Level-of-Processing Effects in Word-Completion Priming

In the public domain

Journal of Experimental Psychology: Learning, Memory, and Cognition 1996, Vol. 22, No. 4,933-947

Level-of-Processing Effects in Word-Completion Priming: A Neuropsychological Study Stephan B. Hatnann

Larry R. Squire

University of California, San Diego

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 MH24600. 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 92161-2002. Electronic mail may be sent via Internet to [email protected].

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.

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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).

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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 131.3 (range = 114-143, 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.

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.

Control Subjects

Method

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.

Amnesic Patients Twelve amnesic patients were tested (Tables 1 and 2). Five patients (4 men and 1 woman) with Korsakoffs syndrome had participated in Table 1 Characteristics of Amnesic Patients Patient

(years)

A.B. P.H. L.J. R.M. H.W. N.A. R.C. N.F. V.F. M.G. P.N. J.W.

55 70 55 77 77 54 76 57 72 60 65 56

M

64.5

WMS-R

WAIS-R

Age

Lesion HF a HF HF HF HF

Dien Dien Dien Dien Dien Dien Dien

IQ

Attention

Verbal

104 115 98 102 109 109 106 94 103 111 99 98

87 117 105 96 97 102 115 91 93 113 81 104

62 67 83 59 84 67 76 62 77 89 77 65

104.0

100.1

72.3

Visual 72 83 60 69 102 89 97 73 65 84 73 70

78.1

General 54 70 69 56 89 68 80 53 67 86 67 57

68.0

Delay .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 AMN 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: AMN 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 AMN 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 AMN group, f (11) = 1.54, p > .10; whereas the effect of LOP was significant in the CON 2 group, .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 AMN, 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 AMN

Liking Vowel FA

Stem completion CON 2 Liking Vowel FA AMN

Liking Vowel FA

M

SEM

.91 .61 .09

.03 .06 .03

.56 .40 .25

.08 .06 .05

.65 .28 .24

.07 .05 .05

.37 .21 .38

.05 .05 .05

Note. Corrected recognition scores are the proportion of hits minus the proportion of false alarms. CON = control; AMN = 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: AMN 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 AMN and CON 1 groups. There was an effect of group, F(l, 22) = 12.97, MSE = 0.07, p < .01, with AMN 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 AMN 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 AMN. In addition, the false-alarm rate was higher in the AMN 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 AMN 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,

LEVEL-OF-PROCESSING EFFECTS IN PRIMING 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 AMN 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 AMN. In addition, the false-alarm rate was marginally higher in the AMN group (.38 ± .05) than in the CON 2 group (.24 ± .05), f(22) = 1.93, p < .07.

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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 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.

Method Amnesic Patients All 12 amnesic patients from Experiment 1 participated in Experiment 2.

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). Thefirstthree 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

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HAMANN AND SQUIRE

session in the case of the second recognition test) or N next to the items they did not remember.

Results Priming 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, AMN = .19 ± .03, t(22) = 0.90, p > .10, or in the fragment-completion task, CON 3 = .25 ± .02, AMN = .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. AMN 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 AMN 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 AMN 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 AMN groups, collapsed across type of task. A mixed-factorial ANOVA (Group: CON 3 vs. AMN 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 AMN group. To determine whether significant LOP effects occurred in each group, we conducted separately planned t tests for the CON 3 group and the AMN 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 AMN, f(ll) = 2M,p < .03. The false-alarm rate was higher in the AMN 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 0.8

m Liking D Vowel

0.7 0.6

g 0.5 S

§* 04 « 0.3 0.2 COM3 Fragment Completion

CONS AMN 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 (AMN, n = 12), who were tested in the same way as CON 3 subjects. Error bars show standard errors of the mean.

0.1 0

CON 3

AMN

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 (AMN, n = 12). Error bars show standard errors of the mean.

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LEVEL-OF-PROCESSING EFFECTS IN PRIMING

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 AMN 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.

Results Word-Completion Priming The priming results for CON 4 and AMN 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

Method Amnesic Patients 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

AMN

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 (AMN, n = 12) given all four priming tasks. Error bars show the standard errors of the mean.

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in the CON 4 and AMN groups, fs(20) < 1.55, p > .10; baseline scores for CON 4 were .27 ± .04, .09 ± .04, .22 ± .03, .17 ± .04 and for AMN 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 AMN groups across all four priming tasks, we conducted a 2 x 2 x 2 x 2 mixed-factorial ANOVA (Group: AMN 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 < 1.20. The Group x LOP interaction indicated that LOP effects were greater in the CON 4 group than in the AMN 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 AMN. Finally, although the CON 4 and AMN groups obtained markedly different priming scores under likingencoding conditions (CON 4 = .47, AMN = .27), t(20) = 3.30, p < .01, (Figure 4), the two groups obtained roughly similar priming scores under vowel-encoding conditions, although the AMN group exhibited a nonsignificantly lower level of priming (CON 4 = .30, AMN = .22), 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 AMN 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. AMN 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 AMN 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

Liking

• Vowel

CON 4

AMN

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 (AMN, n = 12). Error bars show standard errors of the mean.

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CON 4 group (.67 ± .07 vs. .39 ± .06 for the liking-rating and vowel-comparison conditions, respectively) than in the AMN group (.32 ± .04 vs. .20 ± .03 for the liking-rating and vowelcomparison conditions, respectively). Although the LOP effect was smaller in the AMN 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 AMN and CON 4 groups performed similarly in the vowelencoding condition and differently in the liking condition, recognition memory performance was impaired in the AMN 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 AMN 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 AMN groups that collapsed the false-alarm data across the four recognition tasks found a higher false-alarm rate for the AMN group (.37 ± .05) than for the CON 4 group (.19 ± .04), r(18) = 2.74,/? < .05.

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.

Discussion

The Source of the Residual LOP Effect in Amnesic Patients

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

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

WF

LOP

Task

n

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

5.8 9.2 25.0 40.9 40.9 58.0 67.8 72.0 74.1

.03 -.03 .08 .08 .06 .09 .12 .15 .07

Fragment

12 8 8 12 12 9 12 9 12

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.

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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 > .10. 4 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.

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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 (AMN+), 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 AMN+ 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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word-completion priming tasks in addition to the use of explicit retrieval, there is currently insufficient evidence to identify any particular factor as having a clear role. Currently, the best explanation of LOP effects in word-completion priming tests would appear to be that LOP effects simply record an additive influence of explicit retrieval on tasks that are otherwise largely insensitive to explicit memory.

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Received July 24,1995 Revision received September 18,1995 Accepted September 26,1995 •

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