Level-of-Processing Effects in Word-Completion Priming: A Neuropsychological Study

Transcription

1 Journal of Experimental Psychology: Learning, Memory, and Cognition 1996, Vol. 22, No. 4, In the public domain Level-of-Processing Effects in Word-Completion Priming: A Neuropsychological Study Stephan B. Hatnann University of California, San Diego Larry R. Squire Veterans Affairs Medical Center, San Diego, and University of California, San Diego Recent reviews (A. S. Brown & D. B. Mitchell, 1994; B. Challis & D. R. Brodbeck, 1992) concluded that level-of-processing (LOP) manipulations affect priming in perceptual tasks, contrary to earlier suggestions that such tasks are insensitive to LOP. In 3 experiments with amnesic patients and control subjects, the authors examined the effect of LOP manipulations on priming in word-stem and word-fragment completion and on recognition memory. Amnesic patients exhibited reduced or near-zero LOP effects in word-completion priming compared with control subjects. LOP affected recognition memory for both amnesic patients and control subjects, confirming that the LOP manipulation affected explicit memory. When the effect of explicit retrieval on control performance was reduced by using a low-level encoding task, priming was the same for amnesic patients and control subjects. The authors suggest that LOP effects in word-completion priming tasks reflect the influence of explicit retrieval, which can be used usefully by control subjects but much less so by amnesic patients. The distinction between explicit (declarative) and implicit (nondeclarative) memory has been a major focus of recent memory research (for recent reviews, see Schacter, Chiu, & Ochsner, 1993; Schacter & Tulving, 1994; Squire, Knowlton, & Musen, 1993). A widely used method for illuminating explicit and implicit forms of memory has been to demonstrate dissociations in how particular experimental variables affect performance on memory tasks. One striking observation concerns the differential effect on memory performance of varying the encoding strategy (or level of processing [LOP]) that subjects engage in during study. In many experiments, variations in LOP typically exert large effects on measures of explicit memory but little or no effect on measures of implicit memory, notably word priming (Richardson-Klavehn & Bjork, 1988; Roediger & McDermott, 1993; Shimamura, 1986; Tulving & Schacter, 1990).* The absence of LOP effects in priming tasks has come to be considered an important characteristic of implicit memory, even a defining property, that distinguishes implicit from explicit memory. It should be noted that most studies of LOP and priming have involved perceptual tasks in which the stimulus presented at test is a perceptually degraded version of the stimulus Stephan B. Hamann, Department of Psychiatry, University of California, San Diego; Larry R. Squire, Veterans Affairs Medical Center, San Diego, California, and Departments of Psychiatry and Neurosciences, University of California, San Diego. This research was supported by the Medical Research Service of the Department of Veterans Affairs and by National Institute of Mental Health Grant MH We thank Nicole Champagne, Brent Kronenberg, Kamilla Willoughby, and Joyce Zouzounis for research assistance. Correspondence concerning this article should be addressed to Larry R. Squire, Veterans Affairs Medical Center, 3350 La Jolla Village Drive, San Diego, California Electronic mail may be sent via Internet to lsquire@ucsd.edu. presented at study, for example, fragment completion (e e h nt for elephant), stem completion (mot for motel), and perceptual identification (in which test items are presented very briefly). However, priming can also be demonstrated in tasks in which the relationship between studied items and test cues is conceptual (semantic or associative) rather than perceptual. For example, participants can be asked at test to generate category exemplars (e.g., list 8 vegetables) after being presented with a study list that includes some appropriate exemplars (e.g., carrot and spinach). Perceptual and conceptual tasks differ in that LOP effects are readily observed in conceptual tasks (Hamann, 1990; Srinivas & Roediger, 1990; Tulving & Schacter, 1990). Although the prevailing view has been that LOP effects are small or absent in perceptual priming tasks, recent reviews suggest that LOP effects may in fact be typical (Brown & Mitchell, 1994; Challis & Brodbeck, 1992; Chiarello & Hoyer, 1988). The LOP effect often falls short of significance in individual experiments, but when this effect is examined across many studies in a meta-analysis, the LOP effect is numerically small (and much smaller than the effect of LOP on measures of explicit memory), but the LOP effect is statistically significant. Many studies have examined whether LOP effects occur in perceptual priming tasks in normal participants. However, only three studies have examined whether LOP effects occur with word-completion priming tasks in amnesia. The first of these studies (Graf, Squire, & Mandler, 1984) examined the effect of LOP on word-stem completion priming in two experiments (Experiments 1 and 2). In both experiments there 1 Many kinds of implicit (nondeclarative) memory tasks have been studied in addition to priming. These include skill learning, adaptationlevel effects, artificial grammar learning and prototype learning, and simple forms of classical conditioning. In the current study we focused on word priming. 933

2 934 HAMANN AND SQUIRE was an LOP effect of roughly equal size for both control subjects and amnesic patients. The second study (Squire, Shimamura, & Graf, 1987) examined the effect of LOP on priming of both word-stem completion (Experiment 2) and word-fragment completion (Experiment 3). In contrast to the earlier Graf et al. (1984) study, amnesic patients exhibited lower LOP effects than control subjects in both experiments. However, a test for a Subject Group x LOP interaction could not be conducted because it was precluded by the experimental design, in which LOP was manipulated within subject in the amnesic patient group but between subjects in the control subject group (thefindingof lower LOP effects in the amnesic patients was based on separate comparisons between amnesic patients and control subjects in the semantic orienting and nonsemantic orienting conditions). Finally, Carlesimo (1994, Experiment 1) examined the effect of LOP in word-stem completion in amnesic patients and alcoholic control subjects and found a marginal overall effect of LOP and no interaction between subject group and LOP. Notably, the LOP effects were numerically larger in this study for the amnesic patients than for the alcoholic control subjects, a finding contrary to the findings of both previous studies in which amnesic patients always exhibited numerically lower LOP effects than control subjects. However, this finding is likely related to the poor performance of the alcoholic control group, which exhibited lower word-completion performance for both studied (primed) and nonstudied (unprimed) words than the amnesic patients. 2 These studies have, in general, not yielded a consistent pattern of findings and have been inconclusive regarding the factors that determine why LOP effects in word-completion are sometimes present and sometimes absent in amnesia. We revisit these earlier studies in the General Discussion section, in which we propose an explanation to account for the findings of each of these studies in conjunction with the results of the present study. Challis and Brodbeck (1992) identified three possibilities for why LOP effects occur in perceptual priming tasks. First, explicit retrieval from memory may occur during an implicit memory test contrary to the intention of the experimenter. For example, subjects might treat a test of word completion as an explicit test of cued recall. Second, the manipulation of LOP might directly affect perceptual processing of stimuli at the time of encoding. For example, suppose that a shallowencoding task such as attending to vowels or letters with enclosed parts (e.g., o and g) reduces substantially the degree to which stimuli are perceived. In this case, incomplete representations of study items would be established and performance would be impaired on a subsequent test, regardless of whether the test was intended to assess explicit memory or implicit memory. Finally, perceptual priming tasks might contain a conceptual component that is sensitive to LOP. As Challis and Brodbeck (1992) noted, studies of amnesic patients are potentially useful for deciding between these alternatives. Specifically, if LOP effects in perceptual priming tests are absent in amnesia, or are smaller than in control subjects, then LOP effects (when they do occur in normal subjects) are likely due to the contaminating effects of explicit retrieval. If, on the other hand, amnesic patients exhibit LOP effects similar to the LOP effects observed in control subjects, then LOP effects in perceptual priming tasks must be due to some factor other than explicit retrieval, for example, one of the other two alternatives just outlined. Challis and Brodbeck (1992) reviewed the findings of Graf et al. (1984) and Squire et al. (1987) and found them inconclusive. Although control subjects consistently showed more priming than amnesic patients in the semantic rather than the physical condition in the Squire et al. (1987) study, Challis and Brodbeck (1992) noted that "conclusions based on these findings are not on firm ground because, in some cases, amnesics did not show LOP effects on explicit tests so that the absence of priming LOP effects in word-stem completion would not be surprising" (p. 605). In addition, they noted that Graf et al. found significant LOP effects in word-stem completion in both amnesic patients and control subjects, a result which is inconsistent with the Squire et al. (1987) findings. The utility of amnesic patients for studies of LOP effects and perceptual priming depends on the assumption that the impairment exhibited by the patients is limited to explicit memory and that perceptual and conceptual processing is intact. This assumption is supported by a variety of experimental data. In the case of perceptual processing, amnesic patients exhibit intact baseline (unprimed) and studied (primed) performance on a range of perceptual tasks, indicating that their perceptual abilities do not differ from those of healthy control subjects. These tasks include perceptual identification for words (Haist, Musen, & Squire, 1991; Hamann, Squire, & Schacter, 1995) and pronounceable nonwords (Haist et al., 1991), word-stem and word-fragment completion (Graf et al., 1984; Squire et al., 1987), object decisions concerning possible and impossible novel objects (Schacter, Cooper, Tharan, & Rubens, 1991; Schacter, Cooper, & Treadwell, 1993), and perception of briefly presented linefigures (Gabrieli, Milberg, Keane, & Corkin, 1990; Musen & Squire, 1992). Evidence also indicates that amnesic patients exhibit intact conceptual processing. Baseline (unprimed) and studied (primed) performance in priming tasks that require conceptual processing does not differ between amnesic patients and control subjects. For example, amnesic patients are intact in their ability to generate exemplars from common categories and in their ability to generate common associates to cue words (Graf, Shimamura, & Squire, 1985). In addition, amnesic patients obtain scores in the normal range on many cognitive tests, including tests of general intelligence (Squire & Shimamura, 1986; see also the Method sections of this article). In the present study we studied the effect of LOP manipulations on perceptual priming in both amnesic patients and control subjects, focusing on the two most commonly used perceptual priming tasks: word-stem completion and wordfragment completion. In each of three experiments, we examined whether LOP manipulations that markedly affect explicit memory also affect perceptual priming, and in each case we 2 The reason why alcoholic control subjects sometimes perform poorly in word-completion priming tasks is unclear. Poor performance by this subject population was also noted in both the Graf et al. (1984) and Squire et al. (1987) studies, as well as in Carlesimo (1994).

3 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 935 compared the performance of amnesic patients and control subjects. If explicit retrieval contributes to LOP effects in word-completion tasks, an interaction should be observed between subject group and LOP, with lower LOP effects always observed for the amnesic patients. Finally, recognition memory was assessed for study words to verify that the LOP manipulation was effective. Experiment 1 In the first experiment we compared the effect of LOP on priming in fragment completion and stem completion in amnesic patients and two groups of control subjects. The design of the experiment closely matched the design of an earlier study of LOP and word priming (Squire et al., 1987). That study was notable because LOP effects were absent in amnesic patients in a fragment-completion task (Experiment 3); whereas significant LOP effects were obtained for control subjects. In Experiment 1 we reexamined the effect of LOP on fragment-completion and stem-completion priming in amnesic patients and control subjects, using the same procedure for both tasks. To facilitate comparison with the results from the earlier Squire et al. (1987) study, we used the same materials as in that study, and with minor modifications the procedure followed Experiment 3 of that study. If the use of explicit retrieval contributes to LOP effects in word-completion priming tasks, significant LOP effects should occur for the control subjects in both stem-completion and fragment-completion tasks, but these effects should markedly be reduced or absent in amnesic patients. Amnesic Patients Method Twelve amnesic patients were tested (Tables 1 and 2). Five patients (4 men and 1 woman) with Korsakoffs syndrome had participated in quantitative radiographic studies (Shimamura, Jernigan, & Squire, 1988; Squire, Amaral, & Press, 1990; for N. F., unpublished observations), which demonstrated marked reductions in the volume of the mammillary nuclei, reduced thalamic tissue density, and frontal lobe atrophy. N. F. also had bilateral reduction in the size of the hippocampal formation. Of the 6 other patients (4 men and 2 women), M. G. had a bilateral thalamic infarction confirmed by magnetic resonance imaging. P. H., L. J., R. M., and H. W. had bilateral damage to the hippocampal formation identified by magnetic resonance imaging (for P. H., Polich & Squire, 1993; for L. J., R. M, and H. W., unpublished observations). Patient A. B., who was unable to participate in magnetic resonance studies, became amnesic in 1976 after an anoxic episode and was presumed to have hippocampal damage based on this etiology. Finally, patient N. A. became amnesic, primarily for verbal material, following a stab wound to the left diencephalic region with a miniature fencing foil (Squire, Amaral, Zola-Morgan, Kritchevsky, & Press, 1989; Teuber, Milner, & Vaughan, 1968). These 12 patients averaged 64.5 years of age at the time of testing and had 13.3 years of education. Individual IQ scores (Wechsler Adult Intelligence Scale Revised; WAIS-R; Wechsler, 1981) and scores on the Wechsler Memory Scale Revised (WMS-R; Wechsler, 1987) appear in Table 1. Immediate and delayed (12 min) recall of a short prose passage averaged 4.2 and 0 segments, respectively (21 total segments, Gilbert, Levee, & Catalano, 1968). Scores for other memory tests appear in Table 2. Finally, the mean score on the Dementia Rating Scale (Mattis, 1976) was (range = , maximum score = 144), with most of the points lost on the memory subportion of the test (mean points lost = 7.3); the mean score on the Boston Naming Test (Kaplan, Goodglass, & Weintraub, 1983) was 55.9 (range = 51-59, maximum score = 60). In summary, the amnesic patients all had a clinically significant memory disorder as their primary neuropsychological deficit and were otherwise unimpaired on neuropsychological tests of perceptual and cognitive function. Control Subjects Two groups of control subjects were tested: CON 1 (n = 12) and CON 2 (n = 12). The subjects were either employees or volunteers at the San Diego Veterans Affairs Medical Center or were members of the retirement community of the University of California, San Diego. Table 1 Characteristics of Amnesic Patients Patient A.B. P.H. L.J. R.M. H.W. N.A. R.C. N.F. V.F. M.G. P.N. J.W. Age (years) Lesion HF a HF HF HF HF Dien Dien Dien Dien Dien Dien Dien WAIS-R IQ Attention Verbal WMS-R Visual General Delay <50 57 <50 < < M Note. The WAIS-R and the WMS-R indices yield a mean score of 100 in the normal population with a standard deviation of 15. The WMS-R does not provide scores for subjects who score below 50. Therefore, the three scores below 50 were scored as 50 for calculating a group mean. WAIS-R = Wechsler Adult Intelligence Scale- Revised; WMS-R = Wechsler Memory Scale Revised. HF = hippocampal formation; dien = diencephalon. a The lesion site has not radiologically been confirmed but is strongly supported by the etiology of amnesia

4 936 HAMANN AND SQUIRE Table 2 Memory Test Performance of Amnesic Patients A.B. P.H. L.J. R.M. H.W. N.A. R.C. N.F. V.F. M.G. P.N. J.W. Patient M Control (n = 8) Diagram recall Paired associates O-O-l O Word recall (%) Word recognition (%) words faces Note. The diagram recall score is based on delayed (12 min) reproduction of the Rey-Osterrieth figure (Osterrieth, 1944; maximum score = 36). The average score for copying the figure was 28.8, a normal score (Kritchevsky, Squire, & Zouzounis, 1988). The paired-associate scores are the number of word pairs recalled on three successive trials (maximum score = 10/trial). The word recall score is the percentage of words identified correctly on five successive study-test trials (Rey, 1964). The word recognition score is the percentage of words identified correctly by yes-no recognition across five successive study-test trials. The scores for words and faces are based on 24-hr recognition tests of 50 words or 50 faces (modified from Warrington, 1984; maximum score = 50, chance = 25). The mean scores for healthy control and alcoholic, nonamnesic subjects shown for these tests are from Squire and Shimamura (1986). Note that patient N.A. is not severely impaired on the nonverbal memory tests because his brain injury is primarily left unilateral. Each group (CON 1 and CON 2) was selected to match the amnesic patients () with respect to age (CON 1: 64.5 years, range = 50-77; CON 2: 63.2 years, range = 51-76), education (CON 1: 13.9 years; CON 2:14.3 years), and WAIS-R subtest scores for Information (CON 1: 20.9; CON 2: 22.1; and : 20.3) and Vocabulary (CON 1: 56.7; CON 2: 55.5; and : 54.6). Immediate and delayed recall of the short prose passage was 6.5 and 4.9 segments, respectively, for CON 1 and 7.7 and 6.3 segments, respectively, for CON 2. Materials Two word lists of 54 target words each were used for the stemcompletion and fragment-completion tasks. These words were randomly selected from word lists used previously for stem completion (Squire et al., 1987, Experiment 1) and fragment completion (Squire et al., 1987, Experiment 3). The first 3 letters of each of the 54 words in the stem-completion list were common to at least 10 dictionary entries (e.g., MOT for MOTEL). Each of the 54 target words had a different stem. The mean frequency of occurrence per million for the stemcompletion list was 74.1 (Kucera & Francis, 1967). An additional 36 words served as distractors for a separate recognition memory test. Finally, 5 different words were used as fillers during presentation of the study list to reduce primacy and recency effects (3 words at the beginning of each list and 2 words at the end, with the same 5 filler items used for both study lists); and 5 words were used as practice stems at the beginning of the stem-completion test. These 46 additional words ( ) were matched to the target words for frequency of occurrence, and none of them had the same stem as any target word. For the 54 words used in the fragment-completion task, the mean frequency of occurrence per million was 5.8 (Kucera & Francis, 1967). Word fragments were constructed for each word by replacing letters with dashes such that each fragment could be completed to form only one English word. Distractor words for the recognition memory test, filler words for the study list, and practice items were also used, just as in the stem-completion task. Each of the two lists of 54 target words was divided into three lists of 18 words. For each task (stem completion and word-fragment completion), one of these three lists was presented under semantic (liking rating) orienting instructions, another was presented under nonsemantic (vowel comparison) instructions, and the third was not presented but was used in the test phase as a source of nonstudied test items to obtain a baseline completion score. For both the stem-completion and fragment-completion tasks, each 18-item list appeared equally often in the semantic, nonsemantic, and baseline conditions across subjects. All items were printed on white index cards and were presented manually. Procedure The experiment was conducted in two sessions. In the first session the fragment-completion task was presented twice: once in a semantic orienting condition and once in a nonsemantic orienting condition. In the second session, the stem-completion task was presented twice: once in a semantic orienting condition and once in a nonsemantic orienting condition. The same amnesic patients were tested in both sessions. The interval between the two testing sessions for the amnesic patients averaged 9.9 weeks across subjects [range = 4 18 weeks]. CON 1 subjects were tested in the first session, and CON 2 subjects were tested an average of 4.6 weeks later in the second session. Each of the two test sessions began by instructing subjects how to perform one of the two orienting tasks. The order in which the two orienting tasks were assigned to each session was balanced across the subjects in each group. For the semantic orienting task (liking rating), subjects rated individual words on a 5-point scale according to how much they liked each word. An index card showing the numbers from 1 (greatly dislike) to 5 (greatly like) was displayed in front of the subjects. For the

5 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 937 nonsemantic task (vowel comparison), subjects decided whether each successive pair of words in a list shared any of the same vowels. The vowels A, E, I, O, and (/were displayed on an index card in front of the subjects. In all experiments of the current study, the orienting tasks were presented to subjects as isolated tasks, without reference to the memory tests that would follow. Subjects were instructed to concentrate exclusively on performing the orienting tasks accurately. In both orienting tasks subjects responded aloud, and the experimenter recorded their responses. The experimenter presented one at a time the 23 words assigned to that orienting task (3 filler words followed by 18 target words followed by 2 filler words) while the subject performed the orienting task. Stimulus presentation was subject paced (about 3 s/item for both amnesic patients and control subjects). Immediately after the 23-word list had been presented, the words were presented again in the same manner in a new random order. The filler items remained at the beginning and end of the list as in the first presentation. The test phase followed immediately on the second presentation of the study list. Subjects were shown 27 test items (word fragments in the first session and word stems in the second session). Five fillers were placed at the beginning of the test list as practice items, followed in a mixed order by 18 target items that corresponded to study words and 9 items that did not correspond to any study words. These 9 items served to assess baseline completion performance. The 9 baseline test items were either thefirstor last 9 items of the 18-word list that was assigned to the nonstudied condition for that subject. The remaining 9 baseline items from the 18-word list were presented as baseline test items in the test that followed the second encoding task. Across subjects, the first and last 9 baseline test items occurred equally often in the first word-completion test of the session and in the second wordcompletion test of the session. For the test phase of the fragment-completion task, test items were presented at a rate of 5 s/item. Subjects were told to use the word fragments as cues to form the "first English word that comes to mind," excluding proper nouns. The previous study list was not mentioned. Subjects made their responses aloud, and these were recorded by the experimenter. After a 1-min interval, the second list of 23 words that was assigned to the other orienting task was presented for study in the same manner as the first list (i.e., 18 target words were presented twice). After the second presentation of this study list, the 27-item test list corresponding to these study words was presented in the same manner as in the first test phase of that same session. Immediately after the second word-completion task of the session, a 72-item yes-no recognition task was administered, consisting of 36 target items intermixed with 36 distractor items. The 36 target items consisted of the 18 words that had been studied under the semantic orienting condition and the 18 words that had been studied under the nonsemantic orienting condition. Subjects were instructed to mark Y next to items they remembered as having appeared in the study lists and N next to items they did not remember. The recognition memory test was self-paced. The word-stem test and a corresponding recognition memory test were administered in the second test session in the same manner. The procedure for the two sessions was identical, except that the amount of time given to respond to individual test items was greater for the fragment-completion task (5 s) than for the word-stem completion task (3 s) because subjects were slower to complete fragments than stems. In general, the experiment followed the procedure of Experiment 3 of Squire et al. (1987) with the following exceptions: (a) The LOP manipulation was entirely within subjects (to allow a test of the critical Subject Group x LOP interaction), whereas in Squire et al. (1987), LOP was a within-subject variable for the amnesic patients and a between-subjects variable for the control subjects; (b) recognition memory was assessed in a single test at the end of each testing session in a test that assessed recognition for words studied under both semantic and nonsemantic orienting conditions, whereas in Squire et al. (1987), recognition for the words studied under semantic and nonsemantic orienting conditions was tested in separate sessions (because the semantic and nonsemantic orienting tasks were administered in separate sessions). Word-Completion Priming Results The priming results for fragment completion and stem completion for the, CON 1, and CON 2 groups are shown in Figure 1. For each task (fragment completion and stem completion), preliminary analyses werefirstperformed to establish that baseline completion rates did not differ between the amnesic patients and the control subjects and to determine whether significant priming had occurred following the likingand vowel-encoding tasks for each group. After these analyses, the group was compared with the appropriate control group to determine whether the LOP manipulation affected the group and the control group in a similar way. (A combined analysis of variance [ANOVA] including both CON 1 and CON 2 with the group could not be conducted with the in this experiment because CON 1 and CON 2 consisted of separate groups of subjects.) Fragment completion. The proportion of nonstudied (baseline) fragments completed to form list words was similar in the CON 1 (.26 ±.04) and the groups (.30 ±.07), f(22) = 0.5\,p >.10. Significant priming (performance in the studied condition minus performance in the nonstudied baseline condition) occurred following both the liking and the vowelcomparison tasks in both groups. For the group, t(11) = 5.8 for the liking condition, and f(ll) = 4.58 for the vowel condition, ps <.01 for both comparisons. For CON 1, the corresponding comparison yielded t(11) = for the liking CON 1 Fragment Completion CON 2 Liking a Vowel Stem Completion Figure 1. Proportion priming (primed minus baseline performance) for word-fragment and word-stem completion in Experiment 1 following liking (semantic) or vowel (nonsemantic) encoding conditions for control subjects (CON 1, n - 12; CON 2, n = 12) and amnesic patients (, n = 12). Error bars show standard errors of the mean.

6 938 HAMANN AND SQUIRE condition, and t(11) = 7.37 for the vowel condition,/^ <.01 for both comparisons. The level of priming in both word-fragment and word-stem completion in this study was somewhat higher than the level typically found in most word-completion priming studies. This difference was most likely due to the fact that study words were presented twice (not once) at study and that the delay interval was very short. A mixed-factorial ANOVA (Group: vs. CON 1 x LOP: liking-rating task vs. vowel-comparison task), with priming score as the dependent variable, was conducted to compare the effect of LOP on fragment-completion performance in the and CON 1 groups. There was an effect of group, F(l, 22) = 6.97, MSE = 0.04,p <.02, an effect of LOP, F(l, 22) = 11.09, MSE = 0.02, p <.01, and an interaction between group and LOP, F(l, 22) = 6.71, MSE = 0.02, p <.02. This interaction reflected the fact that LOP effects were greater in the CON 1 group than in the group. Planned comparisons using a t statistic revealed no significant effect of LOP in the group, t{\\) = 0.86,p >.10; whereas the effect of LOP in the CON 1 group was significant, r(ll) = 3.28, p <.01. Priming was the same for the and the CON 1 groups following vowel encoding, t(22) = 0.77, p >.10; whereas priming was higher in the CON 1 group following liking-rating encoding, t(22) = 3.81,/? <.01. Stem completion. The proportion of nonstudied (baseline) stems completed to form list words was similar in the CON 2 (.09 ±.02) and groups (.08 ±.01),f(22) = 0.21,/? >.10. Significant priming occurred following both the liking and the vowel-comparison tasks in both groups, as indicated by t tests comparing the proportion of nonstudied items completed (baseline stems) and the proportion of studied items completed. For the group, ((11) = 5.58 for the liking condition, and f(ll) = 4.06 for the vowel condition, p <.01 (all t tests were two tailed). For CON 2, the corresponding comparison yielded t(\l) = 5.9 for the liking condition, and t{\\) = 7.41 for the vowel condition,ps <.01. A mixed-factorial ANOVA (Group: vs. CON 2 x LOP: liking-rating task vs. vowel-comparison task), with priming score as the dependent variable (i.e., the difference score described above), was conducted to compare the effect of LOP on stem-completion performance in the and CON 2 groups. There was an effect of LOP, F(l, 22) = 13.02, MSE = 0.02, p <.01, but no effect of group, F(l, 22) = 1.15, MSE = 0.05, p >.10, and a marginal interaction between group and LOP, F(l, 22) = 3.26, MSE = 0.02, p =.08. Planned comparisons using a t statistic indicated that the effect of LOP was not significant in the group, f (11) = 1.54, p >.10; whereas the effect of LOP was significant in the CON 2 group, <(11) = 3.34,/? <.01. Priming was the same for the and the CON 1 groups following vowel encoding, t(22) = 0.08,/? >.10; whereas priming was numerically higher in the CON 1 group following liking-rating encoding, t(22) = 1.56,/? =.13. Recognition Table 3 shows the corrected recognition scores (the proportion of hits minus the proportion of false alarms) for the, CON 1, and CON 2 groups. It should be noted that in this experiment and in the two that follow, recognition memory was Table 3 Corrected Recognition Scores for Experiment 1 Condition Fragment completion CON1 Liking Vowel FA Liking Vowel FA Stem completion CON 2 Liking Vowel FA Liking Vowel FA M SEM Note. Corrected recognition scores are the proportion of hits minus the proportion of false alarms. CON = control; = amnesic; FA = false alarm. always tested immediately following the end of the wordfragment or word-stem tests. Accordingly, recognition test scores can be expected to benefit from the production of words during the immediately preceding word-completion or wordfragment completion test. We report recognition performance here not as a definitive measure of recognition memory but as a way of comparing recognition memory performance across groups and as a means of verifying that the LOP manipulation did in fact influence explicit memory performance. Fragment-completion target words. A mixed-factorial ANOVA (Group: vs. CON 1 x LOP: liking-rating task vs. vowel-comparison task), with corrected recognition scores (the proportion of hits minus the proportion of false alarms) as the dependent variable, was first conducted to compare the effect of LOP on recognition of fragment-completion target words in the and CON 1 groups. There was an effect of group, F(l, 22) = 12.97, MSE = 0.07, p <.01, with performance worse than that of CON 1; an effect of LOP, F(l, 22) = 43.12, MSE = 0.01, p <.01; and a marginal interaction between group and LOP, F(l, 22) = 4.03, MSE = 0.01,p <.06. Although the Group x LOP interaction was not significant, LOP had a numerically greater effect on recognition memory in the CON 1 group than in the group. Planned comparisons using a t statistic showed that LOP affected recognition performance in both groups: f(ll) = 5.44, p <.01 for CON 1; f (11) = 3.70,/? <.01 for. In addition, the false-alarm rate was higher in the group (.25 ±.05) than in the CON 1 group (.09 ±.03), f(22) = 2.55,p <.05. Stem-completion target words. The same analysis that had been conducted for fragment-completion target words was next conducted for stem-completion target words. There was an effect of group, F(l, 22) = 6.22, MSE = 0.06,/? <.05, with performance worse than that of CON 2; an effect of LOP,F(1, 22) = 66.64, MSE =.01,p <.01; and an interaction between group and LOP, F(l, 22) = 11.13, MSE =.01,

7 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 939 p <.01. The form of this interaction indicated that LOP had a greater effect on recognition memory in the CON 2 group than in the group. Planned comparisons using a t statistic showed that LOP affected recognition performance in both groups: ((11) = 6.9, p <.01 for CON 2; f(ll) =4.36,p <.01 for. In addition, the false-alarm rate was marginally higher in the group (.38 ±.05) than in the CON 2 group (.24 ±.05), f(22) = 1.93, p <.07. Discussion Control subjects exhibited larger LOP effects than amnesic patients in both word-fragment completion priming and wordstem completion priming. The interaction between subject group and LOP was significant in the case of word-fragment completion, though not in the case of word-stem completion. Recognition performance in all groups was affected by LOP, confirming that the encoding task was effective in influencing explicit memory. The interaction between group and LOP in the fragmentcompletion task is consistent with the idea that the use of explicit retrieval by control subjects is the source of LOP effects in perceptual priming tasks. Note, however, that the stem-completion results reported in this experiment provide weaker evidence for this idea than the results for fragment completion because the interaction between group and LOP was not significant (p =.08). The fact that the Group x LOP interaction was significant for word-fragment completion but not for word-stem completion raises the question of whether these two word-completion priming tests might differ in some way in their susceptibility to LOP effects. However, the present experiment did not permit this question to be pursued because as Roediger, Weldon, Stadler, & Riegler (1992) noted, there is an inherent confounding in experiments like the current one. Specifically, the targets in stem completion included words that were on average shorter and of higher frequency than words used as targets in fragment completion. In Experiment 2 we followed the approach advocated by Roediger et al. and compared the effects of LOP on stem completion and fragment completion by using the same set of stimulus materials. Experiment 2 The question of interest was whether, as in Experiment 1, amnesic patients would exhibit lower LOP effects in the stem-completion and fragment-completion tasks than the control subjects. Such a result would provide additional support for the idea that the use of explicit retrieval by control subjects is an important source of LOP effects. Amnesic Patients Method All 12 amnesic patients from Experiment 1 participated in Experiment 2. Control Subjects The 12 control subjects in Experiment 2 (hereinafter referred to as CON 3) matched the amnesic patients with respect to age (64.1 years, range = 56-75), education (16.4 years), and WAIS-R subtest scores for Information (19.8; amnesic patients = 19.1) and Vocabulary (51.8; amnesic patients = 50.0). Immediate and delayed recall of the short prose passage was 7.6 and 6.7 segments, respectively. Materials To permit a direct comparison between tasks (i.e., stem completion and fragment completion), we compiled a list of 108 words that could serve as a source of targets and nonstudied baseline items for both the stem-completion and fragment-completion tests. Thus, the target words were the same for both the stem-completion and fragmentcompletion tests. However, the stem-completion cues (e.g., SOL for SOLDIER ) had several possible completions; whereas the fragmentcompletion cues (e.g., S _LD R for SOLDIER) had only one possible completion. The words and their corresponding word stems and word fragments were randomly selected from the words used in Experiments 2 and 3 of Roediger et al. (1992). Thefirst three letters of each word in the 108-item list were common to at least four dictionary entries (e.g., SOL for SOLDIER), but each word had a different stem. The mean frequency of occurrence per million for the list was 40.9 (Kucera & Francis, 1967). An additional 72 words were used as distractors for the recognition memory test. Another 10 words were used as fillers during the study phase to reduce primacy and recency effects (3 words at the beginning of each list and 2 words at the end). Five of thesefilleritems were used in the stem-completion task, and the other 5 words were used in the fragment-completion task. Finally, 10 other words were used as practice items: 5 for the stem-completion test and another 5 for the fragment-completion test. The list of 108 target words was divided into 6 lists of 18 items each. For each priming task (multiple-completion stem completion or single-completion fragment completion), one of the lists was presented under semantic (liking-rating) orienting instructions, another was presented under nonsemantic (vowel-comparison) instructions, and the third was not presented and was used as a source of nonstudied test items to obtain a baseline completion score. Each 18-item list was used equally often across subjects in the semantic, nonsemantic, and baseline conditions and in the stem-completion and fragmentcompletion tasks. All items were printed on white index cards and were presented manually. Procedure CON 3 was tested in two sessions 4-10 days apart by using the same procedure used in Experiment 1. The exception was that instead of testing recognition memory at the end of each session, recognition memory was assessed only at the end of the second session. The word-completion tests were always administered in the same order, with the fragment-completion test given on the first session and the stem-completion test given on the second session. Two recognition tests were administered at the end of the second session. Thefirstassessed recognition memory for the study words that had been presented earlier in the same session, and the second assessed recognition memory for the words studied in the first session 4-10 days earlier. As in Experiment 1, for each test 36 target items were randomly intermixed with 36 distractor items, and subjects marked Y next to items they remembered as having appeared in the study list of that day's session (or in the study list from the earlier

8 940 HAMANN AND SQUIRE session in the case of the second recognition test) or N next to the items they did not remember. Priming Results The priming results for stem completion and fragment completion for the CON 3 group and the amnesic patients are shown in Figure 2. For each task (stem completion and fragment completion), preliminary analyses established that baseline completion rates did not differ between the groups, either in the stem-completion task, CON 3 =.16 ±.02, =.19 ±.03, t(22) = 0.90, p >.10, or in the fragment-completion task, CON 3 =.25 ±.02, =.28 ±.04, t(22) = 0.79, p >.10. Planned comparisons using a t statistic showed that significant priming occurred in all conditions:/s (11) > 5.03 for the amnesic patients,/? <,05;ft(ll) > 5.30 for CON 3,p <.05. A 2 x 2 x 2 mixed-factorial ANOVA was conducted (Group: CON 3 vs. x Task: stem completion vs. fragment completion x LOP: liking-rating task vs. vowel-comparison task), with priming score as the dependent variable. The analysis found a main effect of LOP, F(l, 22) = 24.44, MSE = 0.02, p <.0001, and an interaction between LOP and group, F(l, 22) = 4.63, MSE = 0.02, p <.05. The form of this interaction indicated that for both stem completion and fragment completion, the LOP effect in the CON 3 group was larger than in the group. The main effect of group was also significant, F{\, 22) = 4.79, MSE = 0.05,p <.05. There were no other main effects or interactions. Planned comparisons using a t statistic indicated that for the CON 3 group, the effect of LOP was significant in both the stem-completion task and the fragment-completion task; t(\\) = 2.9,p <.01, andf(ll) = 5.07,/? <.001, respectively. For the amnesic patients there was a marginal effect of LOP, t{\\) = 2M,p <.07, andf(ll) = 1.80,/> <.10, respectively. For both the stem-completion task and the fragmentcompletion task the CON 3 group exhibited greater priming than the group following semantic encoding conditions, ((22) = 2.43, p <.02, and t(22) = 2.46, p <.02, for stem completion and fragment completion, respectively; whereas priming was the same in both groups following vowel-encoding conditions, t{22) = 1.14,/> >.10,andr(22) = 0.41,/? >.10, for stem completion and fragment completion, respectively. Recognition Figure 3 shows the corrected recognition scores (hits minus false alarms) for the CON 3 and groups, collapsed across type of task. A mixed-factorial ANOVA (Group: CON 3 vs. x LOP: liking-rating task vs. vowel-comparison task), with corrected recognition scores as the dependent variable, revealed a main effect of group, F{\, 22) = 33.52, MSE = 0.03, p <.0001; a main effect of LOP, F(l, 22) = 38.67, MSE = 0.01, p <.0001; and an interaction between group and LOP, F(l, 22) = 8.19, MSE = 0.01, p <.01. The form of this interaction showed that LOP affected recognition performance in the CON 3 group more than in the group. To determine whether significant LOP effects occurred in each group, we conducted separately planned t tests for the CON 3 group and the group, comparing corrected recognition scores in the liking-rating condition and corrected recognition scores in the vowel-comparison condition, collapsed across type of task. Significant effects of LOP were found in each group; for CON 3, f(ll) = 6.24,/? <.0001; for, f(ll) = 2M,p <.03. The false-alarm rate was higher in the group (.38 ±.04) than in the CON 3 group (.20 ±.03),f(22) = 3.48,/> <.01. Discussion The results of Experiment 2 were similar to those of Experiment 1. Control subjects exhibited significant LOP effects in both word-stem completion and word-fragment m Liking D Vowel 0.6 g 0.5 S * 04 «0.3 COM3 Fragment Completion CONS Swirl Corop wtion Figure 2. Proportion priming (primed minus baseline performance) for stem completion and fragment completion in Experiment 2 following liking or vowel-encoding conditions for control (CON 3) subjects (n = 12), who received the fragment-completion task in the first session and the stem-completion task in the second session, and for amnesic patients (, n = 12), who were tested in the same way as CON 3 subjects. Error bars show standard errors of the mean CON 3 Figure 3. Corrected recognition scores (hits minus false alarms) for words presented in liking or vowel-encoding conditions in Experiment 3 for control (CON 3) subjects (n = 12) and amnesic patients (, n = 12). Error bars show standard errors of the mean.

9 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 941 completion; whereas amnesic patients exhibited only marginal (albeit nonzero) LOP effects in both tasks. The significant interaction between subject group and LOP indicated a larger LOP effect in the control subject group than in the amnesic patient group. The absence of a main effect of task or an interaction involving the task factor (stem completion or fragment completion) indicated that the two word-completion tasks behaved in a similar manner, as had been found in an earlier study by Roediger et al. (1992). Recognition performance in both control subjects and amnesic patients was affected by LOP. The finding of larger LOP effects in wordcompletion priming for control subjects than for amnesic patients provides additional support for the idea that the use of explicit retrieval by control subjects is an important source of LOP effects. Experiment 3 In Experiments 1 and 2, the control subjects consistently exhibited significant LOP effects in each stem-completion and fragment-completion task (i.e., on four separate occasions); whereas the amnesic patients did not exhibit LOP effects at conventional levels of significance by two-tailed tests in any of the four word-completion tasks. In addition, there was an interaction involving group and LOP for both the fragmentcompletion task in Experiment 1 and for the combined analysis of the stem-completion and fragment-completion tasks in Experiment 2. Yet, in one case, the interaction between group and LOP was not significant. Specifically, in the comparison involving CON 2 and in the stem-completion task in Experiment 1, the Group x LOP interaction was only marginal (/> =.08). This lack of a significant interaction involving group and LOP for the stem-completion task in Experiment 1 may have resulted from the low power associated with the test for an interaction. The purpose of Experiment 3 was to combine data across the word-completion priming tasks that made up Experiments 1 and 2 to provide an overall picture of the patterns of word-completion and recognition in control subjects and amnesic patients and to permit a more powerful analysis of the critical Group x LOP interaction. In this analysis we compared the same subjects on all four tasks. Thus, amnesic patients and control subjects (CON 4) were compared on the two tests taken by CON 1 and CON 2 in Experiment 1 and the two tests taken by CON 3 in Experiment 3. different groups). The 8 subjects who had already been given one or more of the four tasks were tested only on the tasks they had not yet been given. Two additional subjects were recruited and given all four tasks. The CON 4 group matched the amnesic patients with respect to age (61.8 years, range = 51-77), education (14.2 years), and WAIS-R subtest scores for Information (21.9, amnesic patients = 19.1) and Vocabulary (57.0, amnesic patients = 50.0). Immediate and delayed recall of the short prose passage was 7.0 and 5.3 segments, respectively. Materials The same materials used in Experiments 1 and 2 were used in Experiment 3. Procedure Subjects were tested in a maximum of four separate sessions by using the procedures from Experiments 1 and 2. The order of testing was as follows: (a) Experiment 1: CON 1 procedure, word-fragment completion; (b) Experiment 1: CON 2 procedure, word-stem completion; (c) Experiment 2: the CON 3 procedure. For those subjects who had previously participated in Experiments 1 or 2, this same testing order was followed, but a subject was not tested a second time on a task that had already been given. In this way the CON 4 subjects received all four tasks. Word-Completion Priming Results The priming results for CON 4 and are shown in Figure 4, averaged across the two stem-completion tasks and the two fragment-completion tasks. Preliminary analyses determined that baseline word-completion performance was similar Amnesic Patients Method The 12 amnesic patients had already been tested on all four word-completion tasks, and their data from these tasks were compared with the data obtained with control subjects (CON 4). Control Subjects The control subjects (CON 4, n = 10) were drawn from the same population as those in Experiments 1 and 2. All but 2 had participated more than 1 month earlier in either Experiment 1 or Experiment 2 or in both experiments (1 in CON 1,4 in CON 2,1 in CON 3, and 2 in two CON 4 Figure 4. Proportion priming (primed minus baseline performance) following liking or vowel-encoding conditions in Experiment 3, on the basis of all four priming tasks used in Experiments 1 and 2. The data are for control subjects (CON 4, n = 10) and for amnesic patients (, n = 12) given all four priming tasks. Error bars show the standard errors of the mean.

10 942 HAMANN AND SQUIRE in the CON 4 and groups, fs(20) < 1.55, p >.10; baseline scores for CON 4 were.27 ±.04,.09 ±.04,.22 ±.03,.17 ±.04 and for were.30 ±.17,.08 ±.01,.28 ±.04,.18 ±.03 for the fragment-completion task from Experiment 1, the stem-completion task from Experiment 1, the fragmentcompletion task from Experiment 2, and the stem-completion task from Experiment 2, respectively. Preliminary analyses also established that significant priming occurred in the CON 4 group in each of the four word-completion tasks following both the liking-rating and the vowel-comparison tasks, ft(9) > 4.60, p <.001. To examine the effect of the LOP manipulation on the CON 4 and groups across all four priming tasks, we conducted a 2 x 2 x 2 x 2 mixed-factorial ANOVA (Group: vs. CON 4 x Experiment: Experiment 1 vs. Experiment 2 x Task: stem-completion task vs. fragment-completion task x LOP: liking-rating task vs. vowel-comparison task), with priming score as the dependent variable. There was a main effect of group, F(l, 20) = 8.17, MSE =.10, p <.01; a main effect of LOP, F(l, 20) = 33.24, MSE = 0.02, p <.0001; and an interaction between group and LOP, F(l, 20) = 8.01, MSE = 0.02, p <.01. No other main effects or interactions were obtained: all /"values < The Group x LOP interaction indicated that LOP effects were greater in the CON 4 group than in the group. Nevertheless, a significant effect of LOP was obtained within each group: F(l, 9) = 24.54, MSE = 0.02,p <.0001 for CON 4; F(l, 11) = 6.88, MSE = 0.01,p <.05 for. Finally, although the CON 4 and groups obtained markedly different priming scores under likingencoding conditions (CON 4 =.47, =.27), t(20) = 3.30, p <.01, (Figure 4), the two groups obtained roughly similar priming scores under vowel-encoding conditions, although the group exhibited a nonsignificantly lower level of priming (CON 4 =.30, =.22), <(20) = 1.85,p =.08. Planned comparisons using a t statistic indicated a significant effect of LOP for the CON 4 group in three of the four word-completion tasks that contributed data to Figure 4, fs(9) > 2.65, ps <.05, and a marginally significant effect of LOP for the stem-completion task from Experiment 1, t(9) = 2.16, p <.06. Priming in CON 4 following liking encoding was.52 ±.05,.43 ±.08,.45 ±.05,.47 ±.05, and following vowel encoding was.32 ±.07,.27 ±.06,.29 ±.04,.31 ±.05 for the fragment-completion task from Experiment 1, the stemcompletion task from Experiment 1, the fragment-completion task from Experiment 2, and the stem-completion task from Experiment 2, respectively. For the amnesic patients, there were no LOP effects in the two word-completion tasks from Experiment 1, fs(ll) < 1.54, ps >.10, and marginal LOP effects in the two word-completion tasks from Experiment 2: t(l\) = 2.04 for the stem-completion task, and f(ll) = 1.85 for the fragment-completion task,ps <.10. A series of analyses was next conducted to determine whether the priming results for the CON 4 group replicated the results obtained previously with the CON 1, CON 2, and CON 3 groups in Experiments 1 and 2. Because some of the CON 4 subjects had also been previously tested as subjects in the other control groups (i.e., CON 1, CON 2, or CON 3), comparisons were conducted between the CON 4 group (n = 10) and those subjects in each of the other control groups who were not included in the CON 4 group (10 of 12 subjects remained in CON 1, 7 of 12 remained in CON 2, and 9 of 12 remained in the CON 3 group). Baseline word-completion performance was similar for CON 4 and the other control groups in all four wordcompletion tasks (ts < 1.2,allps >.10). Next, separate mixedfactorial ANOVAs were performed on the priming scores obtained by CON 4 and the other control groups. There were no main effects of group and no interactions involving group and the other variables: LOP or task (stem completion or fragment completion). Thus, the CON 4 group performed similarly to the CON 1, CON 2, and CON 3 groups in the four tasks making up Experiments 1 and 3. Recognition The corrected recognition scores for CON 4 and are shown in Figure 5, averaged across the two stem-completion tasks and the two fragment-completion tasks that were included in Experiments 1 and 2. Because of a testing error, the recognition data for 2 of the 10 control subjects were not available for one task; therefore, the following analysis was conducted with the data from 8 control subjects and 12 amnesic patients. A mixed-factorial ANOVA (Group: CON 4 vs. x Task: four recognition tests from Experiments 1 and 2 x LOP: liking rating task vs. vowel comparison task) was conducted to compare the effect of LOP and task on recognition performance in the CON 4 and groups. This analysis found a main effect of group, F(l, 18) = 20.52, MSE = 0.14, p <.001; a main effect of LOP, F(l, 18) = 49.24, MSE = 0.03,p <.0001; and an interaction between group and LOP, F(l, 18) = 6.99, MSE = 0.03, p <.02. The form of this interaction indicated that the effect of LOP was greater in the CON 4 Liking Vowel Figure 5. Corrected recognition scores (hits minus false alarms) for words presented in liking or vowel-encoding conditions for control (CON 4) subjects (n = 8) and amnesic patients (, n = 12). Error bars show standard errors of the mean.

11 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 943 CON 4 group (.67 ±.07 vs..39 ±.06 for the liking-rating and vowel-comparison conditions, respectively) than in the group (.32 ±.04 vs..20 ±.03 for the liking-rating and vowelcomparison conditions, respectively). Although the LOP effect was smaller in the group than in the CON 4 group, it was statistically significant, f(ll) = 4.34,p <.001. In contrast to the results for priming (Figure 5), in which the and CON 4 groups performed similarly in the vowelencoding condition and differently in the liking condition, recognition memory performance was impaired in the group in both encoding conditions; for the liking-rating condition, f(18) = 5.80, p <.0001, for the vowel comparison condition,/(18) = 2.73,p <.05. Planned comparisons using a t statistic indicated that significant LOP effects occurred in and CON 4 for each of the four corresponding recognition tests that were given together with the four priming tasks used in Experiments 1 and 2, fs ranging from 3.24 to 5.81, p <.01 in each case, with corrected recognition scores as the dependent variable. A comparison of the four false-alarm rates between the CON 4 and groups that collapsed the false-alarm data across the four recognition tasks found a higher false-alarm rate for the group (.37 ±.05) than for the CON 4 group (.19 ±.04), r(18) = 2.74,/? <.05. Discussion In Experiment 3 we found an interaction between group and LOP when results of the four priming tests from Experiments 1 and 2 were compared within the same subject groups. Control subjects (CON 4) exhibited significantly larger LOP effects than amnesic patients across the four priming tasks. For the liking-rating condition, control subjects exhibited higher priming scores than amnesic patients. For the vowel-comparison condition, control subjects and amnesic patients exhibited similar priming scores. Recognition memory performance of both control subjects and amnesic patients was affected by LOP, confirming that the LOP manipulation was effective. Moreover, as expected, amnesic patients exhibited impaired recognition scores in both the liking-rating condition and in the vowel-comparison condition. Finally, the CON 4 results were similar to the corresponding results obtained by the CON 1, CON 2 and CON 3 groups, indicating that thefindingswere replicable and robust. General Discussion In three experiments, we explored the source of LOP effects in word-stem completion and word-fragment completion priming tasks. In Experiments 1 and 2, control subjects consistently exhibited LOP effects in four different word-completion tasks; whereas amnesic patients failed to show significant LOP effects in any task. Finally, when all subjects were given the same four tests (Experiment 3), a Group x LOP interaction confirmed that there were overall larger LOP effects for control subjects than for amnesic patients. The current results support previous suggestions that stem completion and fragment completion are fundamentally similar when the same materials are used for each task (Roediger et al., 1992). In Experiment 2, when materials were equated between the stem-completion and fragment-completion tasks, similar patterns of priming were observed in both tasks in both the control group and the amnesic patient group. These results support the idea that the performance of control subjects in word-completion priming tasks is often benefited by explicit retrieval. LOP effects were reduced in amnesic patients because they could not effectively use explicit retrieval to benefit their performance. Note that although LOP effects were markedly reduced in the amnesic patients, they were not entirely eliminated. Specifically, the results for the amnesic patients in Experiment 3 (which combined results for four different word-completion tests) revealed a small but significant LOP effect (Figure 4). This residual LOP effect could reflect residual explicit memory in the amnesic patient group. Indeed, the above-chance recognition performance of the amnesic patients demonstrated that they did have some residual explicit memory for target words. The possibility should also be considered that this residual LOP effect for the amnesic patients reflects one or both of the other two alternatives previously mentioned, that is, an effect of LOP on conceptual implicit memory or incomplete perceptual processing at encoding. In the following section we consider the evidence for and against all these possibilities. The Source of the Residual LOP Effect in Amnesic Patients If the residual LOP effects exhibited by the amnesic patients in this study (.05, averaged across Experiments 1 and 2) in fact reflect explicit retrieval, then these residual LOP effects should exhibit correlations with variables known to be related to explicit memory. One variable known to affect explicit memory is word frequency. Free recall is positively related to word frequency; whereas recognition is in general negatively related to word frequency (Gregg, 1976; Postman, 1970). Table 4 shows the residual LOP effect for amnesic patients in the nine word-completion experiments taken from the four Table 4 Mean Word Frequency and Mean LOP Effects for Nine Word-Completion Experiments Involving Amnesic Patients Study Exp. 1: Fragment Squire et al., 1987: Exp. 3 Squire et al., 1987: Exp. 2 Exp. 2: Fragment Exp. 2: Stem Grafetal., 1984: Exp. 2 Carlesimo, 1994: Exp. 1 Grafetal., 1984: Exp. 1 Exp. 1: Stem WF LOP Task Fragment Fragment Stem Fragment Stem Stem Stem Stem Stem Note. Experiments not accompanied by citations refer to experiments in the present study. For experiments that included more than one delay condition the data are from the immediate delay condition only. LOP = level-of-processing effect (semantic priming effect minus nonsemantic priming effect); WF = word frequency (number of occurrences per million; Kucera & Francis, 1967); Exp. = experiment; Task: Fragment = fragment-completion priming; Stem = stemcompletion priming. n

12 944 HAMANN AND SQUIRE studies that have examined word-completion LOP effects in amnesia (including the four word-completion tasks in the present study). Table 4 also records the mean word frequency of the target words in each of the nine word-completion experiments. The experimental procedure used in these tasks was the same (with the exception of the Carlesimo [1994] study, which used a similar procedure but presented the words only a single time at study). To avoid the possibly confounding effect of differing retention intervals, we included only the immediate priming test data for those experiments that had both immediate and delayed tests. Table 4 reveals a strong positive relationship between word frequency and the residual LOP effect in studies of amnesic patients, r(7) =.79, p <.01. This relationship is consistent with the idea that the residual word-completion LOP effect in amnesia is attributable to a residual ability of the patients to recall words from the study list. 3 In the current study, the.05 residual LOP effect observed for the amnesic patients (averaged across experiments) would have resulted if the amnesic patients recalled an average of about 1 more item from the semantic (liking rating) encoding list than from the nonsemantic (vowel comparison) encoding list (1 item/18 items in each study list =.056). The correlation between word frequency and the residual LOP effect provides evidence for a role of explicit retrieval in the residual LOP effect for amnesic patients. 4 Neither of the alternative hypotheses available to account for residual LOP effects (conceptual implicit effects or degraded perceptual processing) would predict a positive relationship between word frequency and the LOP effect. For example, to account for a positive correlation between word frequency and the LOP effect, the hypothesis that perceptual degradation occurs at encoding would have to posit that perceptual degradation affects higher frequency words more than lower frequency words, leading to larger LOP effects for higher frequency words. Yet, there is no empirical evidence to indicate that higher frequency words are more affected than lower frequency words by manipulations that degrade perception. Similarly, there is no empirical evidence indicating that LOP effects in conceptual implicit memory differ as a function of word frequency. It can be speculated that high-frequency words might invite more conceptual processing under a likingrating task than a vowel-encoding task and that this difference in conceptual processing might be reflected in larger LOP effects for high-frequency words. However, this possibility is difficult to evaluate because the mechanism by which conceptual implicit effects might operate in the context of a wordcompletion task has not been specified in any detail. Further progress in evaluating the status of the perceptual degradation hypothesis and the conceptual implicit hypothesis will depend on greater elaboration and clarification of these hypotheses. The correlation between word frequency and the residual LOP effect also suggests an explanation for the apparently inconsistent findings of previous studies that have examined word-completion LOP effects in amnesic patients (Carlesimo, 1994; Graf et al., 1984; Squire et al., 1987). Specifically, across all studies that have examined such effects, the residual LOP effect is larger in those studies in which target words of higher average word frequency were used (see Table 4). As mentioned earlier, the correlation between word frequency and residual LOP effects suggests that the pattern of wordcompletion priming performance of amnesic patients in previous studies can be attributed to the patients benefiting more from explicit retrieval in experiments in which higher frequency target words were used than in experiments in which lower frequency target words were used. A second line of evidence that points to explicit retrieval as the source of the residual LOP effects in amnesic patients comes from examination of the results of the fragment completion task in Experiment 1 of the present study. Amnesic patients exhibited very low LOP effects in this task (.03), which may have been related to the fact that the target words for this task were the lowest in frequency (5.8) of any of the four word-completion tasks in the present study. The negligible LOP effect in Experiment 1 was consistent with the idea that the source of residual LOP effects in amnesic patients is explicit retrieval because by that view it should be possible to observe cases in which the LOP effect is entirely absent. In contrast, if residual LOP effects are due even partly to implicit conceptual effects or perceptual effects, it should not be possible to eliminate them entirely in amnesic patients (when control subjects exhibit LOP effects) because implicit memory and perceptual and conceptual processing capacity are intact in amnesic patients. Consistent with the explicit retrieval hypothesis, the.03 LOP effect for the amnesic patients in the fragment-completion task in Experiment 1 was not significant and was markedly reduced from the LOP effect observed for CON 1 (.22). One possible concern is whether the experimental power was sufficient in Experiment 1 to permit an adequate test of the idea that a.03 3 We also calculated the corresponding correlation between word frequency of target items and LOP effects for the six studies in Table 4 that involved normal controls. The nonsignificant positive correlation wasr(4) =.31,p > We also examined the correlation between the magnitude of word-completion LOP effects (in the four word-completion experiments in Experiments 1 and 2) and two additional measures of explicit memory (first measure: proportion corrected recognition [hits minus false alarms] across the three recognition tests in which amnesic patients and control subjects exhibited above-floor recognition performance, that is, both recognition tests in Experiment 1 and the recognition test for stem-completion target words in Experiment 2; second measure: the LOP effect observed in these same three recognition tests). The correlation between the LOP effect in recognition and the LOP effect in word-completion for control subjects was marginal, r(6) =.67, p =.07; the remaining correlations were all nonsignificant (allrvalues <.35,p >.10). These correlational data are not inconsistent with a relationship between explicit memory and LOP effects on word-completion tasks, though they provide only weak support for such a relationship. However, it should be noted that for both subject groups, each recognition test was preceded by a word-completion priming test that would likely have introduced noise into the correlations between the magnitude of LOP effects in the word-completion tasks and recognition performance. In addition, in the case of amnesic patients, the range of LOP effects observed in word-completion priming was restricted by floor effects, which would have worked against finding correlations for this group.

13 LEVEL-OF-PROCESSING EFFECTS IN PRIMING 945 LOP effect is in fact negligible. To permit a more powerful analysis, we combined the data from 4 amnesic patients who had participated in the Squire et al. (1987) Experiment 3 (the experiment that was replicated in Experiment 1 of our study) with the data from the 12 amnesic patients of our Experiment 1 to form a group of 16 amnesic patients. 5 In this larger group (+), the LOP effect remained nonsignificant at.03, t(15) = 0.82, p >.21 (one-tailed test). A power analysis indicated that 61 patients would be required to detect this.03 difference with a one-tailed test at the.50 power level, and 138 patients would be required at the.80 power level (Cohen, 1977). Furthermore, the within-subject design for the test of the LOP effect was particularly powerful because of the high correlation between levels of priming in the semantic and nonsemantic orienting conditions for each patient, r(14) =.84; the power of the test for the LOP effect increased as the correlation between paired scores increased. In summary, even a relatively powerful within-subject test could not detect a significant LOP effect in the + group (n = 16); whereas the CON 1 group (n = 12) tested under the same conditions exhibited a large (.22) LOP effect. This finding of negligible LOP effects in the amnesic patients accords well with the results of the earlier Squire et al. (1987) Experiment 3, in which the LOP effect for the amnesic patients (n = 8) was also negligible and was in fact slightly negative (.03). Together, these findings support the conclusion that the source of the residual LOP effect for amnesic patients is explicit retrieval. Relation to Studies of Normal Subjects The findings of the present study are consistent with the results of other studies of the effect of LOP in wordcompletion tasks that have been conducted with normal subjects. For example, Roediger et al. (1992) found that under conditions that minimized the possibility that subjects would use explicit memory to benefit their performance (one presentation at study, delay intervals of several minutes, and many distractor items at test), there was no LOP effect for either stem completion or fragment completion. Another study of stem-completion priming with normal subjects (Toth, Reingold, & Jacoby, 1994) used the processdissociation procedure (Jacoby, Toth, & Yonelinas, 1993) to estimate the contribution of consciously controlled processes (R, equated with explicit retrieval in their study) and the contribution of automatic processes (A) in the context of a manipulation of LOP at study. There was a significant LOP effect in the stem-completion task, but when the data were analyzed with the process-dissociation procedure, the contribution of automatic influence was estimated to be equal in the semantic and nonsemantic orienting conditions. Accordingly, the LOP effect was considered not to contribute to automatic influences (equated with perceptual priming in the Toth et al. study). The conclusions of the Toth et al. (1994) study are similar to those of the current study; namely, that the LOP effect reflects explicit retrieval when it occurs on an implicit test. Although the Toth et al. study provides additional support for the findings of the current study, it should be noted that an assumption critical to the validity of the process-dissociation procedure used in Toth et al., that the R and A estimates are stochastically independent, has recently been challenged in several experiments (Curran & Hintzman, 1995). These results should therefore be regarded with caution until the status of the independence assumption is better understood in the conditions of the Toth et al. study. Multiple Determinants of LOP Effects in Word-Completion Tasks The results of the present study suggest that explicit retrieval is one important source of LOP effects in wordcompletion tasks. Other variables have also been suggested to influence LOP effects in word-completion tasks: test awareness (Bowers & Schacter, 1990), between-subjects vs. withinsubject experimental designs and the manipulation of LOP in which either a blocked versus a random manner of presentation of deep- and shallow-encoding conditions was used (Challis & Brodbeck, 1992), and presence or absence of a prior free-recall test (Graf et al., 1984). It is possible that these variables are all effective because they influence explicit retrieval. Perhaps these considerations help account for why LOP effects in word-completion tasks have been so variable (see review by Challis & Brodbeck, 1992). It is worth emphasizing that, in our study, LOP effects did lead to a difference in priming between amnesic patients and control subjects, but this difference appeared only in the liking-rating condition, not in the vowel-encoding condition. In the vowel-encoding condition, amnesic patients exhibited intact priming. These results presumably reflect the fact that nonsemantic encoding conditions rendered explicit retrieval much less beneficial to control subjects. Accordingly, nonsemantic encoding conditions should usually be preferable to semantic encoding conditions for studying the nature of priming. Nevertheless, in other priming tasks such as perceptual identification, which is more purely perceptual and presumably less subject to the influence of explicit retrieval, priming can be intact in amnesia, even when semantic encoding tasks are used (Haist et al., 1991; Hamann et al., 1995). Conclusion Consistent with the conclusion of recent reviews (Brown & Mitchell, 1994; Challis & Brodbeck, 1992), performance on word-stem completion and word-fragment completion tests was unmistakably affected by LOP manipulations. However, LOP effects were much reduced in amnesic patients, consistent with the idea that explicit retrieval contributes substantially to LOP effects. We suggest that the effect of explicit retrieval is substantial in control subjects and much reduced but still detectable in amnesic patients, in proportion to their residual capacity for explicit (declarative) memory. Although it remains possible that other factors contribute to LOP effects in 5 The analysis involved all 12 of the amnesic patients from the present study together with the 4 patients from the Squire et al. (1987) study who did not also participate in the present study.

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