Psychopharmacology (1996) 128 : 139 149 Springer-Verlag 1996 ORIGINAL INVESTIGATION Sherry H. Stewart George F. Rioux John F. Connolly Sandra C. Dunphy Michael D. Teehan Effects of oxazepam and lorazepam on implicit and explicit memory: evidence for possible influences of time course Received: 28 January 1996 / Final version: 13 June 1996 Abstract The effects of oxazepam (30 mg), lorazepam (2 mg), and placebo on implicit and explicit memory were studied in two testing cycles, 100 and 170 min after drug administration. Thirty healthy volunteers were randomly assigned to one of three groups (placebo, oxazepam, or lorazepam) in a double-blind, independent groups design. Drug groups were equivalent prior to drug administration on a variety of cognitive measures. Following drug administration, both oxazepam and lorazepam equally impaired performance on a cued-recall explicit memory task relative to placebo, at both testing cycles. Relative to placebo, lorazepam markedly impaired priming on a word-stem completion implicit memory task, at both testing cycles. Consistent with previous work, oxazepam failed to produce impairments in priming on the word-stem completion task at 100 min post-drug administration. However, oxazepam was found significantly to impair priming on this latter task relative to placebo, at close to theoretical peak plasma concentration (i.e., 170 min post-drug administration). Explanations for the observed detrimental effect of oxazepam on implicit memory task performance are considered, including: possible time-dependent effects related to the relative rate of absorption of these two benzodiazepines (BZs); and potential contamination of the implicit memory task by explicit memory strategies during the second testing cycle. Key words Benzodiazepine Explicit memory Implicit memory Lorazepam Oxazepam Priming S.H. Stewart (*) G.F. Rioux J.F. Connolly S.C. Dunphy Department of Psychology, Dalhousie University, Life Sciences Centre, 1355 Oxford Street, Halifax, Nova Scotia, Canada B3H 4J1 S.H. Stewart J.F. Connolly M.D. Teehan Department of Psychiatry, Dalhousie University, Life Sciences Centre, 1355 Oxford Street, Halifax, Nova Scotia, Canada B3H 4J1 Introduction Previous research suggests that benzodiazepines (BZs) impair performance on a variety of traditional tests of memory (see reviews by Curran 1986, 1991). Most previous studies on the amnesic effects of BZs, however, have utilized tasks such as free recall, cued recall, and recognition, in which subjects are explicitly instructed to remember previously presented material. Although BZs clearly impair subjects performance on such explicit memory tasks, their effects on other memory processes remain less well understood. Recently, memory researchers have become interested in the effects of BZs on implicit memory tasks. In contrast to explicit memory tasks, implicit tasks do not involve directing subjects consciously to recollect previous events or experiences. Instead, such tasks assess the non-conscious effects of prior exposure to a stimulus in facilitating its later generation on ostensibly unrelated tests (Richardson-Klavehn and Bjork 1988; Roediger et al. 1989). For example, prior exposure to words has repeatedly been shown to facilitate completion of word-stems with previously studied items above chance levels, when subjects are asked to complete the stems with the first word that comes to mind (Greene 1986); this subsequent facilitation is known as priming (e.g., Tulving and Schacter 1990). Much evidence supports the view that explicit and implicit memory performance are subserved by distinct memory systems. For example, amnesic patients who display profound impairments on traditional explicit memory tasks show priming effects comparable to nonamnesic controls (Graf et al. 1984; Squire et al. 1987). In studies with healthy volunteers, certain drugs such as alcohol have been shown to exert detrimental effects on explicit memory task performance, while leaving intact, priming effects on implicit memory tasks (Hashtroudi et al. 1984; Lister et al. 1991). Few studies to date have contrasted the effects of BZs on explicit versus implicit memory task performance.
140 Early research suggested that like alcohol, BZs induced a dissociation between explicit and implicit memory systems, with BZs such as diazepam exerting a detrimental effect on performance on explicit memory tasks while leaving priming effects intact (e.g., Fang et al. 1987; Danion et al. 1989). However, later research suggested that whereas all BZs can impair performance on explicit memory tasks, implicit memory performance may also be impaired depending on the BZ in question. More specifically, several studies showed that one BZ lorazepam impaired performance on implicit memory measures (e.g., Brown et al. 1989; Knopman 1991; Curran 1992; Curran and Gorenstein 1993). For example, Curran and Gorenstein (1993) compared the effects of lorazepam (2 mg), oxazepam (30 mg), and placebo on performance on two cycles of implicit (word-stem completion) and explicit (free recall) tasks at 120 min post-drug administration. One testing cycle involved incidental encoding, and the other intentional encoding of target words. The results were similar at both testing cycles: while both BZs impaired performance on the free recall task, only lorazepam impaired performance on the word-stem completion task, leading the authors to conclude that both BZs impair explicit memory but only lorazepam impairs priming. Until recently, it was believed that something particular to lorazepam (e.g., its relatively high potency, its potentially differential cortical distribution primarily in occipital regions, and/ or its potential binding to a second population of GABA-ergic BZ receptors) was responsible for this BZ s additional impairments of implicit memory performance (Ghoneim and Mewaldt 1990; Curran 1991; Curran and Gorenstein 1993; Bishop and Curran 1995). However, more recent research suggests that still another BZ diazepam may also exert impairments on implicit memory tasks. Although early studies suggested that diazepam spared priming effects (Fang et al. 1987; Danion et al. 1989, 1990 ), more recent studies have reported a deleterious effect of this drug on implicit memory performance (Sellal et al. 1992; Vidailhet et al. 1994; Legrand et al. 1995). Reasons for this discrepancy remain unclear, but may be related to when, following drug administration, implicit memory is tested (Legrand et al. 1995). These recent findings suggest the possibility that other BZs may also exert detrimental effects on priming, depending on the time course of the effects of the particular BZ in question. The purpose of the present experiment, as part of a larger study on cognitive effects of BZs (Stewart et al. 1995a), was to compare the acute effects of two BZs on implicit and explicit memory tasks at two time points (i.e., 100 and 170 min post-drug) in a placebo-controlled study with healthy volunteers, modelled as an extension of the study by Curran and Gorenstein (1993). As in Curran and Gorenstein (1993), the implicit memory task utilized was a wordstem completion task, which was given following a standard semantic orienting task (Graf et al. 1984). We used a larger set of target stimuli during the initial semantic orienting task than did Curran and Gorenstein (1993) to reduce the likelihood of explicit memory contamination of the implicit task, as recommended by Roediger and McDermott (1988). In contrast to Curran and Gorenstein (1993), who used free recall as their explicit memory measure, we used a cued recall test to allow direct comparison with performance on the implicit word-stem completion task. This approach satisfies Roediger and McDermott s (1988) retrieval intentionality criterion (i.e., when directly comparing performance on implicit and explicit tasks, the tasks should differ only in the instructions presented to subjects). Finally, we included two cycles of testing in the present study (i.e., 100 and 170 min post-drug administration) to assess the time course of the BZinduced impairments in explicit and implicit memory performance. The BZs chosen for the present study were those examined by Curran and Gorenstein (1993). Lorazepam was chosen because previous studies have shown that it impairs priming, and oxazepam was chosen as a chemically similar compound (lorazepam is a chlorinated derivative of oxazepam; Curran and Gorenstein 1993). These two BZs have similar plasma elimination half-lives, but in comparison to lorazepam, oxazepam has a lower receptor affinity (potency; Curran and Gorenstein 1993) and a slower absorption rate (Greenblatt et al. 1981). The doses used in the present study (i.e., 30 mg oxazepam and 2 mg lorazepam) replicated those used by Curran and Gorenstein (1993); these particular doses were chosen based on previous findings that they induce equivalent levels of attentional and psychomotor impairment (Curran et al. 1987). Based on the results of the study by Curran and Gorenstein (1993) it was hypothesized that at the first testing cycle (100 min post-drug administration) both oxazepam and lorazepam would exert equally detrimental effects on subjects performance on the cued recall (explicit memory) task compared to placebo. Also based on the results of Curran and Gorenstein (1993), it was hypothesized that at the first testing cycle, lorazepam, but not oxazepam, would impair priming on the implicit memory task, relative to placebo. The second testing cycle was included in order to examine the time course of the effects of these BZs on implicit and explicit memory task performance. It was hypothesized that the detrimental effects of oxazepam and lorazepam on the explicit memory task relative to placebo, would persist at the second testing cycle (170 min post-drug administration), and that the lorazepam-induced impairments in priming relative to placebo would also persist at the second testing cycle. Based on recent findings that other BZs may also impair priming in a time-dependent fashion (e.g., Legrand et al. 1995), we also hypothesized that oxazepam would impair implicit memory performance relative to
141 placebo, but only at the second testing cycle (170 min post-drug a time point close to oxazepam s theoretical peak plasma concentration; Greenblatt et al. 1981). Materials and methods Subjects Thirty volunteers (18 female and 12 male) aged between 18 and 32 years (mean = 23) were recruited from Dalhousie University. All were heathy, taking no contraindicated medication, and with no history of exclusionary medical illness. None had a history of alcohol or drug abuse, and none was a chronic BZ user. Subjects were instructed to abstain from alcohol and other CNS drugs for 24 h prior to the study. Subjects were allowed a light breakfast, with their normal amounts of caffeine, on the morning of the test day. Written informed consent was obtained; subjects were provided $20 as compensation. The study had ethical committee approval. Experimental design and drugs Subjects were randomly assigned to one of three groups, consisting of ten subjects each: a placebo group, an oxazepam group (30 mg), and a lorazepam group (2 mg). Drug tablets were given orally following a double-blind procedure. On the testing day, subjects were tested in the morning before, and again 100 and 170 min after, drug administration. Tasks and procedure Beginning 30 min prior to drug administration, subjects completed a variety of self-report measures to ensure initial group equivalence on relevant control measures. Subjects provided demographic information (age, gender, number of years of post-secondary education), and completed a brief version of the Michigan Alcoholism Screening Test (Brief MAST; Pokorny et al. 1972) and the Drug Abuse Screening Test (DAST; Skinner 1982), as measures of degree of alcohol and drug problems, respectively. Subjects also provided information on their usual levels of alcohol consumption (alcoholic drinks per week; Stewart et al. 1995b) given evidence of crosstolerance between alcohol and BZs (see review by Stewart et al. 1992). Finally subjects body weights and heights were measured for use in the calculation of body mass index (BMI). To check for possible pre-drug group differences in cognitive performance, subjects were asked for immediate and delayed recall of a prose passage, using the logical memory test from the Wechsler memory scale revised (WMS-R; Wechsler 1987) before drug administration. Subjects were also asked to complete the Vocabulary Test of the Shipley Institute of Living (SIL) Scale (Shipley 1940) to assess baseline (pre-drug) verbal knowledge. Baseline objective sedation (psychomotor speed) was evaluated using the Finger Tapping Test (FTT; Frith 1967). The subjective sedative effects of the drugs were monitored using five visual analogue scales (VASs); subjects were asked to rate their current state by placing a mark on each of five 100 mm lines, anchored with the descriptors: alert-drowsy; excited-calm; clear headed-fuzzy; energetic-lethargic; and quick-slow (Danion et al. 1989). The five VASs were administered at three time points (predrug; post-drug time 1; post-drug time 2) always immediately prior to the administration of cognitive tasks in each test cycle. VAS ratings were linearly scaled from each 100 mm vertical line; the average on the five scales (in mm) served as the dependent measure of subjective sedation at each time point. Stimuli for all post-drug experimental memory tasks were constructed from a pool of 184 words, each of which had a unique three-letter stem. These words were taken from previous studies by Graf and Williams (1987) and Stewart et al. (unpublished). The 184 words were divided into eight sets of 23 words. The eight sets were balanced for length (number of letters), frequency of usage in the English language ( Thorndike and Lorge 1944), number of possible stem completions (Carroll et al. 1971), and base-rate completion of the stem with the target, according to norms obtained with independent samples of drug-free subjects (Graf and Williams 1987; Rioux and Stewart, unpublished). Four word sets were used in the first testing cycle, and the other four word sets in the second testing cycle. These sets were used to create four balanced encoding sets of 46 words each, two of which were used in the first testing cycle, and the other two of which were used in the second testing cycle. In the first testing cycle, which began 100 min post-drug administration, subjects were presented with one of the two 46 word encoding sets, with incidental learning instructions. Presentation of the two encoding sets were counterbalanced across subjects within each drug group. Subjects were shown each word on a computer screen for 5 s and asked to read it and rate how much they liked or disliked it on a five-point scale (Brown et al. 1989; Knopman 1991 Curran and Gorenstein 1993). The subjects provided their ratings of the words verbally during the 3-s inter-stimulus interval. Subjects were then given a 90-s psychomotor speed task (Digit Symbol Test; Wechsler 1981) which also served as a distractor task. Following this, a word-stem completion task presented one of two sets of 46 word-stems, 23 of which could begin words from half of the target set previously shown ( primed stems) and the other 23 of which could begin words from a non-target set ( unprimed stems). The administration of the two word-stem sets was counterbalanced across subjects within each drug group. Subjects were instructed to complete the stems in the order that they appeared on the form with the first word that came into their minds, excluding proper nouns. They were told to take as much time as they needed, but to work as quickly as they could. After the completion task, subjects were given a cued recall task which presented one of two sets of 23 word-stems which could begin words from the remaining half of the target set to which they had been exposed during the word-rating encoding task. Subjects were instructed that the three letter stems were the beginnings of some of the words they had seen in the encoding list. They were asked to try to recall the words from the encoding list, to guess if necessary, to complete the stems in any order, and not to complete the stems with proper nouns. They were allowed 5 min to complete the task. The second testing cycle, which began 170 min post-drug administration, followed exactly the same procedure as the first post-drug cycle with two exceptions. First, to maintain consistency with the methods used by Curran and Gorenstein (1993), the initial encoding task was given with the addition of intentional learning instructions to rate the words and try to remember them. Order of encoding instructions was not counterbalanced across the two testing cycles because the intentional instructions had to be administered second in order to preserve the integrity of the incidental encoding instructions used in the first testing cycle (Curran and Gorenstein 1993). Second, the 90-s objective sedation (psychomotor speed) task, which also served as a distracter task between the encoding and word-stem completion tasks in the second testing cycle, was the Symbol Digit Test (Smith 1973), a task very similar to the Digit Symbol Test used in the first testing cycle, except that subjects were asked quickly to substitute symbols with corresponding numbers, rather than the reverse (Smith 1973). Results Subject characteristics Scores on the control variables were analyzed in a series of one-way (Drug Group) Analyses of Variance
142 (ANOVAs), which revealed no significant effects of Drug Group for any of the variables. Thus, the three groups (placebo, oxazepam, and lorazepam) were equivalent in age, gender composition (% female), education level (years post-secondary education), BMI, MAST scores, DAST scores, and weekly alcohol use (drinks per week). Means (and SDs) for these control variables are illustrated in Table 1 as a function of Drug Group. Pre-drug cognitive functioning Measures of vocabulary ability, immediate and delayed memory for prose passages, and subjective and objective sedation, were assessed prior to drug administration to ensure pre-drug equivalence of the groups. These cognitive measures were subjected to a series of one-way (Drug Group) ANOVAs, which revealed no significant effects of Drug Group. Thus, the three groups were equivalent in baseline (pre-drug) cognitive abilities. Means (and SDs) for the pre-drug cognitive measures are displayed in Table 2 as a function of Drug Group. Memory To maintain consistency with the data analytic techniques used by Curran and Gorenstein (1993), separate ANOVAs were conducted on the memory measures at each post-drug testing cycle, rather than analyzing them together in repeated measures ANOVAs. Direct comparisons of memory performance across the two testing cycles were precluded, since order of encoding instructions (incidental versus intentional) was not counterbalanced across the two testing cycles (Curran and Gorenstein 1993). Explicit memory Cued recall performance at each post-drug testing cycle was scored as the total number of stems correctly completed with target words from the encoding list. Cued recall scores at post-drug times 1 and 2 were analyzed in a set of one-way (Drug Group) ANOVAs. Mean (and SD) cued recall scores at post-drug times 1 and 2 are displayed in Figs 1a and 1b, respectively, as functions of Drug Group. Table 1 Means (and SDs) on the control measures as a function of drug group. BMI Body Mass Index; Brief MAST Brief version of the Michigan Alcoholism Screening Test (Pokorny et al. 1972); DAST Drug Abuse Screening Test (Skinner 1982) Measure Drug group Placebo Oxazepam Lorazepam (n = 10) (n = 10) (n = 10) Age 23.60 23.30 23.10 (in years) (4.48) (3.09) (3.93) Education level 5.00 4.30 3.30 ( years post-secondary education) (2.75) (2.75) (1.77) BMI 23.32 21.74 24.34 [weight (kg)/height (m) 2 ] (1.14) (2.72) (5.33) Alcohol problems 0.00 0.40 0.00 (Brief MAST scores) (0.00) (0.84) (0.00) Drug problems 1.60 1.20 1.70 (DAST scores) (1.51) (1.32) (1.25) Alcohol use 4.86 6.98 4.76 (Drinks per week) (4.67) (9.90) (5.57) Gender composition 50.00 60.00 70.00 (% female) Table 2 Means (and SDs) on the measures of pre-drug cognitive functioning, as functions of drug group. WMS-R Wechsler Memory Scale revised (Wechsler 1987); SIL Shipley Institute of Living Scale (Shipley 1940); Subjective sedation Mean of five 0 100 visual analogue scales ( VASs) of subjective sedation (Danion et al. 1989); Objective sedation Mean intertap interval (ITI) in ms, on Finger Tapping Test (FTT; Frith 1967) Test Drug group Placebo Oxazepam Lorazepam (n = 10) (n = 10) (n = 10) Immediate memory (IM) 25.00 26.30 28.30 (WMS-R logical memory IM) (8.38) (7.73) (4.50) Delayed memory (DM) 23.50 23.90 26.00 (WMS-R logical memory DM) (8.03) (7.15) (3.80) Vocabulary knowledge 30.50 31.10 31.00 (SIL Vocabulary Test score) (5.06) (4.07) (3.80) Subjective sedation 45.76 43.16 40.82 (Mean VAS rating) (16.55) (12.18) (26.37) Objective sedation 199.81 201.59 218.88 (Mean ITI (ms) on FTT) (35.85) (25.14) (33.18)
143 The ANOVA on post-drug time 1 cued recall scores revealed a significant main effect of Drug Group [F(2, 27) = 8.03, P < 0.005]. Newman-Keuls multiple comparisons revealed higher cued recall scores among the placebo as compared to both the lorazepam [Q(3, 27) = 5.36, P < 0.01] and the oxazepam [F(2, 27) = 4.25, P < 0.01] groups; the cued recall performance of the lorazepam and oxazepam groups failed to differ significantly [Q (2,27) = 1.11, ns] (see Fig. 1a). The ANOVA on post-drug time 2 cued recall scores also revealed a significant main effect of Drug Group [F(2,27) = 11.16, P < 0.0005]. Newman-Keuls comparisons again revealed higher cued recall scores for the placebo compared to both the lorazepam [Q(3, Fig. 1a, b Explicit memory performance at a post-drug time 1 (100 min post-drug) and b post-drug time 2 (170 min post-drug): mean correct recall of targets on the Cued Recall Test, as functions of Drug Group (n = 10 per group). Bars represent standard deviations. Asterisks (*) indicate drug conditions that are significantly different from placebo 27) = 6.49, P < 0.01] and the oxazepam [Q (2,27) = 4.62, P < 0.01] groups; the cued recall performance of the lorazepam and oxazepam groups again failed to differ significantly [Q (2, 27) = 1.88, ns] (see Fig. 1b). Although the changes in instructional set during encoding precluded direct statistical comparison of the memory performances at post-drug times 1 and 2, a visual inspection of the means revealed that subjects on the whole did not perform as well on the cued recall task in the second session compared to the first (cf. Figs. 1a and 1b). Implicit memory Word-stem completion performance at each post-drug testing point was scored as the total number of primed and unprimed stems completed with target words from the encoding list. At both post-drug testing points, subjects in all three drug groups performed the task at ceiling levels: no subject completed less than 44 of the 46 word-stems. Number of stems completed with the targets at post-drug times 1 and 2 were analyzed in a set of 3 2 (Drug Group Priming Level) ANOVAs with repeated measures. Means (and SDs) for the number of stems completed with targets at post-drug times 1 and 2 are displayed in Figs 2a and 2b, respectively, as functions of Drug Group and Priming Level. The ANOVA on post-drug time 1 stem completion scores revealed main effects of Drug Group [F(2, 27) = 5.09, P < 0.05] and Priming Level [F(1,27) = 65.76, P < 0.0001], and a Drug Group Priming Level interaction [F(2,27) = 4.42, P < 0.05]. The Priming Level main effect was due to an overall higher rate of completion of stems with the targets for the primed (old) versus unprimed (new) words; the Drug Group main effect was due to an overall higher rate of completion of stems with targets for the placebo compared to the lorazepam group, with the performance of the oxazepam group falling midway between that of the other two groups (see Fig. 2a). To explore the two-way interaction further, simple effects of Drug Group were examined at each level of Priming. For the unprimed stems, there was no simple main effect of Drug Group [F(2,27) = 1.16, NS]; drug administration did not affect subjects chance performance in generating unprimed targets (see Fig. 2a). However, for the primed stems, a simple main effect of Drug Group was revealed [F(2, 27) = 5.46, P < 0.05], which was due to impairment in the lorazepam group only. Newman-Keuls comparisons showed that the number of primed stems completed with target words was significantly lower in the lorazepam compared to both placebo [Q(3,27) = 4.59, P < 0.01] and oxazepam [Q(2,27) = 3.03, P < 0.05) groups; the performance of the oxazepam group did not differ from the placebo group [Q (2,27) = 1.57, ns] (see Fig. 2a). Completion of primed stems with targets was three to four times chance levels in the placebo
144 explore further the two-way interaction, simple effects of Drug Group were examined at each level of Priming. For the unprimed stimuli, there was no simple main effect of Drug Group [F(2,27) = 0.49, ns]; again, drug administration did not affect subjects chance performance in generating unprimed targets (see Fig. 2b). However, for the primed stems, a simple main effect of Drug Group was revealed [F(2,27) = 16.58, P < 0.0001], which was due to impairment in both the lorazepam and oxazepam groups. Newman-Keuls showed that the number of primed stems completed with targets was higher in the placebo compared to both the lorazepam [Q(3,27) = 7.74, P < 0.01] and oxazepam [Q(2,27) = 6.07, P < 0.01] groups; the performance of the oxazepam and lorazepam groups did not differ significantly [Q(2, 27) = 1.67, ns] (see Fig. 2b). Completion of primed stems with targets was about two-and-a-quarter times chance levels for placebo, but only one to one-and-a-half times chance for the lorazepam and oxazepam groups (see Fig. 2b). Although the changes in instructional set during encoding precluded direct statistical comparison of the memory performances at post-drug times 1 and 2, a visual inspection of the means reveals that subjects on the whole did not perform as well in priming levels (i.e., completing primed versus unprimed stems with targets) in the second session compared to the first (cf. Figs. 2a and 2b). Sedative effects Subjective sedation Fig. 2a, b Implicit memory performance at a post-drug time 1 (100 min post-drug) and b post-drug time 2 (170 min post-drug): mean number of stems completed with targets on the Word-Stem Completion Test, as functions of Priming Level and Drug Group (n = 10 per group). Bars represent standard deviations. Asterisks (*) indicate drug conditions that are significantly different from placebo and oxazepam groups, but less than two times chance in the lorazepam group (see Fig. 2a). The ANOVA on post-drug time 2 stem completion performance again revealed main effects of Drug Group [F(2, 27) = 8.50, P < 0.005] and Priming Level [F(1, 27) = 28.92, P < 0.0001], and a Drug Group Priming Level interaction [F(2, 27) = 12.16, P < 0.0005]. The Priming Level main effect was again due to an overall higher rate of completion of stems with the targets for the primed (old) versus unprimed (new) words; the Drug Group main effect was again due to an overall higher rate of completion of stems with targets for the placebo compared to the lorazepam group, with the performance of the oxazepam group falling midway between that of the other two groups (see Fig. 2b). To Mean subjective sedation ratings at each of the three time points (pre-drug; post-drug time 1; post-drug time 2) were submitted to a 3 3 (Drug Group Time) ANOVA with repeated measures. The ANOVA revealed a main effect of Time [F(2,26) = 18.10, P < 0.0001) and a Drug Group Time interaction [F(4,52) = 2.57, P < 0.05]. Mean (and SD) subjective sedation ratings are illustrated in Table 3a as functions of Drug Group and Time. Simple main effects of Time were examined in each Drug Group. No simple main effect of Time was revealed for the placebo group [F(2,26) = 2.41, ns]; subjective sedation ratings of placebo subjects did not change significantly across the course of testing (see Table 3a). Simple main effects of Time were found for the lorazepam [F(2,26) = 17.46, P < 0.0001] and the oxazepam [F(2, 26) = 3.77, P < 0.05] groups. The pattern of drug effects and levels of significance on the subjective sedation ratings remained virtually identical using the conservative Hundt-Feldt adjusted degrees of freedom for repeated measures analyses: the simple main effects of Time remained non-significant in the placebo group [F(1.57,42.40) = 1.60, ns], and remained significant in the lorazepam [F(1.57, 42.40) = 7.28, P < 0.005] and oxazepam [F(1.57,
145 Table 3 Means (and SDs) on the measures of drug-induced sedation. a Subjective sedation, and b objective sedation, as functions of drug group and time. Subjective sedation Mean of five 0 100 visual analogue scales ( VASs) of subjective sedation (Danion et al. 1989); Objective sedation Digit Symbol Test (Wechsler 1981) and Symbol Digit Test (Smith 1973) scores at post-drug times 1 and 2, respectively. For the subjective sedation measures, comparisons are within drug groups, across time; for the objective sedation measures, comparisons are across drug groups Time measure Drug group Placebo Oxazepam Lorazepam (n = 10) (n = 10) (n = 10) a Subjective sedation Pre-drug (30 min pre-drug) 45.76 43.16 b,c 40.82 a (Mean VAS rating) (16.55) (12.18) (26.37) Post-drug time 1 (100 min post-drug) 57.03 58.40 b 66.32 a,b (Mean VAS rating) (16.08) (10.37) (19.37) Post-drug time 2 (170 min post-drug) 54.97 61.82 c 51.50 b (Mean VAS rating) (18.88) (19.89) (25.22) b Objective sedation Post-drug time 1 (100 min post-drug) 67.20 a,b 58.90 b,c 49.60 a,c (Mean digit symbol test score) (7.50) (10.79) (7.78) Post-drug time 2 (170 min post-drug) 55.40 b 53.50 c 44.70 b,c (Mean symbol digit test score) (7.49) (10.70) (8.56) Means with similar superscripts are significantly different from one another ( a P < 0.01; b P < 0.05; c P < 0.05) 42.40) = 4.38, P < 0.05] groups. Newman-Keuls showed that for the oxazepam group, subjective sedation ratings at post-drug time 1 were greater than predrug ratings [Q(2,26) = 3.21, P < 0.05], ratings at post-drug time 2 were greater than pre-drug ratings [Q(3,26) = 3.93, P < 0.05], but ratings at post-drug time 1 did not differ from ratings at post-drug time 2 [Q(2,26) = 0.72, ns] (see Table 3a). For the lorazepam group, sedation ratings at post-drug time 1 were greater than pre-drug ratings [Q(3,26) = 5.37, P < 0.01], ratings at post-drug time 1 were greater than post-drug time 2 ratings [Q(2,26) = 3.12, P < 0.05], but ratings at post-drug time 2 did not differ from pre-drug ratings [Q(2,26) = 2.25, ns] (see Table 3a). Objective sedation Number of items correctly completed on the Digit Symbol (post-drug time 1) and Symbol Digit (postdrug time 2) tests were scored as measures of objective sedation (i.e., psychomotor speed). Since different objective sedation measures were used at each postdrug testing point, scores on each measure were subjected to separate one-way (Drug Group) ANOVAs. Mean (and SD) scores on the two psychomotor tests are displayed in Table 3b as functions of Drug Group. The ANOVA on the Digit Symbol scores revealed a main effect of Drug Group [F(2,27) = 9.98, P < 0.001]. Newman-Keuls comparisons revealed that the lorazepam group was impaired relative to the placebo group [Q(3,27) = 6.32, P < 0.01], the oxazepam group was impaired relative to the placebo group [Q(2,27) = 2.98, P < 0.05], and the lorazepam group was impaired relative to the oxazepam group [Q(2, 27) = 3.34, P < 0.05] (see Table 3b). The ANOVA on the Symbol Digit scores also revealed a main effect of Drug Group [F(2,27) = 4.01, P < 0.05]. Newman- Keuls revealed that the lorazepam group remained impaired relative to the placebo group [Q(3,27) = 3.75, P < 0.05], the lorazepam group remained impaired relative to the oxazepam group [Q(2,27) = 3.09, P < 0.05], but the oxazepam group no longer differed from the placebo group [Q(2,27) = 0.67, ns] (see Table 3b). Covariation between sedative effects and performance on implicit explicit memory tasks A series of Analyses of Covariance (ANCOVAs) were carried out, separately covarying scores on the subjective and objective measures of sedation at post-drug times 1 and 2, from the scores on the implicit and explicit memory tasks at post-drug times 1 and 2, respectively. Covarying subjective sedation did not change the pattern of results or levels of significance of Drug Group differences in performance on either the implicit or explicit memory tasks, at either postdrug testing point. Similarly, covarying the objective sedation measures (Digit Symbol scores, post-drug time 1; Symbol Digit scores, post-drug time 2) did not change the pattern of results and did not tend to change the levels of significance of Drug Group differences in performance on either the implicit or explicit memory tasks, at either post-drug testing point. The only exception was at post-drug time 2, where covarying Symbol Digit scores slightly reduced the level of significance of the Drug Group simple main effect for the primed items on the word-stem completion task [F(2, 26) = 14.78, P < 0.0005]. The significant simple main effect of Drug Group for completion of primed stems revealed in the ANCOVA continued to be due to impairment in both the lorazepam and oxazepam groups, after controlling for the effects of psychomotor speed (Symbol Digit Test scores). Newman-Keuls comparisons of the covariateadjusted means showed that the number of primed stems completed with targets continued to be higher in
146 the placebo group compared to both the lorazepam [Q(3, 26) = 7.94, P < 0.01] and oxazepam [Q(2, 26)= 6.04, P < 0.01] groups. The performance of the oxazepam and lorazepam groups still did not differ significantly [Q(2, 26) = 1.90, ns], following adjustment of the means for the psychomotor speed covariate. Relation between stem-completion and cued recall A priming index was calculated for each subject at each post-drug testing point by subtracting the number of unprimed stems completed with targets from the number of primed stems completed with targets. Priming index scores (the implicit memory measure) were correlated with the number of correctly recalled targets on the cued recall task (the explicit memory measure), for the entire sample (n = 30). At both post-drug times 1 and 2, these correlations failed to reach significance using a Bonferroni-adjusted P-value of 0.025 (0.05/ 2) (i.e., rs = 0.356 and 0.259, for post-drug times 1 and 2, respectively), indicating that priming performance bore no significant relation to explicit memory performance at either post-drug testing point. Discussion Results obtained at the first cycle of testing replicated and extended those of Curran and Gorenstein (1993) in demonstrating that, at a certain point following drug administration (i.e., 100 min post-drug), the two BZs have different effects on an implicit memory task, but similar effects on a comparable explicit memory task (i.e., cued recall). These two tasks can be compared directly because they meet the retrieval intentionality criterion they differed only in the instructions given to subjects at the time of memory testing (Greene 1986; Roediger and McDermott 1988; Bishop and Curran 1995). Although both BZs equally impaired explicit memory performance (as evidenced by the equivalently lower scores of the oxazepam and lorazepam groups compared to the placebo group on the cued recall task), only lorazepam impaired completion of stems for primed words on the word-stem completion (implicit memory) task, at post-drug time 1. These findings parallel those of several previous studies in demonstrating lorazepam-induced implicit memory impairments (e.g., Brown et al. 1989; Knopman 1991; Curran 1992; Bishop and Curran 1995). However, our results at post-drug time 2 (170 min post-drug administration) challenge previous assertions that only lorazepam impairs priming (e.g., Curran and Gorenstein 1993). At this later time point of testing, the lorazepam and oxazepam groups were again equally impaired relative to the placebo group on a measure of explicit memory (i.e., cued recall), but at this time point both drug groups were also equally impaired relative to the placebo group, on a comparable implicit memory measure (i.e., word-stem completion): both oxazepam and lorazepam equally impaired subjects completion of stems for primed words on this implicit measure, relative to placebo. This finding is consistent with recent findings that BZs other than lorazepam may also impair performance on implicit memory measures: studies by Legrand et al. (1995), Sellal et al. (1992), and Vidailhet et al. (1994) have shown that diazepam can impair implicit memory, depending on factors such as the time point relative to drug administration at which encoding and memory testing take place. The present study provides the first demonstration that oxazepam may similarly impair priming in a time-dependent fashion. The possibility that BZs other than lorazepam may impair implicit as well as explicit memory performance is consistent with emerging data on the mechanisms underlying the various cognitive effects of BZs (Bishop and Curran 1995; Giersch et al. 1995; Legrand et al. 1995). Although it was previously suggested that the additional impairments of lorazepam on implicit memory performance were due to some pharmacologically distinct property of this particular BZ (e.g., its binding to a second unique population of GABA-ergic BZ receptors), Bishop and Curran (1995) have recently shown that administration of the BZ antagonist flumazenil attenuated lorazepam-induced impairments on both implicit and explicit memory tasks identical to those used in the present study. Thus, the authors concluded that the effects of lorazepam on implicit memory tasks are in fact mediated by the same BZ receptor complex as mediates lorazepam s other cognitive effects. Given the present findings that oxazepam may exert impairments in implicit memory performance at 170 min post-drug administration, it would be useful for future research to examine whether flumazenil attenuates this oxazepam-induced priming impairment. Given other recent findings of diazepaminduced priming impairments (Sellal et al. 1992; Vidailhet et al. 1994; Legrand et al. 1995), it would be instructive similarly to examine the pharmacological mechanisms underlying these diazepam-induced deficits. There was a virtual absence of priming in subjects administered lorazepam at post-drug times 1 and 2, and in subjects administered oxazepam at post-drug time 2: primed target completion levels were very nearly identical to unprimed target completion levels (i.e., all less than two times chance levels) among these groups. Moreover, in contrast to some previous studies (e.g., Brown et al. 1989; Knopman 1991) the drug effects on implicit memory performance in the present study could be interpreted unambiguously as demonstrating implicit memory impairments, since drug administration produced no significant deficits in subjects completion of unprimed stems with targets at either time
147 point. Consistent with previous findings that BZs do not exert impairments in access to semantic memory (Curran 1991), chance completion rates in the present study were equivalent across groups. The simplest explanation for the observed delayed impairment in priming among the oxazepam group is the slower absorption rate of oxazepam compared to lorazepam (Greenblatt et al. 1981). Thus, results obtained in the present study may reflect differences in absorption rates, rather than differences in the cognitive effects of the two drugs, per se. In Curran and Gorenstein s (1993) study, memory was tested at 120 min post-drug administration. Their chosen time point of memory testing may not have allowed sufficient time post-drug administration for an oxazepaminduced impairment in priming to be detected, given that oxazepam only reaches theoretical peak plasma concentration at 160 min post-administration (Greenblatt et al. 1981), closer to the time 2 testing point of 170 mins. post-drug administration used in the present study. However, in addition to differences in relative rates of absorption, several other explanations for the delayed impairment in priming seen in the oxazepam group in the present study should be considered. First, different encoding instructions were used at post-drug time 1 and post-drug time 2, to maintain consistency with the methods of Curran and Gorenstein (1993). At post-drug time 1 subjects encoded target words with incidental task instructions, whereas at post-drug time 2 subjects encoded target words with intentional task instructions. Thus, the observed priming impairment in the oxazepam group at 170 min post-drug administration may represent a relative sensitivity of this drug to intentional memory instructions in affecting later priming levels. Several lines of evidence make this interpretation unlikely. First, previous research suggests that while instructional set (incidental versus intentional) may exert significant influences on explicit memory performance, this manipulation does not influence priming on implicit memory measures (Greene 1986; Schacter et al. 1993). In addition, the manipulation of instructional set used in the present study was identical to that used by Curran and Gorenstein (1993) who found that independent of instructions, only lorazepam, but not oxazepam, exerted impairments in priming relative to placebo at 120 min post-drug. Second, relative differences in potency (i.e., receptor affinity) between the two BZs used in the present study may have contributed to the findings. Lorazepam and oxazepam differ not only in relative rate of absorption, but also in relative affinity for binding to the GABAergic BZ receptor, with lorazepam being a more potent drug than oxazepam (Curran and Gorenstein 1993). Thus, the delayed impairment in priming observed in the oxazepam group in the present study may merely suggest that BZs with relatively stronger receptor affinities exert influences on implicit memory performance more quickly. Evidence from one previous study makes this latter interpretation unlikely. Using triazolam, a more potent BZ even than lorazepam in terms of its affinity for the GABA-ergic BZ receptor complex, Weingartner et al. (1992) found no impairments in priming at 60, 180, or 480 min post-drug administration. However, Weingartner et al. (1992) did use a different implicit memory task than the word-stem completion task used in the present study. The most likely alternative interpretation of the delayed effects of oxazepam on priming is the potential contamination of the implicit memory task at postdrug time 2, with subjects use of explicit memory strategies. Since the same implicit task (word-stem completion) was used at both post-drug times 1 and 2, subjects may have caught on to the relation between the implicit test and the prior study list, and thus used explicit strategies in an attempt to enhance their implicit memory performance at time 2 (Schacter et al. 1993). Thus, the additional oxazepam-induced impairment in priming at post-drug time 2 observed in the present study may merely reflect increased use of explicit memory strategies by subjects at time 2, resulting in poorer performance of both active BZ groups compared to placebo at the second testing point. Consistent with this implicit memory contamination interpretation, levels of priming in both the lorazepam and oxazepam groups decreased markedly from postdrug time 1 to post-drug time 2. However, it should be noted that subjects as a whole did not do as well on either memory test in the second testing session compared to the first, most likely due to the use of repeated memory testing (i.e., learning from the first cycle interfering with learning in the second cycle), plus the additional impairment produced by oxazepam at time 2. Concerns about implicit memory task contamination by explicit strategies have led some memory researchers to counsel against the use of implicit memory tasks as repeated measures (e.g., Roediger and McDermott 1988). It should also be noted that designs such as the one used in the present study where implicit and explicit tasks are examined in a within-subjects fashion, especially those in which both implicit and explicit memory tasks are used as repeated measures, run the risk not only of contamination of the implicit task by explicit memory processes, but vice versa (i.e., contamination of the explicit task by implicit memory processes). Two lines of evidence argue against the implicit memory contamination interpretation of the present findings, however. First, Curran and Gorenstein (1993) also used the word-stem completion test as a repeated measure in their study and found that lorazepam, but not oxazepam, impaired priming relative to placebo at both testing cycles, 120 min post-drug administration. Second, correlational results from the present study argue against the possibility that subjects were making
148 increased use of explicit strategies on the implicit task at time 2. If this had been the case, priming performance on the implicit task at time 2 should have been strongly positively related to performance on the explicit memory cued recall task at time 2. However, correlations between performance on the two memory measures revealed no significant relation between priming and cued recall performance at either testing point, suggesting that subjects did not appear to be making significant use of explicit memory strategies on the implicit task at either time 2 or time 1. Some studies on the effects of BZs on memory have examined the issue of stochastic (in)dependence to assess the relation between stem completion and explicit memory task performance (e.g., Curran and Gorenstein 1993). Using this approach, researchers can examine the production of particular words from the encoding list on the implicit and/ or explicit tasks. If the implicit task is indeed contaminated by explicit strategies, one would expect a strong stochastic dependence between particular words produced on both the implicit and explicit tasks. Curran and Gorenstein (1993), for example, examined stochastic (in)dependence as a function of drug group. They found that the relation between performance on the explicit and implicit tasks was stronger for placebo subjects than for subjects in the oxazepam or lorazepam groups at both testing points, 120 min post-drug administration, suggesting that implicit memory contamination by explicit strategies was stronger for placebo than BZtreated subjects. Moreover, levels of stochastic dependence did not increase across the two memory testing cycles for any drug group, providing no support for the notion that subjects had caught on to the relation between the implicit test and the prior study list due to repeated memory testing. However, procedural differences across the two studies (e.g., the fact that Curran and Gorenstein (1993) used word-stem completion as a repeated measure at 120 min post-drug, whereas the present study used the same task at 100 and 170 min post-drug), preclude ruling out implicit memory task contamination in the present study on the basis of Curran and Gorenstein s (1993) results. Unfortunately, stochastic dependence could not be evaluated in the present study because the implicit and explicit memory tests used different stems (i.e., each test used balanced sets of stems from only half the encoded targets). It has been argued that BZ-induced disruptions in memory performance could be secondary to changes in arousal (Curran 1991). Since drug administration exerted significant impairments in both measures of sedation used in the present study (i.e., subjective and objective sedation), ANCOVAs were used to determine whether the various memory impairments persisted after the sedative effects of the drugs were controlled in the statistical analyses. Consistent with Curran and Gorenstein (1993), subjective sedation ratings did not relate to any aspect of memory performance assessed in the present study. Similarly, the drug group effects on the implicit and explicit memory tasks in the present study persisted at both post-drug testing points, after the scores on the objective sedation (psychomotor speed) measures were covaried out. Covarying out objective sedation (Symbol Digit scores) at post-drug time 2 did reduce the level of significance of the drug group effect on performance on the implicit memory task, implying a partial contribution of psychomotor sedation to this effect. However, this latter drug group effect still remained significant (P < 0.0005), demonstrating that the two BZs induced significant impairments in priming at time 2, over and above these drugs objective sedative effects. Thus, the BZ-induced memory disruptions observed in the present study do not appear attributable to changes in either subjective or objective arousal. In summary, the present study demonstrates that both lorazepam and oxazepam induce significant impairments in explicit memory, and that these effects persist up to 170 min post-drug administration. A different pattern of results emerged in terms of druginduced impairments on a test of implicit memory: while impairments in priming were observed in the lorazepam-treated subjects at both 100 and 170 min post-drug administration, an additional oxazepaminduced impairment was observed only at 170 min post-drug. The differential time-course of the effects of oxazepam on implicit versus explicit memory processes provides additional support for the hypothesized distinction of these two types of remembering. The additional oxazepam-induced impairment in priming at 170 min may be related to this drug s relatively slower rate of absorption, or to contamination of the implicit task by explicit strategies at the second testing cycle. This latter interpretation could be ruled out in future research by testing subjects only at 170 min post-drug administration to determine if oxazepam-induced priming impairments persist without prior exposure to the implicit task. It would also be useful to follow these drug effects over a longer time course and to consider more than a single dose of each of these BZs. Clearly, future studies on the effects of various BZs on a range of implicit memory tasks are needed, especially since implicit memory functions are themselves differentiated (i.e., perceptual versus conceptual implicit memory; Tulving and Schacter 1990). Such research would have both theoretical implications for the understanding of memory organization (Ghoneim and Mewaldt 1990), as well as clinical implications, since priming is a form of nonconscious memory which people probably rely on frequently in their day-to-day lives and since the disruption of this form of memory by certain drugs may severely compromise people s ability to function at their full potential (Tulving and Schacter 1990).