2011. Vol.2, No.3, 143-149
Copyright © 2011 SciRes. DOI:10.4236/psych.2011.23023
Does a BIS-Guided Maintenance of Anesthetic Depth Prevent
Implicit Memory?
Mehmet Sertac Ozcan1, Scott Douglas Gronlund2, Ryan Trojan3, Qaiser Khan3,
Jorge Cure3, Carson Wong 4
1Department of Anesthesiology, College of Medicine, University of Illinois at Chicago, Chicago, USA;
2Department of Psychology, College of Arts and Sciences, the University of Oklahoma, Norman, USA;
3Department of Anesthesiology, College of Medicine, the University of Oklahoma Health Sciences Cen-
ter, Oklahoma City, USA;
4Department of Urology, College of Medicine, the University of Oklahoma Health Sciences Center,
Oklahoma City, USA.
Received January 6th, 2011; revised February 28th, 2011; accepted April 1st, 2011.
Recent studies investigating the relationship between depth of anesthesia and implicit memory have conflicting
results. Limitations of these studies include lack of standardization in surgical procedures and failure to control
depth of anesthesia prospectively. We assessed implicit memory function at two different (BIS-guided and pro-
spectively controlled) anesthetic depths during surgical stimulus. A list of words was played via headphones to
37 patients during orthopedic surgery under general anesthesia. The Bispectral (BIS) Index was monitored and
patients were randomized to remain in a deeper (BIS 40 - 45) or lighter (BIS 55 - 60) plane of surgical anesthe-
sia during word presentation. Postoperatively, implicit memory performance was tested using a simple auditory
word-stem completion test for presented as well as non-presented words. Absence of explicit memory was
evaluated by asking four standard questions regarding intraoperative awareness. All patients received sevoflu-
rane and fentanyl for general anesthesia. There was no evidence of implicit memory in either study group. Hit
rates for presented and non-presented words were 0.23 ± 0.14 and 0.25 ± 0.09, respectively. No explicit memory
was reported by any patient. Depth of anesthesia did not affect implicit memory formation in anesthetized pa-
tients undergoing surgery. General anesthesia, even at a higher BIS range, appears to abolish implicit memory as
assessed by a simple word-stem completion test.
Keywords: Implicit Memory, General Anesthesia, Depth of Anesthesia
Memory during anesthesia continues to be an interesting
subject for anesthesiologists and psychologists alike. A review
of the recent literature suggests a continuing interest in the sub-
ject in the last decade (e.g., Andrade & Deeprose, 2007). This
coincides with recent advances in both anesthesiology and cog-
nitive psychology. Depth of anesthesia monitors such as the
Bispectral Index (BIS) are being increasingly used in clinical
practice and as well as in memory studies (Bowdle, 2006). On
the psychology front, Schacter (1987) has classified episodic
memory into explicit (or conscious) and implicit (or unconscious)
types. These advances inspired investigators to ask whether
explicit and implicit memory might be preserved under general
BIS is a patient monitor that quantifies the depth of the hyp-
notic component of general anesthesia (Bowdle, 2006). It uses
electroencephalogram recorded over the forehead and computes
a number (ranging from 0 to 99) using a proprietary algorithm.
This number is displayed on the monitor during an anesthetic
and it can help the anesthesiologist to titrate the anesthetic
agents. A BIS range of 40 to 60 has generally been accepted to
represent an ideal anesthetic depth of where explicit recall is
extremely unlikely (Bowdle, 2006).
Formation of explicit memories during general anesthesia is
associated with intraoperative awareness. This has been recog-
nized since the early days of clinical anesthesia practice (Wright
& Aldrete, 1987). In one survey (Stoelting, 1999), more than
one third of anesthesiologists ranked intraoperative awareness
as a major patient-safety concern. It has been associated with
morbidity such as post-traumatic stress disorder (Sandin, 2006).
Implicit memory during anesthesia has been recognized more
recently. Because there is no conscious recollection of implicit
memories, they can only be demonstrated by special tests. Pre-
senting a list of spoken words (priming) under anesthesia and
subsequent administration of a word-stem completion (WSC)
test after emergence from anesthesia is a common method used
for this purpose. In WSC, subjects are asked to complete the
word stem with the first word that comes to mind. The WSC
test has been demonstrated as a sensitive method of identifying
implicit memory under anesthesia (Andrade & Deeprose, 2007;
Deeprose, Andrade, Varma, & Edwards, 2004).
Alternatively, a process dissociation procedure (PDP) can be
used to detect and eliminate the contribution of explicit mem-
ory from true implicit memory effects (e.g., Jacoby, 1991). PDP
has inclusion and exclusion conditions. The inclusion condition
requires the subject to complete the word-stem with either a
word that was recalled from the intraoperative period, or with
the first word that comes to mind if there is no recall of a word
heard intraoperatively. In contrast, the exclusion condition re-
quires the subject to not complete the word-stem with a word
that can be recalled from the intraoperative period and instead
come up with another word that completes the stem. That is, the
subject excludes consciously recalled words. Simple algebraic
manipulations and subtractions result in estimates for the ex-
plicit an d implicit memory contributions (given the contentious
assumption of independence between these contributions). Al-
though using PDP purportedly increases the specificity of de-
tecting implicit memory effects, it is more complex, makes
controversial assumptions, and is awkward for the patient dur-
ing the perioperative period because the patient is asked to try to
recall a word from a period when they report not recalling any-
thing. Besides these concerns, PDP as a method for detecting
intraoperative implicit memory has been questioned for poten-
tially generating false positive results (Hadzidiakos et al., 2009).
Implicit memory formation without explicit recall during
general anesthesia has been demonstrated in a number of stud-
ies (e.g., Deeprose et al., 2009; Dobrunz, Jaeger, & Vetter,
2007; Iselin-Chaves et al, 2005; Lubke, Kerssens, Phaf, & Se-
bel, 1999; Stonell, Leslie, He, & Lee, 2006). However, there
are other studies in which these results were not replicated (e.g.,
Kerssens, Ouchi, & Sebel, 2005; Lequeux et al., 2005; Lequeux,
Cantraine, Levarlet, & Barvais, 2003; Munte et al. 2001; Rus-
sell & Wang, 2001). A review of these studies suggests that
depth of anesthesia and the presence of surgical stimulus as
factors that might modulate implicit memory formation. How-
ever, there are conflicting findings regarding both of these po-
tential factors. For instance, Stonell et al. reported that implicit
memory was present only when the BIS was greater than 50.
Iselin-Chaves et al. demonstrated that the incidence of implicit
memory was similar at BIS ranges of 41 - 60 and 61 - 80. On
the other hand, Kerssens et al. found no evidence of implicit
memory when BIS was between 40 and 60 during word prim-
ing. Unfortunately, only some of those studies conducted the
word priming procedure during surgical stimulus. Moreover,
the type of surgery varied (Iselin-Chaves et al., 2005; Kerssens
et al., 2005) or was not reported (Stonell et al., 2006). In addi-
tion, most of these articles did not specify whether the word
priming coincided with a consistently painful period during a
surgical procedure (e.g., immediately after incision) in all pa-
tients. In short, factors facilitating or preventing the formation
of implicit memory during general anesthesia are not clear from
the available data, because few studies effectively and prospec-
tively controlled these potential factors.
The aim of the present study was to investigate the effect of
depth of anesthesia on formation of implicit memory during
orthopedic procedures. We hypothesized that implicit memory
might be better preserved at a lighter plane of anesthesia com-
pared to a deeper anesthetic plane. To test our hypothesis, we
designed a study in which depth of anesthesia was prospec-
tively controlled and maintained in patients randomized into
two groups (BIS ranges of 55 to 60 and 40 to 45). A list of
auditory words was presented at the same phase of the proce-
dure (i.e., immediately starting with surgical instrumentation of
the bone) in all subjects. After emergence from anesthesia, a
word-stem completion (WSC) test was conducted to detect
implicit memory.
An Institutional Review Board at the University of Okla-
homa Health Sciences Center approved the study before any
consent was obtained. All patients signed an informed consent
before participating. Inclusion criteria included being a literate,
native English speaker and having been scheduled to have an
orthopedic procedure. Patients with hearing impairment, history
of cognitive dysfunction, or alcohol or other recreational drug
abuse within the previous six months were excluded.
Sixty-four subjects were enrolled upon meeting the inclusion
criteria but 9 of those were dropped from the study before any
study intervention was made (surgery was cancelled in three
cases, two patients decided not to proceed with the study, study
equipment was unavailable in two cases, and exclusion criteria
were discovered on the day of surgery in two other cases). Ex-
posure phase was completed in 53 subjects. Subsequently,
seven subjects were excluded from the BIS 55 - 60 group due to
inability to maintain BIS within the target range (average BIS
during word priming were < 50 in these seven subjects). Four
subjects were excluded from BIS 40 - 45 group, one of whom
remained intubated for three days postoperatively, another was
too sedated to complete the test phase, and the remaining two
had unacceptably poor audio quality of the recorded WSC test.
Data from the remaining 37 subjects were included in the final
analysis, 20 of which were in the BIS 40 - 45 group and 17 in
the BIS 55 - 60 group.
Table 1 shows patient demographics, distribution of surgical
procedures, and intraoperative variables. Demographic vari-
ables were analyzed using independent sample t-tests. There
were no significant differences in demographic variables be-
tween the two groups.
Patients admitted to the hospital postoperatively were tested
in the morning of the first post-operative day. Others that had a
same-day procedure were tested upon recovery from anesthesia,
immediately before their discharge from the hospital. Table 2
details the timing of the WSC test in relation to priming, emer-
gence from anesthesia, and the day of surgery. The ratio of
patients who had the WSC test on the day of surgery was 35%
in both groups.
Anesthetic Technique
All patients were premedicated with 2 mg IV midazolam
immediately before being transferred to the operating room.
Standard anesthesia monitors (including EKG, non-invasive
blood pressure, pulse oximetry, temperature, and end-tidal gas
monitoring) were applied in the operating room. Bispectral
index (BIS) (S/5 BIS module, DSC XP, Datex-Ohmeda, Hel-
sinki, Finland) was monitored in all patients starting from be-
fore the induction until the end of the intraoperative study pro-
A bolus of fentanyl (2 g. kg–1) was given intravenously im-
mediately before preoxygenation. After preoxygenation for two
minutes, anesthesia was induced with propofol (1.5 to 2 mg.
kg–1) in a dose sufficient to abolish the eyelash reflex. Sux-
amethonium (1 mg. kg–1) or cis-atracurium (0.15 mg. kg–1) was
used to facilitate tracheal intubation. Anesthesia was main-
tained with sevoflurane and air 50% in oxygen. Ventilation was
controlled artificially to attain an end-tidal partial pressure of
carbon dioxide of 33 to 38 mmHg.
Immediately following tracheal intubation, patients were
randomly assigned into one of two groups: maintenance of
anesthesia in a lighter (BIS range of 55 to 60) or deeper (BIS
M. S. OZCAN ET AL. 145
Table 1.
Demographics and Cli n i cal Variables.
BIS 40 - 45
(n = 20) BIS 55 - 60
(n = 17) p
Age (years) 43 ± 14 43 ± 14 1.0
Gender (Male/Female) 14/6 10/7
Duration of Anesthesia ( min) 303 ± 134 297 ± 123 0.89
Duration of Surgery (min) 231 ± 120 222 ± 102 0.81
BIS during w o rd presen tation 39 ± 5 55 ± 7 < 0.001
etSev during word presentation (%) 1.5 ± 0.4 1.3 ± 0.4 0.14
Fentanyl dose
Until word presentation completed 75 ± 83 198 ± 111 < 0.001
Total 225 ± 155 308 ± 114 0.08
Mean BP during word presentation (mmHg) 85 ± 16 90 ± 15 0.34
etSev: end-tidal Sevoflurane; BP: blood pressure. Data are presented as mean ± standard deviation and compared using a t-test. p < 0.05 was consid ered statistically sig-
Table 2.
Timing of the WSC Test in Relation to Priming, Emergence from Anesthesia, and Day of Surgery.
BIS 40 - 45
(n = 20) BIS 55 - 60
(n = 17) p
Word Presentation to WSC t est (hours) 22.2 (2.6-27.1) 19.9 (2-28) 0.60
Emergence to WSC test (hours) 20.5 (1.4-26.4) 17 (1-26.5) 0.27
Timing of WSC testing
Same Day of Surgery 35% 35%
Next day of Surgery 65% 65%
WSC: W ord stem completion. Data are presented as medians (range) and compared using a Mann-Whitney Rank Sum test.
range of 40 to 45) plane. BIS values were closely monitored
and sevoflurane vaporizer output was continually adjusted to
maintain the target BIS range. Additional fentanyl was given
during surgery to provide analgesia at the discretion of the at-
tending anesthesiologist. Additional cis-atracurium was used to
prevent movement as indicated.
Amount of opioids administered until the completion of
word presentation were significantly greater in the BIS 55 - 60
group (198 ± 111 vs 75 ± 83, p < 0.001). End-tidal concentra-
tions of sevoflurane were not significantly different between the
groups (1.5 ± 0.4 vs. 1.3 ± 0.4, p = 0.14).
A pool of 80 two-syllable words was selected. All the words
had limited co-articulation between syllables such that the
stems, when pronounced in isolation, maintained the same
pronunciation as they did within a word (e.g., MUS in MUS-
TARD, rather than a different pronunciation, e.g., MUS in
MUSHROOM). The 80 words consisted of 40 words classified
as negative and 40 words classified as neutral on the trait of
emotionality (John, 1988). This was done because Iselin-
Chaves et al. (2005) suggested a potential role for the amygdala.
Deeprose et al. (2004) found implicit memory during, but not
before surgical stimulation, and suggested that this might be
due to the body’s stress reaction and the concomitant contribu-
tion of the amygdala. We hypothesized that negative words
might be more likely to be implicitly preserved than neutral
words, due to the amygdala’s role in emotion and memory
(Phelps, 2004). The scale for emotionality was from –6 (very
negative feeling) to +6 (very positive feeling), with 0 being a
neutral feeling. The negative group’s mean was –2.89 (SD =
0.98) and the neutral group’s mean was +0.96 (SD = 0.88). For
each word, a number of possible two-syllable completions ex-
isted for the first syllable stem. For example, the word stem
“win” has ten possible two-syllable completions (e.g., window,
windy, winner, winter, etc.).
Randomization of words to conditions was accomplished in
the following manner: First, 20 negative and 20 neutral words
were randomly selected from the initial pool of 80 words. One
patient would be presented these words while under anesthesia
while a patient paired to this patient would have the other 40
words presented while under anesthesia. This ensured that each
word occurred equally often in the role of a presented and an
unpresented stimulus. If both of the se patients were assigned to
the BIS level of 40 - 45, a third and fourth patient would be
presented with the same two sets of words at the BIS level of
55 - 60. The 40 selected words were randomly ordered for each
patient. However, due to a programming error, only 10 of the
negative words and 10 of the neutral words actually were pre-
sented during surgery. Consequently, the remaining 10 negative
and 10 neutral words were treated like not presented words in
the analyses that follow.
The experiment comprised an exposure phase (word priming)
and a test phase (WSC test). The exposure phase consisted of
the auditory presentation of a list of words via closed head-
phones. The auditory stimuli were digitized and presented un-
der the control of the E-Prime software version 1.0 (Psychology
Software Tools, Inc., Pittsburgh, PA) running on a notebook
computer. Auditory presentation began after the target BIS
level was achieved, and within 5 minutes of the beginning of
the surgical instrumentation of bone. Each auditory file (i.e.
spoken word) was 2 seconds in length and was repeated 20
times consecutively. Thus, the duration of the exposure phase
(word priming) was 13 minutes and 20 seconds.
To begin the test phase, patients were asked four free-re-
sponse questions to elicit evidence of explicit recollection of
intraoperative events (‘What is the last thing you remember
before falling asleep?’, ‘What is the first thing you remember
about waking up?’, ‘Did you dream while you were asleep?’,
and ‘Did you hear any words while you were asleep?’). The
questions were similar to those used in previous studies (e.g.,
Russell & Wang, 1997).
Implicit memory was tested using a WSC test. Auditory clips
of the word stems were prepared using sound-editing software;
each word (i.e., auditory file) was truncated to preserve only the
first syllable of the word. A computer was used to present the
audio clips of the word stems. The testing set-up was explained
to the patients and the computer’s screen was positioned to face
the patient at a comfortable reading distance. Two practice
stems (not from the 80-word pool) were used to verify satisfac-
tory testing conditions.
Patients were presented with auditory stems of all 80 words
from the word pool, of which 20 had been presented as com-
plete words during surgery. The stems were presented in a ran-
dom order. Each audio clip was played twice, 2 seconds apart.
The letters making up each stem appeared on the computer
screen at the same time that the stem was heard. However, pa-
tients were told that their completion did not need to match the
letters they saw but only needed to match the sound that they
heard. Patients were instructed to complete the word-stem with
the first word that came to mind, but that all could be com-
pleted by two-syllable words. Nevertheless, the first response of
the patient was recorded as their answer, regardless of whether
it was a two-syllable completion. The experimenter was blinded
as to which words during test had been presented during the
Two independent coders subsequently listened to the re-
cordings and transcribed the patients’ responses. These coders
were blinded to the study hypotheses as well as to which words
a particular patient had heard while under anesthesia. In case of
a disagreement between the coders, they met to discuss and
resolve any transcriptions about which they disagreed.
BIS values during word presentation in each group are de-
tailed in Figure 1. Average BIS values for the entire priming
period in the BIS 40 - 45 and BIS 55 - 60 groups were 39 ± 5
and 55 ± 7, respectively (mean ± SD). Average BIS values
were significantly different between two groups (t (36) = 8.09,
p < 0.0001). The two groups also significantly differed in their
BIS values at every minute during priming (see Figure 1).
Figure 1.
BIS values during the auditory word presentation in 37 patients. Word presentation started at minute 1 and ended between minutes 14 and 15. Open
() and solid () circles represent the means for BIS 55 - 60 group (n = 17) and BIS 40 - 45 group (n = 20), respectively. Error bars represent stan-
dard deviations.
M. S. OZCAN ET AL. 147
There was no evidence of explicit memory formation during
surgery. No patient had any recollection of the intraoperative
events when asked the series of free-response questions. For all
patients, the last memory before falling asleep was related to
the application of monitors prior to anesthesia induction, and
the first memory after awakening was being in the recovery
room. None of the patients reported dreaming or hearing words
during surgery.
The WSC test results were analyzed using a dependent sam-
ples t-test because patients heard and were tested with both
presented and nonpresented word stems. An extreme studen-
tized deviate (ESD) method (Grubbs’ test) was used to detect
outliers of the individual subjects’ WSC test results. When hit
rates for individual subjects were analyzed, no outliers were
detected. The subject with biggest difference in hit rates for
presented and non-presented words was in the BIS 40 - 45
group, and had a value of –0.27 (Z = 2.29). This value was not
detected as a significant outlier (p > 0.05, Z > 3.00 cut-off for
There was no evidence of implicit memory formation. Words
that were presented to the patients while under anesthesia were
no more likely to be completed during the test phase than words
that were not presented (Table 3). No statistical significance
was found between words presented and not presented for ei-
ther BIS level separately or even when combined across BIS
level (t (36) = –1.62, p = 0.11). Note that this marginal effect
was in the opposite direction from what would be expected if
implicit memories were being formed to the presented words.
Typical statistical tests only allow the null hypothesis to be
rejected or not. But sometimes, the null hypothesis is the cor-
rect hypothesis and not rejecting it is the correct decision.
Rouder et al. (2009) recently developed a Bayes factor (BF)
alternative to the conventional t-test that allows researchers to
express the degree of preference for the null hypothesis. Prior
to the availability of this BF methodology, researchers were
unable to specify the degree of evidence in support of the null
hypothesis. The BF is interpreted as an odds ratio. For example,
a BF value of 3 signals that for a set of results, the null is three
times as likely to be true as the alternative. To compute the BF,
the researcher need only provide the sample size, the t-value,
and an estimate of the effect size. (Software is available at The researcher must choose
an effect size that reflects the paradigm being used. Because the
literature shows that the implicit priming effect is small, we
used an effect size of r = 0.1, which assumes that e ffect size s of
0.3 or larger are uncommon. This small value of r also is ap-
propriate when small differences are of theoretical importance,
which would be the case if information presented while a pa-
tient is under anesthesia actually were preserved.
The techniques Rouder et al. (2009) developed for express-
ing the BF were for two-tailed hypotheses only. However, we
cast our alternative hypothesis as a one-tailed test because the
predicted result was that WSC in the presented condition would
be superior to WSC in the not presented condition. That meant
that the null hypothesis was supported if the completion rate in
the not presented condition was equal to or better than in the
presented condition. For the BIS 40 - 45 condition, the BF was
3.63 (i.e., the null was 3.63 times as likely to be true than the
alternative hypothesis). For the BIS 55 - 60 condition, BF was
2.68. A meta-analytic BF, computed by multiplying these two
BF values, was 9.73. In other words, given our data, the null
was 9.73 times as likely to be true than the alternative hypothe-
sis. This is strong support against the existence of implicit
memory being formed while under anesthesia.
There was no evidence of a greater likelihood for implicit
preservation of negative words. The proportion of completions
did not differ between the negative versus neutral presented
words (negative = 0.20 ± 0.08, neutral = 0.28 ± 0.1).
We found no evidence for implicit memory at lighter or deeper
anesthetic planes in patients undergoing orthopedic procedures.
Mean completion rates for presented and non-presented words
in the BIS 55 - 60 group were 0.24 ± 0.11 and 0.26 ± 0.08,
respectively. In the BIS 40 - 45 group, completion rates for
presented and non-presented words were 0.21 ± 0.15 and 0.25 ±
0.1, respectively. In fact, Bayes factor methodology showed
that the null hypothesis of “no implicit memory during anesthe-
sia” for all subjects was 9.73 times more likely to be correct
than the alternative hypothesis. It is possible that there might be
a few subjects demonstrating implicit memory (i.e. outliers),
which could have been obscured in an analysis of the whole
group. This was ruled out by a Grubbs’ test that did not detect
any outliers among the study population. Explicit memory also
was absent, indicated by the responses given to a s t a n dard set of
Our results were consistent with the findings of a recent
study by Hadzidiakos et al. (2009) that used a WSC test and an
enhanced PDP procedure, neither of which showed an implicit
memory effect. In that study, the patients had a median BIS
value of 32.5, which corresponded to an anesthetic depth where
implicit memory was abolished even in studies with positive
findings (e.g., Iselin-Chaves et al., 2005; Stonell et al., 2006).
Our findings extend those of Hadzidiakos et al. by showing that
implicit memory was absent even at higher BIS ranges.
Table 3.
Proportion of Successf ully Completed Words i n the WSC Test.
BIS 40-45
(n = 20) BIS 55-60
(n = 17) Overall
(n = 37)
Presented 0.21 ± 0.15 0.24 ± 0.11 0.23 ± 0.14
Not Presented 0.25 ± 0.10 0.26 ± 0.08 0.25 ± 0.09
Data shown as mean ± SD. Comparison between Presented vs. Not Presented words using paired t-test.
Our study had several strengths that support the evidence in
favor of the null hypothesis. The main strength was the relative
uniformity of the surgical stimulus and depth of anesthesia at
the time of word presentation. For all subjects, presentation of
the words started within 2 - 3 minutes of bone instrumentation.
Depth of anesthesia for each subject was achieved before the
word presentation was started and maintained for the duration
of priming. To our knowledge, no other study maintained depth
of anesthesia in as tight a target interval.
One weakness of the study was that the timing of the mem-
ory test was performed on the day of surgery for some subjects
but on the next day in others. Ideally, all WSC test should have
been conducted after the same time interval because Dobrunz,
Jaeger, and Vetter (2007) found a difference in WSC related to
timing of the test. It is also possible that the level of the surgical
stimulus might not have been strong enough to facilitate im-
plicit learning. It has been proposed that surgical stimulus en-
ables learning during anesthesia. Although there is no direct
evidence from human studies, epinephrine has been shown to
enable fear conditioning in rats (Gold, Weinberger, & Sternberg,
1985). The majority of studies that have found implicit memory
under anesthesia had ongoing surgical stimulus during the
priming phase. Conversely, studies on healthy volunteers under
anesthesia in the absence of a surgical stimulus were unable to
detect implicit memory. Therefore, it is possible that the level
of anesthesia in our study, even in the BIS 55 - 60 group, might
have been sufficient to prevent implicit learning given the sur-
gical procedures our subjects were exposed to. We also pre-
medicated our subjects with midazolam, which has been
avoided in most other studies. Benzodiazepines are known to
have anterograde amnestic properties on the formation of ex-
plicit memories, but their effect on implicit memory is not es-
tablished. Lastly, our methods might be not sensitive enough to
detect implicit memory in this setting. Intraoperative word
priming followed by a postoperative WSC test as a method of
detecting implicit memory under anesthesia is still evolving.
Many factors such as timing of the WSC test, choice of target
and distractor words, and whether PDP is utilized to remove
possible contamination by explicit memories, are still being
debated. No method has been established as a standard for test-
ing implicit memory in the perianesthetic state.
In summary, we were unable to demonstrate implicit mem-
ory in two groups of patients under different anesthetic depths
(BIS 40 - 45 and BIS 55 - 60) during bone instrumentation.
According to our results, maintaining the BIS below 60 is an
adequate strategy to prevent the formation of implicit as well as
explicit memories. This conclusion is 9.73 times more likely to
be true than the alternative hypothesis that implicit memories
can be formed under anesthesia.
We gratefully acknowledge the support of the Vice President
for Research from the University of Oklahoma for this project.
We would like to thank Jeffrey N. Rouder and Michael S.
Pratte for their assistance with the statistical analysis.
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