The effects of level of processing on long-term recognition memory in retarded and nonretarded persons

INTELLIGENCE 17, 205-221 (1993)

The Effects of Level of Processing on Long-Term Recognition Memory in Retarded and Nonretarded Persons PAMELA WOODLEY-ZANTHOS UniversiO' of Alabama

Two experiments compared recognition memory of nonmentally retarded (NMR) and mildly mentally retarded (MR) adolescents immediately and 1 week following presentation of word stimuli with either semantic incidental, nonsemantic incidental, or intentional orienting instructions. In Experiment 1 the task was subject-paced. Recognition memory for all subjects improved following semantic encoding instructions; both intelligence-level groups performed comparably in all encoding conditions at both delay intervals. Encoding times for MR persons were approximately twice those of NMR persons. In Experiment 2 encoding times were equated for the two groups. Both groups recognized more words following semantic instructions; NMR persons recognized more words than MR persons in the semantic condition but not in the nonsemantie condition. Results suggest similar processing of words in MR and NMR persons; however, MR persons require longer encoding times in order to equalize recognition-memory performance of NMR persons when orienting instructions are semantic. There were no intelligence-level differences in longterm forgetting in either experiment.

B r o w n (1974) referred to recognition m e m o r y as an involuntary, passive process that m i n i m i z e s the necessity for active strategy use, It is this nonstrategic nature o f the recognition task that is b e l i e v e d to account for the narrowed discrepancy (or no discrepancy) b e t w e e n mentally retarded (MR) and nonmentally retarded ( N M R ) persons' p e r f o r m a n c e on tasks that use a r e c o g n i t i o n - m e m o r y paradigm. H o w e v e r , several investigators (Ellis, Meador, & Bodfish, 1985; Fagan, 1984; McCartney, 1987) found M R - N M R differences on tasks where recognition m e m o r y was measured. Differential processing o f different types o f stimuli may account, in part, for the equivocal findings thus far obtained in studies of intelligence and recognition memory. Ellis et al. (1985) used computer-generated words and faces as stimuli; Fagan (1984) used novel patterns and cartoon faces; M c C a r t n e y (1987) used photographs o f laces. In all of those studies, recognition

This research was supported in part by NICHD Grants HD07262 to the University of Alabama and HD07226 to Vanderbilt University. Correspondence and requests for reprints should be sent to Pamela Woodley-Zanthos, John F. Kennedy Center. Box 154, Peabody College. Vanderbilt University, Nashville, TN 37203. 205

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performance of the NMR was superior to that of the MR persons. Harris and Fleer (1972) presented pictures of either whole or half faces and found that recognition performance of MR persons was equivalent to that of NMR persons when the stimuli were whole faces, but NMR performance was superior when half faces were used. Although it is not necessary for an individual to adopt an encoding strategy deliberately in order to perform well on most recognition tasks, it has been demonstrated that experimenter-induced orienting instructions can influence recognition performance when the stimuli are words, and that semantic orienting instructions result in better recognition memory than physical or acoustic orienting instructions (Chow, Currie, & Craik, 1978; Craik & Tulving, 1975; Fisher & Craik, 1977). However, in all of those studies, subjects were college students. Tests to determine the effects of semantic orienting instructions on recognition memory of MR persons have thus far been inconclusive (e.g., see Dulaney & Ellis, 1991; McFarland & Sandy, 1982; Schultz, 1983; Stan & Mosley, 1988). McFarland and Sandy, Schultz, and Stan and Mosley varied encoding instructions and compared recognition memory for words in NMR and MR persons. McFarland and Sandy and Stan and Mosley found that memory performance improved for NMR persons, but not for MR persons, following semantic encoding instructions. Schultz found that recognition memory for both intelligence-level groups improved similarly when semantic processing was induced. Dulaney and Ellis used photographs in a picturebook as stimuli, varied encoding instructions, and compared performance of MR and NMR persons on a task that tested recognition memory for items as well as for spatial location. Recognition memory was tested at 0, 1-day, and 1-week delay intervals, thus providing a measure for long-term memory (LTM) as well as for short-term memory (STM). They found that when instructions were semantic, there were no group differences in forgetting. However, in the nonsemantic condition, MR persons recognized fewer items than NMR persons after 1 week. It may be that retention is affected differentially for MR and NMR persons when semantic processing is induced. NMR persons have a richer, more complex associative network of word knowledge than MR persons; therefore, they may be, as McFarland and Sandy (1982) indicated, facilitated more than MR persons by semantic encoding when words are stimuli. However, if NMR persons naturally process information at a deeper semantic level, and MR persons naturally process information at a more superficial level, then both the amount of information retained and its durability would be lessened in MR persons; consequently, they would be able to benefit more than NMR persons from experimentally induced semantic processing. To test the latter hypothesis, two experiments were conducted comparing recognition memory of the two intelligence-level groups immediately and 1 week following presentation of word stimuli with either semantic incidental, nonsemantic incidental, or intentional orienting instructions.

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EXPERIMENT 1 Method

Subjects. Subjects were 45 M R and 45 N M R adolescents recruited from the public school systems of Tuscaloosa, Alabama. MR persons came from special education classes that serve mildly retarded students. The mean IQ of the retarded group was 65.8 (SD = 7.62). The mean chronological age (CA) was 15.8 (SD = 1.60). Five M R subjects were excluded from the study: 4 (IQs = 65, 61, 58, and 53) were unable to read the words; 1 (IQ = 75) had a severe visual impairment. These subjects were replaced. The NMR group came from regular classes in the same school or school system as the M R subjects. The mean CA of the N M R group was 13.8 (SD = 0.70). All subjects received small compensations (a choice of T-shirt, candy, caps) for each day of their participation. Stimuli and Apparatus. A total of 320 unique nouns from the Thorndike and Lorge (1944) word list constituted the stimuli. Approximately 65% of the words were A or A A rated; approximately 30% were rated 30 to 49; and approximately 5% were rated less than 30, but were selected subjectively as being readable and understandable by MR persons. The 320 words were separated into three categories: words that should receive a definite "yes" response to the semantic encoding question, "Have you seen one of these today?" (e.g., hair, shoe, and door); words that should receive a definite "no" response (e.g., king, fox, and war); and words to which the response would depend on the subject's own experiences (e.g., baby, gate, and church). From these lists, words were randomly selected to constitute two unique study lists of 160 words each. Of the 160 study words on each list, approximately 40% were definite yes response words, 40% definite no response words, and 20% idiosyncratic yes or no response words. In each study list there were 8 buffer words, 4 at the beginning and 4 at the end of the list. Of the remaining 152 words on each list, 72 comprised the target words, and 80 were used as "filler" words. The filler words were used to lengthen the study list, thus minimizing potential ceiling effects. Neither buffer nor filler words were tested. Words were presented, one at a time, on an IBM PC monitor. Of the 72 target words on each study list, 36 were randomly selected as target words for the immediate test and 36 were target words on the l-week delay test. The only constraint to random selection was that each test list had an approximately equal distribution of words previously labeled as definite yes, definite no, and idiosyncratic yes/no words. Because a subject saw only one study list, targets from the not-seen study list were used as distractors in each of the test lists. The order of presentation of the test words (targets and distractors) was determined randomly with one constraint: not more than three targets or three distractors were shown in succession. This constraint was to minimize the possi-

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bility of a response set, particularly in MR subjects. Each of the test lists oc curred equally often at the immediate and delay-test intervals. The insert and delete keys on the computer keyboard were covered with brigh yellow tape and marked YES (insert key) and NO (delete key). These keys were used to answer both the semantic encoding questions and the recognition tesl questions. A computer printout was obtained that included yes/no responses tc each of the stimulus items in the semantic condition, latency of responses to the study items, and recognition test responses. Procedure. The 45 subjects from each intelligence group were randomly and equally assigned to the semantic incidental, nonsemantic incidental, or intentional encoding conditions. Participants were tested individually. A cover task was used to minimize the possibility that subjects in the two incidental conditions would suspect a memory test. Subjects were told that we were testing a belief that the ability of people to work with numbers is not as good after they have done "a lot of reading or working with words." They were given a sheet of random numbers and asked to circle all the "threes" they could find for 3 min. They were told that they would be given a similar task after reading a list of words so that we could compare how they did on the number task before and after they read the words. Subjects in the intentional condition were given the same instructions but also were told that there would be a recognition test following the second number task, and that they should try to remember the words. Subjects in the nonsemantic incidental and intentional conditions were asked to read aloud each word as it appeared on the screen, and to press the space bar after reading a word so that a new word would appear. In the semantic condition, subjects were told to read each word aloud and to answer the question, "Have you seen one of these today?" Subjects in this condition pressed the yes or no key that reflected their answers to the encoding question; the depression of a key caused the next word to appear. After subjects viewed all 160 words, they were asked to circle all the "fours" on a page of numbers for 3 min. Following the distractor task, subjects were given an additional 2 min to select an item from the array of compensations offered them for participation in the study; thus, there was a 5-min interval between the study and test phases of the task. The computer signalled an end to the interval and instructions for the recognition test appeared on the screen. The experimenter read these instructions aloud as the subject read them silently. Subjects then viewed the 72 test words, which were presented one at a time on the computer screen, and responded yes or no to the recognition test question. Each test word remained on the screen until the subject had pressed a response key. The l-week delay test was identical to the first test with the exception that a different set of 36 target and 36 distractor words were used, At the end of this session, subjects were thanked, debriefed, and given another small compensation

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for their participation. Four of the MR subjects were absent from school on the 7th day after initial testing; they were retested on the 8th day.

Results

Word Recognition. Table l presents proportion of hits, proportion of false alarms (FA), and recognition scores (d'). Figure I depicts d' scores for individual subjects in each instruction condition at each delay. A 2 (Groups) x 3 (Instructions) x 2 (Delay) mixed analysis of variance (ANOVA) was performed on the d' data. There was a main effect for encoding instructions, F(2, 84) : 17.06, p < .01. Subjects who received semantic encoding instructions recognized more TABLE 1 Means and Standard Deviations of Recognition Scores (d'), Hits, and False Alarms (FA) for Each Group, Instruction Condition, and Retention Interval in Experiment I

Retention Interval Instruction and Measure Retarded Semantic d' Hits FA Nonsemantic Incidental d' Hits FA Intentional d' Hits FA

Nonretarded Semantic d' Hits FA Nonsemamic Incidental d' Hits FA Intentional d' Hits FA

Immediate M (SD)

l-Week M (SD)

2.33 (.89) .84 (.09) .17 (.18)

.53 (.38) .64 (. 17) ,48 (,21)

1.66 (.92) ,74 (. 18) .24 [.19)

.45 (.40) .57 (.20) .38 (.18)

1.43 (.45) .74 (.I 1) .24 1".09)

.44 (.30) .59 (,13) .43 (.16)

2.34 (.72) .86 (.72) .15 (.10)

.81 (.47) .60 (. 18) .32 (.18)

1.19 (.57) .59 [.10) .19(.[1)

.29 (.28) .51 (.16) .43 (.18)

1.37 (.63) ,67 (.13) .21 (.13)

.43 (.27) .49 (.14) .35 (.15)

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Figure 1. Recognition scores (d') for individual subjects in each instruction condition at each delay in Experiment 1.

words than subjects who received nonsemantic incidental or intentional instructions (1.50 v s . . 9 0 and .92, respectively). The difference in recognition performance between persons in the nonsemantic incidental and intentional conditions was not significant (F < 1). There was a delay main effect, F(1, 84) = 293.08, p < .01; in all instruction conditions recognition accuracy decreased at the 1-week delay interval. There was also a significant Instruction × Delay interaction, F(2, 84) = 9.36, p < .01. A decomposition of the interaction showed that subjects who received semantic encoding instructions recognized more words than subjects in either of the other conditions at both the immediate test, F(2, 87) = 16.40, p < .01, and the delay test, F(2, 87) = 5.63, p < .01. An examination of the mean differences between immediate and delay scores in each instruction condition (M = 1.67, 1.05, and .97 for semantic, nonsemantic, and intentional conditions, respectively) indicates that forgetting was relatively greater in the semantic condition than in the other two conditions. There was no effect of intelligence group on recognition memory scores at either the immediate or the 1-week delay interval.

Response l~pe (Hits-to-Yes/Hits-to-No). When Craik and Tulving (1975) were developing the levels of processing (LOP) model of information processing, they determined that when subjects responded yes to an encoding question (e.g., Does [given word] fit in [given sentence]?), retention of the word was greater than when the response was no. They concluded that when a yes response

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was appropriate to a semantic encoding question, the context of the sentence frames was encoded along with the target word, providing a more enriched, specifiable code for the stimulus. The finding led to a modification of the original model that included the concept of elaboration. That is, within each processing domain (structural, acoustic, and semantic), the stimulus could be elaborated in many ways. In the semantic domain, for example, there could be minimal or more elaborate semantic analyses of stimuli; furthermore, a minimal semantic analysis would result in better retention than an elaborate structural analysis. Subsequently, response type (yes or no answer to an encoding question) was used as an index of elaboration. In the encoding phase of the current task, subjects in the semantic condition were asked to respond to the question, "Have you seen one of these today?" The mean percentages o f yes and no responses were identical for MR and NMR persons. Both groups responded yes to 47% of the words and no to 53% of the words (SD = . 12 for M R persons and .08 for N M R persons). Data were collected to determine what proportion o f words given a yes response in the encoding phase were recognized in the test phase (hits-to-yes), and what proportion of words given a no response in the encoding phase were recognized in the test phase (hits-to-no). There was no significant intelligence-related difference in these data (F < l). The main effects for delay, F(1, 84) = 79.60, p < .01, and response type, F ( 1 , 84) = 16.57, p < .01, were qualified by a Delay x Response type interaction, F ( 1 , 84) = 9.46, p < .01. At the immediate test, the mean hits-to-yes and hits-to-no proportions were, respectively, .85 (SD = . 10) and .85 (SD = . 10) for M R persons, and .88 (SD = .09) and .84 (SD = . 14) for N M R persons; the response type effect was not significant, F ( l , 28) = 1.55. At the 1-week delay interval, the mean hits-to-yes and hits-to-no proportions were, respectively, .74 (SD = . 18) and .55 (SD = .24) for M R persons, and .69 (SD = . 16) and .51 (SD = .20) for N M R persons. Both intelligence groups recognized more words to which the encoding response was yes than no, F(1, 28) = 20.07, p < .01.

Encoding Time. The task was subject-paced and encoding time was measured in seconds from the time a word appeared on the screen until the subject pressed a response key. When a response key was pressed, the next word appeared on the screen. Mean encoding times for M R persons were 2.96 (SD = 1.01) in the semantic condition, 2.56 (SD = 1.39) in the nonsemantic incidental condition, and 2.09 (SD = 1.06) in the intentional condition. For the N M R persons, mean encoding times were 1.57 (SD = .35), 1.14 (SD = . 14), and 1.44 (SD = .95) in the semantic, nonsemantic incidental, and intentional conditions, respectively. An ANOVA of these data yielded a main effect for intelligence, F(1, 84) = 35.08, p < .01. Although the encoding times for all subjects were slightly higher in the semantic condition, the instruction effect was not significant, F(2, 84) = 2.56. Table 2 presents correlation coefficients (Pearson rs) obtained between d'

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TABLE 2

Correlations (Pearson r) Between Recognition Scores and Response Latencies for MR and NMR Groups in Each Instruction Condition at Each Delay Interval in Experiment l Retention Interval Immediate 1-Week Encoding Condition

MR

NMR

MR

NMR

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- . 13

.50**

.07

.38

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.00 -.28

,60"** .62****

.63***** .00

.49* .16

*p .057. **p .051. ***p * * * * p = .012. * * * * * p = .01.

.016.

scores and encoding times for both intelligence groups, in each instruction condition, at each retention interval. The meaning of the significant correlation for MR persons in the nonsemantic condition at the delay interval is equivocal. Of more interest is that for NMR persons, at the immediate test, there was a systematic relation between encoding time and recognition scores, but for MR persons the relation was not systematic. Pearson r coefficients were also obtained for 1Q scores and encoding times, and IQ scores and d' scores for MR persons. These correlations did not approach significance.

Discussion Subjects who received semantic encoding instructions recognized more words than those who did not. Recognition memory did not differ between subjects who received nonsemantic incidental instructions and intentional instructions. The interaction between encoding instructions and delay interval showed that subjects recognized more words following semantic instructions at both the immediate test and the 1-week delay test, but the loss of information was relatively greater in the semantic condition at the 1-week interval. Because recognition scores were higher in the semantic condition at the immediate test, the relatively greater loss at the delay interval was to be expected; that is, there was more "room" for a decrease in this instruction condition than in the other two conditions. There were no differences in recognition memory between the two intelligence groups at either test interval in any of the encoding instruction conditions. A comparison of encoding times yielded the most apparent difference between the two intelligence-level groups. Encoding times for MR persons were approximately twice those of N M R persons; this was probably due to the slower reading rates of the MR persons. Of more interest was the finding that for NMR persons, encoding times were positively correlated with recognition scores. Such was not the case with MR persons. The difference may reflect differential "use" of encod-

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ing time. It could be argued that the similar recognition performance for the two groups in the semantic condition occurred not because of similarities in processing, but because of differences in encoding time. That is, longer encoding time plus semantic processing in MR persons may have equalized semantic processing alone in NMR persons. EXPERIMENT2 The relative effects of semantic encoding on recognition memory in the two groups may have been confounded by differential encoding time in Experiment 1. In Experiment 2, a word-reading rate was determined for each subject prior to the study phase of the task; a constant "processing" time was then added to the individual reading rate. The purpose was to equalize encoding time for the two groups while controlling for differential reading rates. In Experiment 1 subjects' attention to the words in the nonsemantic condition was insured by having them read the words aloud; however, a conventional shallow encoding task was not required. If there were differences due to intelligence in covert processing of the words in the nonsemantic incidental condition, requiring a task known to induce shallow processing should minimize the differences. Experiment 2 included a conventional shallow processing task: subjects in the nonsemantic instruction condition were told to respond yes or no to the question, "Does the word have the letter ' E ' in it?" Experiment 2 used a 2 (Groups: MR and NMR) x 2 (Encoding Instructions: Semantic and Nonsemantic) x 2 (Delay: Immediate and 1-Week) mixed design. Experiment 2 differed from Experiment 1 in that (a) there was no intentional condition, (b) the nonsemantic condition required a conventional shallow processing task ("E" checking), and (c) a constant processing time was added to individual reading rates. The purpose was to separate the effects of the semantic encoding variable and differential encoding times. Both groups were expected to recognize more words following semantic encoding instructions. However, if longer encoding times offered an advantage to MR persons in Experiment 1, then NMR persons would be expected to recognize more words than MR persons in the semantic condition in Experiment 2. Otherwise, the predictions for Experiment 2 were the same as for Experiment 1. Method Subjects. Subjects were 40 mildly MR and 40 NMR children from the same school system as those in Experiment 1. None of the subjects in Experiment 1 participated in Experiment 2. The mean IQ for the MR subjects was 66.3 (SD = 7.41); the mean CA for this group was 15.8 (SD = 1.80). Four MR subjects were excluded from participation in the experiment: 3 (IQs = 61, 55, and 49) were unable to read the words; 1 (IQ = 69) did not understand the directions. The

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mean CA for the NMR subjects was 14.3 (SD = 0.71). Two MR and 4 NMR persons were unavailable for testing on the 7th day and were tested on the 8th day. Two MR persons were tested on the 6th day due to a known schedule conflict on the 7th day. Compensation for all subjects was a choice of candy or T-shirts.

Stimuli and Apparatus. Stimulus materials and apparatus were the same as in Experiment 1 except that 40 additional words, rated A or AA, from the Thorndike and Lorge (1944) word list appeared simultaneously on the computer screen prior to the study phase of the task. Procedure. The 40 subjects from each intelligence group were assigned randomly and equally to each instruction condition. The same cover story and task were used as in Experiment 1. Prior to the study phase of the task, 40 words appeared on the computer screen in five columns of 8 words each. Subjects were asked to read these words aloud beginning with the first word of the first column, reading vertically down the columns. When the last word was read, the experimenter pressed the space bar. The computer recorded the reading time and calculated the per-word reading rate for each subject. A constant of 2.5 s was then added to each subject's reading rate for each of the 160 words in the study phase of the task; there was a 500-ms interval between words. Subjects were told to read each of the study words aloud and to answer the question, "Have you seen one of these today?" (semantic condition), or, "Does the word have the letter 'E' in it?" (nonsemantic condition). During the 5-min interval between the study and test phases, subjects circled "fours" on a sheet of paper for 3 min and selected their compensations for 2 rain as in Experiment 1. The test phase was identical to that in Experiment 1. Results

Word Recognition. Table 3 presents proportion of hits, proportion of FAs, and d'. Figure 2 depicts the d' scores for individual subjects. A mixed ANOVA was performed on the d' data. There was an instruction main effect, F(1, 76) = 74.14, p < .01; subjects from both intelligence groups who received semantic encoding instructions recognized more words than subjects who received nonsemantic encoding instructions, but this was qualified by interaction effects. There was an Intelligence x Instruction interaction, F(1, 76) = 8.56, p < .01. A decomposition of the interaction showed that, in the semantic condition, there was an intelligence effect, F(1, 38) = 9.30, p < .01, with the NMR persons recognizing more words than the MR persons. In the nonsemantic condition, there was no intelligence effect (F < 1). There was a main effect for delay, F(1, 76) = 266.12, p < .01, and an Instruction × Delay interaction, F(1, 76) = 26.9, p < .01. As in Experiment 1, subjects who received semantic encoding instructions recognized more words at

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TABLE 3 Means and Standard Deviations of Recognition Scores (d'), Hits, and False Alarms (FA) for Each Group, Instruction Condition, and Retention Interval in Experiment 2

Instruction and Measure Retarded Semantic d' Hits FA Nonsemantic d' Hits FA Nonretarded Semantic d' Hits FA Nonsemantic d' Hits FA

Retention Interval Immediate M (SD)

1-Week M (SD)

2.19 (.71) .82 (.19) .15 (.10)

.79 (.45) .59 (.19) .33 (.15)

1.27 (.63) .69 (. 13) .25 (.14)

.60 (.37) .54 (. 16) .34 (.18)

2.74 (.69) .90 (.04) .10 (.10)

1.19 (.43) .61 (.15) .21 (.13)

1.26 (.42) .68 (. 10) .24 (.14)

.41 (.39) .58 (. 12) .43 (.16)

both the immediate test, F(1, 78) = 67.33, p < .01, and the 1-week delay test, F(1, 78) = 24.41, p < .01, but the relative loss of information was greater for subjects in the semantic condition.

Response Type (Hits-to.Yes/Hits-to-No). In Experiment 2, subjects in both the semantic and nonsemantic conditions answered yes or no to the encoding questions for each condition. A mixed ANOVA, with intelligence level and instructions as between-subject variables and response type (yes or no) and delay (immediate and 1-week) as within-subject variables, was performed on the data. There were main effects for instruction, F(1, 76) = 13.12, p < .01, delay, F ( 1 , 2 2 8 ) = 191.85, p < .01, and response type, F ( 1 , 2 2 8 ) = 35.93, p < .01. These were qualified by an Instruction x Delay interaction, F ( 1 , 2 2 8 ) = 27.47, p < .01, and a Delay x Response type interaction, F ( 1 , 2 2 8 ) = 10.39, p < .01. Decomposition of the interactions showed first that, at the immediate test, there was a main effect for instructions, F(1, 78) = 38.17, p < .01. Subjects in the semantic condition had a higher proportion of hits than subjects in the nonsemantic condition (.89 v s . . 7 1 ) . Second, there was a main effect for response

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Figure 2. Recognition scores (d') for individualsubjects in each instructioncondition at each delay in Experiment 2. type, F(1, 78) = 7.77, p < .01; subjects recognized more words to which they had responded yes than to which they had responded no (.82 v s . . 7 8 ) . At the l-week delay test, there was no effect for instructions (F < 1), but there was a main effect for response type, F(1, 78) = 47.39, p < .01. Subjects in both instruction conditions recognized more words to which they had responded yes (.66) than to which they had responded no (.52).

Reading Rate. In the semantic and nonsemantic instruction conditions, respectively, the mean reading rates (in seconds) for MR persons were 1.16 (SD = .46) and 1.31 (SD = .56), and, for N M R persons, .57 (SD = . 10) and .55 (SD = .09). The effect for intelligence level was significant, F ( I , 76) = 65.71, p < .01. When the 2.5-s constant was added to the reading rates, mean encoding times in the semantic and nonsemantic conditions were, respectively, 3.66 (SD = .46) and 3.80 (SD = .57) for MR persons, and 3.07 (SD = . 10) and 3.05 (SD = .09) for NMR persons. When reading rates were correlated with d' scores, the only correlation (Pearson r) that approached significance was for MR persons in the nonsemantic condition at the immediate test, r(17) = - . 4 4 , p = .052. The correlation for MR persons in the semantic condition at the immediate test was - . 2 0 . Pearson correlation coefficients were converted to Fisher Z coefficients, averaged for each intelligence group at each delay interval, and converted back to Pearson rs. The mean correlation for MR persons at the immediate test interval was - . 3 3 ; with two variables and 36 degrees of freedom, this correlation was significant at the .05 level. For NMR persons, the mean correlation at the imme-

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217

diate test was .01. For MR and NMR persons, respectively, the mean correlations at the delay interval were - . 1 2 and .14. None of these approached significance. G E N E R A L DISCUSSION Experiments 1 and 2 both confirmed the hypothesis that word-recognition memory for all subjects would improve following semantic encoding instructions. This finding is consistent with the word-recognition study of Schultz (1983), the automaticity studies of Ellis and his colleagues (Ellis & Allison, 1988; Ellis, Katz, & Williams, 1987; Katz & Ellis, 1991), and the recall study of Boyd and Ellis (1985). Those studies, in addition to this one, suggest that memory in MR and NMR persons improves similarly following semantic encoding instructions regardless of whether effortful memory processes (such as free recall) or passive, involuntary, or automatic processes (such as recognition, memory for spatial location, and memory for frequency of occurrence) are tested. The contrasting findings of McFarland and Sandy (1982) and Stan and Mosley (1988) most likely are due to methodological differences. McFarland and Sandy (1982) compared performance of MR and CA-matched NMR adolescents on a word-recognition task that had three encoding conditions: semantic, acoustic, and intentional/no strategy. Subjects read target words aloud, then, in the semantic condition, assigned a subjective rating from 1 to 5 regarding how "good" or "bad" each word was, supplied a rhyme for the target word in the acoustic condition, and simply read the word aloud in the intentional/no strategy condition. The task was subject-paced. After a 1-min interval, subjects were given an oral word-recognition test. The same words that were tested at the immediate test were retested after a 24-hour delay. MR persons showed greater memory loss in the semantic condition than did NMR persons, but this finding was obscured by a ceiling effect on the immediate test for NMR persons in this condition. It may be that the NMR subjects benefited more from the second study phase (i.e., seeing the words a second time during the immediate test) than did the MR subjects. In addition, the rating task may have involved two responses: a primary encoding response (i.e., an emotional response to whether the word was good or bad), followed by a secondary response ("let me think about how good or bad on a scale of 1 to 5"). Of these two responses, it is the second that is more likely to lead to more elaborate processing. It is also the second that is less likely to be engaged in (or even understood) by the nonstrategically oriented MR persons. In the Stan and Mosley study (1988) there were three conditions: LOP, distinctiveness of encoding, and a control group. The l O P condition included physical, acoustic, and semantic encoding instructions. The different types of lOP questions were intermixed and asked of each subject (MR and CA-matched NMR adults) as each of 75 target words appeared on a screen. Presentation of the words was experimenter-paced at 2 s per word. No data were provided that sepa-

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rated the effects of the different types of LOP questions; thus, the differential effects of the different types of questions could not be determined. Because recognition performance was superior in the NMR group under all conditions, Stan and Mosley concluded that NMR persons benefit from semantic encoding and MR persons do not. There were no comparisons of group processing times in either study. The hypothesis that recognition memory for MR persons should improve relatively more than that of NMR persons following semantic encoding instructions was not supported by either of the experiments here. The major inconsistent finding between the two experiments was that in Experiment 1 recognition memory for both groups improved similarly following semantic instructions, but in Experiment 2, recognition memory improved relatively more for NMR persons after semantic encoding. This difference apparently was due to the manipulation of encoding time. In the first experiment, encoding time for MR persons was approximately twice that of NMR persons, a difference that was attributed to the slower reading rates of the MR persons. To equate this difference, reading rates for each subject were obtained in the second experiment and a constant "processing" time of 2.5 s was added to each individual's reading rate. This may have inadvertently provided an advantage for the NMR persons. In the semantic condition in Experiment 1, MR subjects had a mean encoding time of 2.96 s and in Experiment 2, with the 2.5 s added to the reading rate, the mean encoding time for this group was 3.66 s. This suggests that encoding time "provided" by the experimenter increased by 20% the encoding time MR subjects were using when the task was subject-paced. Mean encoding time for NMR persons in the semantic condition in Experiment 1 was 1.57 s, but in Experiment 2, the encoding time was 3.07 s, or about a 50% increase from the first experiment. Again, inequitable encoding time may have influenced the results of this experiment. It would be difficult to overcome this problem. One alternative may have been to add a constant percentage in Experiment 2 to the mean encoding times for each group in Experiment 1. However, doing so would not allow adjustment for individual differences in reading rates, which were more variable in MR persons (SD = .46) than in NMR persons (SD = . 10). Nevertheless, processing times were equated for the groups in absolute terms and, under those conditions, NMR persons recognized more semantically encoded words than MR persons, but the two groups recognized a comparable number of words when encoding instructions were nonsemantic. It was predicted that the forgetting rates for the two groups would be comparable in the semantic condition, but that MR persons would forget more than NMR persons after nonsemantic instructions. There were no differences in forgetting attributable to intelligence in any instruction condition in either experiment. As can be seen from Figures 1 and 2, there were no ceiling effects at the immediate test in either experiment that might obscure differential forgetting rates. Three studies have compared LTM in MR and NMR persons and reported no ceiling effects (Klausmeier, Feldhusen, & Check, 1959; Scott, 1971; Stinnett & Prehm,

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1970). Of these three, this study supports the findings of Klausmeier et al., and Scott, who found no LTM differences in the two intelligence groups. The study here is in contrast with the Stinnett and Prehm study, which suggested that there were LTM differences in MR and NMR persons. Of the two studies that reported no improvement in recognition memory in MR persons following semantic encoding instructions (McFarland & Sandy, 1982; Stan& Mosley, 1988), neither used d' as the dependent variable. McFarland and Sandy used corrected proportion scores; Stan and Mosley used proportion of hits. To determine how the results of the study here would compare to results of those studies if the same measure had been used, the current data from Experiment 2 were transformed to corrected proportion scores and an ANOVA was performed on the transformed scores; an ANOVA was also performed on the proportion of hits from Experiment 2. A comparison of the ANOVAs on corrected proportion and d' scores yielded minimal differences in F ratios, and the significance levels were identical for all effects. Thus, interpretation of the resuits would remain the same whether d' or corrected proportion was the dependent variable. In comparing the results of this study to those of previous studies (Boyd & Ellis, 1985; Dulaney & Ellis, 1991; McFarland & Sandy, 1982), both discrepancies and similarities can be found. With the exception of the McFarland and Sandy study, all of the studies found improved memory in MR persons following semantic encoding instructions; it is the degree of improvement relative to that of NMR persons that differs. Boyd and Ellis (1985) used pictures as stimuli and tested free recall. The instruction effect for both intelligence groups was similar, but NMR persons recalled more pictures than MR persons regardless of instructions. Results were discussed in terms of differences in spread of encoding in the two groups. Through administration of a word-association test, MR persons were shown to have an impoverished semantic associative network that was assumed to have led, in turn, to less semantic elaboration and less recall facilitation following semantic encoding instructions. In the study here, both groups demonstrated almost identical effects of spread of encoding as indexed by the hits-to-yes/hits-tono measure; thus, there was no evidence to suggest that MR persons do not elaborate. The different interpretations of spread of encoding in the Boyd and Ellis study and this one may be viewed in terms of differences in the wordassociation test and response type (hits-to-yes/hits-to-no) as measures of spread of encoding. Quantitatively, MR persons may have fewer semantic associates than NMR persons, but they may process meaningful information in a qualitatively similar manner. That is, even if MR persons have fewer stored associates per stimulus item, they may still process the context along with the item when the encoding response is yes (see Craik & Tulving, 1975, Experiment 6). The study here suggests that any processing differences between the two intelligence groups are quantitative rather than qualitative. When the task was subject-paced, as in Experiment 1, MR persons had a longer encoding time, but

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recognized as many words as N M R persons regardless of encoding instructions. In Experiment 2, the task was experimenter-paced but N M R persons inadvertently may have had relatively more encoding time than MR persons. A more complex associative network may have provided an advantage for the NMR persons under these circumstances. For example, when words with several meanings (such as "trunk") were presented, N M R persons may have processed several meanings (elephant's trunk, footlocker, car trunk) before responding, whereas MR persons may have processed only one meaning of the stimulus word. Dulaney and Ellis (1991) used pictures as stimuli and tested recognition memory. They found that semantic encoding instructions facilitated picturerecognition memory in MR, but not in N M R persons. Marks (1989) also found that picture-recognition memory was not facilitated by semantic instructions for NMR persons; Marks did not compare performance of NMR and M R persons. If there are qualitative differences in the way in which MR and N M R persons process meaningful information, they may be more apparent when pictures are stimuli. The results of these experiments suggest similar processing of words in MR and NMR persons as reflected by manipulation o f encoding instructions. There were no differences in word-recognition memory in the two groups in either the nonsemantic incidental or intentional conditions, which did not differ from each other; thus, intent to remember facilitated recognition memory in neither group. When the task was subject-paced, induced meaningful processing facilitated memory similarly in both groups, and the groups performed comparably. The NMR recognized more words following semantic instructions than did the M R persons when the task was experimenter-paced, but the longer encoding times may have provided more of an advantage for NMR persons. The hits-to-yes/hitsto-no data suggest that recognition memory for both groups was facilitated similarly by semantic elaboration. There was also no evidence to suggest differential forgetting rates in the two groups.

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