levels of processing and single word priming in amnesic and control subjects

LEVELS OF PROCESSING AND SINGLE WORD PRIMINGIN AMNESIC AND CONTROL SUBJECTS

Valerie Jenkins1, Riccardo Russo2 and Alan J. Parkin1

(1Laboratory of Experimental Psychology, University of Sussex, Brighton; 2Department of Psychology, University of Essex, Colchester)

ABSTRACT

This paper describes an experiment which examined how levels of processing (LOP)affected word fragment completion in a group of Wernicke-Korsakoff patients, a group ofpatients with closed head injury, and matched controls. The data showed that both thememory-impaired groups and the controls showed a LOP effect but that the effect waslarger in controls. Data from other studies are reviewed and, in conjunction with the presentfindings, it is concluded that LOP effects obtained when memory-impaired individuals aretested using implicit memory tasks arise mainly from the contribution of lexical processingof targets and from contamination by explicit recollection.

Key words: amnesia, word priming

INTRODUCTION

Recently, the Levels of Processing (LOP) framework has been applied to thearea of implicit memory research. A number of studies have shown that levels ofprocessing manipulations at study produce a dissociation between explicit andimplicit memory (e.g. Challis and Brodbeck, 1992; Craik, Moscovitch andMcDowd, 1994). A typical LOP manipulation involves rating the meaningfulness ofa word (semantic processing), compared to counting the number of vowels containedin a word (physical processing). There are numerous demonstrations that encodingmanipulations, while producing a strong effect on performance of direct/explicittests, do not affect priming on indirect/implicit tests in normal and elderly subjects(e.g. Jacoby and Dallas, 1981; Schacter, 1985; Java and Gardiner, 1991).

Reliable priming effects have also been found on indirect tasks involvingsemantic knowledge, for example generating category exemplars followingexposure to a list of items (e.g. Gardener et al., 1973). In addition, a number ofstudy manipulations appear to have a dissociative effect on perceptual versusconceptual forms of indirect tasks. Srinivas and Roediger (1990) demonstratedthat category association (an indirect conceptual test) responded like free recallby showing a levels of processing effect whereas fragment completion did not.

According to the processing view (e.g. Roediger, Weldon and Challis, 1989),the majority of direct memory tests and conceptually driven indirect memorytests rely on conceptually driven processing for their completion, whereas datadriven indirect memory tests (e.g. fragment completion) depend on perceptualprocesses. For this reason, an indirect perceptual task such as fragmentcompletion should not show a LOP effect. The explanation given is that

Cortex, (1998) 34, 577-588

semantic orienting tasks should not promote more perceptual processing thanorienting tasks focusing on the structural characteristics of the targets, thereforea similar amount of perceptual processing is assumed to occur for both semanticand physical encoding tasks. On the other hand, given that conceptually driventasks rely on elaborative processing, LOP should affect performance on bothdirect and conceptual priming tests. However, while it is generally suggestedthat indirect memory tests are more sensitive to data driven processing anddirect memory tests to conceptually driven processing, Roediger and Blaxton(1987), point out that no necessary correlation between processing mode andtype of test exists. Rather, as Srinivas and Roediger (1990) state, it is moreuseful to acknowledge that tests may involve both types of processes.

Challis and Brodbeck (1992) investigated the claim that LOP does not affectpriming in perceptual forms of indirect tests. They recorded the effect ofdifferent levels of processing on a word fragment completion test in normalsubjects. In the experiments, subjects studied words under semantic and physicallearning conditions and, following a five minute interval, were instructed tocomplete each fragment so that it made an English word. The results revealed areliable priming effect and, unlike other studies (e.g. Graf et al., 1982; Schacter,1985), a LOP effect occurred on this indirect memory task. These findings ledChallis and Brodbeck to review the literature on LOP effects in tests of wordfragment completion, word stem completion and perceptual identification.

In their review, Challis and Brodbeck (see also Brown and Mitchell, 1994;Thapar and Greene, 1994) reported that a number of studies demonstratedsignificant LOP effects in perceptual indirect tests of memory (e.g. Squire,Shimamura and Graf, 1987). In other cases the amount of priming, although notsignificantly different, was greater in the semantic than in the physical condition.Challis and Brodbeck proposed three explanations for the findings. The first wasthat performance on the indirect perceptual task was contaminated by explicitretrieval. According to this explanation, when completing the indirect tasknormal subjects may realise that some of the test items had appeared at study.The subjects then continue to complete the task as one of cued recall. Thiscomplements a study by Bowers and Schacter (1990) with test aware and testunaware subjects. Bowers and Schacter obtained a LOP effect with test awaresubjects and suggested that these subjects had adopted an explicit retrievalstrategy. One way to try and resolve the issue of whether indirect tasks arecontaminated by explicit retrieval strategies is to work with amnesic patients.The absence of a LOP effect in this subject group would strengthen the viewthat normal subjects use explicit memory strategies to complete indirect tasks.

The second explanation proposed by Challis and Brodbeck (1992) is thatLOP affects perceptual processes during encoding. This assumption is derivedfrom Weldon (1991), who suggested that lexical processing facilitates repetitionpriming in indirect perceptual tests. Therefore, in the shallow processingconditions, for example counting vowels in a word, less priming should occurbecause the task discourages lexical processing. The third explanation follows onfrom the second and rests on the assumption that indirect perceptual tests reflectconceptual as well as perceptual processes. It is known that some tasks aresensitive to semantic or conceptual encoding procedures, for example, generating

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a target item from a conceptual cue produces priming on a perceptualidentification and a word fragment completion task (Weldon, 1991). If primingtasks reflect different aspects of encoding, then one would expect to find a LOPeffect on those that are more sensitive to the conceptual nature of the task.

The following experiment was developed to examine whether single wordpriming in word fragment completion is affected by different levels of processing.A prediction that follows from the previous account is that if LOP effects inperceptual implicit tasks are a by-product of explicit memory contamination, thenone may expect to observe a LOP effect in control subjects but not in amnesicpatients. In the following experiment the performance of two groups of memoryimpaired subjects was compared with two matched control groups. The controlgroups differed significantly by age but not IQ, which was similar to the amnesicgroups. The inclusion of two control groups allowed for the comparison of testperformance and specifically examined whether normal controls would show asemantic enhancement effect on this indirect memory task.

MATERIALS AND METHODS

Subjects

The subjects were nine Wernicke Korsakoff (WKS) and nine Closed Head Injured (CHI)adult patients. All the Korsakoff patients had a history of chronic alcoholism, were unable torecall day-to-day events, and had extensive retrograde amnesia. The closed head injuredsubjects had all sustained their injuries through road traffic accidents, which left them withdiffering degrees of memory loss.

Table I gives the mean performance of each group in terms of age, IQ and memoryimpairment. The groups did not differ significantly from one another on scores of both current and pre-morbid intelligence (NART; Nelson and O’Connell, 1978). However,the closed head injured subjects were significantly different from the Korsakoff group onthree counts, age [t (16) = 3.83, p < 0.001], degree of memory loss – General MemoryIndex of WMS - [t (16) = 2.57, p < 0.02], and recognition ability words [t (14) = 2.62, p < 0.02]. Therefore the WKS were both older and had a more profound amnesia than theCHI group.

Two control groups were also used, matched to each amnesic group by age and IQ (using the NART). The control subjects were recruited from the University of Sussex subject pool and were paid a small amount of money for their time. The Korsakoffcontrol group did not differ from the Korsakoff subjects by age [t (16) = 1.35, p > 0.10]or NART score [t (16) = 0.72, p > 0.10] and neither did the Closed Head Injured and their

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TABLE I

Comparison of the Two Patient Groupsin Terms of Age, Intelligence and Severity of Memory Impairment

WKS (n = 9) CHI (n = 9)Mean (S.D) Mean (S.D)

AGE 58.0 (1.1) 33.77 (12.6)NART 103.33 (12.5) 103.4 (14.4)FSIQ 88.22 (10.04) 91.37 (7.81)GMI 57.44 (10.40) 73.88 (15.69)RMT (words) ~ 31.42 (5.5) 39.0 (5.83)

NART = National Adult Reading Test; FSIQ = Full Scale Intelligence Quotient; GMI = General Memory Index; RMT= Recognition Memory Test (Warrington, 1984); ~ = mean based on seven subjects.

control group [t (16) = 1.16, p > 0.10 for age; t (16) = 1.42, p > 0.10 for NART).

Materials and Design

A list of forty words was extracted from the set used by Blaxton (1989). The wordsvaried in length from five to eight letters and the corresponding fragments could becompleted with only one English word. The mean frequency of the words was 34.5 permillion (Kucera and Francis, 1967). The forty words were divided into four equal sets andlabelled A, B, C, and D. Three sets were presented at study and each incorporated a differentorienting task in the incidental study of the target words. The tasks were:

1: Rating pleasantness (Semantic) 2: Counting syllables (Syllable Judgement) 3: Counting ascending and descending letters (Physical).The study list comprised three sets of words presented in a blocked fashion with, the

orienting tasks counterbalanced across the study lists, for example, A1B2C3, B2C3D1, withthe unstudied set forming the baseline in each condition. The test list comprised thefragments of all forty words presented in a random order, plus four easy to completefragments at the start. This was to encourage the amnesic subjects to get into a positiveframe of mind, because as a group they are easily discouraged by failure.

Procedure

Subjects were tested individually and the stimuli were presented one at time on an AppleMacintosh Powerbook using the “Experimenter” package (Wathanasin, Birkett, Russell etal., 1991). Study words and test fragments were presented in lower case and the orientingtask commands were given at the start of each set of words. In the semantic study condition,the subjects were asked to:

“Please read the word aloud and rate the pleasantness of the word using a seven pointscale. Seven indicates that you find the meaning of the word very pleasant and one veryunpleasant.”

In the syllable judgement condition the subjects were asked to:“Please read the word aloud and count the number of syllables contained in each word,

for example, the word egg contains one syllable and butter contains two”.In the physical condition the subjects were asked to:“Please read the word aloud and count the number of ascending and descending letters

present in each word. These are the letters that extend above or below the main body ofthe word.”

An example was given to clearly explain this point. The word biology was presentedvisually on a piece of paper and the instructions were repeated. A short trial wasadministered to each subject, comprising six words, to ensure that they understood theinstructions.

Each word was presented for ten seconds with a three second inter-stimulus interval.The distractor phase lasted five minutes during which time the amnesic subjects completedthe Wisconsin Card Sort Task. The control groups were asked to try and name a series ofphotographs of famous faces throughout the decades.

The test phase of the experiment comprised the presentation of the studied and nonstudied word fragments in a random order and the subjects were presented with thefollowing instructions:

“Please try and complete each fragment with the first word that comes to mind”.The test was indirect in that no reference was made to the study episode. The fragments

were presented in lower case and appeared on the screen for ten seconds, then were replacedby the next fragment. There was an inter-stimulus interval of three seconds between eachfragment presentation. The study and test conditions were maintained equally throughoutfor both the amnesic and control groups.

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RESULT

Fragment Completion

Table II shows the percentage of fragments correctly completed for thewords studied under the three orienting tasks and for the baseline words.

Priming

A measure of priming was calculated for each subject by subtracting thebaseline measure from each score for each condition, and these results areillustrated in Figure 1. In order to assess the affect of LOP on priming a 4 × 3mixed analysis of variance (ANOVA) and a series of planned comparisons wasperformed on these scores. The between subjects factor had four levelscorresponding to the four groups (wks, chi, control-wks, control-chi), and thewithin subjects factor had three levels corresponding to the semantic, syllabicand physical processing conditions.

This analysis showed no differences in the overall fragment completionperformance across the four groups, F (3, 32) < 1, neither a significantdifference in the overall performance between patients and controls, F (1, 32) =2.05, (Mean Square Error) MSe = .142, p > .10. The LOP effect was significant(semantic = 29.4%, (Standard Error) SE = 4.5%; syllabic = 26.4%, SE = 4.3%;physical = 12.5%, SE = 3.9%), F (2, 64) = 11.69, MSe = .025, p < .01, andpriming was significantly larger than chance in all study conditions, ts (35) >3.2, ps < .01 (one-tail).

There was a significant interaction, F (6, 64) = 2.33, MSe = .025, p < .05,suggesting that the LOP effect differed in the various groups. More specificallya planned contrast indicated, in line with the predictions, that the LOP effectwas larger among controls compared to patients, F (2, 64) = 5.86, MSe = .025,p < .01 (see Figure 1).

Further analyses were carried out to assess the magnitude of the LOP effectin both controls and patients.

A 2 (control-wks vs control-chi) × 3 (processing levels) mixed ANOVAproduced only a significant main effect of LOP, F (2, 32) = 11.96, MSe = .033,p < .01 (semantic = 41.7%, SE = 5.5%; syllabic = 30.0%, SE = 7.2%; physical= 12.2%, SE = 5.9% - priming was significantly larger than chance in all studyconditions, ts (l7) > 2.06, ps < .05, one-tail).

Moreover it appeared that priming under syllabic processing was significantly

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TABLE II

Scores from the Word Fragment Completion Task (percentage correct)

Groups Semantic Syll.Judg. Physical Baseline

WKS 35% 47% 35% 22%Controls-WKS 63% 47% 33% 17%CHI 40% 38% 31% 18%Controls-CHI 65% 57% 36% 27%

larger than under physical processing, F (1, 17) = 7.49, MSe = .038, p < .025,and that priming under semantic processing was larger than under syllabicprocessing, F (1, 17) = 6.46, MSe = .019, p < .025. The remaining main effectand interaction were not significant, Fs < 1, overall indicating that an equivalentLOP effect occurred in both control groups.

A similar analysis was performed on the patient groups. Neither the maineffect of group nor the interaction were significant, Fs < 1.22, ps > .10.However the LOP effect approached significance, F (2, 32) = 2.64, MSe = .017,p = .087 (semantic = 17.2%, SE, = 6.1%; syllabic = 22.8%, SE = 4.8%; physical = 12.8%, Se = 5.0% - priming was significantly larger than chance inall study conditions, ts (17) > 2.54, ps < .05, one-tail). It also appeared thatpriming under syllabic processing was significantly larger than priming underphysical processing, F (1, 17) = 5.67, MSe = .016, p < .03. Priming undersemantic processing did not differ from priming under physical processing, F (1, 17) = 1.11, MSe = .016, p > .10, or syllabic processing, F (1, 17) = 1.38,MSe = .02, p > .10. A comparison of the overall priming supported by the twodeepest processing conditions (i.e. semantic and syllabic) with the amount ofpriming obtained under the physical processing condition showed a marginallysignificant advantage of deeper vs shallow processing, F (1, 17) = 4.30, MSe =.054, p = .054.

Using a slightly different way to assess the effect of processing on priming,it appeared that no differences occurred between controls and patients in theirpriming scores for item studied at physical or syllabic processing, Fs < 1, while

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Fig. 1 – Graph showing the interaction between subject groups (Amnesics vs Controls) and levelsof processing. Keys: sem = semantic processing, syll = syllabic processing, phys = physicalprocessing. The dependent variable used is the difference in the percentage of completed targetfragments minus new fragments.

sem

syll

phys

Amnesics Controls

.5

.45

.4

.35

.3

.25

.2

.15

.1

.05

.0

controls showed larger priming scores compared to patients under semanticprocessing, F (1, 34) = 8.87, MSe = .06, p < .01.

GENERAL DISCUSSION

Summarising, it appeared that both controls and amnesic patients showed areliable priming effect in a word fragment completion task. A reliable overallLOP effect was also detected indicating that deeper levels of processing at studyinduced larger priming. However, the LOP effect was significantly larger in thecontrols than in the amnesic groups. Since it has been shown that there are nodifferences in encoding abilities between amnesics and controls (e.g. Mayes,Downes, Shoqeirat et al., 1993) the presence of a larger LOP effect in thecontrol groups is unlikely to be due to better semantic or lexical processing oftargets, but it is more likely to be a by-product of an explicit memorycontribution to a nominally implicit memory task. As suggested by Challis andBrodbeck (1992) this type of result may reflect the use by normal subjects ofexplicit memory strategies to complete the indirect task. The words that hadbeen encoded at a “deeper” level would be more accessible to the subject,especially if he or she became aware of the relationship between the studyepisode and test situation. Test awareness was not specifically examined in thecontrol groups, but a couple of subjects commented that toward the end of thetask they recognised some of the words. However, Bowers and Schacter (1990)realised during their own study that they could not determine whether testawareness was a necessary condition for priming or a consequence of priming(for a similar view see also Richardson-Klavehn, Gardiner and Java, 1994).

The substantial LOP effect shown by control subjects, i.e. 29.5% differencebetween semantic and physical processing conditions, is by no means an isolatedfinding. Differences in performances between physical and semantic processingequal or larger than 25% have been already reported in other studies measuringLOP effects in implicit memory as measured by fragment completion (e.g.Hamman and Squire, 1996, Experiment 1; Squire et al., 1987, Experiment 3).These large effects, as the present one, could be interpreted as an indication thatsubjects used the available explicit memory of the learning episode to aid theword completion task. We think that this is a plausible suggestion for tworeasons. First, we employed a relatively small number of target items (i.e.,thirty), and 68% of the items in the test list were targets from the learning phase(similar figures apply also to Hamman and Squire, 1996; and Squire et al., 1987,studies). As suggested by Roediger and McDermott (1993) under theseconditions it is likely that healthy normal controls use explicit strategies toperform implicit memory tasks. Second, we tested a group of twelveundergraduates in a pilot experiment comparing an implicit version with anexplicit version of the word fragment completion task used in the previouslydescribed experiment. The only difference between the two versions of thecompletion task was in the test instructions. In the explicit version subjects wereasked to use the provided fragments as cues to recall the previously processeditems. Results indicated, apart from a lower performance for new/baseline

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fragments in the explicit task (i.e., .07 implicit vs .41 explicit), the presence ofa significant LOP effect of an equivalent magnitude in both implicit and explicittests. The difference between semantic and physical processing conditionscompletion rate was .11 in the implicit task, t (11) = 2.59, p <.05, and .14 in theexplicit task, t (11) = 2.84, p < .05. The LOP effect did not differ significantlyin the two tasks, t (11) < 1. In addition, it must also be stressed that the essentialpoint here is that large LOP effects can only be obtained in fragment completionpriming when subjects have intact explicit memory. This may arise as a directconsequence of explict influences on task performance or because the braindamage found in the memory-impaired group somehow impairs factors whichallow a LOP type of influence on implicit memory for fragment completion.This point, however, must await future research – not in the least thedevelopment of an unambiguous means of assessing implicit memory free ofputative explicit influences.

A marginally significant LOP effect was also present in the amnesic subjects.This may be caused, as suggested in the case of normal control subjects, by anexplicit memory contribution to the implicit task. This would also explain thereduced LOP effect in the amnesics compared to the control subjects. Sinceamong the amnesics the available explicit memory for the study episode isgenerally reduced compared to controls, and given that this occurs especially fordeep/semantic level of processing (e.g. Graf, Squire and Mandler, 1984), anexplicit contribution to fragment completion is expected to be reduced comparedto controls.

Another possible explanation for the significant LOP effect in the amnesicpatients may relate to the view that lexical processing is a factor that positivelyaffects priming (e.g. Weldon, 1991; Richardson-Klavehn and Gardiner, 1998).Lexical access for each target was induced at study by the request to read aloudeach presented item. However, syllabic and semantic orienting tasks, focusingthe analysis on the items as a whole, may have boosted lexical processing oftargets compared to the physical orienting task that could be performed withoutfurther analysing target words in their entirety. Therefore this extra lexicalprocessing in the syllabic and semantic orienting tasks could be held responsiblefor the LOP effect in the amnesics.

The present data seem to rule out the third possible explanation suggested byChallis and Brodbeck (1992) to explain LOP effects in implicit tasks. Accordingto this account semantic processing should boost the conceptual analysis ofitems, which should then benefit priming in perceptual implicit tasks. Thatconceptual processes could have positively affected priming in the amnesicssubjects over and above lexical processing seems unlikely. This is suggestedbecause priming under semantic processing did not benefit any extra incrementover that provided by lexical/syllabic processing at study.

A fuller account of LOP effects in implicit memory tasks among memoryimpaired patients can arise from a closer analysis of the amnesics’ data in thecurrent study, and from a review of the relevant published studies on this topic.To this respect it is important to note that while our amnesic patients showed aLOP effect in the present study, such an effect was different from the onedisplayed by normal controls. In fact, while normal controls’ completion

584 Valerie Jenkinsand Others

performance was at its best after semantic processing, intermediate after syllabicprocessing, and lowest after physical processing, the performance of amnesicpatients in the semantic condition was not better than in the syllabic condition.This finding suggests that the LOP effect displayed by our amnesic sample wasmore likely due to lexical processing of the stimuli than due to an explicitmemory contribution to the implicit task, as in the case of the control subjects.In fact, if explicit memory had primarily contributed over and above lexicalprocessing to the LOP effect in the amnesic sample, then amnesics should haveperformed significantly better in the semantic than in the lexical processingcondition. This pattern of performances should have occurred in the case of anexplicit contribution to the word completion test because semantic orientingtasks normally induce better explicit memory performance than lexical orientingtasks. A comparable performance after lexical and semantic orienting taskssuggests that lexical processing of stimuli, equivalently activated by the twoprocessing tasks, should be held responsible for the observed LOP effect amongour amnesics (for a similar discussion and findings on normal subjects seeRichardson-Klavehn and Gardiner, 1998).

The above conclusion does not preclude the relevance of an explicit memorycontribution to LOP effect in implicit memory tasks among amnesics.Indications that this can be the case can be gathered from a review of therelevant literature on this topic. There are six published studies where the LOPmanipulation was used in conjunction with amnesic patients (Graf et al., 1984;Squire, Shimamura and Graf, 1987; Carlesimo, 1994; Carlesimo, Marfia,Loasses and Caltagirone, 1996; Brunfaut and d’Ydewalle, 1996; Hamman andSquire, 1996). In these studies patients were tested in either word fragment orword stem completion tasks only after physical and semantic processing atstudy. No lexical processing tasks were used.

In the first study (Graf et al., 1984), two experiments assessed the effect ofLOP in a word stem completion task using amnesics. The results of the firstexperiment are, however, difficult to interpret. Implicit testing occurred afterlearning, but a free recall task on the targets used in the implicit task wasadministered just before stem completion. Since amnesics showed a LOP effectin both the implicit and the explicit task it is difficult to rule out an explanationin term of explicit memory contamination in the implicit task.

In the second experiment the intervening free recall was removed and stemcompletion was assessed at three different study test delays: no delay, 15 min,and 2 hours. No statistical analyses are provided on the LOP effect in theamnesics. However, inspecting Figure 2 at p. 171, it appears that a LOP effectof about 5% was present at zero delay, while the LOP effect basicallydisappeared at 15 min and at 2 hours delays. A plausible interpretation of thisresult could be that a LOP effect occurred only when amnesics were capable oflinking the test phase of the fragment completion task to the study phase. Atlonger delays the link between the study and test might not have been apparentto the amnesics, given their poor memory for past episodes, therefore theprobability of explicit contamination in the implicit tasks was reduced.Concurrent to this the LOP effect in the implicit task disappeared.

Congruent with this interpretation of the LOP effect in implicit tasks is a

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study by Squire et al. (1987). In two experiments using word stem and wordfragment completion a LOP effect of 8% (no statistical analysis are available) inthe amnesic patients was only present when testing was carried out at no delay,and this only in the word stem completion task. When the delay between studyand test was either two hours or four days no LOP effect was present. It isrelevant to notice that amnesic patients showed a LOP effect of comparablemagnitude in a yes/no recognition memory task at immediate and two hoursdelay. This suggests that conceptual and lexical processing of targets, assumingthe relevance of lexical processing to recognition memory, should have beensimilarly available at both intervals to support the LOP effect in the implicittask. However such an effect was absent at the two-hour delay in the implicittasks (– 10% in stem completion and 0% in fragment completion). This resultrules out conceptual and lexical processing as factors supporting LOP effects inimplicit word completion tasks at least in Squire et al. study.

The similar in magnitude LOP effect in recognition memory among amnesicsat immediate and two hour delays is compatible with an account of the LOPeffect in the implicit task performance being due to explicit memorycontamination. In fact, even if a similar amount of explicit memory for the studyepisode was available to amnesics immediately and after a two hour delay, it islikely that, given the nature of the implicit task (i.e. say the first word that popto mind in response to a cue), only after no delay the link between the study andtest would be apparent, thus allowing patients to use the available explicitmemory of the learning episode in the implicit task. Congruent with this accountof LOP effects in implicit tasks in amnesia are the results obtained by Carlesimo(1994), Carlesimo, Marfia, Loasses et al. (1996), and Hamman and Squire(1996). In these studies amnesics were either tested immediately or 5 min afterlearning using word stem and/or word fragment completion tasks. Here toolarger priming effects, often marginally significant, were obtained undersemantic than physical processing of targets. Finally, Brunfaut and d’Ydewalle(1996) tested a sample of Korsakoff patients using a word stem completion task.No apparent LOP effect was detected in both amnesic and control subjects. It isrelevant to note that Korsakoff patients did not show a LOP effect in a cuedmemory task where word stems acted as cues. This finding is compatible withthe view that LOP effects in implicit memory, when present, arise from thecontribution of explicit recollection.

From the above review of the relevant literature on LOP effects in implicitmemory tasks among amnesic patients it then appears that explicit memorycontamination is a plausible factor in contributing to the appearance ofsignificant LOP effects in implicit word completion tasks among amnesics (for asimilar view see also Hamman and Squire, 1996). It is however important tonote that none of the reviewed studies used a lexical processing task inconjunction with semantic and physical orienting tasks. Therefore, it is somehowdifficult to assess the differential contribution of lexical processing and explicitmemory contamination on LOP effects in implicit tasks (but see the abovediscussion in relation to the methodology employed by Squire et al, 1987). Ifafter a lexical orienting task patients had displayed an equivalent amount ofrepetition priming as under the semantic orienting task, then the role of explicit

586 Valerie Jenkinsand Others

memory contamination in originating the LOP effect among amnesics wouldhave been undermined.

The present study in conjunction with Graf et al.’s (1984), Squire et al.’s(1987), Carlesimo (1994), and Hamman and Squire (1996) studies showed thatLOP effects in implicit memory tasks are present not only in normal controls(for reviews see Challis and Brodbeck, 1992; Brown and Mitchell, 1994) butalso in amnesic subjects. A closer analysis of the results of the presentexperiment suggests that the lexical processing of targets is the most plausiblecontributing factor the LOP effect in word completion implicit tasks. On theother hand a literature review suggests that explicit memory contamination canalso have an important role in the origin of LOP effects in implicit memorytasks among amnesic patients. Limitations of the methodology used in thereviewed studies did not allow to clearly assess the contribution of lexicalprocesses to the LOP effects detected in the reviewed studies.

While the results obtained in the present experiment provide evidence for acritical role of lexical processing in originating the LOP effect in implicitmemory tasks among amnesic patients, we recognise the need of further studiesto clearly assess the respective role of lexical processing and explicit memorycontamination as causes of LOP effects in implicit memory tasks amongamnesics. However, as this study suggests (see also Richardson-Klavehn andGardiner, 1998), only the use of a lexical orienting task in combination withsemantic and physical orienting tasks will provide clearer evidence on the roleof the above factors in LOP effects in implicit memory tasks among amnesics.

Finally we would like to point out that our discussion of LOP effects inimplicit tasks applies when LOP is manipulated between subjects or withinsubjects in a blocked way. Further studies are required to explain the effect ofLOP in implicit tasks when different orienting tasks are randomly mixed in thestudy list (see Thapar and Green, 1994).

Acknowledgements.Valerie Jenkins was supported by a scholarship and Alan Parkinwas supported by a grant (G9433211N) from the UK Medical Research Council. RiccardoRusso was supported by a grant from The Nuffield Foundation.

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Alan J.Parkin, Laboratory of Experimental Psychology, University of Sussex, Brighton BN1 9QG. E-mail: alp@epunix.biols.susx.ac.ukDr. Riccardo Russo, Dept.of Psychology, University of Essex, Colchester C04 3SQ, VK. E-mail: rrusso@essex.ac.uk

(Received 28 December 1996; accepted 2 December 1997)

588 Valerie Jenkinsand Others