- Email: [email protected]
UNCONSCIOUS PERCEPTION DURING GENERAL ANAESTHESIA
BRITISH JOURNAL OF ANAESTHESIA
1334
B. BONKE
P. I. M. SCHMTTZ F. VERHAGE
Rotterdam A. ZWAVELING
Leiden REFERENCES Bonke, B., Schmitz, P. I. M., Verhage, F., and Zwaveling, A. (1986). Clinical study of so-called unconscious perception during general anaesthesia. Br. J. Anaesth., 58, 957. Editorial (1986). Advertising during anaesthesia? Lancet, 1, 1019. Jones, J. G., and Konieczko, K. (1986). Hearing and memory in anaesthetised patients. Br. Med. J., 292, 1291. Levinson, B. W. (1965). States of awareness during general anaesthesia. Br. J. Anaesth., 37, 544. (1967). States of awareness during general anaesthesia; in Hypnosis and Psychosomatic Medicine (ed J. Lassner), pp. 201, 202. Berlin: Springer. Pearson, R. E. (1961). Response to suggestions given under general anesthesia. Am. J. Clin. Hypn., 4, 106. Sir,—Bennett, Davis and Giannini (1985) described a study to examine non-verbal response to intraoperative conversation. While anaesthetized, 11 patients were presented with the taperecorded suggestion that, in a postoperative interview, they would pull their ear to indicate that they had heard the intraoperative message. A control group of 22 patients was presented with operating room sounds. Subsequently, no patient had any conscious recall of intraoperative events, but the test patients were reported to be significantly more likely to touch their ear during the interview, and to spend longer total time in ear-pulling (experimental group made 66 ear-pulls for 667 s; control group made 18 ear-pulls for 98 s). While the authors claim that the data are evidence for a non-verbal response to intraoperative conversation (i.e. memory for events during anaesthesia), one should be guarded about the reliability of the results for the following reasons. The primary difficulty concerns the methodology, where there is an omission of any pre-surgical assessment of "earpulling" in order to determine the baseline or natural frequency of the behaviour. Had such an assessment been
made, one might then constrast the frequency of the behaviour before and after surgery. However, without such a baseline assessment one cannot determine unequivocally whether the post-surgical difference between the groups is the result of the intra-anaesthetic suggestion or of chance allocation to the experimental group of patients with a naturally higher frequency of the behaviour. With such small sample sizes and the biased allocation of patients to groups (2:1; control v. experimental), the latter probability is high. The second major difficulty stems from the small size of the experimental group, where extreme reactions by one or two patients may give a misleading result. This is well illustrated by the measure of total time spent ear-pulling: two patients each spent 300 s pulling their ears, accounting between them for 600 s of the total 667 s spent by the 11 members of the group in ear-pulling. (Note that the 655 s total time cited by Bennett and co-workers in the text of the paper is incorrect both in the present Journal and in another report of the study published elsewhere (Bennett, Davis and Giannini, 1984); their table I provides data for each experimental patient and sums to 667 s). These same two patients also account for 29 of the 66 separate ear-pulls carried out by the experimental group. If the latter two patients were excluded from the analysis, it is unlikely that the experimental and control groups would differ significantly. The reader might make such an analysis for himself, but Bennett, Davis and Giannini (1985) provided raw data only for the experimental group. The significance of the results is also thrown in doubt by the authors' use of a completely inappropriate statistical test of tetrachoric correlation between the variables of patientgrouping (experimental v. control) and the ear-touch response. That test is appropriate when both variables are continuous but, in the study concerned the variable of patient grouping is obviously discrete. The correct test to use in this case, where one variable is discrete and the other continuous, is the point biserial correlation. Again, the reader is unable to make such an analysis because the control group data are omitted from the report. One might also point out that a Fischer exact test carried out by the authors on the same data gave a value of P = 0.05. Clearly, by this measure, the difference the groups lies very much on the borderline between a significant treatment effect and chance alone. Finally, the authors regard as significant the fact that an anaesthetized patient, within whose presence a surgeon made highly pessimistic comments concerning the quality of the bone graft being performed, took longer to recover and required more analgesics than other patients. The implicit conclusion is that exposure to the surgeon's comments had exerted a subconscious adverse influence upon recovery. However, given the surgeon's objective assessment of the unsatisfactory surgical procedure, it is surely more appropriate (and logical) to conclude that the patient's extended and painful recovery was a direct consequence of physical discomfort arising from imperfect surgery, rather than of subconscious trauma. In conclusion, while one would not deny that there is evidence for subconscious registration of traumatic and neutral material during anaesthesia (for example Levinson (1965) and Millar and Watkinson (1983)), one would suggest that failings in the methodology, analysis and inference of the study by Bennett, Davis and Giannini (1985) render their results somewhat inconclusive. K. Glasgow
Downloaded from http://bja.oxfordjournals.org/ at University of California, Santa Barbara on June 29, 2015
providing an overall effect estimate adjusted for age, as Drs Wilson and Spiegelhalter proposed, had become meaningless. Of course, "final" evidence will, as usual, not be established until future studies have confirmed our findings. We would also like to add a few comments about the often cited study by Levinson (1965). In 1967, Levinson published a slightly more extensive report on his controversial study with hypnotizable, young patients. He reported that, apart from the four patients who recalled the pseudo-crisis almost verbatim, another four "displayed a severe degree of anxiety while reliving the operation .. ., woke from hypnosis and refused to participate further." Not only are these reactions to hypnosis quite uncommon—certainly in such a high proportion of subjects—but it also seems inappropriate to ascribe these responses to "leading questions" or knowledge of the nature of the feigned crisis, by the hypnotist. Levinson's results may not have provided completely adequate evidence for so-called unconscious perception during general anaesthesia, but doing away with them by implying that his work can be ignored, is like throwing the baby out with the bathwater.
CORRESPONDENCE
1335
REFERENCES Bennett, H. L., Davis, H. S., and Giannini, J. A. (1984). Nonverbal response to intraoperative conversation. Anesth. Analg., 63, 185. (1985). Non-verbal response to intraoperative conversation. Br. J. Anaeith., 57, 174. Lcvinson, B. W. (1965). States of awareness during general anaesthesia. Br. J. Anaesth., 37, 544. Millar, K., and Watkinson, N. (1983). Recognition of words presented during general anaesthesia. Ergonomics, 26, 585.
TABLE I. Data from experimental and control groups showing number of ear touches observed in postoperative interview and duration of total ear touches
Experimental group
Control group
Patient No.
No. ear touches
Duration
Patient No.
No. ear touches
Duration
00
1 2 3 4 5 6 7 8 9 10 11
1 2 1 17 9 0 0 6 14 4 12
15 2 1 300 25 0 0 6 14 4 300
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
0 0 3 0 2 0 1 0 2 0 0 0 0 1 4 0 0 2 2 0 1
0 0 11 0 4 0 1 0 2 0 0 0 0 60 12 0 0 5 2 0 1
00
Downloaded from http://bja.oxfordjournals.org/ at University of California, Santa Barbara on June 29, 2015
Sir,—I respond first to the points made by Dr Millar concerning our research on assessment of intraoperative responses to operating room conversation (Bennett, Davis and Giannini, 1985). Dr Millar asks that readers question the reliability of our results which indicated that patients can be instructed during operation on to carry out a behaviour after operation. Reliability is necessary to validity, and his is therefore a serious charge. In response, three answers are necessary: first, the study was our second. In the first (Bennett, Davis and Giannini, 1984) we believe we had established a reliable nonverbal indicator of intraoperative hearing and thus proceeded with several refinements to examine this specifically with the second study. Second, the response has been replicated by Goldman (1986) using chin touching. Third, we have a series of 48 patients that continue to show that the response can be elicited as long as highly motivating consent interviews arc given to agreeable patients along with convincing and personally meaningful intraoperative appeals to engage in a consciously unmonitored behaviour after operation. Thus
while healthy scepticism is warranted, my position on the topic must remain with what the data state. In his critique, Millar suggests a "control count" of preoperative ear touching was necessary. While perhaps desirable, such a count is no more necessary than the degree of preoperative ear wax might be. By random assignment, behaviours, attributes, and differences among all extraneous variables are assumed to vary equally. There is no reason to expect a priori favouritism towards ear touching. Giannini, the third author of our original paper, for his doctoral dissertation, counted preoperative ear touching behaviour in his replication attempt. The behaviour was present to a minor degree and was equally spread over experimental and control groups. Preoperative ear touching did not affect the statistical analysis of the data. Our own more recent studies support this: preoperative behaviours are not a confounding factor in eliciting the response. In the next paragraph, Millar wishes to exclude patients showing interesting data (large numbers of ear touching behaviours in the postoperative interview). Then the author wishes we had included raw data from the control group of 21 patients which describes ear touching during the postoperative interview. We had not anticipated such interest in our data and merely included summary frequencies for the control group at the bottom of figure 1 of the original article. The raw control group data are presented now (table I); it was our error not to include them originally. After these promising results, we have since looked more closely at this response, a non-verbal response to an intraoperative message addressed to the patient, and found it to be robust (Bennett, Boyle and Willits, unpublished observations). To Dr Millar's next point, we acknowledge the preference of one type of correlational analysis over another. Our use of a
1334
B. BONKE
P. I. M. SCHMTTZ F. VERHAGE
Rotterdam A. ZWAVELING
Leiden REFERENCES Bonke, B., Schmitz, P. I. M., Verhage, F., and Zwaveling, A. (1986). Clinical study of so-called unconscious perception during general anaesthesia. Br. J. Anaesth., 58, 957. Editorial (1986). Advertising during anaesthesia? Lancet, 1, 1019. Jones, J. G., and Konieczko, K. (1986). Hearing and memory in anaesthetised patients. Br. Med. J., 292, 1291. Levinson, B. W. (1965). States of awareness during general anaesthesia. Br. J. Anaesth., 37, 544. (1967). States of awareness during general anaesthesia; in Hypnosis and Psychosomatic Medicine (ed J. Lassner), pp. 201, 202. Berlin: Springer. Pearson, R. E. (1961). Response to suggestions given under general anesthesia. Am. J. Clin. Hypn., 4, 106. Sir,—Bennett, Davis and Giannini (1985) described a study to examine non-verbal response to intraoperative conversation. While anaesthetized, 11 patients were presented with the taperecorded suggestion that, in a postoperative interview, they would pull their ear to indicate that they had heard the intraoperative message. A control group of 22 patients was presented with operating room sounds. Subsequently, no patient had any conscious recall of intraoperative events, but the test patients were reported to be significantly more likely to touch their ear during the interview, and to spend longer total time in ear-pulling (experimental group made 66 ear-pulls for 667 s; control group made 18 ear-pulls for 98 s). While the authors claim that the data are evidence for a non-verbal response to intraoperative conversation (i.e. memory for events during anaesthesia), one should be guarded about the reliability of the results for the following reasons. The primary difficulty concerns the methodology, where there is an omission of any pre-surgical assessment of "earpulling" in order to determine the baseline or natural frequency of the behaviour. Had such an assessment been
made, one might then constrast the frequency of the behaviour before and after surgery. However, without such a baseline assessment one cannot determine unequivocally whether the post-surgical difference between the groups is the result of the intra-anaesthetic suggestion or of chance allocation to the experimental group of patients with a naturally higher frequency of the behaviour. With such small sample sizes and the biased allocation of patients to groups (2:1; control v. experimental), the latter probability is high. The second major difficulty stems from the small size of the experimental group, where extreme reactions by one or two patients may give a misleading result. This is well illustrated by the measure of total time spent ear-pulling: two patients each spent 300 s pulling their ears, accounting between them for 600 s of the total 667 s spent by the 11 members of the group in ear-pulling. (Note that the 655 s total time cited by Bennett and co-workers in the text of the paper is incorrect both in the present Journal and in another report of the study published elsewhere (Bennett, Davis and Giannini, 1984); their table I provides data for each experimental patient and sums to 667 s). These same two patients also account for 29 of the 66 separate ear-pulls carried out by the experimental group. If the latter two patients were excluded from the analysis, it is unlikely that the experimental and control groups would differ significantly. The reader might make such an analysis for himself, but Bennett, Davis and Giannini (1985) provided raw data only for the experimental group. The significance of the results is also thrown in doubt by the authors' use of a completely inappropriate statistical test of tetrachoric correlation between the variables of patientgrouping (experimental v. control) and the ear-touch response. That test is appropriate when both variables are continuous but, in the study concerned the variable of patient grouping is obviously discrete. The correct test to use in this case, where one variable is discrete and the other continuous, is the point biserial correlation. Again, the reader is unable to make such an analysis because the control group data are omitted from the report. One might also point out that a Fischer exact test carried out by the authors on the same data gave a value of P = 0.05. Clearly, by this measure, the difference the groups lies very much on the borderline between a significant treatment effect and chance alone. Finally, the authors regard as significant the fact that an anaesthetized patient, within whose presence a surgeon made highly pessimistic comments concerning the quality of the bone graft being performed, took longer to recover and required more analgesics than other patients. The implicit conclusion is that exposure to the surgeon's comments had exerted a subconscious adverse influence upon recovery. However, given the surgeon's objective assessment of the unsatisfactory surgical procedure, it is surely more appropriate (and logical) to conclude that the patient's extended and painful recovery was a direct consequence of physical discomfort arising from imperfect surgery, rather than of subconscious trauma. In conclusion, while one would not deny that there is evidence for subconscious registration of traumatic and neutral material during anaesthesia (for example Levinson (1965) and Millar and Watkinson (1983)), one would suggest that failings in the methodology, analysis and inference of the study by Bennett, Davis and Giannini (1985) render their results somewhat inconclusive. K. Glasgow
Downloaded from http://bja.oxfordjournals.org/ at University of California, Santa Barbara on June 29, 2015
providing an overall effect estimate adjusted for age, as Drs Wilson and Spiegelhalter proposed, had become meaningless. Of course, "final" evidence will, as usual, not be established until future studies have confirmed our findings. We would also like to add a few comments about the often cited study by Levinson (1965). In 1967, Levinson published a slightly more extensive report on his controversial study with hypnotizable, young patients. He reported that, apart from the four patients who recalled the pseudo-crisis almost verbatim, another four "displayed a severe degree of anxiety while reliving the operation .. ., woke from hypnosis and refused to participate further." Not only are these reactions to hypnosis quite uncommon—certainly in such a high proportion of subjects—but it also seems inappropriate to ascribe these responses to "leading questions" or knowledge of the nature of the feigned crisis, by the hypnotist. Levinson's results may not have provided completely adequate evidence for so-called unconscious perception during general anaesthesia, but doing away with them by implying that his work can be ignored, is like throwing the baby out with the bathwater.
CORRESPONDENCE
1335
REFERENCES Bennett, H. L., Davis, H. S., and Giannini, J. A. (1984). Nonverbal response to intraoperative conversation. Anesth. Analg., 63, 185. (1985). Non-verbal response to intraoperative conversation. Br. J. Anaeith., 57, 174. Lcvinson, B. W. (1965). States of awareness during general anaesthesia. Br. J. Anaesth., 37, 544. Millar, K., and Watkinson, N. (1983). Recognition of words presented during general anaesthesia. Ergonomics, 26, 585.
TABLE I. Data from experimental and control groups showing number of ear touches observed in postoperative interview and duration of total ear touches
Experimental group
Control group
Patient No.
No. ear touches
Duration
Patient No.
No. ear touches
Duration
00
1 2 3 4 5 6 7 8 9 10 11
1 2 1 17 9 0 0 6 14 4 12
15 2 1 300 25 0 0 6 14 4 300
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
0 0 3 0 2 0 1 0 2 0 0 0 0 1 4 0 0 2 2 0 1
0 0 11 0 4 0 1 0 2 0 0 0 0 60 12 0 0 5 2 0 1
00
Downloaded from http://bja.oxfordjournals.org/ at University of California, Santa Barbara on June 29, 2015
Sir,—I respond first to the points made by Dr Millar concerning our research on assessment of intraoperative responses to operating room conversation (Bennett, Davis and Giannini, 1985). Dr Millar asks that readers question the reliability of our results which indicated that patients can be instructed during operation on to carry out a behaviour after operation. Reliability is necessary to validity, and his is therefore a serious charge. In response, three answers are necessary: first, the study was our second. In the first (Bennett, Davis and Giannini, 1984) we believe we had established a reliable nonverbal indicator of intraoperative hearing and thus proceeded with several refinements to examine this specifically with the second study. Second, the response has been replicated by Goldman (1986) using chin touching. Third, we have a series of 48 patients that continue to show that the response can be elicited as long as highly motivating consent interviews arc given to agreeable patients along with convincing and personally meaningful intraoperative appeals to engage in a consciously unmonitored behaviour after operation. Thus
while healthy scepticism is warranted, my position on the topic must remain with what the data state. In his critique, Millar suggests a "control count" of preoperative ear touching was necessary. While perhaps desirable, such a count is no more necessary than the degree of preoperative ear wax might be. By random assignment, behaviours, attributes, and differences among all extraneous variables are assumed to vary equally. There is no reason to expect a priori favouritism towards ear touching. Giannini, the third author of our original paper, for his doctoral dissertation, counted preoperative ear touching behaviour in his replication attempt. The behaviour was present to a minor degree and was equally spread over experimental and control groups. Preoperative ear touching did not affect the statistical analysis of the data. Our own more recent studies support this: preoperative behaviours are not a confounding factor in eliciting the response. In the next paragraph, Millar wishes to exclude patients showing interesting data (large numbers of ear touching behaviours in the postoperative interview). Then the author wishes we had included raw data from the control group of 21 patients which describes ear touching during the postoperative interview. We had not anticipated such interest in our data and merely included summary frequencies for the control group at the bottom of figure 1 of the original article. The raw control group data are presented now (table I); it was our error not to include them originally. After these promising results, we have since looked more closely at this response, a non-verbal response to an intraoperative message addressed to the patient, and found it to be robust (Bennett, Boyle and Willits, unpublished observations). To Dr Millar's next point, we acknowledge the preference of one type of correlational analysis over another. Our use of a