Propofol and midazolam inhibit conscious memory processes very soon after encoding: An event related potential study of familiarity and recollection in volunteers

Abstract

Background

Intravenous drugs active via gamma-aminobutyric acid receptors produce memory impairment during conscious sedation. Memory function was assessed using event related potentials (ERPs) while drug was present.

Methods

The continuous recognition task measured recognition of photographs from working (6 seconds) and long term (27 seconds) memory while ERPs were recorded from Cz (familiarity recognition) and Pz electrodes (recollection recognition). Volunteer participants received sequential doses of one of placebo (n=11), propofol 0.45 and 0.9 ug/ml (n=10), midazolam 20 and 40 ng/ml (n=12), thiopental 1.5 and 3 ug/ml (n=11), or dexmedetomidine 0.25 and 0.4 ng/ml (n=11). End of day yes/no recognition 225 minutes after the end of drug infusion tested memory retention of pictures encoded on the continuous recognition tasks.

Results

Active drugs increased reaction times and impaired memory on the continuous recognition task equally, except for a greater effect of midazolam (p<0.04). Forgetting from continuous recognition tasks to end of day was similar for all drugs (p=0.40), greater than placebo (p<0.001). Propofol and midazolam decreased the area between first presentation (new) and recognized (old, 27 seconds later) ERP waveforms from long term memory for familiarity (p=0.03) and possibly for recollection processes (p=0.12). Propofol shifted ERP amplitudes to smaller voltages (p<0.002). Dexmedetomidine may have impaired familiarity more than recollection processes (p=0.10). Thiopental had no effect on ERPs.

Conclusion

Propofol and midazolam impaired recognition ERPs from long term, but not working memory. ERP measures of memory revealed different pathways to end of day memory loss as early as 27 seconds after encoding.

Introduction

When a high enough dose of an anesthetic agent is administered, information from the outside world is not processed by the patient and, thus, no conscious memories are formed. In this sense, all anesthetic agents are amnesic at a certain dose. Subsequently it became known that benzodiazepines at lighter levels of sedation also produced a lack of later recall. ¹ This was the first group of drugs to exemplify the difference of amnesia from sedation. The term ‘conscious amnesia’ is thus a more appropriate description of this drug effect. Our work has demonstrated that propofol produced virtually identical conscious amnesia as the benzodiazepine midazolam. ^2-4 Characteristics of conscious amnesia included relatively minor sedation, apparently normal cognition, and most notably, a lack of memory for events occurring when effective concentrations of propofol or midazolam were present. Conscious memory is also referred to as explicit or episodic memory, and is characterized by recollection of events or items within a context of time and place. ⁵ This is in contrast to unconscious or implicit memories observable indirectly by changes in behavioral responses to re-experience of the full or degraded stimulus. ⁶

The range of drug dose from conscious amnesia without sedation to memory impairment from sedation is very narrow and not predictable in a given individual. ² Thus, a key problem in the study of conscious amnesia is the ability to quantify the degree of sedation present. Reaction time and error rate measures are useful, but of more relevance is the ability to perform the task correctly when drug is present, allowing the effect of a drug on the memory processes necessary for task performance to be measured. The continuous recognition task (CRT) measures the functioning of conscious memory while simultaneously providing measures of sedation.

Learning (encoding) of items into memory on the CRT depends on two different types of memory function, determined by the interval between first presentation and recognition of the item. (see Fig 1) Working memory, defined as the short term retention of information no longer present in the environment, supports recognition of items presented some seconds before. ⁷ In the current study the participant viewed a new color photograph every few seconds during the CRT. Thus pictures held in working memory were quickly replaced by new ones. ^8,9 Recognition at relatively longer intervals was supported by long term memory. This term is used even though the time period of retention was less than a minute. After completion of the CRT, drug was stopped and memory was tested again after longer intervals to assess the impact of the drug's presence during the CRT on forgetting of newly formed memories. ⁴

Fig 1.

The Continuous Recognition Task (CRT) is ideal to differentiate working and long term memory processes. Recognition of pictures was tested at either a 6 second interval from working memory or a 27 second one from long term memory during the CRT. Note that the term “long” refers to a memory process different from working memory, even though the interval is less than a minute. Recognition at 27 seconds cannot be from working memory, as by that time the contents of working memory have been replaced by new material that must be attended to in performance of the CRT. In the current study, the CRT consisted of a standardized series of photographic pictures from the International Affective Picture System, with one presented every 3 seconds. Most pictures were repeated to maximize the efficiency of the task. The volunteer categorized each picture as either old (i.e. a repeat) or new (first presentation) using a button press. Measures of the effect of sedation on task performance included changes in reaction times and errors in the correct categorization of pictures.

We have shown that propofol and thiopental affected working and long term memory processes somewhat independently on the CRT.⁴ Excessive sedation impaired working memory so that recognitions from both working and long term memory were poor and, not surprisingly, delayed recognition at the end of the study day was also poor. Thus, participants did not encode memories in the first place. More relevant to the study of conscious amnesia were participants whose recognitions on the CRT from long term memory were unaffected when drug was present. These participants forgot memories faster than those who received placebo. Propofol produced a greater effect than thiopental, suggesting a different mechanism of memory impairment for propofol. In our previous studies, differentiation of the effects of drugs on memory was difficult to establish by behavioral measures. ^2,4 One approach to increase the reliability of this differentiation would be to obtain more direct measures of brain activity supporting memory operations during the CRT. ¹⁰

Recognition of an item in conscious memory can be supported by at least two differing memory processes, familiarity and recollection. ^11,12 Familiarity represents an early, undifferentiated recognition of an item without contextual information, typified by the ‘I can remember the face, but not the name’ response. Recollection represents a richer type of retrieval because it embodies both the item and contextual details. These memory processes can be measured by event related potentials (ERPs) obtained to recognized items. ¹³ This ERP measure is descriptively termed the old/new effect and is dependent on episodic memory function, being smaller when the strength of memory is less. ^14-16 Correct, conscious recognitions produce an ERP with more positive voltage or potential. Familiarity occurs early, from 300-400 msec after item presentation, which explains the undifferentiated nature of recognition. It takes slightly longer to recognize memories with contextual detail, and recollection occurs 400-600 msec after item presentation. Though sequential in time, familiarity and recollection processes are independent and possess different topographies. ^13,17 (see Fig 2) ^*

Fig 2.

The old/new or episodic memory effect is a very robust measure of memory processes, as measured by effect sizes in the current study. If a stimulus was correctly recognized as having been experienced before, the event related potential (ERP) produced by recognition processes was more positive (green and red lines, scale is positive down). The largest differences between ERP waveforms occurred from 400-600 msec in the parietal region, close to the Pz electrode. This represents recognition of a memory in a context of time and place, i.e. episodic memory. An earlier recognition processes occurred with an undifferentiated sense of familiarity (the ‘fez is familiar...’, without recall of specific details). Familiarity produced an ERP that was maximal centrally, near the Cz lead from 300-400 msec after item presentation. The continuous recognition task (CRT), which differentiates working and long term memory processes, allows ERPs of these memory functions to be readily obtained. In this figure arrows point to areas between old (recognition during the CRT, green and red) and new waveforms (first presentation of the picture on the CRT, blue). Results from the placebo group are displayed, with areas between waveforms being similar for working (green) and long term memory (red). Similar areas indicated that recognition of pictures from working and long term memory were supported by recognition processes that were functioning equally well. Two categories of ERP response variables were measured in the current study. These were the average ERP amplitudes for old and new waveforms in the associated time windows (thus, 2 data points being the average over the time window of the blue and either red or green lines), and the area between old and new waveforms in the time window (one data point being the area indicated by the arrowhead).

Recognition of a memory can be made using recollection, familiarity or both processes as revealed by ERP measures. An example of this occurs in the elderly. At similar levels of recognition performance as the young, the elderly rely more on undifferentiated familiarity responses for recognition. ²³ The lack of recollection could represent the presence of weaker memory traces, and thus explain the lack of durability of these memories over time. If, as in the elderly, memory processes supporting recognition on the CRT were attenuated by propofol then this could explain why memories were lost at a higher rate.

The goal in the current study was to administer propofol and midazolam at sufficient doses to produce a significant effect on recognition memory during the CRT, while maintaining adequate task performance (i.e. sufficient encoding into long term memory to track subsequent memory decay or forgetting). Thiopental was included as an active control drug, as it does not have properties of conscious amnesia similar to propofol or midazolam. Dexmedetomidine was also included in the current study as little is known about its effects on episodic memory function. Though active at alpha-2 receptors, dexmedetomidine produces sedation via gamma-aminobutyric acid receptors in the ventrolateral pre-optic nucleus, producing a different quality of sedation. ²⁴ Thus, dexmedetomidine could test the ability of ERPs to differentiate mechanisms of drug action on memory. A placebo group allowed determination of whether the study drugs were present at high enough concentrations to be active, as well as providing a control for fatigue effects on the ERP.

As mentioned, the difference in dose that produces conscious amnesia from one that produces memory impairment from sedation is quite small. In order to ensure a memory effect despite individual variability, two sequential doses of drug were administered to each participant, bracketing a predicted 50% probability of memory impairment on a visual recognition task. ²⁵ A key question in the current study is whether drugs produced equivalent degrees of impairment amongst themselves. If this were the case, then differences in ERPs would point to mechanistic differences among the different drugs. Equivalency among active drugs was determined by comparisons which excluded the placebo group.

The hypothesis tested in the current study was that ERP measures would differentiate the effects of the study drugs on memory processes supporting recognition from long term memory during the CRT. ERP measures of working memory were used to quantify the presence of sedation in addition to reaction time and error rate measures.

Materials and Methods

Participants

Healthy volunteers were recruited by internet advertisement (CraigsList^†) and word of mouth. The study was approved by the Institutional Review Board of Memorial Sloan-Kettering Cancer Center, New York, New York, United States. Volunteers gave their informed consent and were paid $200 for their participation.

Seventy-nine healthy volunteers were recruited with 12 withdrawing their consent to participate, thus 67 volunteers participated. Of these, 55 are included in the current study. In the remaining 12 volunteers, 6 did not remain awake or failed to perform the memory task adequately during drug administration (1 thiopental, 1 propofol, 1 midazolam, 3 dexmedetomidine) and in 6 the electroencephalogram was not recorded for technical reasons (1 placebo, 2 thiopental, 2 propofol, and 1 dexmedetomidine participant).

Inclusion criteria included evidence of good health, normal vision, age between 18-50, and being right-handed, of normal weight for height, a high school graduate, and fluent in English.

Exclusion criteria were evidence of medical, neurological or psychiatric disease, currently taking Central Nervous System active medication, visual impairment, body mass index ≥ 30, history of alcohol or drug abuse, and (for females) pregnancy. Recruitment was followed by a telephone interview and in-person orientation session, where volunteers were given a brief physical exam and trained on the experimental tasks. Volunteers who did not perform the experimental task adequately were excluded from study. Volunteers were screened for handedness using the Edinburgh Handedness Inventory ²⁶.

Experimental Design

Participants were randomized to one of five experimental groups who received placebo (n=11), thiopental (n=11), propofol (n=10), midazolam (n=12), or dexmedetomidine (n=11). The first target dose of drug was predicted to produce 40% memory impairment (40AmnDose), estimated from a previous study. ²⁵ The first set of CRTs was then administered, with stimuli delivered in 3 equal blocks of approximately 7 minutes duration each, with 5 minutes rest between blocks (see Table 1, timeline). Following the low dose condition, a somewhat higher dose was given, estimated to provide 60% memory impairment (60AmnDose), and the CRT repeated with new stimuli. Drug infusion was then discontinued, and repeated recognition testing was performed over the subsequent 3 hours and 45 minutes, the detailed results of which will be reported in a companion manuscript. Only the final recognition task at 225 minutes (end of day) is reported in the current manuscript, with data analyses focused on results from the CRTs, when drug was present.

TABLE 1.

Timeline of study

8:40 AM	Preparation: EEG1 cap applied, venous catheter inserted, saline administered intravenously, practice tasks.
	Dose Level	Tasks
t=0	Baseline	VVLT2, PANAS	PANAS (Positive and Negative Affect Schedule) mood questionnaire #1, Visual Verbal Learning Task List #1
t+ 12 min		EEG	Check EEG, record EEG eyes closed, record EEG eyes open, DR (Delayed Recognition) VVLT List #1
t+ 52 min			Drug Infusion Begins at low dose to target 40% memory impairment.
t+ 68 min	40% predicted amnesic dose	VVLT, PANAS	VVLT List #2, PANAS level 1 #1
t+ 79 min		CRT3 Block 1	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 98 min		CRT Block 2	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 112 min		CRT Block 3	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 120 min		PANAS, VVLT	PANAS Level 1 #2, Dr VVLT list #2
t+ 125 min			Infusion increases to a higher dose, estimated to provide 60% memory impairment.
t+ 146 min	60% predicted amnesic dose	VVLT, PANAS	VVLT List #3, PANAS level 2 #1
t+ 153 min		CRT Block 1	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 177 min		CRT Block 2	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 190 min		CRT Block 3	Categorize the stimulus as either new, or a repeat of a previously seen image.
t+ 197 min		PANAS, VVLT	PANAS Level 2 #2, DR VVLT List #3
t+ 199 min		BREAK	STOP Drug Infusion
t+ 205 min to t+ 321 min		Rest period alternating with recognition tasks (see companion manuscript)
t+ 424 min		EOD4 recognition	Photo seen previously? (yes/no, equal number of distractors)

¹EEG- Electroencephalogram

²VVLT- Visual Verbal Learning Task

³CRT- Continuous recognition test

⁴EOD- End of day

Procedures

After verification of nil per os (nothing by mouth) status, electroencephalogram electrodes were applied using collodion adhesive. A 20g IV was started using D5½NS running at approximately 125 ml/hr. Including electrode application and removal, the session lasted from 8 am until about 4:30 pm. Volunteers were provided a light lunch after the end of drug administration.

Continuous Recognition Task (CRT)

Participants were informed that they would view a series of pictures, in which some pictures would be repeated. Each picture was shown twice during the CRT, with recognition either 6 or 27 seconds (but not both) after first presentation. The CRT was designed for maximal efficiency, and almost all pictures were shown twice with a small number of additional neutral pictures acting as filler items. Participants categorized the stimulus as either new (first presentation), or a repeat of a previously seen image (old) by pressing a mouse button. Reaction times (RT) were collected on each trial, with ones less than 100 msec being excluded.

Delayed Recognition Task

Recognition tasks contained an equal number of new and old pictures that had been previously presented during one of the CRTs. Thus, each picture was shown twice on the CRT and a third time during recognition testing following drug discontinuation.

Electroencephalogram Recording

19 channels of the electroencephalogram signal (plus vertical, horizontal electrooculography and left and right mastoid) from 1-75 Hz were sampled at 1 kHz using Scan Synamps amplifiers in the standard 10/20 configuration with impedances of less than 5 Kohms (Compumedics Neuroscan, Charlotte, NC). A ground electrode was placed on the forehead. All leads were wrapped and shielded to decrease 60 Hz noise. After accrual of approximately 1/3 of participants, a large 2” × 4” disposable electrode (Viasys Healthcare, Madison WI) was placed on the left shoulder to provide additional grounding to decrease 60 Hz noise. During acquisition, electroencephalogram data were referenced to the left mastoid, but were re-referenced to linked mastoids for ERP analysis.

In offline processing, electroencephalogram was epoched to 2500 msec, with a pre-stimulus baseline of 199 msec. Vertical and horizontal Electrooculography artifacts were reduced using the Neuroscan regression algorithm (Compumedics Neuroscan Scan 4.3, Charlotte, NC). Epoched files were visually inspected and artifacts removed (after automated rejection of amplitudes exceeding ± 75 uV). Baseline correction, re-referencing and averaging over the three CRT tasks presented at each dose level was then performed.

For each participant, electrodes with less than 10 valid sweeps were eliminated from the analysis. Most commonly, invalid sweeps represented an incorrect recognition response, but could also include excessive artifact. In all participants in the current study, recordings from the Cz and Pz electrodes were of good technical quality. ERP waveforms were filtered with a 20-point smoothing algorithm (equivalent to a low-pass filter of 6.9 Hz) for graphical presentation only. Unfiltered values were used in statistical analyses.

Material

Pictures used on the CRT were selected from the International Affective Picture System. ²⁷, with all tasks counterbalanced for valence and arousal criteria. Pictures were presented in color in full screen mode, displayed for 1.5 seconds, with a 3 second inter-stimulus interval. 140 unique pictures were presented at each dose level, divided over 3 blocks in counterbalanced order. Subjects were seated in a semi-reclining posture on a gurney, about 2 ½ feet from the screen of a CRT or (after 33 participants) an LCD monitor.

Drug Administration

Drugs were infused using STANPUMP,^‡ which administers a continuous infusion of drug adjusted for body weight and age using the appropriate pharmacokinetic models (thiopental – Shafer, propofol – Schnider, midazolam – Greenblatt, and dexmedetomidine – Markku Anttila). Two sequential drug concentrations use were: thiopental 1.5 and 3 mcg/ml, propofol 0.45 and 0.9 mcg/ml, midazolam 20 and 35 ng/ml, dexmedetomidine 0.2 and 0.4 ng/ml. Placebo solutions were matched in appearance to active drugs (thiopental – multivitamin solution, propofol – intralipid, midazolam and dexmedetomidine – normal saline), and the corresponding STANPUMP parameters were used for infusion. Testing was started after predicted effect site concentrations were within 5% of the target concentration.

Statistical Analysis

Results of memory tasks are expressed as percent correct recognitions, excluding trials where no response was made. Only correct recognitions were used in producing the average ERP waveforms.

ERP Analysis

Note that the CRT was divided into 3 segments at each dose level, to avoid fatigue effects of one long CRT. The ERPs from these 3 segments were analyzed together at each dose level. ERPs were averaged by stimulus type (new (first presentation), old (second presentation)) and interval (6 or 27 seconds). Figure 2 validates Cz and Pz electrodes as measuring familiarity and recollection memory processes respectively. Average ERP amplitudes at the Cz and Pz electrodes for old (red and green waveforms) and new (blue waveforms) pictures, over the time windows 300-400 msec and 400-600 msec respectively, were calculated. Similarly, the area between old and new ERP waveforms was determined as the sum of differences for each msec (data were collected at 1 kHz) over the time window of interest (uV × sec). Thus, at Pz the areas are about twice as large as at Cz, as the time window at Pz is 200 msec.

Statistical Software

Data analyses were carried out using linear mixed-effects models. ²⁸ Bagiella and colleagues summarized the main advantages of mixed-effects models over repeated-measures ANOVA in psychophysiology research ²⁹. An important advantage is that casewise deletion of missing observations is no longer necessary, which allows the experimenters to analyze all available data with higher statistical power by comparison. Mixed-effects models also better handle the correlation structures of repeated measures nested within participants, which circumvents the need to check for heteoskedasticity and spherericity assumptions and make statistical adjustments thereafter. Moreover, there is no need to use the quasi-F statistic to test random effects associated with experimental materials. ³⁰ The mixed effects model has the ability to include both categorical and continuous data simultaneously. ³¹

For the current study, the factors of drug (placebo, thiopental, propofol, midazolam, dexmedetomidine), dose (40AmnDose, 60AmnDose), interval (6 or 27 seconds) and stimulus type (old, new) were incorporated as fixed factors. Participant (ID1-55) was used as a random effect in the model. To calculate the unbiased p-values in testing experimental effects ³⁰, we used the Markov-Chain Monte Carlo method in the statistical computer language R. ^§ Specifically, the lme4 package in R was used to implement the mixed-effects models analyses. ^** The Markov-Chain Monte Carlo p-values were calculated using pvals.fnc and aovlmer.fnc functions available in the languageR package^††. The Markov-Chain Monte Carlo simulations were repeated 10,000 times to reach a stable burn in. p-values were based on t statistics when model co-efficient estimates were compared with the referent (e.g. placebo group), or the F statistic when fixed factors in the model were tested. Confidence intervals (95%) of model coefficient estimates were obtained from Markov-Chain Monte Carlo and provided whenever appropriate. As an estimate of effect size, coefficients from the model fit (for the effect of drug, in comparison with placebo) were divided by the standard error for the given effect.

Results

Participants

Fifty-five cases were available for analysis (36 M, 19 F; age 29.1 ± 8.1 years, weight 71.5 kg ± 12.4 kg, body mass index of 24.5 ± 5.7; 51% Caucasian, 22% African-American: 16% Asian, and 11% Hispanic). All but two individuals continued their education beyond high school, with 47.3% being college graduates and a further 18.1% having post-graduate education. As tested by one-way ANOVAs groups did not differ on any variable (see Table 2).

Table 2.

Participant variables: mean (standard deviation). Differences between groups were tested by one-way ANOVA. The groups did not differ significantly on any variable.

GROUP	Gender (M, F)	Age (years)	BMI1	WAIS-R2 Vocabulary	Edinburgh Handedness Inventory	Hearing	Sleep (hours)
PLACEBO (n=11)	7, 4	30.12 (5.8)	23.3 (2.2)	81.1 (20.0)	83.3 (17.9)	18.4 (2.4)	6.2 (1.1)
MIDAZOLAM (n=12)	7, 5	27.5 (8.1)	22.8 (3.5)	74.3 (26.2)	80.1 (26.6)	19.4 (4.4)	6.2 (1.1)
PROPOFOL (n=10)	7, 3	24.5 (4.6)	27.1 (11.9)	76.2 (25.8)	80.9 (16.6)	20.9 (4.0)	6.2 (1.1)
THIOPENTAL (n=11)	8, 3	31.0 (8.3)	25.5 (2.8)	82.4 (17.9)	84.7 (25.0)	17.6 (2.0)	6.3 (0.6)
DEXMEDETOMIDINE (n=11)	7, 4	32.1 (11.2)	24.3 (2.6)	87.3 (10.2)	84.5 (13.4)	18.5 (2.0)	6.0 (0.8)
TOTAL (N=55)	36, 19	29.1 (8.1)	24.5 (5.6)	79.6 (20.7)	82.7 (20.0)	18.9 (3.2)	6.2 (1.0)

¹BMI- Body Mass Index

²WAIS-R- Wechsler Adult Intelligence Scale-Revised

Behavioral Effects of Drugs

Testing was performed to assess whether drugs were administered in sufficient concentration to produce a measurable effect on behavioral responses (factor of drug), while still allowing adequate performance on the CRT (recognition memory). The similarity of drug effect was tested by excluding the placebo group. The factor of interval (recognition 6 or 27 seconds after presentation) was used to contrast working and long term memory function, while the factor of stimulus type (old,new) was used to contrast reaction to pictures recognized from memory versus new pictures. The difference between correct recognitions on the CRT and delayed recognition at the end of the study day (end of day recognition) measured the degree of forgetting.

Drugs impaired performance in a dose related fashion. Impairment was equal among active drugs, except for a greater effect of midazolam

Reaction Times

With the placebo group included, there was a significant effect of drug on reaction times (F(4,432) = 2.88, p = 0.002), with midazolam having RTs 187 (70.1 – 301.2) msec slower than placebo (t = 3.2, p = 0.001, see Fig 3A, Table 3). Excluding placebo, there was little or no difference among drugs on RTs (F(3,345) = 1.96, p = 0.12). Thus all drugs slowed processing to a similar extent.^‡‡ As dose increased, reaction times were slower, with the 60AmnDose slowing RTs by 76.7 (59.1 – 94.8) msec compared with the 40AmnDose (placebo group included, F(1,432) = 69.69, p < 0.001, effect size 8.3).

Fig 3.

The factor of interval contrasted the difference between working and long term memory function. All drugs increased reaction times and decreased correct recognitions in comparison to placebo, but for the most part had similar effects amongst themselves (except for a greater effect of midazolam, indicated with *). All drugs increased the error rate in categorizations (false alarms, panel B), but there was no difference amongst drugs. Behavioral responses revealed normal function of working memory in the presence of drug (panel A, 22 msec faster recognitions of pictures at 6 seconds, F=5.83). The functioning of working and long term memory to support recognitions during the continuous recognition task (CRT) was equivalent (panel C, factor of interval n.s.). In other words, the CRT was successful in encoding long term memories in the presence of sedation. Midazolam produced more impairment than other drugs (significantly increased reaction times (p=0.013 vs. placebo, n.s. vs other drugs) and decreased memory function (p=0.003 vs, placebo, lower dose; p<0.001 vs. placebo higher dose, p<0.04 vs. other drugs, higher dose)). Notably, in terms of memory function, dexmedetomidine was no different from placebo even at the higher dose (panel C). All drugs increased forgetting of pictures encoded during the CRT compared with placebo (panel D). The degree of forgetting was the same amongst active drugs, as the calculation (recognition CRT – recognition end of day) normalized the starting point for forgetting. There was less forgetting of pictures recognized from long term memory than those recognized from working memory (as shown by the factor of interval F=8.72). Preferentially better long term memory may have been exaggerated in the case of thiopental (shown by the differences in the third and fourth bars in each cluster), indicating a lack of a conscious amnesic effect (a drug by interval interaction suggested by p=0.10, with thiopental being different from placebo (p=0.02)).

Table 3.

Behavioral measures of Continuous Recognition Task (CRT) performance and loss of memory to end of day (EOD), means (standard deviation)

1: Reaction Time CRT (msec)
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
40% Amn Dose Working Memory (6 sec)	824 (198)	866 (168)	845 (107)	950 (213)	833 (110)
40% Amn Dose Long Term Memory (27 sec)	837 (159)	880 (163)	864 (121)	972 (188)	863 (101)
60% Amn Dose Working Memory (6 sec)	810 (130)	936 (173)	991 (148)	1070 (321)	884 (167)
60% Amn Dose Long Term Memory (27 sec)	833 (118)	957 (153)	1038 (121)	1059 (185)	934 (174)

2: False Alarm Rate (%)
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
40% Amn Dose	3 (1)	5 (5)	5 (4)	8 (6)	5 (3)
60% Amn Dose	4 (3)	12 (7)	16 (7)	14 (12)	14 (6)

3: CRT % Correct Recognition
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
40% Amn Dose Working Memory (6 sec)	95 (5)	93 (8)	94 (5)	87 (13)	94 (4)
40% Amn Dose Long Term Memory (27 sec)	96 (4)	91 (8)	93 (6)	85 (12)	93 (7)
60% Amn Dose Working Memory (6 sec)	97 (3)	84 (14)	86 (12)	73 (21)	88 (6)
60% Amn Dose Long Term Memory (27 sec)	96 (3)	86 (14)	84 (11)	72 (19)	88 (9)

4: Forgetting (% of CRT Recognition)
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
40% Amn Dose Working Memory (6 sec)	34 (19)	55 (22)	58 (18)	57 (20)	44 (16)
40% Amn Dose Long Term Memory (27 sec)	35 (17)	43 (14)	49 (15)	56 (11)	45 (18)
60% Amn Dose Working Memory (6 sec)	34 (15)	65 (17)	63 (21)	61 (21)	55 (16)
60% Amn Dose Long Term Memory (27 sec)	32 (24)	47 (20)	60 (23)	53 (19)	50 (16)

False Alarms

A false alarm (FA) occurred when a new picture was incorrectly recognized as being old. Increased FA rates may represent changes in attention or a change in bias. In other words, an increase in FA rate may represent a lower confidence level in being correct when the participant was uncertain. Overall, drugs increased FA rates by 6.2 (1.5 – 11.0)% when compared to the placebo group (effect of drug F(4,104) = 3.07, p = 0.02), except possibly thiopental (t=1.95, p=0.054 vs. placebo, see Fig 3 B). As with reaction times there was a large effect of dose, increasing FA rates by 6.5 (4.8-8.3)% (F(1,104) = 60.79, p < 0.001, effect size 7.8). Excluding placebo, there was no drug by dose interaction (the amount of increase in FA rate as dose increased was the same for each drug, F(3,80) = 1.04, p = 0.38). Thus, all drugs had similar effects on FA rates amongst themselves.

Participants performed the CRT task well – drugs did not impair working memory nor differentially inhibit encoding into long term memory

Recognitions of pictures from working memory (6 seconds after presentation) was 22 (3.8-39.9) msec faster than recognition after 27 sec (F(1,432) = 5.83, p = 0.002) (see Fig 3A). This indicates that at 6 seconds the picture was in working memory and thus easier to match than at 27 seconds when the picture would be gone from working memory, requiring a search in long term memory. There was no drug by interval interaction (F(4,428) = 0.44, n.s.), i.e. there was no difference with any drug from placebo in the relationship of RT from working versus long term memory.

In contrast to reaction times, there was no effect of interval on recognition memory (F(1,213) = 0.48, n.s., see Fig 3, panel C). Thus, participants recognized pictures in working and long term memory equally well in the presence of drug, despite the presence of sedation as measured by overall increased reaction times.

Pictures were encoded into conscious memory, with retrieval being faster for thiopental

Overall, participants recognized old pictures 108.8 (91.3-127.6) msec faster than new ones (F(1,432) =140.35, p < 0.001, effect size 11.8), indicative of a repetition effect. ³² This indicates that it was easier to match a picture already in memory (i.e. the recognized old picture), than to search for one that was not there. There was a drug by stimulus type (old,new) interaction (F(4,428) = 3.22, p = 0.01). Pairwise testing revealed that thiopental increased the difference between these categories of stimuli by 69.3 (12.8 – 126.3) msec more than in the placebo group (t = 2.43, p=0.02). In other words, there was no indication that the presence of drug delayed retrieval of pictures in memory in relation to new presentations.

Increased drug dose impaired recognitions on the CRT, similar to dose effects on RTs; Midazolam impaired recognition more than other drugs, with its effect on memory being greater than on RTs

Correct recognitions of pictures on the CRT revealed similar effects for the factor of dose as for reaction times (Fig 3 C showing the inverse effect of panel A). There were 6.8 (5.13 – 8.42)% fewer recognitions as dose increased from 40AmnDose to 60AmnDose (F(1,213) = 66.69, p < 0.001, effect size 8.17 compared with the reaction time effect size 8.35). The interaction of drug by dose was significant for all drugs (F(4,209)=9.34, p<0.001, all drugs p<0.02 compared with placebo). In other words, all drugs decreased recognitions on the CRT as dose increased, in contrast to the placebo group, who maintained performance on the second set of CRTs similar to the first. As memory function was the primary behavioral response of interest, data from both doses were analyzed separately.

At the lower 40AmnDose there was a significant effect of drug (F(4,104) = 2.66, p = 0.04), with participants receiving midazolam having 9.8 (3.1 – 16.3)% less success in recognition than placebo (t = − 3.02, p=0.003). Excluding placebo, there was similarity of drug effect (F(3,83) = 2.07, p = 0.11), though this level of significance suggested that midazolam may have had 7.6 (0.6 – 15.2) % lower recognitions than propofol (t = 2.10 p = 0.04) and 7.5 (0.2 – 14.5)% fewer recognitions than dexmedetomidine (t = 2.13, p = 0.04).

At the higher 60AmnDose, drug impaired memory compared with placebo overall (factor of drug F(4,104) = 5.60, p<0.001). Dexmedetomidine was not different from placebo, but other drugs had significantly lower recognitions (midazolam 23.8 (13.4-33.8)%, t = −4.63, p<0.001, propofol 11.4 (0.5 – 22.1)%, t = −2.10, p=0.04, and thiopental 11.1 (0.6 – 21.6)%, t = −21.2, p=0.04 less than placebo). The midazolam group had lower recognition scores than the other drugs (excluding placebo, factor of drug F(3,83) = 3.04, p=0.03, all drugs p<0.04 compared with midazolam)

The effect of midazolam on correct recognitions on the CRT (effect size 4.25) was larger than the increase in reaction times (effect size 3.23), demonstrating a greater effect on memory than processing speed, i.e. conscious amnesia.

Memory impairment was present on the CRT. Memory Loss from CRT to End of Day Recognition (Forgetting) was greater than placebo, but not different amongst drugs

After completion of the CRTs, drug infusion was stopped, and forgetting of newly learned information was measured by the difference from recognitions on the CRT to the end of the study day (end of day recognition). ^§§

Pictures encoded at the higher dose were forgotten 4.4% (0.8 – 8.2) more than at the lower dose (factor of dose, F(1,213)=5.45, p=0.02), demonstrating that forgetting was a drug related effect. The effect size of 2.34 for the factor of dose on forgetting is smaller than for its effect on CRT recognitions, 8.17. This indicates that forgetting was normalized to a certain degree among different starting points, i.e. any memory effect present on the CRT was discounted in the forgetting measure.

There was significant forgetting in the placebo group alone (t=6.14, p < 0.001), with a loss of memory for 33.9 (SD 18.3)% of pictures initially recognized on the CRT. All drugs had a greater forgetting rate than placebo (F(4,213) = 5.14, p < 0.001, see Fig 3 D). This difference ranged from 14.7 (3.2 – 27.1)% greater than placebo for dexmedetomidine (t = 2.47, p=0.01) to 23.4 (11.1 – 35.2)% for propofol (t = 3.85, p<0.001).

As opposed to recognitions on the CRT, midazolam was no different than other drugs in terms of forgetting (excluding placebo, factor of drug F(3,120) = 0.98) Thus, after the discontinuation of drug infusion, all active drugs had similar impact on the rate of forgetting. This measure normalized different starting points, including the difference of the midazolam group with other active drugs.

Pictures recognized from long term memory were forgotten less readily

Note that separate forgetting rates were available for pictures recognized from either working memory (6 seconds) or long term memory (27 seconds after presentation) on the CRT. Pictures recognized from long term memory were forgotten 5.6 (2.1 – 9.4) % less than overall forgetting (F(1,213) = 8.72, p = 0.004, effect size 2.95). Thus, if a participant could recognize a picture at 27 seconds, then they were more likely to recognize it at the end of the study day despite a large degree of forgetting. Recall that there was no effect of interval on correct recognitions during the CRT (Fig 3, Panel C). Thus, proportionally more pictures were forgotten from working than long term memory. In other words, pictures encoded into long term memory represented stronger memories. Excluding placebo, the effect of interval was larger (F(1,170) = 10.01, effect size 3.16), indicating that this differential forgetting effect for pictures recognized from working memory was exaggerated in the presence of drug. There was no interaction with drug by interval, in other words all drugs had the same effect (F(4,209) = 1.74, p=0.1). If one accepts that a significance of p = 0.1 was suggestive of an effect, then this was due to thiopental (t = −2.40, p = 0.02 compared with placebo). In other words, the relatively good retention of stimuli in long term memory on the CRT may have been exaggerated for thiopental (compare third and fourth bars in thiopental cluster, Fig 3, panel D). Note also that thiopental decreased the time for retrieval of items from memory in comparison to other drugs (factor of stimulus type (old,new)). These results point to thiopental lacking a conscious amnesic effect on memory, as represented by the other active drugs.

ERP waveforms underlying successful recognition on the CRT

A more positive recognition ERP to a second stimulus presentation documents that this stimulus is present either in working (6 seconds) or long term (27 seconds) memory during performance of the CRT. The factor of stimulus type (old,new) was tested to determine the presence of conscious memories. The difference between old/new waveforms (i.e. area between waveforms) is a quantification of conscious memory processes.^15,16 Note that recognitions from working or long term memory could involve the use of familiarity, recollection or both memory processes. ERP measures of these processes were analyzed separately. The effect of drug (placebo, thiopental, propofol, midazolam and dexmedetomidine) and interval (6 or 27 seconds) and their interactions were tested to determine the effect of different drugs on familiarity and recollection processes used in recognition of pictures from working and long term memory. ERPs were obtained only for correct categorizations, and thus represent memory processes supporting successful recognition.

ERPs prove encoding of pictures into conscious (episodic) memory

The difference between old and new ERP waveforms was highly significant, as shown by the slope of the line connecting average amplitudes in the associated time windows for new (first) and old (second) picture presentations on the CRT (Fig 4, Table 4). For recollection, measured at the Pz electrode (400-600 msec), there was a 2.67 uV (2.28 – 3.02) more positive potential waveform for the second (old) presentation of the picture (F(1,423) = 200.53, p < 0.001, effect size 14.2), i.e. the slopes of the lines in Fig 4 are up-going for all drugs in all conditions.

Fig 4.

As shown in Fig 2, the old/new effect is a robust measure of recognition memory. The event related potential (ERP) amplitudes averaged over the time window 400-600 msec at the Pz electrode are shown here categorized by stimulus type (old/new), drug (line and symbol), dose (increasing from A to B, and C to D) and interval (A,B working memory, C,D long term memory). Note that all lines have positive slopes in all conditions, indicating the presence of the old/new effect in the presence of drug. Notably, propofol shifted ERP amplitudes to less positive voltages in a dose related fashion. Midazolam may have had similar effects. For active drugs as a whole, recognitions from long term memory produced ERP amplitudes with less positive voltage. This may indicate less confidence in the decision, as indicated by the increase in false alarm rates in the presence of drug. Note that drugs had no effect on working memory (panels A and B), except for propofol shifting ERPs to a less positive voltages. Despite this shift for propofol, the area between the old and new waveforms remained the same (as shown by the slope of the lines in panel A and B), i.e. working memory processes that come into play in recognition of an old picture, representing the sum of the potentials of the neuronal populations involved in recognition processes, were active to the same extent as in the placebo group. This indicated that the degree of sedation, as measured by increases in reaction times and error rates, did not affect working memory function.

Table 4.

Average amplitudes of old and new waveforms for recollection, measured from 400-600 msec at the Pz electrode in uV, mean (standard deviation)

	A	B	C	D
	40% Amn Dose Working Memory (6 Sec)	60% Amn Dose Working Memory (6 Sec)	40% Amn Dose Long Term Memory (27 Sec)	60% Amn Dose Long Term Memory (27 Sec)
PLACEBO
Old	5.99 (1.95)	6.09 (1.58)	5.66 (2.51)	5.34 (1.59)
New	2.67 (2.73)	2.77 (1.79)	1.79 (2.71)	3.39 (2.35)
THIOPENTAL
Old	4.14 (2.49)	4.45 (3.65)	3.61 (2.77)	3.31 (2.86)
New	1.48 (3.04)	1.75 (2.72)	1.99 (2.30)	.54 (3.02)
PROPOFOL
Old	6.64 (2.11)	3.32 (3.84)	5.51 (3.22)	1.69 (3.37)
New	2.34 (3.08)	.20 (3.61)	3.20 (3.27)	.33 (3.08)
MIDAZOLAM
Old	5.81 (3.42)	5.16 (3.17)	5.75 (3.67)	3.61 (2.81)
New	2.35 (4.36)	1.61 (3.68)	2.97 (4.24)	2.95 (3.62)
DEXMEDETOMIDINE
Old	5.78 (5.71)	5.17 (4.30)	4.20 (4.57)	4.47 (3.83)
New	2.5 (4.41)	2.86 (3.56)	2.49 (3.27)	1.97 (3.93)

As with recollection, the old / new difference was robustly present for familiarity measured at the Cz electrode (time window 300-400 msec), having a 2.24 (1.73 – 2.74) uV more positive potential for old pictures (F(1,420) = 77.86, p < 0.001, effect size 8.8) (see Table 5).

Table 5.

Average amplitudes of old and new waveforms for familiarity, measured from 300-400 msec at the Cz electrode in uV, mean (standard deviation)

	A	B	C	D
	40% Amn Dose Working Memory (6 sec)	60% Amn Dose Working Memory (6 Sec)	40% Amn Dose Long Term Memory (27 Sec)	60% Amn Dose Long Term Memory (27 Sec)
PLACEBO
Old	0.49 (2.89)	0.25 (2.47)	0.31 (2.96)	0.12 (2.10)
New	−.32 (7.08)	−.47 (8.42)	−3.82 (2.95)	−1.65 (3.27)
THIOPENTAL
Old	1.41 (4.99)	0.77 (4.07)	−.42 (4.29)	0.23 (3.57)
New	−2.25 (2.94)	−1.68 (4.48)	−2.33 (3.57)	−2.80 (3.76)
PROPOFOL
Old	−2.35 (4.82)	−4.86 (5.09)	−3.92 (4.80)	−6.66 (5.54)
New	−7.03 (5.71)	−7.51 (5.81)	−6.07 (5.43)	−8.31 (4.97)
MIDAZOLAM
Old	.14 (3.54)	−.25 (3.61)	−.24 (5.13)	−2.08 (5.09)
New	−3.49 (6.59)	−3.65 (4.99)	−2.89 (5.79)	−2.77 (4.62)
DEXMEDETOMIDINE
Old	−1.08 (3.65)	−1.51 (3.86)	−2.49 (2.33)	−2.08 (3.34)
New	−3.08 (3.32)	−2.46 (3.98)	−4.50 (3.39)	−2.78 (2.53)

Thus, ERP waveforms confirmed the presence of memories in conscious or episodic memory, indicating that the CRT was successful in encoding memories.

Drugs did not affect ERP measures of Working Memory

The functioning of working memory was measured by the area between old and new waveforms to correctly recognized pictures 6 seconds after presentation (tabulated in Table 6; in Fig 5 comparisons being made among values on the left side of the graphs).

Table 6.

Area Between old/new waveforms (uV × msec)

1: Familiarity, Cz Electrode; (old-new) every 1 msec, summed from 300-400 msec
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
Working Memory (6 seconds recognition)	253.90 (229.44)	253.78 (237.39)	414.54 (172.61)	351.75 (384.68)	148.13 (395.45)
Long Term Memory (27 seconds recognition)	325.84 (208.38)	220.64 (207.03)	186.63 (241.29)	175.51 (228.80)	157.92 (211.67)

2: Recollection, Pz Electrode; (old-new) every 1 msec, summed from 400-600 msec
	PLACEBO	THIOPENTAL	PROPOFOL	MIDAZOLAM	DEXMEDETOMIDINE
Working Memory (6 seconds recognition)	527.61 (320.71)	423.27 (617.69)	799.07 (341.74)	657.89 (649.70)	613.40 (475.07)
Long Term Memory (27 seconds recognition)	569.71 (344.52)	397.18 (372.33)	391.82 (374.29)	353.23 (520.84)	395.21 (534.98)

Fig 5.

Only correctly recognized pictures on the continuous recognition task (CRT) were used to obtain event related potentials (ERP), thus areas between waveforms represented memory processes underlying successful recognition from working and long term memory. The areas between old and new waveforms are plotted for familiarity recognitions, measured at Cz, and recollection recognitions, measured at Pz. No drug affected the function of working memory, i.e. all the points on the left side of the graphs are equivalent statistically. There was a significant decrease in area when recognition was from long term as opposed to working memory. The change in area was different amongst drugs, shown by a significant drug by interval interaction (this interaction is revealed in the graph by different slopes among groups). This interaction was likely due to the effects of propofol and midazolam (bold triangles), and revealed an effect of propofol and midazolam on memory processes supporting recognition within seconds after encoding. The effect of propofol and midazolam on recognition from long term memory achieved significance in the case of familiarity and weaker evidence was present for recollection. Inclusion of the dexmedetomidine group in the analysis likely decreased the power to detect differences among drugs. There was no effect of the factor of dose on areas between waveforms (Pz, F(1,209)=1.35, p=0.25, Cz F(1,207) = 3.40, p=0.07), which implies that some effect on ERPs was present at the lower doses of drug. Propofol decreased ERP areas similarly to midazolam, but at a dose where memory was affected less. In other words, the propofol group revealed that impaired recognition memory processes were present before conscious amnesic effects were evident in the behavior of decreased recognitions from long term memory on the CRT. As the time window measured at Pz was 200 msec vs. 100 msec at Cz, those areas were roughly twice as large.

There was no effect of dose either on familiarity (at Cz, F(1,101) = 0.33, n.s.) or recollection (at Pz, F(1,102) = 0.34, n.s.), and thus, areas at each electrode were collapsed across dose. There was no effect of drug on area associated with working memory for either familiarity (Cz, F=2.06 (df 4,101), p = 0.09) or recollection (Pz, F=1.25 (df 4,102), p = 0.29). If one accepts that p=0.09 is suggestive of an effect of drug on familiarity, then this resulted from a larger area at Cz for propofol (t=1.56 vs. placebo, p=0.12). These are key results, as they showed that working memory in participants receiving drug functioned equivalently to placebo participants, with no decrement at higher doses of drug, corroborating behavioral results. Thus, any sedation effect was only evident by increased reaction times and errors, namely false alarm rates and less correct recognition. When recognition was correct, as was true for a majority of pictures, working memory processes supporting recognition were intact.

Recollection of Long Term Memory

Recollection of a memory in a context of time and place is the memory process most visibly affected by midazolam and propofol. In this section the impact of the study drugs on ERP measures of recollection, which occur in the parietal region 400-600 msec after picture presentation, is presented. The impact of drugs on the familiarity processes is discussed in the next section.

Recollection from Long Term Memory was more difficult than from Working Memory

There was a shift of ERP amplitudes to less positive potentials as the interval of recognition increased from 6 to 27 seconds (see Fig 4). ERP waveforms became less positive by 0.41 (0.04 – 0.78) uV (F(1,423) = 4.53, p = 0.03, effect size 2.2), with no interaction with drug for this effect (F(4,420) = 0.42). In other words, all drugs shifted ERP potentials to less positive values equally when recognition occurred from long term as compared to working memory. This shift may represent a decrease in the efficacy of recognition processes from long term memory, even though successful recognition occurred at the same rate as from working memory (though with longer reaction times). Recall that ERPs were obtained only to correctly recognized pictures, and thus these ERP changes are independent of error rates. We will interpret this finding in terms of the ‘pedestal’ effect in the discussion.

Propofol shifts Recollection ERPs to less positive potentials

At the Pz electrode, there were shifts in ERP amplitudes to less positive values as drug dose increased, with ERP amplitudes being 0.75 (0.39 – 1.13) uV less positive (effect of dose, F(1,423) = 15.21, p < 0.001, effect size 3.9; dose increases from panels A and C to B and D in Fig 4). This relationship was different amongst drugs, and there was a significant drug by dose interaction (F(4,420) = 6.81, p < 0.001). Propofol was the only drug to reduce ERP potentials by 3.50 (2.06 – 4.92) uV as dose increased (t = −4.81, p < 0.001, propofol is highlighted in Fig 4). A similar, but smaller, effect may have been present for midazolam (t = −1.90, p = 0.06). A shift in ERP potentials towards less positive values is shown in Fig 4 by the position of the lines connecting new and old amplitudes moving towards the x-axis (amplitude = zero) as propofol, and possibly midazolam, doses increased.

Propofol and midazolam at these doses probably impair recollection processes from long term memory at 27 seconds

A quantifiable measure of episodic memory processing is the area between old and new ERP waveforms. ^15,16 This area can be approximated by the slope of the lines in Fig 4, with zero slope indicative of the absence of an old/new effect (e.g. propofol in panel D of Fig 4 has a smaller slope than in Panel A, and thus a smaller area between waveforms). Area values are displayed in Fig 5B. Though there were dose-related effects on ERP amplitudes (seen in Fig 4), there was no dose-related effect on the areas between waveforms at Pz (F=1.35, p=0.25). Thus, data were collapsed across doses.

There was a decrease of approximately 30% in area between waveforms when pictures were recognized 27 seconds after presentation compared to 6 seconds (F(1,209) = 8.34, p=0.004, effect size 2.9). Thus, the size of the effect of interval on area was slightly larger than on the shift in ERP amplitudes to less positive voltages, whose effect size was 2.2.

Importantly, there was a suggestion of a drug by interval interaction, i.e. the decrease in area when recognition occurs from long term versus working memory may have been different among groups (F(4,205) = 1.86, p = 0.12). Using pairwise comparisons for individual drugs with placebo, a highly significant drug by interval interaction was found for propofol (F(1,79) = 9.30, p = 0.003, effect size 3.1), with weaker evidence present for midazolam (F(1,83) = 2.91, p = 0.09, effect size 1.7). In other words, the decrease in area from 6 to 27 second recognitions seemed to be larger for propofol than for placebo, as represented by the slopes of the lines for individual drugs in Fig 5B.***

Recognition without context – familiarity

The undifferentiated recognition response of familiarity occurs 300-400 msec after item presentation, maximal at the Cz electrode (see Fig 2). This recognition represents one without full context, in other words the stimulus is familiar, but details about when and where it was acquired are not retrieved at this early time point.

Propofol shifts familiarity recognition ERPs to more negative values

Similar to effects on recollection, there was a shift towards more negative ERP amplitudes in the presence of drug (factor of drug F(4,420) = 3.13, p = 0.01), with propofol being responsible for this effect. It produced 5.1 (1.73 – 8.21) uV more negative amplitudes (t = −3.10 vs. placebo, p = 0.002, see Fig 6). The voltages at Cz became 0.75 (0.23 – 1.24) uV more negative when recognition was from long term compared with working memory (F(1,420) = 8.71, p=0.003, effect size 3.0). Different from the recollection response at Pz, familiarity ERP amplitudes at Cz were not affected by dose (F(1,420) = 1.19, p = 0.28).

Fig 6.

As with recollection (measured at Pz), propofol decreased the area between event related potential (ERP) waveforms for recognition from long term memory (red waveform) in comparison to working memory (green waveform) by some 50% for familiarity recognitions. In addition, propofol shifted ERP amplitudes to more negative voltages, compared to placebo. As ERPs are averaged only to correctly recognized pictures, these ERPs represent memory processes used in successful recognition of pictures in the presence of drug. Dexmedetomidine seemed to have different effects on familiarity and recollection processes (in Fig 5, compare slopes at Cz, being flat, with Pz, where a decrease with interval was present). In a pairwise comparison with placebo, the dexmedetomidine group decreased the area for familiarity (p = 0.02, though the overall factor of drug was borderline, F(4,207) = 1.96, p = 0.10, due to the large number of groups in the current study).

Propofol and midazolam impair familiarity processes in long term memory

Similar to recollection processes, there was only a marginal effect of increased dose on the area between waveforms (F(1,207) = 3.40, p = 0.07), with no drug by dose interaction. In other words, any effect of dose was equivalent amongst drugs (drug by dose interaction, F(4,203) = 0.49, n.s.). Thus, areas were collapsed across dose.

There likely was a significant decrease in area between the old/new waveforms by approximately 25% when recognition was from long term as opposed to working memory (F(1,207) = 3.76, p = 0.054, see Fig 5A)†††. As can be seen by the differences in slopes of the lines in Fig 5A, there were differences between drugs on the effect of interval, with an interaction of drug by interval (F(4,203) = 2.69, p = 0.03). Propofol (t = −2.70, p = 0.008) and midazolam (t = −2.34, p = 0.02) were different from placebo. In other words, there was a larger decrease in area as recognition interval increased for propofol and midazolam than for other drugs or placebo, reflecting the suggestion of a similar effect present at Pz.

Dexmedetomidine may have had different effects on familiarity and recollection processes

Dexmedetomidine might have decreased the areas between waveforms for familiarity at Cz in a dose related fashion (see Fig 6). There was a suggestion of an effect of drug (F(4,207) = 1.96, p = 0.10), and in comparison with placebo, dexmedetomidine did have a smaller area (t = −2.29, p = 0.02).‡‡‡ There was no effect of interval on familiarity with dexmedetomidine, whereas weak evidence for this may have been present for recollection (Fig 5B, t = −1.36, p = 0.17 compared with placebo).

Summary of ERP results

In summary, propofol and midazolam decreased areas between old and new recognition ERP waveforms by some 50% when a picture was recognized correctly from long term memory 27 seconds after presentation during the CRT. There was no effect on recognitions from working memory. Both drugs affected familiarity and recollection processes used for recognition from long term memory to a similar extent. There was a suggestion that dexmedetomidine had different effects on familiarity and recollection processes, with familiarity processes impacted more. Dexmedetomidine was more similar to propofol and midazolam in its effect on recollection processes. Propofol shifted both recollection and familiarity ERP waveforms to more negative values, with a dose effect on recollection, but not familiarity processes. The effect size for propofol was larger at the Pz than at Cz electrode, and thus the power to detect the effects of increasing propofol dose may have been better at Pz.

Discussion

As revealed by ERPs, successful recognition of items on the CRT was supported by memory processes that were affected in different ways by the presence of the study drugs. These differences occurred during recognition of pictures from long term memory, as no drug affected working memory processes. Though processing was slowed by the presence of drug in a dose related fashion, access to items in working memory continued to be consistently faster than items in long term memory, and ERP waveforms were preserved at 6 second recognitions. Propofol and midazolam, drugs possessing conscious amnesic properties, impaired long term memory processes as measured by the reduced area between ERP waveforms 27 seconds after encoding. As the size of this area relates to memory strength, this indicates that memory was weaker in the presence of propofol and midazolam even at this early time. ^15,16 Thiopental had no effect on ERPs, despite memory impairment being present at the end of the day. Thus, propofol and midazolam affected memory processes much sooner than previously appreciated. Two previous studies have shown decreased area between old/new waveforms when benzodiazepines were given, but at longer time intervals than in the current study. ^33,34

Differentiation of drug effects on memory function was revealed more readily by ERP measures than by behavioral responses. No differential memory impairment of propofol in comparison with thiopental was apparent, yet robust decreases in areas between ERP waveforms were present, with ERP potentials being shifted to smaller amplitudes. The presence of conscious amnesia with midazolam was difficult to ascertain by behavioral measures. Recognition from working memory was no better than from long term memory (no effect of interval, see Fig 3 panel C). Thus, it could be argued that any decrement in long term memory on the CRT was a function of a sedation effect on working memory. By these measures, midazolam's effects on memory could not be differentiated from thiopental. Conscious amnesia from midazolam was only evident in the greater effect of dose on recognition memory as opposed to reaction time. In contrast to these subtle behavioral changes, differences in recognition ERPs from long term memory were similar to the effects of propofol. Thus, ERP measures documented the influence of propofol and midazolam on memory at drug concentrations just below or at those producing conscious amnesia. ERP changes seemed to presage changes in behavioral performance that would become apparent if the dose were increased or time passed. Though propofol had the same rate of forgetting as thiopental in the current study, there may have been a differential effect present sometime before the end of day recognitions. In a recent imaging study, we showed that the effect of propofol on recognition memory became apparent between 18 and 42 minutes after encoding. ³⁵

Once begun, the effects of propofol and midazolam on memory continue following discontinuation of the drug after completion of the CRTs. Forgetting of memories encoded in the presence of drug continued at a higher rate than placebo for all drugs. Previously we showed an increased rate of forgetting for propofol compared with thiopental. ⁴ In the current study forgetting was determined only for pictures proven to be present in memory by correct recognition on the CRT, different from the previous study where overall recognition rates were measured. A companion manuscript will describe in more detail the memory decay in the period between the CRT and final delayed recognition, where the same memory end-point was reached for all drugs. The current study provides ERP evidence that, for propofol and midazolam, the stage for memory loss is set almost immediately after a memory is formed.

Changed ERP recognition waveforms so soon after encoding raise the question as to whether encoding was affected, and this was the reason why memory was lost so readily. One difficulty is to define the exact time point at which encoding occurs. ³⁶ The current study demonstrated normal functioning of working memory at 6 seconds. Propofol and midazolam affected memory processes within 30 seconds, which may point to an effect on ‘encoding’ into long term memory. However it should be noted that ERPs were averaged only from correct responses. Thus changes in ERP waveforms were uncontaminated by responses that were incorrect or missed. In other words, ERPs represent memory processes underlying successful recognition in the presence of drug. In this sense, these memories are encoded as they are available for retrieval, but the memory processes supporting successful recognition were impaired. We have previously shown that encoding is not inhibited to a major degree by propofol over a time period of approximately 12 minutes, as measured by changes in cerebral blood flow. ³⁵ As there were relatively fewer participants in that study, the sensitivity to detect effects may not have been as good as the current study. Due to the difficulty in separating encoding from recognition processes, ³⁷ an alternative approach would be to consider the effect of a drug on memory in terms of its influence on a particular portion of the cascade of processes put into play once an item is in memory, i.e. consolidation. ^38-41 The effect of propofol or midazolam on some of these yet to be defined processes at an early time point sets the stage for loss rather than retention of a memory.

ERPs are excellent tools to isolate a particular temporal window of drug action on memory. A repeated observation that no drug has been demonstrated to have a retrograde amnesic effect in humans supports the notion that propofol and midazolam inhibit memory processes very early after the acquisition of information, in contrast to observations in animals. ^42-44 In other words, if the critical effect of drug on memory was at a significantly later time in memory consolidation, then memories ‘in the pipeline’ would be lost, as occurs for example when hippocampal function is lost due to hypoxia. ^45,46 The period of retrograde amnesia is defined by the time between acquisition of a memory and the critical memory process affected by the drug. If the critical effect is earlier in the consolidation process, then retrograde amnesia would be of smaller magnitude. This seems to be the case in humans receiving midazolam or propofol.

How can the larger effects on ERPs be reconciled with smaller effects on behavior? In other words, why is recognition successful when recognition memory ERPs were impaired? Even though there was a decrease in area between waveforms during recognition, the area was not zero. In other words, there continued to be a difference between old and new waveforms. To critically test this, the difference between these waveforms for propofol and midazolam at the higher dose for recognition from long term memory was tested (panel D in Fig 5). Indeed, there was a robust difference (F(1,39) = 5.11, p = 0.03, effect size 2.3), with no effect of drug (F(1,39) = 2.87, p = 0.10). Thus, recognition was supported by the remnants of the old/new effect.

Propofol had an additional effect on ERPs from midazolam, producing dose related shifts of ERP potentials to smaller amplitudes. As the relative dosing of midazolam was greater than propofol, any tendency of midazolam to shift ERP amplitudes was clearly less, as shown in Fig 5. Smaller ERP potentials could represent decreased confidence of decisions, decreased strength of memory trace, or less discriminability of items. ²¹ One informative way to conceptualize memory-related ERP potentials is to consider recognition processes as resting on a pedestal, the ERP waveform to new items, which is larger or smaller under certain conditions. The memory processes brought into play by neural generators activated by recognition sum their potentials to cause a deflection from this pedestal, i.e. the old/new effect. Thus, shifts in ERP potentials may be as important as decreases in the displacement between waveforms. For example, subsequent recognition success in the elderly was related to the size of the pedestal at encoding. ⁴⁷ Thus, propofol had two different actions on long term memory processes, whereas midazolam seemed to have one.

Dexmedetomidine, an alpha-2 adrenoceptor agonist drug, produces a downstream effect on gamma-aminobutyric acid receptors, a different mechanism of action from the other study drugs. ²⁴ This difference seemed to be echoed in the ERPs. Though definitive conclusions can't be drawn from the current study, the data are suggestive. One problem with the current study design was that multiple drugs were studied simultaneously, which decreased the power to detect true effects of any given drug due to the number of comparisons required. Behavioral data reveal very little effect of dexmedetomidine in comparison with placebo, even at the higher dose (see Fig 3). During conduct of the study, however, we noted substantial sedation in these participants. As others have noted, the transition from sedation to functioning well was quite rapid, even in the presence of constant doses of dexmedetomidine. ²⁴ This seems to be reflected in the behavioral data of the current study. Despite high cognitive functioning, suggestive results indicated that dexmedetomidine affected ERPs differently from the other drugs. The familiarity response for dexmedetomidine may have been preferentially suppressed in comparison with the recollection, as shown by a difference in slopes at Cz and Pz in Fig 5 and smaller area in Fig 6. As mentioned, thiopental did not seem to affect recognition ERPs in any way, even though it produced significant effects on memory at the end of the study day. In other words, the sedation effect of thiopental increased reaction times and the error rate, but normal memory processes were used for recognition of items in long term memory.

In summary, the effects of propofol and midazolam on long term memory began within 30 seconds of encoding. Both drugs impaired familiarity and recollection processes in recognition from long term memory at this early time. Propofol produced a shift in ERPs to smaller amplitudes, an effect absent for midazolam. Thus, despite the great similarity of conscious amnesic effects and actions on gamma-aminobutyric acid receptors, it appears that these drugs affected memory by somewhat different mechanisms. In the past we have used the term “drug induced amnesia” to describe the sedation independent effects of these drugs on memory. However, “amnesia” is a nebulous term, and we wish to differentiate temporarily induced drug effects on memory from organic amnesias, including post operative cognitive dysfunction. As pointed out in the introduction, all anesthetic drugs will produce amnesia but, as demonstrated in the current study, via different mechanisms. The first signs of accelerated memory decay induced by propofol and midazolam were inhibition of recognition ERPs from long term memory. This effect on ERPs differentiated this mechanism of action from thiopental, which also produced accelerated memory decay compared with placebo. Thus, conscious amnesia produced by propofol and midazolam is characterized by early electrophysiologic changes which presage accelerated memory decay. Dexmedetomidine may have still different actions on memory, with greater effects on familiarity than recollection processes. Thiopental had no discernable effect on recognition ERPs, even though significant impairment of memory occurred. Thus, to fully understand how anesthetic drugs impair memory, measures of memory function other than behavioral ones are needed. The current study demonstrated the utility of memory related ERPs to fulfill this goal. Additionally, as ERPs seemed to presage behavioral effects, the possibility exists to develop an electroencephalogram based measure of memory function during light levels of anesthesia.