG trace with the visual cortical evoked potential trace. A straight line EOG indicated very little or no eye movement. A rise then dip in the EOG trace would indicate consistent eye movement. and, if the EOG is superimposed over the visual cortical evoked potential trace, the effect would be revealed by rises and dips in the potential at the same temporal position. None of our trials were repeated due to EOG contamination of the visual cortical evoked potential. Continuous monitoring of EEG during trials was accomplished with a Tektronix Oscilloscope, with storage capacity, and recordings were notably consistent and free of eye movement, general movement artifact or electrical noise contamination.
Results The mean amplitudes ( p )and latencies (msec.) of VEP components were computed from the X-Y plotter tracings. A component identified as N1 was considered to be the first negative dip in the VEP trace which occurred 50 msec. after the stimulus. The amplitude of the N1 component was measured as the vertical distance from "baseline" to the trough of this first
J. L. ANDREASSI, ET AL.
374
depression. The baseline was determined by the horizontal X-Y trace to the point where the first continuous negative excursion began. The P1 component was measured as the vertical distance from the N1 peak to the peak of the first positive component, while N2 was the vertical distance from P1 to the trough of the second major depression, and so on. Latencies (or time after stimulus presentation) were measured to the midpoints of each positive and negative peak. If the "peak was flat and appeared as a plateau, the midpoint of the plateau was taken as the latency measure. The main analyses of this experiment are for the most consistent and largest components (PI, N2, and P2). The mean amplitudes for the various components under the three conditions are presented in Table 1. Preliminary examination of Table 1 indicates slightly higher amplitudes under Condition A (suggested brightness) than B (suggested dimness) and C (no suggestion). The amplitude data were analyzed for significance by the Wilcoxon T test for matched pairs (Siegel, 1956). All possible pairs for P1, N2 and P2 components were tested, i.e., A vs B, A vs C, and B vs C, and none were significant ( p > .05, 2-tailed criterion used throughout). The latency data also are presented in Table 1. It can be seen in Table 1 that the latencies of the visual cortical evoked potential under the three conditions were very similar. Their high degree of similarity was confirmed by Wilcoxon Ts, to test for significance ( p > .05 for all latency comparisons). T o answer the question regarding the degree to which the suggestion was accepted by subjects, their protocols were analyzed to determine how many times they used the cernis "bright," "dim," or "no comment" under the three conditions. For Condition A, the stimulus was termed "bright" in 75% of the cases, while brightness was not commented upon 25% of the time. The response of "dim" was used in 40% of the responses for B and not commented upon the rest of the time. It seemed as though the suggestion of brightness was TABLE 1 MEANAMPLITUDE ( p V ) AND MEANLATENCY (MsEc.) OF COMPONENTS OF VISUAL CORTICAL EVOKED POTENTIALS UNDERTHREECONDITIONS( n = 1 2 ) Component of Visual Cortical Evoked Potential
Condition A
B
Mean Amplitude ( p v ) 4.75 3.89 9.37 8.57 6.69 6.63 6.27 6.25 Mean Latency (msec.) 88 86
C 4.39 8.58 6.29 6.08 90
SUGGESTION AND VISUAL CORTICAL EVOKED POTENTIAL
375
accepted more readily than the one of dimness. In the no-suggestion condition (C), "bright" was mentioned in 40% of the responses and "dim" in 12% of the records. The superimposed traces for one of our subjeccs (E.G.) are shown in Fig. 1. The traces reveal a slightly higher amplitude under Condition A as compared to B and C. However, this trend was seen to be reversed in other subjects, resulting in a lack of significant amplitude differences using group data. A second experiment was conducted to determine whether a different approach would provide results similar to those of Exp. I. It was decided to divide the experiment into two sessions, each one conducted on a separate day, to prevent possible carry-over effects from a hypnotic induction condition to one in which no hypnosis was used.
FIG. 1. VEP traces superimposed for one subject (E.G.) under Conditions A, B, and C. Negativity is downward.
0 MI PI
0
100 2 0 0 300 4 0 0 TIME IN MILLISECONDS
500
EXPERIMENTI1 Method Subjects.-The subjects were one male and five female students at Baruch College of the City Universiry of New York. The subjects had no visual-system defects. Apparatu~and procedure.-The apparatus and recording conditions were the same as in Exp. I. The EEG was recorded from 0. and EOG from above the left eye. The selection of the subjects and process of hypnotic induction were the same as in the previous experiment Major changes for the present study involved ( 1 ) rewording of instruction to s u b jects; ( 2 ) reduction from three to two conditions; ( 3 ) increased number of trials spread
J. L. ANDREASSI, ET AL.
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out over two days; and ( 4 ) alternating hypnotic and non-hypnotic experimental sessions. The following represent the revised conditions used in the preeent study: Condition B: "Keep your eyes closed and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. The light flashes will be very d i n , but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. You will see them as u w y dim. Count them silently, to yourself, as they are presented. (The hypnotist repeated the phrase 'you will see them very d i n ' three more times for a total of 5 times.)" Condition C: "Keep your eyes closed and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you Watch the light flashes and count each flash with your eyes closed. Count them silently, to yourself, as they are presented. (The hypnotist repeated the phrase 'count each flash' three more times for a total of 5 times.)" The third and fourth changes dealt with the number of trials and testing days and alternating hypnotic and non-hypnotic sessions. Six VEP traces were obtained for each person under the two conditions. The six subjects were divided into two equal groups. The first group of three subjects were tested under hypnosis the first day and then under normal (non-hypnotic) conditions the second day, and vice versa for the second group of three subjects. After the presentation of each condition, subjects were encouraged to continue relaxing and told that they were doing well. Also, each subject was asked to report the number of flashes they saw. In addition, they were asked to indicaie how the light seemed to them, as in Exp. I. These verbal reports were then recorded by the experimenter. The two conditions were counterbalanced across six subjects and two days. Each subjcct was presented with each condition six times during the experiment, for a total of 12 trials and 12 traces of the visual cortical evoked potential from 0.. In all trials, as in Exp. I, the light intensity did not vary.
Resalts The amplitude and latency data were obtained in the same manner as for Exp. I. Table 2 presents the mean amplitude data for the six individuals under the two conditions. There was a tendency for the components to have a slightly
MEAN AMPLITUDE(
p ~ AND )
TABLE 2 MEAN LATENCY (MSEC.)
OF
COMPONENTSOF VISUAL AND C ( n = 6 )
CORTICALEVOKED POTENTIALS UNDERCOND~TIONSB Component of Visual Cortical Evoked Potential
Condition B C Mean Amplitude
N1 P1 N2 P2 N1
5.45 10.97
4.60
8.40 6.00 Mean Latency 88
8.67 6.29
9.55
89
SUGGESTION A N D VISUAL CORTICAL EVOKED POTENTIAL
377
higher amplitude for Condition B. However, Wilcoxon matched-pairs tests indicated no significant amplitude differences for Conditions B vs C ( P I , N2, P2 ), using a two-tailed criterion ( p > .05 ). The latency data in Table 2 indicate little difference between conditions. The Wilcoxon values were all insignificant ( p > 05 ) . The subjective reports were in accord with the suggestion of dimness in 44% of the responses, while under Condition C a response of dimness was obtained in 33% of the reports. The superimposed VEP traces for one subject (D.C.) are presented in Fig. 2.
FIG. 2. VEP traces superimposed for one subject (D.C.) under Conditions B and C. Negarivity is downward.
R
C MI
- -
0
100
--
200 300 400 500
TIME IN MILLISECONDS
DISCUSSION The results of Erps. I and I1 indicate that a hypnotic suggestion of stimulus intensity change was not sufficient to elicit a reduction or increase in amplitude or latency of the visual cortical evoked potentials. W e are not aware of prior studies relating hypnotic suggestion to the evoked cortical potential and can only compare our results with those studies using some other physiological response measure. Our results appear to be in agreement with those of Schoenfeld, et al. (1973) in which a suggestion that the test stimulus was either "very loud" or "very soft" did not alter stapedial reflex threshold, although the experience of the two subjects was in accord with the suggestions. The studies reviewed by Barber (1973) which gave positive results in line with the hypnotic suggestions involved instructions which specified the type of change which was to take place. In our approach the subject was never instructed to alter evoked potential amplitude or latency. In fact, subjects were naive as to the nature of evoked potentials and the purpose of the experiment.
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378
Perhaps a more efficient approach would be to direct the subject to produce a change in his brain's response (either an increase or decrease). However, the question still remains as to whether a subject would know how to alter a VEP in the desired direction. It appears from several studies reviewed by Barber (1973) than an important factor in a study attempting to influence physiological variables through hypnotic suggestion is not whether an individual is truly hypnotized but that the suggestion given is accepted. The acceptance of the suggestion by our subjects was good for Condition A in Exp. 1 ( 7 5 % ) but only fair for B ( 4 0 % ) . Again, in Exp. 11, the acceptance of the "dim" suggestion was only fair for B ( 4 4 % ) . Thus, in Exp. I, the subjects did not vary in their visual cortical evolced potentials even though they accepted the suggestions for the most part under Condition A. In Exp. I1 the lack of changes was accompanied by a slight tendency to accept the suggestion given. The two experiments, as a whole, indicate some tendency to accept the suggestions but no change in the nervous system response to the stimulation. W e would have to conclude that, under the conditions of the present study, hypnotic suggestion of stimulus intensity change did not result in a change in the visual cortical evoked potential. REFERENCES
BARBER, T. X. Physiological effects of "hypnosis." Psychological Bulletin, 1961, 58, 390-419.
BARBER, T. X. Experimental hypnosis. In B. B. Wolrnan (Ed.), Handbook of general psychology. Englewood Cliffs, N . J.: Prentice-Hall, 1973. Pp. 942-963. CHERTOK,L., & KRAMARZ, P. Hypnosis, sleep and electroencephalography. ]ournal of Nervous and Mental Disease, 1959, 128, 227-238.
JASPER,H. H. Report of the committee on methods of examination
in electroencephalography. Electroencephalography and Clinic61 Neurophysiology, 1958, 10, 370375. KELLEY, C. R. Psychological factors in myopia. Unpublished doctoral dissertation, New School for Social Research, New York, 1958. SCHOENFELD, L. S., COOPER, J. C., JR., & MARTIN, F. N . Recruitment, loudness and the stapedial threshold under hypnosis. Pezceptual and Motor Skills, 1973, 36, 420422. SIEGEL,S. Nonparamet+ic statircics for the behavioral sciences. New York: McGrawHill, 1956. VAUGHAN, H. G., JR., COSTA, L. D., & GILDEN,L. The functional relation of visual evoked response and reaction time to stimulus intensity. Vision Resemch, 1966, 6, 645-656. VAUGHAN, H. G., JR., & HULL, R. C. Functional relation between stimulus intensicy and photically evoked cerebral responses in man. Nature, 1965, 206, 720-722.
Accepted November 24, 197.5.
@ Perceptual and Motor Skills 1976
HYPNOTIC SUGGESTION OF STIMULUS CHANGE A N D VISUAL CORTICAL EVOKED POTENTIAL1 J. L. ANDREASSI, B. BALWSKY, J. A. GALLICHIO,
J. J. DE SIMONE, B. W. MELLERS Baruch College, C i ~ yUniversity of New York' Summary.-Two experiments were conducted to determine whether the hypnotic suggestion of stimulus intensity change would affect the physical parameters (latencies, amplitudes) o f the visual cortical evoked potential. Stimulus intensity was kept constant as visual cortical evoked potentials were measured under conditions in which it was suggested that stimuli were "bright" or "dim." The results of both experiments led to the conclusion that, under the present condition, hypnotic suggestion of stimulus intensity change did not result in changes in visual cortical evoked potentials.
The problem that concerned us in this study was whether hypnotic suggestion could alter the amplitude of a physiological response, i.e., the visual cortical evoked potential (VEP). Studies of hypnotic suggestion and physiological responses, in general, seem to have produced varied results. For example, stapedial reflex threshold was not changed under hypnotic instruction that sounds were "loud" or "soft" although suggestion altered the experience of the two Ss tested in the expected direction (Schoenfeld, et al., 1973). Barber (1973) mentions several studies in which as many control subjects as hypnotized subjects produced the suggested change in physiological activity. For example, suggestions of heart-rate change were effective with the control group as with the hyponotized group (Klemrner, 1963, as cited in Barber, 1973). Visual acuity was improved in myopic individuals, both in hypnotic and control groups, through suggestion (Kelley, 1958) and allergic responses were altered in both control and hypnotized subjects (Ikemi & Nakagawa, 1962, as cited in Barber, 1973). In an earlier review, Barber (1961) concluded that "Physiological functions vary in the same way during 'hypnosis' as they do during 'waking' behavior" ( p . 411). With regard to brain activity in particular, it has been demonstrated that the hypnotized person shows a similar electroencephalogram (EEG) as when awake (Chertok & Kramarz, 1959). It is well known that amplitude of the visual evoked potential (VEP) is related to stimulus intensity, such that higher intensity stimuli produce greater amplitude responses and shorter latencies (e.g., see Vaughan & Hull, 1965; Vaughan, et a/., 1966). The present two experiments were conducted in an attempt to answer the question, "Will a suggested change of stimulus intensity result in a change in the visual evoked potential?" Three conditions were used 'This research was supported by the Environmental Physiology Branch of the Physiology Program, Office of Naval Research, under O N R Contract N00014-72-A-0406-0006, and ONR Contract Authority No. N R 201-053. '155 East 24th Street, New York, New York 10010.
372
J. L. ANDREASSI, ET AL.
in Exp. I to explore the effects of suggested "brightness," suggested "dimness," or no suggestion at all, about the intensity of invariant light flashes. Method Subjects.-The subjects were 7 male and 5 female undergraduate students at Baruch College of the City University of New York, ranging in age from 19 to 37 yr. The subjects had no visual system defects other than myopia (corrected to at least 20/30). A p p a ~ a c uand ~ p~ocedu~e.-Subjects were seated in an electrically shielded, soundattenuated room (IAC Chamber) during testing. All experimental sessions were conducted with the lights out in the chamber. In order to obtain the averaged cortical evoked potential, the electroencephalogram (EEG) of each subject was recorded from 0. ("Ten-Twenty" system; Jasper, 1958), with s silver cup electrode referenced to a silver clip electrode on the subject's left ear lobe. The subject was grounded by means of a clip electrode attached to the right ear lcading to "patient ground" of a Beckman RM Dynograph. The 9806A coupler of the Dynograph was used to condition the EEG signal (bandpass set at 0.5 to 32.0 H Z ) . The filtered and amplified signal was then fed into a Mnemotron Computer of Average Transients (CAT 1000). The elecuo-oculograrn (EOG) was measured by a separate channel of the Beckman Dynograph and averaged by the CAT to check on possible distortion of the visual evoked potential (VEP) due to excessive eye movement. A "start" signal from a Photo-stimulator triggered the CAT to take EEG and EOG samples of 500 msec. duration following the presentation of each stimulus to the subject. A Grass Model PS2 Photo-stimulator was used to present flashing light stimuli. The Photo-stimulator was set at Intensity 1 with an interstimulus interval of 1 sec. for all conditions. The PS2 was mounted at the subject's eye level outside the chamber at a distance of 71 cm. (28 in.). A Bausch and Lomb chin rest was used to keep the subject's head in a fixed position. After 100 stimulus presentations the summated VEP responses from CAT memory were plotted by a Hewlett-Packard X-Y Plotter. Selection of subjects was based on a group test of postural sway by the second author in rwo of his classes. Potential subjects were asked to close their eyes, extend their arms and sway while in a standing position. The degree of swaying, readiness to cyose eyes prior to being told, and the immediate closing of the eyes after instruction to do so were used as indicators of hypnotizability. Observance of these indicators was accomplished by the other authors. As each subject arrived for his/her appointment, it was explained that the purpose of the experiment was to measure brain responses to a flashing light while he/she was under hypnosis. Electrode leads were then applied to the appropriate locations and the subject was seated in the IAC Chamber. While the subject was in the chamber, a hypnotic state was induced by one of the experimenters (B.B.). The following is a summarized version of the hypnotic procedure: Initially, subjects were told to relax and that they would be placed in a very special state. They were instructed to listen very carefully to the hypnotist's voice and to disregard all other sounds they might hear. Each subject was asked to breathe deeply throughout the induction procedure. When the subject's eyes appeared to be closing, explicit instruction was given for them to close their eyes. The hypnotist would tell the subject that he/she feels very relaxed, very blissful, free of all worries-like sleeping on a cloud. A test of arm rigidity and sensitivity to pain (hand pinch) was then administered to indicate whether subjea was in a hypnotic state. The subjea was given the instruction once again to relax and listen only to the hypnotist's voice. The individual was then instructed to rest, place his chin and to relax while await-
SUGGESTION AND VISUAL CORTICAL EVOKED POTENTIAL
373
ing further instructions. The person was then read the instructions for Conditions A, B or C as in the following. Condition A : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash of light and tell me how many were presented when I ask you. The light flashes will be very bright, but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. The flashes are very bright. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'the flashes are very bright' three more times for a total of 5 times.)" Condition B : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. The light flashes will be very din,but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. The flashes are very dim. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'the flashes are very dim' three more times for a total of 5 times.)" Condition C : "You will close your eyes and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. Watch the light flashes and count each flash with your eyes closed. Count them silently, to yourself, as they are presented. (Hypnotist repeated the phrase 'count each flash' three more times for a total of 5 times.)" After the presentation of each condition, subjects were encouraged to continue relaxing and told they were doing well. Each subject was asked to report the number of flashes they counted. In addition, they were asked to indicate how the light seemed to them. If this did not elicit a comment about the intensity of the flash, they were further asked "what was the intensity like?" This was done to determine the degree to which the hypnotic suggestion was accepted. These verbal reports were recorded. The three conditions were completely counterbalanced across the 12 subjects in a Latin-square design. Each subject was presented with each condition two times in the experimental session, for a total of six trials and six visual cortical evoked potential traces from 0.. The EOG was measured from over the subject's left eye and averaged by the CAT to monitor possible distortion of the visual cortical evoked potential trace due to excessive eye movement or blink. Inspection for eye-movement contamination involved a comparison of the averaged EOG trace with the visual cortical evoked potential trace. A straight line EOG indicated very little or no eye movement. A rise then dip in the EOG trace would indicate consistent eye movement. and, if the EOG is superimposed over the visual cortical evoked potential trace, the effect would be revealed by rises and dips in the potential at the same temporal position. None of our trials were repeated due to EOG contamination of the visual cortical evoked potential. Continuous monitoring of EEG during trials was accomplished with a Tektronix Oscilloscope, with storage capacity, and recordings were notably consistent and free of eye movement, general movement artifact or electrical noise contamination.
Results The mean amplitudes ( p )and latencies (msec.) of VEP components were computed from the X-Y plotter tracings. A component identified as N1 was considered to be the first negative dip in the VEP trace which occurred 50 msec. after the stimulus. The amplitude of the N1 component was measured as the vertical distance from "baseline" to the trough of this first
J. L. ANDREASSI, ET AL.
374
depression. The baseline was determined by the horizontal X-Y trace to the point where the first continuous negative excursion began. The P1 component was measured as the vertical distance from the N1 peak to the peak of the first positive component, while N2 was the vertical distance from P1 to the trough of the second major depression, and so on. Latencies (or time after stimulus presentation) were measured to the midpoints of each positive and negative peak. If the "peak was flat and appeared as a plateau, the midpoint of the plateau was taken as the latency measure. The main analyses of this experiment are for the most consistent and largest components (PI, N2, and P2). The mean amplitudes for the various components under the three conditions are presented in Table 1. Preliminary examination of Table 1 indicates slightly higher amplitudes under Condition A (suggested brightness) than B (suggested dimness) and C (no suggestion). The amplitude data were analyzed for significance by the Wilcoxon T test for matched pairs (Siegel, 1956). All possible pairs for P1, N2 and P2 components were tested, i.e., A vs B, A vs C, and B vs C, and none were significant ( p > .05, 2-tailed criterion used throughout). The latency data also are presented in Table 1. It can be seen in Table 1 that the latencies of the visual cortical evoked potential under the three conditions were very similar. Their high degree of similarity was confirmed by Wilcoxon Ts, to test for significance ( p > .05 for all latency comparisons). T o answer the question regarding the degree to which the suggestion was accepted by subjects, their protocols were analyzed to determine how many times they used the cernis "bright," "dim," or "no comment" under the three conditions. For Condition A, the stimulus was termed "bright" in 75% of the cases, while brightness was not commented upon 25% of the time. The response of "dim" was used in 40% of the responses for B and not commented upon the rest of the time. It seemed as though the suggestion of brightness was TABLE 1 MEANAMPLITUDE ( p V ) AND MEANLATENCY (MsEc.) OF COMPONENTS OF VISUAL CORTICAL EVOKED POTENTIALS UNDERTHREECONDITIONS( n = 1 2 ) Component of Visual Cortical Evoked Potential
Condition A
B
Mean Amplitude ( p v ) 4.75 3.89 9.37 8.57 6.69 6.63 6.27 6.25 Mean Latency (msec.) 88 86
C 4.39 8.58 6.29 6.08 90
SUGGESTION AND VISUAL CORTICAL EVOKED POTENTIAL
375
accepted more readily than the one of dimness. In the no-suggestion condition (C), "bright" was mentioned in 40% of the responses and "dim" in 12% of the records. The superimposed traces for one of our subjeccs (E.G.) are shown in Fig. 1. The traces reveal a slightly higher amplitude under Condition A as compared to B and C. However, this trend was seen to be reversed in other subjects, resulting in a lack of significant amplitude differences using group data. A second experiment was conducted to determine whether a different approach would provide results similar to those of Exp. I. It was decided to divide the experiment into two sessions, each one conducted on a separate day, to prevent possible carry-over effects from a hypnotic induction condition to one in which no hypnosis was used.
FIG. 1. VEP traces superimposed for one subject (E.G.) under Conditions A, B, and C. Negativity is downward.
0 MI PI
0
100 2 0 0 300 4 0 0 TIME IN MILLISECONDS
500
EXPERIMENTI1 Method Subjects.-The subjects were one male and five female students at Baruch College of the City Universiry of New York. The subjects had no visual-system defects. Apparatu~and procedure.-The apparatus and recording conditions were the same as in Exp. I. The EEG was recorded from 0. and EOG from above the left eye. The selection of the subjects and process of hypnotic induction were the same as in the previous experiment Major changes for the present study involved ( 1 ) rewording of instruction to s u b jects; ( 2 ) reduction from three to two conditions; ( 3 ) increased number of trials spread
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out over two days; and ( 4 ) alternating hypnotic and non-hypnotic experimental sessions. The following represent the revised conditions used in the preeent study: Condition B: "Keep your eyes closed and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you. The light flashes will be very d i n , but you will still be able to count them. Watch the light flashes and count each one with your eyes closed. You will see them as u w y dim. Count them silently, to yourself, as they are presented. (The hypnotist repeated the phrase 'you will see them very d i n ' three more times for a total of 5 times.)" Condition C: "Keep your eyes closed and flashes of light will be presented to you through your closed eyelids. You will silently, to yourself, count each flash and tell me how many were presented when I ask you Watch the light flashes and count each flash with your eyes closed. Count them silently, to yourself, as they are presented. (The hypnotist repeated the phrase 'count each flash' three more times for a total of 5 times.)" The third and fourth changes dealt with the number of trials and testing days and alternating hypnotic and non-hypnotic sessions. Six VEP traces were obtained for each person under the two conditions. The six subjects were divided into two equal groups. The first group of three subjects were tested under hypnosis the first day and then under normal (non-hypnotic) conditions the second day, and vice versa for the second group of three subjects. After the presentation of each condition, subjects were encouraged to continue relaxing and told that they were doing well. Also, each subject was asked to report the number of flashes they saw. In addition, they were asked to indicaie how the light seemed to them, as in Exp. I. These verbal reports were then recorded by the experimenter. The two conditions were counterbalanced across six subjects and two days. Each subjcct was presented with each condition six times during the experiment, for a total of 12 trials and 12 traces of the visual cortical evoked potential from 0.. In all trials, as in Exp. I, the light intensity did not vary.
Resalts The amplitude and latency data were obtained in the same manner as for Exp. I. Table 2 presents the mean amplitude data for the six individuals under the two conditions. There was a tendency for the components to have a slightly
MEAN AMPLITUDE(
p ~ AND )
TABLE 2 MEAN LATENCY (MSEC.)
OF
COMPONENTSOF VISUAL AND C ( n = 6 )
CORTICALEVOKED POTENTIALS UNDERCOND~TIONSB Component of Visual Cortical Evoked Potential
Condition B C Mean Amplitude
N1 P1 N2 P2 N1
5.45 10.97
4.60
8.40 6.00 Mean Latency 88
8.67 6.29
9.55
89
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377
higher amplitude for Condition B. However, Wilcoxon matched-pairs tests indicated no significant amplitude differences for Conditions B vs C ( P I , N2, P2 ), using a two-tailed criterion ( p > .05 ). The latency data in Table 2 indicate little difference between conditions. The Wilcoxon values were all insignificant ( p > 05 ) . The subjective reports were in accord with the suggestion of dimness in 44% of the responses, while under Condition C a response of dimness was obtained in 33% of the reports. The superimposed VEP traces for one subject (D.C.) are presented in Fig. 2.
FIG. 2. VEP traces superimposed for one subject (D.C.) under Conditions B and C. Negarivity is downward.
R
C MI
- -
0
100
--
200 300 400 500
TIME IN MILLISECONDS
DISCUSSION The results of Erps. I and I1 indicate that a hypnotic suggestion of stimulus intensity change was not sufficient to elicit a reduction or increase in amplitude or latency of the visual cortical evoked potentials. W e are not aware of prior studies relating hypnotic suggestion to the evoked cortical potential and can only compare our results with those studies using some other physiological response measure. Our results appear to be in agreement with those of Schoenfeld, et al. (1973) in which a suggestion that the test stimulus was either "very loud" or "very soft" did not alter stapedial reflex threshold, although the experience of the two subjects was in accord with the suggestions. The studies reviewed by Barber (1973) which gave positive results in line with the hypnotic suggestions involved instructions which specified the type of change which was to take place. In our approach the subject was never instructed to alter evoked potential amplitude or latency. In fact, subjects were naive as to the nature of evoked potentials and the purpose of the experiment.
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Perhaps a more efficient approach would be to direct the subject to produce a change in his brain's response (either an increase or decrease). However, the question still remains as to whether a subject would know how to alter a VEP in the desired direction. It appears from several studies reviewed by Barber (1973) than an important factor in a study attempting to influence physiological variables through hypnotic suggestion is not whether an individual is truly hypnotized but that the suggestion given is accepted. The acceptance of the suggestion by our subjects was good for Condition A in Exp. 1 ( 7 5 % ) but only fair for B ( 4 0 % ) . Again, in Exp. 11, the acceptance of the "dim" suggestion was only fair for B ( 4 4 % ) . Thus, in Exp. I, the subjects did not vary in their visual cortical evolced potentials even though they accepted the suggestions for the most part under Condition A. In Exp. I1 the lack of changes was accompanied by a slight tendency to accept the suggestion given. The two experiments, as a whole, indicate some tendency to accept the suggestions but no change in the nervous system response to the stimulation. W e would have to conclude that, under the conditions of the present study, hypnotic suggestion of stimulus intensity change did not result in a change in the visual cortical evoked potential. REFERENCES
BARBER, T. X. Physiological effects of "hypnosis." Psychological Bulletin, 1961, 58, 390-419.
BARBER, T. X. Experimental hypnosis. In B. B. Wolrnan (Ed.), Handbook of general psychology. Englewood Cliffs, N . J.: Prentice-Hall, 1973. Pp. 942-963. CHERTOK,L., & KRAMARZ, P. Hypnosis, sleep and electroencephalography. ]ournal of Nervous and Mental Disease, 1959, 128, 227-238.
JASPER,H. H. Report of the committee on methods of examination
in electroencephalography. Electroencephalography and Clinic61 Neurophysiology, 1958, 10, 370375. KELLEY, C. R. Psychological factors in myopia. Unpublished doctoral dissertation, New School for Social Research, New York, 1958. SCHOENFELD, L. S., COOPER, J. C., JR., & MARTIN, F. N . Recruitment, loudness and the stapedial threshold under hypnosis. Pezceptual and Motor Skills, 1973, 36, 420422. SIEGEL,S. Nonparamet+ic statircics for the behavioral sciences. New York: McGrawHill, 1956. VAUGHAN, H. G., JR., COSTA, L. D., & GILDEN,L. The functional relation of visual evoked response and reaction time to stimulus intensity. Vision Resemch, 1966, 6, 645-656. VAUGHAN, H. G., JR., & HULL, R. C. Functional relation between stimulus intensicy and photically evoked cerebral responses in man. Nature, 1965, 206, 720-722.
Accepted November 24, 197.5.
