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Adult age differences in short-term forgetting
Acta Psychologica North-Holland
x3
60 (1985) 83-101
ADULT AGE DIFFERENCES
IN SHORT-TERM
Stanley R. PARKINSON, Vaughan Stephen E. DANNENBAUM * Arizona Stare Uniuersitv, Accepted
January
W. INMAN
FORGETTING
and
USA
1985
Two experiments are reported in which young and old adults performed in a Brown-Peterson task. In the first experiment young adults recalled with greater accuracy than old adults and the difference between age groups was greater in delayed than in immediate recall. Performance varied inversely with interpolated task difficulty in the delayed recall condition, but this effect did not interact with age. In the second experiment an attempt was made to equate immediate recall performances of old and young adults to determine if age differences in the rate of forgetting are independent of age differences in registration. Each participant was pre-tested to determine the number of stimulus repetitions needed to achieve a minimum of 83% correct in immediate serial recall of 6-letter sequences. The number of repetitions an individual required in pre-testing was then used in a subsequent Brown-Peterson task. No significant age differences in delayed recall were obtained when immediate recall differences were minimized by differential repetition of to-be-remembered sequences, The results of these experiments suggest that age differences in forgetting rates arise from age-related differences in encoding and storage.
Introduction Comparisons between young and old adults have revealed age-related differences in a variety of short-term memory tasks including memory span (Botwinick and Storandt 1974; Kausler and Puckett 1979; Parkinson et al. 1982) running span (Parkinson 1980; Talland 1968) free recall (Craik 1968; Parkinson et al. 1982) and dichotic memory (Inglis and Caird 1963; Parkinson et al. 1980). In addition, age-related differences in retention have been found in dual task research where one of the tasks measures forgetting over brief intervals (Wright 1981). Whereas the results of many studies have been consistent in demonstrating age differences, little consensus has been achieved regarding the * Mailing address: 85287. USA.
OOOl-6918/85/$3.30
S.R. Parkinson.
Dept.
of Psychology.
0 1985. Elsevier Science Publishers
Arizona
State
University,
B.V. (North-Holland)
Tempe.
AZ
84
S. R. Parkrnson et al. / Edxamination of uge differences
processing and (or) storage mechanisms which decline with age. Some researchers have stressed the importance of age differences in registration, hypothesizin g age-related deficits in encoding (Craik and Simon 1980; Eysenck 1974; Smith 1980) and storage (Craik 1977; Parkinson et al. 1983). Others have emphasized differential forgetting with age differences resulting from increased susceptibility to interference (Welford 1958) or decay (Fraser 1958). The Brown-Peterson (Brown 1958; Peterson and Peterson 1959) distractor procedure has been used extensively in studies of encoding, storage and short-term forgetting with young adults and would seem to provide a useful framework for delineating factors underlying age differences. In the Brown-Peterson procedure a distractor task (e.g., counting backwards by 3s) is interpolated between presentation and recall of to-be-remembered items. Forgetting functions are obtained by varying the length of the retention interval in which the distractor task is performed. These functions are generally characterized by a rapid decline in recall which reaches asymptote within 10 set (e.g., Dillon and Reid 1969). In a recent experiment conducted in our laboratory (Inman and Parkinson 1983) substantial age-related differences in BrownPeterson recall were obtained. In that study 46 young (mean age = 20.5 years) and 27 old (mean age = 72.4 years) adults were shown memory sets consisting of 3, 4 or 5 letters presented at a rate of 2 letters/set. Memory sets were followed by the presentation of pairs of digits (1, 3, 5 or 7) and participants were instructed either to read the digits as they appeared (easy distractor task), or to report the sum of each digit pair and whether the sum was odd or even (difficult distractor task). Pacing the distractor tasks ensured that participants of both age groups processed the same amount of information between presentation and recall of memory letters. The results showed that old adults recalled with lower accuracy.than young adults (both free and serial recall scoring) and that the magnitude of the difference between age groups was positively related to retention interval. Whereas the age by retention interval interaction in the Inman and Parkinson study suggested more rapid information loss by the elderly, that interpretation was precluded by the finding that there were age differences after only one interpolated digit pair, particularly with larger memory sets under the more difficult interpolated condition. Thus, rate of forgetting differences could not be separated from potential differences in registration. An attempt was made in the present
S.R. Parkinson et al. / Examination
of age differences
85
research to distinguish between age differences in registration and rate of forgetting in the Brown-Peterson task. Accordingly, in the first experiment, groups of old and young adults received memory sets followed by either 0 or 7 interpolated digit pairs (immediate and delayed recall, respectively). If recall of old and young adults were equivalent at immediate recall but different after the delay, the age by retention interval interaction would indicate an age-related difference in rate of forgetting which could be attributed to increased susceptibility to interference (Welford 1958) or decay (Fraser 1958). Conversely, if recall of old and young adults were different in both conditions, rate of forgetting differences could not be distinguished from registration differences. During recent years there has been an increasing tendency to attribute lower levels of recall performance by the elderly to a deficit in central processing capacity (e.g. Craik and Simon 1980; Wright 1981). Participants of both age groups in the Inman and Parkinson study recalled with greater accuracy following an easy than difficult interpolated task thus replicating a previous finding (Dillon and Reid 1969) and supporting the hypothesis that central processing demands vary as a function of the amount of information reduction required (Posner and Rossman 1965). However, in contrast to the hypothesis that aging is associated with a reduction in central processing capacity, the magnitude of the difference between age groups did not vary significantly with interpolated task. A second purpose of the first experiment was to provide another test of age differences in central processing capacity as a factor in BrownPeterson task performance. Therefore, distractor task difficulty was manipulated as in the Inman and Parkinson study.
Experiment 1 Method
Participants Thirty young adults (17 males, 13 females; mean age = 19.2 years; range 17-23) and 30 old adults (9 males, 21 females; mean age = 74.2 years; range 67-84) served as participants in the first experiment. The young adults were undergraduates at Arizona State University. Participation of the undergraduates served as partial fulfillment of introductory psychology course requirements. The old group consisted of a number of
86
S. R. Parkinson et 01. / Examnatmn
of age differences
highly educated individuals: The group possessed an average of 16.8 years of formal education and included former lawyers, engineers, academicians and school teachers. Elderly participants were recruited from a local retirement community offering full life care and all reported themselves to be in good health at time of testing. The mean Wechsler Memory Scale (WMS) raw score of the old group was 58.9 (SD = 7.6, range 43.5 to 70) which is approximately equal to the 50th percentile norm of Wechsler’s 40 to 44 year old group (Wechsler 1945). The mean and standard deviation obtained by our old group is also approximately equal to that obtained by Hulicka’s (1966) 40 to 49 year age group (mean = 58.8, SD = 7.1). Our group was significantly superior to Hulicka’s 70 to 79 year age group. t(74) = 4.40, the former having a mean raw score 8 units higher than the latter. In previous studies with groups of old and young adults drawn from the same sources we have found significant differences between age groups in vocabulary (favoring the elderly) and memory span (favoring the young adults). Memory spans of the present groups were consistent with the pattern evident in our previous work. Immediate memory spans of letters were assessed for all participants. Letters were presented visually at the rate of 1.2 letters/set and participants were instructed to recall the letters in the exact order in which they were presented. Five span estimates were obtained for each participant and an individual’s span was determined to be the mean of the five estimates (Inman and Parkinson 1983). The mean letter span of the young adults (mean = 5.86; SD = 0.77) was significantly greater than that of the old adults (mean = 5.17; SD = 0.74) F(1, 58) = 12.34. p < 0.001. 0’ = 0.159. Procedure
All participants were tested individually. Each participant received 60 trials in the Brown-Peterson task. 20 each with memory sets of 3, 4 and 5 letters. Adams et al. (1969) found that sequential presentation of memory letters in a Brown-Peterson task resulted in the formation of fewer natural language mediators than simultaneous presentation. As mediators benefit recall and as older adults generate fewer mediators spontaneously (Hulicka and Grossman 1967), sequential presentation was used in the present study to minimize encoding bias against the elderly. Memory letters were presented on a video terminal controlled by a TRS-80 microcomputer. Memory letters were 0.7 cm in height and were presented sequentially in the same location in the center of the display at a rate of 2 letters/set. Each letter was displayed for 220 msec. A new series of letters was presented on each trial. Letters were selected at random from the alphabet, excluding the letter V, with the restriction that no letter be repeated within a trial or on immediately adjacent trials. Participants were requested to pronounce each letter aloud during presentation. As the number of letters presented was a factor in the present study, the use of sequential presentation necessitated a decision between presenting lists for the same amount of time, regardless of length, or covarying presentation time with list length. In the former case the length of time available to form associations between letters is confounded with list length, whereas in the latter, the time between the onset of the first item and recall is confounded with list length. It was assumed that recall of lists of all lengths would be nearly perfect at the beginning of the retention interval and that the retention interval was therefore more critical than the duration of the stimulus
S.R. Parkinson et al. / Examination of age differences
87
presentation in this procedure. Thus, presentation time of each letter was held constant for all list lengths. Memory letters were followed by either 0 or 7 pairs of digits. Digit pairs were presented in the center of the display at a rate of 1 pair/2 sec. On immediate recall trials the word ‘RECALL’ followed the offset of the last memory letter by 750 msec and remained on the screen for 8 sec. In delayed recall, the recall signal followed the last digit pair by 750 msec. Trials were blocked on the basis of memory set. Within a block, 10 trials were immediate recall and 10 tested recall following 7 interpolated pairs. The order of 0 and 7 digit pair trials was randomized within a block. Twelve seconds were allowed for recall. As in the Inman and Parkinson study two interpolated tasks were used. These tasks differed in the amount of information reduction required (Posner and Rossman 1965). Participants in the difficult distractor condition (15 young adults and 15 old adults) were instructed to report aloud the sum of the digit pairs and whether it was odd or even. The instructions for the easy distractor condition (15 young adults and 15 old adults) were to read the digits aloud as they appeared. All participants were instructed that when the ‘RECALL’ message appeared they were to write the letters they had read in left to right order in the boxes of a prepared response sheet. Participants were instructed further to guess or to place dashes in boxes if they could not recall letters. Results Only trials in which letters had been read aloud correctly, and in which interpolated task instructions were followed, were included in data analyses. Errors in reading the to-be-remembered letters resulted in the exclusion of 7.4 and 1.6 percent of trials of old and young participants, respectively. Failure to complete interpolated digit pair reading or transformation resulted in the exclusion of an additional 3.7 and 1.3 percent of trials to old and young participants, respectively. Whereas participants were instructed to recall letters in the exact order in which they were presented, scoring was done with both serial and free recall criteria. Analyses on serial and free recall are presented separately. Serial recall analysis
Because of the unequal distribution of the sexes between the two age groups a preliminary analysis of variance was conducted on the arcsin transform of the proportion of letters recalled correctly averaged over serial positions. Sex, age group, and interpolated task were between-group variables in this analysis and retention interval and memory set (3, 4 or 5 letters) were within-group variables. The main effect of sex was not significant, F(1, 52) = 1.21, p > 0.27, w2 < 0.001, and no interaction between sex and other factors was found (all ps > 0.24). Therefore sex was not included in the remaining analyses reported here. In fig. 1 the proportion of letters recalled correctly, averaged over serial positions, is displayed as a function of memory set, age, retention interval, and interpolated task. An analysis of variance performed on the arcsin transform of the proportion correct revealed significant main effects for all factors: old participants recalled with lower accuracy than young participants, F(1, 56) = 19.64, p < 0.0001, w2 = 0.042; more
88
S. R. Parkinson loo-
q
of age differences
Ed (I/. / Exnminrrtion
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Fig. 1. Percent correct recall as a function of number of memory letters, retention interval and interpolated task. Solid lines describe performances of young adults and dashed lines describe performances of old adults.
letters were recalled correctly when participants were required to read than sum digit pairs, F(1, 56) = 24.27, p < 0.0001, u2 = 0.053; and recall varied inversely with both number of memory letters, F(2, 112) = 194.019, p < 0.0001, CJ*= 0.177, and retention interval, F(1, 56) = 287.90, p < 0.0001, w* = 0.327.
S. R. Parkinson et al. / Examination of age differences
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Three second-order interactions were significant: retention interval interacted with interpolated task, F(1, 56) = 46.95, p < 0.0001, w2 = 0.052, number of memory letters. F(2, 112) = 16.57, p < 0.0001, a2 = 0.015, a n d age, F(1, 56) = 11.92, p < 0.002, w2 = 0.012. In each case ‘he difference in accuracy between levels of a factor was greater following an interpolated task than with immediate recall. Free recall analysis A very similar pattern of results was obtained when recall was scored according to a free recall criterion. Old adults recalled with lower accuracy than young adults, F(1, 56) = 23.22, p i 0.0001, wz = 0.050; r ea d’mg digit pairs resulted in greater accuracy than summing digit pairs, F(1, 56) = 29.12, p < 0.0001, w2 = 0.063, and recall varied inversely with both number of memory letters, F(2, 112) = 106.78, p < 0.0001, w2 = 0.097, and retention interval, F(1, 56) = 292.87, p < 0.0001, w2 = 0.365. As with serial recall, all two-way interactions with retention interval were significant (for all of which p < 0.0001).
Discussion The two major functions of the first experiment were to (a) provide information regarding immediate recall in an attempt to distinguish between registration and rate of forgetting differences, and (b) conduct another test of age differences in tasks thought to require different amounts of processing capacity. Concerning the latter, although the effect of interpolated task was significant, neither the first-order age by interpolated task interaction, nor any higher-order interactions involving these components reached significance (for all serial recall comparisons, F-C1.0; for free recall all ps > 0.12). Because an interaction between interpolated task and age appears visually in fig. 1, especially with memory sets of 4 and 5 letters, four further analyses were conducted. Univariate ANOVAs were performed in which age and interpolated task were grouping variables and delayed recall of 4 and 5 letters were dependent variables. In no case by either serial or free recall criteria did the age by task interaction reach significance (Fsc 1.0). This finding replicates that of Inman and Parkinson (1983) and again fails to support the hypothesis that age differences in Brown-Peterson recall result from a reduction in central processing capacity in the elderly. In his discussion of acquisition and retention, Underwood (1966) maintained that differences following a retention interval could be interpreted in terms of rate of forgetting only if acquisition levels were equivalent and potential differences in acquisition were not masked by
90
S. R. Parkinson
et al. / Exmnination
of age
differences
ceiling effects. Only in the conditions with 5 memory letters was the latter criterion met as obvious ceiling effects were found for the 3- and 4-letter conditions with immediate recall. In the immediate recall condition with 5 letters young adults averaged 94.29% and old adults 87.75% and this difference was significant, F(1, 56) = 5.38, p < 0.03, w2 = 0.069. Less than perfect performance in the immediate recall of short lists is often attributed to misperceptions. However, it should be remembered that participants in the present study pronounced each letter aloud during presentation and that only those trials in which all items were pronounced correctly were included in data analyses. Whereas these data are interesting, still unresolved are the nature of the mechanisms yielding age differences in immediate recall and whether there are age differences in rate of forgetting which are independent of registration differences. One aspect of the present procedure and that of Inman and Parkinson (1983) which might have led to poorer registration on the part of the elderly was the rate of presentation of to-be-remembered letters. In both studies memory letters were presented at the relatively fast rate of 2/set and as processing rate is slower in old adults than young adults (Birren 1974; Lindholm and Parkinson 1983) the presentation rate might have differentially affected registration in the elderly. That the rapid presentation rate, per se, was not responsible for age group differences in immediate recall is suggested by another study from our laboratory. In that study, thirty-six young (mean age = 22.9 years) and eighteen old (mean age = 69.5 years) adults received sets of five memory letters presented at the rate of 1 letter per 2 sec. The same interpolated tasks used in experiment 1 were examined and retention was tested following 2, 4, 7 and 13 digit pairs. While there was no immediate recall test in that study, retention was measured following 7 interpolated digit pairs, a data point in common with both Inman and Parkinson (1983) and experiment 1 of the present research. Whereas it is difficult to compare results across different experiments, participants in these experiments were drawn from the same subject populations and were tested with similar procedures and the same equipment. Included in fig. 2 are the recall data for five memory letters from all three experiments. As can be seen, the asymptotic levels achieved by participants of both age groups are quite consistent across experiments. Given the comparable recall values found following 7 interpolated
S.R. Parkinson et al. / Examinatton
of agedifferences
91
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INTERPOLATED
,
I
7
13
DIGIT PAIRS
Fig. 2. Percent correct recall of 5-letter memory sets for young and old adults as a function of interpolated task and retention interval. Data shown are from three experiments. Performances of old adults are indexed by circles and young adults by squares. Solid lines are from experiment 1, Lines shown as combinations of dots and dashes are from Inman and Parkinson (1983). Dashed lines are from the study described in the text with a presentation rate of 1 letter/2 sec.
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S. R. Parkinson et al. / Examination
of age differences
items in the three studies, it would appear that any age differences in registration in the previous experiments were not ameliorated by decreasing presentation rate. Another possible factor mediating age group differences in both immediate and delayed recall is storage capacity. In a series of studies we have found reliable differences between young and old adults in immediate memory span (Inman and Parkinson 1983; Parkinson et al. 1983). The span difference is on the order of one item (digit, word or letter) and the span distributions of the elderly are truncated at the high end. The letter spans of old and young adults who participated in experiment 1 are combined with span data from Inman and Parkinson and are displayed in fig. 3. Again, while it is difficult to compare results across experiments, participants were sampled from the same sources and identical procedures were used to assess span. The data were combined to increase the reliability of the distributions. The span difference between young and old adults has important consequences for delayed recall as Melton (1963) hypothesized that rate of forgetting varies as a function of the number of ‘chunks’ in store. Melton predicted a family of functions with gradual forgetting rates for sub-span lists and rapid decline for span-length sequences. Data generated in his laboratory were consistent with predictions. The relationship between number of items and forgetting rate has also been confirmed in a probe recall task (Carey and Okada 1973). As memory spans of old adults are shorter than those of young adults, old adults would be expected to exhibit more rapid forgetting by virtue of the fact that a given number of items would occupy a greater proportion of the available storage space among members of that group. In order to estimate the extent to which letter span can predict recall following retention intervals of interpolated activity, interpolated task, letter span, and age group were stepwise regressed on the arcsin transform of delayed serial recall probability, averaged over 4- and Netter memory sets. As can be seen in table 1, letter span provides considerable prediction of delayed recall performance, F(1, 57) = 35.94, p -e 0.0001. After the variance attributable to task and span is accounted for, age group remains a significant predictor, F(1, 56) = 8.29, p < 0.01, though its additional contribution to prediction is small. These findings are in accord with the assertion that individual differences in rates of forgetting are a function of the proportion of storage capacity required. As delayed recall is related to memory span and as span is negatively
S.R. Parkinson et al. / Examination
93
of age differences
YOUNG 40
N=76 x=5.9 Y a w
20-
E
lo-
4
30-
5
6
MEAN
I
7
LETTER
9
SPAN
OLD
&
5 3 20-
N=57 X=5.2
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4
6LETTER ;PAN
5
8
9
MEAN Fig. 3. Distributions of mean letter spans for old and young adults. Individual based on five estimates.
letter spans are each
correlated with age, comparisons such as those in experiment 1 are not optimal for examining age differences in rate of forgetting. A better test of rate differences could be achieved if old and young adults showed equivalent accuracy in immediate recall. In the next experiment we attempted to equate immediate recall performances of young and old adults to determine if age differences in delayed recall are independent of, or dependent on, differences in immediate recall. The method used to equate participants was to vary the number of stimulus repetitions. In previous studies of repetition effects in the Brown-Peterson task, repetition of to-be-remembered
Table 1 Stepwise regression
analysis
Variables
Correlations
for delayed
Proportion
Interpolated task Letter span Age group
serial recall of 4 and 5 letters.
corrt’ct
Interpolated task
- 0.608 0.478 - 0.410
0.027 0.000
Increase , in RL
Letter span
-0.419
Adjusted
.’ ,I i 0.0001;
R R’ R’
0.369’ 0.244 ” 0.050 h 0.814 “ 0.663 0.645
h p
material has been shown to enhance immediate recall and retard the rate of forgetting (Hellyer 1962). Melton (1963) hypothesized that the effect of repetition was to reduce the number of ‘chunks’ in the to-be-remembered unit. By varying the number of repetitions on an individual basis we hoped to equate age groups on immediate recall and observe the influence on delayed recall performance. Each participant was pre-tested prior to engaging in the Brown-Peterson task. The pre-test yielded an estimate of the number of times it was necessary to present a letter sequence in order for the participant to achieve a minimum of 83% accuracy in immediate recall. This number of presentations was then used in a Brown-Peterson task in which letters were presented for either immediate recall or recall following three interpolated digit pairs. Only the more difficult (sum) interpolated task instruction was used. Initially, we attempted to equate young and old adults with five-letter memory sets. Twelve old adults and 12 young adults were tested. However, as in experiment 1, young adults recalled with greater accuracy than old adults in immediate recall, F(1, 22) = 4.58, p < 0.05,and following a delay of three interpolated digit pairs, F(1, 22) = 5.38, p < 0.04. The failure to equate recall performances of old and young adults in immediate recall apparently resulted from the size of the memory set. Ten of 12 young adults attained criterion in pre-testing with only 1 presentation. and the other two young participants needed only 2 presentations. Thus, recall accuracy of young participants was
S.R. Parkinson et al. / Examination ofagedifferences
exceptionally high in the immediate experiment 2 we increased the length sequences to six letters.
95
condition (95%). Therefore in of the to-be-remembered letter
Experiment 2 Method Participants Thirty-six female adults participated in the second experiment. The decision to test only female adults stemmed from the consistent difficulties we have experienced in attempting to recruit elderly males for participation and from the lack of sex effects in Brown-Peterson performance found in the first experiment. The 26 young participants (mean age = 21 years; range 17 to 28) were undergraduates at Arizona State University and their participation served as partial fulfillment of introductory psychology course requirements. Ten old adults (mean age = 68 years; range 62 to 80) were recruited from the same source as in experiment 1. Procedure Testing was conducted individually and 22 participants (12 young adults and 10 old adults) were given the acquisition level test prior to the administration of the Brown-Peterson task. The remaining 14 young participants were given the standard Brown-Peterson task with only a single presentation of to-be-remembered sequences. The performance of this group served as a reference for assessing the effect of repetitions. Acquisition level test. Letters were presented sequentially in the same manner as in experiment 1, except that the rate of presentation was 1.8 letters/set with each letter on the display for 340 msec. The letter presentations were from sets of 6-letter lists; one set for each repetition level, 1 through 8. The lists were generated randomly from the alphabet, excluding the letters E and V’, with the following constraints: (a) no letter was repeated within a list, and (b) no letter was repeated in adjacent lists. The instructions were to read each letter aloud as it was presented and to recall the letters aloud when the word ‘RECALL’ appeared. A guess or the word ‘blank’ was to be substituted for forgotten letters. At least three practice trials preceded testing at each repetition level. Acquisition testing began with a single stimulus presentation and testing continued until the participant recalled ten or more letters incorrectly, or until ten trials had been completed. If the ten trials had been completed with fewer than ten errors (i.e., greater than 83 percent accuracy) testing proceeded to the Brown-Peterson task. If ten or more errors occurred, acquisition testing continued with each letter sequence presented twice with a 0.93 set pause between the offset of the last letter in the sequence and the repetition of the first letter. Acquisition testing continued in a similar manner with additional presentations, if necessary, until fewer than ten errors occurred at a repetition level. Testing then proceeded to the Brown-Peterson task. Trials on which reading errors occurred were discarded and replaced.
96
SERIAL RECALL
NUMBER OF INTERPOLATED Fig. 4. Percent correct of retention interval.
FREE RECALL
PAIRS
NUMBER OF INTERPOLATED
PAIRS
serial (left panel) and free (right panel) recall in experiment 2 as a function
Brow,n- Peterson
task. A minimum of 6 practice trials followed by 40 test trials was run with each participant receiving the number of repetitions used for the last set presented in acquisition testing. Recall was written and the procedure was identical to that for experiment 1 except as noted. The letters E and V were excluded from the stimulus set and only sequences of 6 letters were presented. if the letter sequences were repeated a 0.93-set inter-seqence interval was used. To-be-remembered letters were followed by either zero or three pairs of digits and only the difficult (sum) interpolated task instruction was used. As can be seen in fig. 2. stable age differences were obtained with three interpolated pairs in previous studies. Three pairs rather than seven were used in the present study to reduce session time. Participants were instructed to say each letter aloud as it was presented. Trials on which letter reading or interpolated task errors occurred were discarded and replaced.
Young adults required 2.25 repetitions (range 1 to 3) to reach acquisition criterion whereas old adults averaged 2.80 repetitions (range I to 6). and this difference was significant, t(20) = 2.32. p < 0.05. Results of the BrownPeterson task are displayed in fig. 4. Data shown in the left panel of the figure are based on serial recall scoring whereas those in the right panel are based on free recall scoring. Seriul recull
Statistical analyses were conducted on the arcsin transform of the proportion of letters recalled correctly. averaged over serial positions. Two a priori contrasts were
S. R. Parkinson et al. / Examination of age differences
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included in the ANOVA; recall of young adults with repetitions was contrasted with recall of old adults with repetitions and repetition groups together were contrasted with the no-repetition group. As can be seen, the equating procedure produced very similar performances in immediate recall for old (mean = 87.67%) and young (mean = 89.99%) adults and the difference remained very small after the delay. Neither the difference between age groups nor the age group by retention interval interaction was significant when only the repetition groups were considered (both Fs(l, 33) < 1.0). The contrast between the repetition groups and the no-repetition group indicated that repetitions yielded greater recall accuracy, F(1, 33) = 11.19. p < 0.003, wz = 0.077. and that the difference between groups with and without repetitions was greater following the delay, F(1, 33) = 6.46, p i 0.016, w* = 0.009. Immediate recall was. of course, superior to delayed recall F(1, 33) = 377.69, p < 0.0001, CJ*= 0.596. Free recall
The free recall analysis was conducted on the arcsin transform of the proportion of letters recalled correctly without regard to serial order. Again there was no significant difference between the age groups which received repetitions in either immediate recall, F(1, 33) = 1.28, p > 0.26, CJ*= 0.002, or delayed recall, F(1. 33) < 1.0. Those receiving repetitions recalled with greater accuracy than those who did not, F(1, 33) = 8.41, p < 0.007, w* = 0.004, and the difference between these groups was greater following the delay, F(1, 33) = 12.72, p < 0.002, w2 = 0.023.
Discussion Two main results were found in the present experiments. First, age comparisons in a Brown-Peterson task with immediate and delayed recall conditions revealed a main effect of age and an age by retention interval interaction. A subsequent test showed that young adults were significantly more accurate than old adults in the immediate recall of five-letter memory sets. Second, under conditions where immediate recall differences between young and old were minimized by repetition of to-be-remembered sequences, no significant effects of age were found in delayed recall. These results suggest that age deficits in Brown-Peterson recall are not the result of increased susceptibility to interference from distractor items or more rapid decay during the retention interval; rather, age deficits apparently stem from problems at input, either in encoding or storage or both. It is not possible to define a clear separation between the processes of encoding and storage. In experiment 1 and in the research reported by Inman and Parkinson, however, old participants committed more reading errors than young participants suggesting that the elderly
experienced greater difficulty encoding to-be-remembered letters. In that light it is also interesting that a reduction in presentation rate from 2 letters/set to 1 letter/2 set failed to alter the pattern of age differences. If the results of Inman and Parkinson and experiment 1 derived from the use of a presentation speed which prevented elderly participants from forming adequate representations, there should have been some convergence of performances of old and young adults when rate was reduced by a factor of four. These results indicate that the difficulties elderly individuals experience with registration are not eliminated by simply giving them more time. In previous research (e.g., Parkinson 1982; Parkinson et al. 1980; Parkinson et al. 1982) we pursued the working hypothesis that aging is associated with a reduction in storage capacity. We have viewed age differences in memory span as an index of the storage deficit. The present results fit nicely within this framework. By definition, the reduction in storage capacity hypothesis accounts for the difference in immediate recall, and when combined with Melton’s (1963) hypothesized relationship between storage and rate of forgetting. a reduction in storage capacity leads to expected differences in retention following a delay. Moreover, the metaphor of reduced storage capacity is consistent with the observation in the present research that age differences are invariant over large differences in presentation rate. In the present study and in Inman and Parkinson, manipulations of the processing capacity requirements of the interpolated task failed to yield interactions with age. Whereas interpolated task performance was monitored for compliance with instructions in both studies, accuracy of performance was not recorded. Failure of the two groups to achieve equal accuracy in the interpolated transformations could be a reason for the failure to obtain an interaction, although monitoring of a sample of the trials revealed *few errors by participants of either age group. Failure to obtain an interaction could also have stemmed from combinations of ceiling and floor effects so the null results do not provide a stringent test of processing capacity differences between old and young adults. The question of whether processing capacity or storage capacity decrements are responsible for age group differences in recall performance can be well framed by invoking the working memory model of Baddeley and Hitch (1974). That model includes a memory buffer of limited storage capacity and a central executive of limited processing
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capacity. The central executive is responsible for coordinating ongoing mental processes including control processes regulating the contents of the memory buffer. When memory load is low, few demands are placed on the central processor. When buffer capacity is exceeded, some central processing capacity may be allocated to storage of the buffer overflow. Up to the beginning of the retention interval task, age group differences in either buffer storage capacity or processing capacity could account for lower recall performance by the older adults in experiment 1, and the need for more stimulus repetitions by the older adults in experiment 2. If buffer storage capacity were reduced with age, then fewer items could be stored before heavy demands were placed on processing capacity. If processing capacity were reduced with age then approaching or exceeding buffer capacity would be more costly for older adults. The finding from experiment 1 and Inman and Parkinson that increasing the amount of information reduction required during the retention interval increased the rate of forgetting, suggests that processing demands during retention do affect rate of information loss. This assumes that our retention interval tasks required little storage capacity, and can be viewed primarily as demands for processing capacity. Within the working memory model the forgetting rate difference for the two tasks can be ascribed either to dismption of control processes which maintain information in the buffer, or to a decrement in the capacity of the central executive to handle buffer overflow. If reducing the amount of processing capacity available increases the rate of forgetting then reductions in processing capacity as a result of aging should also produce an increase in the rate of forgetting. This increase was not observed in experiment 1, and no differences in rate of forgetting, were observed when age groups were equated for initial levels of retention in experiment 2. Thus processing demands during the retention interval do not appear critical to age differences in Brown-Peterson recall performance. It is possible that the effect of stimulus repetition is to reduce the demand on processing capacity during retention through the formation of more easily maintained traces, and thus age differences in processing may still explain the observed age differences in performance. It is also possible that the effect of repetition is to decrease, perhaps through chunking, the demands on limited buffer storage capacity and that the observed age differences were the result of storage capacity differences.
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of age differences
In either case our results suggest that the question of whether storage capacity, processing capacity, or both decline with age will best be addressed by closer examination of memory item registration rather than retention. In summary, the results of the present research indicated that differences in rate of forgetting between old and young adults were dependent on age differences in immediate recall. These results suggest that age comparisons in encoding and storage offer a promising direction for delineating information processing mechanisms which decline with age.
References Adams. J.A., H.I. Thorsheim and J.S. McIntyre, 1969. Item length, acoustic similarity, and natural language mediation as variables in short-term memory. Journal of Experimental Psychology 80. 39-46. Baddeley, A.D. and C.J. Hitch, 1974. ‘Working memory’. In: G.H. Bower (ed.), The psychology of learning and motivation: advances in research and theory, Vol. 8. New York: Academic Press. Birren. J.E., 1974. Psychophysiology and speed of response. American Psychologist 29, X08-815. Botwinick, J. and M. Storandt, 1974. Memory, related functions, and age. Springfield, IL: Charles C. Thomas. Brown, J.. 1958. Some tests of the decay theory of immediate memory. Quarterly Journal of Experimental Psychology 10, 12-21. Carey, ST. and R. Okada, 1973. Part-list recall following part-whole learning. Memory and Cognition 1, 172-176. Craik. F.I.M., 1968. Two components in free recall. Journal of Verbal Learning and Verbal Behavior 7, 996-1004. Craik, F.I.M., 1977. ‘Age differences in human memory’. In: J.E. Birren and K.W. Schaie (eds.). Handbook of the psychology of aging. New York: Van Nostrand Reinhold Company. Craik. F.I.M. and E. Simon, 1980. ‘Age differences in memory: the roles of attention and depth of processing’. In: L.W. Poon, J.L. Fozard. L.S. Cermak, D. Arenberg and L.W. Thompson (eds.), New directions in memory and aging. Hillsdale. NJ: Lawrence Erlbaum. Dillon, R.F. and L.S. Reid, 1969. Short;term memory as a function of information processing during the retention interval. Journal of Experimental Psychology 81, 261-269. Eysenck. M.W.. 1974. Age differences in incidental learning. Developmental Psychology 10. 936-941. Fraser, D.C.. 1958. Decay of immediate memory with age. Nature 182. 1163. Hellyer, S.. 1962. Frequency of stimulus presentation and short-term decrement in recall. Journal of Experimental Psychology 64, 650. Hulicka, 1.. 1966. Age differences in Wechsler memory scale scores. Journal of Genetic Psychology 109, 135-145. Hulicka, I.M. and J.L. Grossman, 1967. Age group comparisons for the use of mediators in paired-associate learning. Journal of Gerontology 22, 46-51. Inglis. J. and W.K. Caird, 1963. Age differences in successive responses to simultaneous stimulation. Canadian Journal of Psychology 17, 98-105.
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Inman, V.W. and S.R. Parkinson. 1983. Differences in Brown-Peterson recall as a function of age and retention interval. Journal of Gerontology 38, 58-64. Kausler, D.H. and J.M. Puckett, 1979. Effects of word frequency on adult age differences in word memory span. Experimental Aging Research 5, 161-169. Lindholm, J.M. and S.R. Parkinson, 1983. An interpretation of age-related differences in letter matching performance. Perception & Psychophysics 33, 283-294. Melton, A.W., 1963. Implications of short-term memory for a general theory of memory. Journal of Verbal Learning and Verbal Behavior 2. 1-21. Murdock, B.B., Jr., 1974. Human memory: theory and data. New York: John Wiley and Sons. Parkinson. S.R., 1980. Aging and amnesia: a running span analysis. Bulletin of the Psychonomic Society 15, 215-217. Parkinson, S.R., 1982. ‘Performance deficits in short-term memory tasks: a comparison of amnesic Korsakoff patients and the aged’. In: L.S. Cermak (ed.), Human memory and amnesia. Hillsdale, NJ: Lawrence Erlbaum. Parkinson. S.R., J.M. Lindholm and V.W. Inman, 1982. An analysis of age differences in immediate recall. Journal of Gerontology 37, 425-431. Parkinson, S.R., J.M. Lindholm and T. Urell, 1980. Aging, dichotic memory and digit span. Journal of Gerontology 35, 87-95. Peterson, L.R. and M.J. Peterson, 1959. Short-term retention of individual verbal items. Journal of Experimental Psychology 58, 193-198. Posner, MI. and E. Rossman, 1965. Effect of size and location of informational transforms upon short-term retention. Journal of Experimental Psychology 70. 496-505. Smith, A.D., 1980. ‘Age differences in encoding, storage and retrieval’. In: L.W. Poon, J.L. Fozard, L.S. Cermak, D. Arenberg and L.W. Thompson (eds.), New directions in memory and aging. Hillsdale, NJ: Lawrence Erlbaum. Talland, G.A., 1968. ‘Age and the span of immediate recall’. In: G.A. Talland (ed.), Human aging and behavior. New York: Academic Press. Underwood, B.J., 1966. Experimental psychology (2nd ed.). New York: Appleton-Century-Crofts. Wechsler, D.. 1945. A standardized memory scale for clinical use. Journal of Psychology 19, x7-95. Welford, A.T., 1958. Aging and human skill. London: Oxford University Press. Wright, R.E., 1981. Aging, divided attention, and processing capacity. Journal of Gerontology 36. 6055614.
x3
60 (1985) 83-101
ADULT AGE DIFFERENCES
IN SHORT-TERM
Stanley R. PARKINSON, Vaughan Stephen E. DANNENBAUM * Arizona Stare Uniuersitv, Accepted
January
W. INMAN
FORGETTING
and
USA
1985
Two experiments are reported in which young and old adults performed in a Brown-Peterson task. In the first experiment young adults recalled with greater accuracy than old adults and the difference between age groups was greater in delayed than in immediate recall. Performance varied inversely with interpolated task difficulty in the delayed recall condition, but this effect did not interact with age. In the second experiment an attempt was made to equate immediate recall performances of old and young adults to determine if age differences in the rate of forgetting are independent of age differences in registration. Each participant was pre-tested to determine the number of stimulus repetitions needed to achieve a minimum of 83% correct in immediate serial recall of 6-letter sequences. The number of repetitions an individual required in pre-testing was then used in a subsequent Brown-Peterson task. No significant age differences in delayed recall were obtained when immediate recall differences were minimized by differential repetition of to-be-remembered sequences, The results of these experiments suggest that age differences in forgetting rates arise from age-related differences in encoding and storage.
Introduction Comparisons between young and old adults have revealed age-related differences in a variety of short-term memory tasks including memory span (Botwinick and Storandt 1974; Kausler and Puckett 1979; Parkinson et al. 1982) running span (Parkinson 1980; Talland 1968) free recall (Craik 1968; Parkinson et al. 1982) and dichotic memory (Inglis and Caird 1963; Parkinson et al. 1980). In addition, age-related differences in retention have been found in dual task research where one of the tasks measures forgetting over brief intervals (Wright 1981). Whereas the results of many studies have been consistent in demonstrating age differences, little consensus has been achieved regarding the * Mailing address: 85287. USA.
OOOl-6918/85/$3.30
S.R. Parkinson.
Dept.
of Psychology.
0 1985. Elsevier Science Publishers
Arizona
State
University,
B.V. (North-Holland)
Tempe.
AZ
84
S. R. Parkrnson et al. / Edxamination of uge differences
processing and (or) storage mechanisms which decline with age. Some researchers have stressed the importance of age differences in registration, hypothesizin g age-related deficits in encoding (Craik and Simon 1980; Eysenck 1974; Smith 1980) and storage (Craik 1977; Parkinson et al. 1983). Others have emphasized differential forgetting with age differences resulting from increased susceptibility to interference (Welford 1958) or decay (Fraser 1958). The Brown-Peterson (Brown 1958; Peterson and Peterson 1959) distractor procedure has been used extensively in studies of encoding, storage and short-term forgetting with young adults and would seem to provide a useful framework for delineating factors underlying age differences. In the Brown-Peterson procedure a distractor task (e.g., counting backwards by 3s) is interpolated between presentation and recall of to-be-remembered items. Forgetting functions are obtained by varying the length of the retention interval in which the distractor task is performed. These functions are generally characterized by a rapid decline in recall which reaches asymptote within 10 set (e.g., Dillon and Reid 1969). In a recent experiment conducted in our laboratory (Inman and Parkinson 1983) substantial age-related differences in BrownPeterson recall were obtained. In that study 46 young (mean age = 20.5 years) and 27 old (mean age = 72.4 years) adults were shown memory sets consisting of 3, 4 or 5 letters presented at a rate of 2 letters/set. Memory sets were followed by the presentation of pairs of digits (1, 3, 5 or 7) and participants were instructed either to read the digits as they appeared (easy distractor task), or to report the sum of each digit pair and whether the sum was odd or even (difficult distractor task). Pacing the distractor tasks ensured that participants of both age groups processed the same amount of information between presentation and recall of memory letters. The results showed that old adults recalled with lower accuracy.than young adults (both free and serial recall scoring) and that the magnitude of the difference between age groups was positively related to retention interval. Whereas the age by retention interval interaction in the Inman and Parkinson study suggested more rapid information loss by the elderly, that interpretation was precluded by the finding that there were age differences after only one interpolated digit pair, particularly with larger memory sets under the more difficult interpolated condition. Thus, rate of forgetting differences could not be separated from potential differences in registration. An attempt was made in the present
S.R. Parkinson et al. / Examination
of age differences
85
research to distinguish between age differences in registration and rate of forgetting in the Brown-Peterson task. Accordingly, in the first experiment, groups of old and young adults received memory sets followed by either 0 or 7 interpolated digit pairs (immediate and delayed recall, respectively). If recall of old and young adults were equivalent at immediate recall but different after the delay, the age by retention interval interaction would indicate an age-related difference in rate of forgetting which could be attributed to increased susceptibility to interference (Welford 1958) or decay (Fraser 1958). Conversely, if recall of old and young adults were different in both conditions, rate of forgetting differences could not be distinguished from registration differences. During recent years there has been an increasing tendency to attribute lower levels of recall performance by the elderly to a deficit in central processing capacity (e.g. Craik and Simon 1980; Wright 1981). Participants of both age groups in the Inman and Parkinson study recalled with greater accuracy following an easy than difficult interpolated task thus replicating a previous finding (Dillon and Reid 1969) and supporting the hypothesis that central processing demands vary as a function of the amount of information reduction required (Posner and Rossman 1965). However, in contrast to the hypothesis that aging is associated with a reduction in central processing capacity, the magnitude of the difference between age groups did not vary significantly with interpolated task. A second purpose of the first experiment was to provide another test of age differences in central processing capacity as a factor in BrownPeterson task performance. Therefore, distractor task difficulty was manipulated as in the Inman and Parkinson study.
Experiment 1 Method
Participants Thirty young adults (17 males, 13 females; mean age = 19.2 years; range 17-23) and 30 old adults (9 males, 21 females; mean age = 74.2 years; range 67-84) served as participants in the first experiment. The young adults were undergraduates at Arizona State University. Participation of the undergraduates served as partial fulfillment of introductory psychology course requirements. The old group consisted of a number of
86
S. R. Parkinson et 01. / Examnatmn
of age differences
highly educated individuals: The group possessed an average of 16.8 years of formal education and included former lawyers, engineers, academicians and school teachers. Elderly participants were recruited from a local retirement community offering full life care and all reported themselves to be in good health at time of testing. The mean Wechsler Memory Scale (WMS) raw score of the old group was 58.9 (SD = 7.6, range 43.5 to 70) which is approximately equal to the 50th percentile norm of Wechsler’s 40 to 44 year old group (Wechsler 1945). The mean and standard deviation obtained by our old group is also approximately equal to that obtained by Hulicka’s (1966) 40 to 49 year age group (mean = 58.8, SD = 7.1). Our group was significantly superior to Hulicka’s 70 to 79 year age group. t(74) = 4.40, the former having a mean raw score 8 units higher than the latter. In previous studies with groups of old and young adults drawn from the same sources we have found significant differences between age groups in vocabulary (favoring the elderly) and memory span (favoring the young adults). Memory spans of the present groups were consistent with the pattern evident in our previous work. Immediate memory spans of letters were assessed for all participants. Letters were presented visually at the rate of 1.2 letters/set and participants were instructed to recall the letters in the exact order in which they were presented. Five span estimates were obtained for each participant and an individual’s span was determined to be the mean of the five estimates (Inman and Parkinson 1983). The mean letter span of the young adults (mean = 5.86; SD = 0.77) was significantly greater than that of the old adults (mean = 5.17; SD = 0.74) F(1, 58) = 12.34. p < 0.001. 0’ = 0.159. Procedure
All participants were tested individually. Each participant received 60 trials in the Brown-Peterson task. 20 each with memory sets of 3, 4 and 5 letters. Adams et al. (1969) found that sequential presentation of memory letters in a Brown-Peterson task resulted in the formation of fewer natural language mediators than simultaneous presentation. As mediators benefit recall and as older adults generate fewer mediators spontaneously (Hulicka and Grossman 1967), sequential presentation was used in the present study to minimize encoding bias against the elderly. Memory letters were presented on a video terminal controlled by a TRS-80 microcomputer. Memory letters were 0.7 cm in height and were presented sequentially in the same location in the center of the display at a rate of 2 letters/set. Each letter was displayed for 220 msec. A new series of letters was presented on each trial. Letters were selected at random from the alphabet, excluding the letter V, with the restriction that no letter be repeated within a trial or on immediately adjacent trials. Participants were requested to pronounce each letter aloud during presentation. As the number of letters presented was a factor in the present study, the use of sequential presentation necessitated a decision between presenting lists for the same amount of time, regardless of length, or covarying presentation time with list length. In the former case the length of time available to form associations between letters is confounded with list length, whereas in the latter, the time between the onset of the first item and recall is confounded with list length. It was assumed that recall of lists of all lengths would be nearly perfect at the beginning of the retention interval and that the retention interval was therefore more critical than the duration of the stimulus
S.R. Parkinson et al. / Examination of age differences
87
presentation in this procedure. Thus, presentation time of each letter was held constant for all list lengths. Memory letters were followed by either 0 or 7 pairs of digits. Digit pairs were presented in the center of the display at a rate of 1 pair/2 sec. On immediate recall trials the word ‘RECALL’ followed the offset of the last memory letter by 750 msec and remained on the screen for 8 sec. In delayed recall, the recall signal followed the last digit pair by 750 msec. Trials were blocked on the basis of memory set. Within a block, 10 trials were immediate recall and 10 tested recall following 7 interpolated pairs. The order of 0 and 7 digit pair trials was randomized within a block. Twelve seconds were allowed for recall. As in the Inman and Parkinson study two interpolated tasks were used. These tasks differed in the amount of information reduction required (Posner and Rossman 1965). Participants in the difficult distractor condition (15 young adults and 15 old adults) were instructed to report aloud the sum of the digit pairs and whether it was odd or even. The instructions for the easy distractor condition (15 young adults and 15 old adults) were to read the digits aloud as they appeared. All participants were instructed that when the ‘RECALL’ message appeared they were to write the letters they had read in left to right order in the boxes of a prepared response sheet. Participants were instructed further to guess or to place dashes in boxes if they could not recall letters. Results Only trials in which letters had been read aloud correctly, and in which interpolated task instructions were followed, were included in data analyses. Errors in reading the to-be-remembered letters resulted in the exclusion of 7.4 and 1.6 percent of trials of old and young participants, respectively. Failure to complete interpolated digit pair reading or transformation resulted in the exclusion of an additional 3.7 and 1.3 percent of trials to old and young participants, respectively. Whereas participants were instructed to recall letters in the exact order in which they were presented, scoring was done with both serial and free recall criteria. Analyses on serial and free recall are presented separately. Serial recall analysis
Because of the unequal distribution of the sexes between the two age groups a preliminary analysis of variance was conducted on the arcsin transform of the proportion of letters recalled correctly averaged over serial positions. Sex, age group, and interpolated task were between-group variables in this analysis and retention interval and memory set (3, 4 or 5 letters) were within-group variables. The main effect of sex was not significant, F(1, 52) = 1.21, p > 0.27, w2 < 0.001, and no interaction between sex and other factors was found (all ps > 0.24). Therefore sex was not included in the remaining analyses reported here. In fig. 1 the proportion of letters recalled correctly, averaged over serial positions, is displayed as a function of memory set, age, retention interval, and interpolated task. An analysis of variance performed on the arcsin transform of the proportion correct revealed significant main effects for all factors: old participants recalled with lower accuracy than young participants, F(1, 56) = 19.64, p < 0.0001, w2 = 0.042; more
88
S. R. Parkinson loo-
q
of age differences
Ed (I/. / Exnminrrtion
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Fig. 1. Percent correct recall as a function of number of memory letters, retention interval and interpolated task. Solid lines describe performances of young adults and dashed lines describe performances of old adults.
letters were recalled correctly when participants were required to read than sum digit pairs, F(1, 56) = 24.27, p < 0.0001, u2 = 0.053; and recall varied inversely with both number of memory letters, F(2, 112) = 194.019, p < 0.0001, CJ*= 0.177, and retention interval, F(1, 56) = 287.90, p < 0.0001, w* = 0.327.
S. R. Parkinson et al. / Examination of age differences
89
Three second-order interactions were significant: retention interval interacted with interpolated task, F(1, 56) = 46.95, p < 0.0001, w2 = 0.052, number of memory letters. F(2, 112) = 16.57, p < 0.0001, a2 = 0.015, a n d age, F(1, 56) = 11.92, p < 0.002, w2 = 0.012. In each case ‘he difference in accuracy between levels of a factor was greater following an interpolated task than with immediate recall. Free recall analysis A very similar pattern of results was obtained when recall was scored according to a free recall criterion. Old adults recalled with lower accuracy than young adults, F(1, 56) = 23.22, p i 0.0001, wz = 0.050; r ea d’mg digit pairs resulted in greater accuracy than summing digit pairs, F(1, 56) = 29.12, p < 0.0001, w2 = 0.063, and recall varied inversely with both number of memory letters, F(2, 112) = 106.78, p < 0.0001, w2 = 0.097, and retention interval, F(1, 56) = 292.87, p < 0.0001, w2 = 0.365. As with serial recall, all two-way interactions with retention interval were significant (for all of which p < 0.0001).
Discussion The two major functions of the first experiment were to (a) provide information regarding immediate recall in an attempt to distinguish between registration and rate of forgetting differences, and (b) conduct another test of age differences in tasks thought to require different amounts of processing capacity. Concerning the latter, although the effect of interpolated task was significant, neither the first-order age by interpolated task interaction, nor any higher-order interactions involving these components reached significance (for all serial recall comparisons, F-C1.0; for free recall all ps > 0.12). Because an interaction between interpolated task and age appears visually in fig. 1, especially with memory sets of 4 and 5 letters, four further analyses were conducted. Univariate ANOVAs were performed in which age and interpolated task were grouping variables and delayed recall of 4 and 5 letters were dependent variables. In no case by either serial or free recall criteria did the age by task interaction reach significance (Fsc 1.0). This finding replicates that of Inman and Parkinson (1983) and again fails to support the hypothesis that age differences in Brown-Peterson recall result from a reduction in central processing capacity in the elderly. In his discussion of acquisition and retention, Underwood (1966) maintained that differences following a retention interval could be interpreted in terms of rate of forgetting only if acquisition levels were equivalent and potential differences in acquisition were not masked by
90
S. R. Parkinson
et al. / Exmnination
of age
differences
ceiling effects. Only in the conditions with 5 memory letters was the latter criterion met as obvious ceiling effects were found for the 3- and 4-letter conditions with immediate recall. In the immediate recall condition with 5 letters young adults averaged 94.29% and old adults 87.75% and this difference was significant, F(1, 56) = 5.38, p < 0.03, w2 = 0.069. Less than perfect performance in the immediate recall of short lists is often attributed to misperceptions. However, it should be remembered that participants in the present study pronounced each letter aloud during presentation and that only those trials in which all items were pronounced correctly were included in data analyses. Whereas these data are interesting, still unresolved are the nature of the mechanisms yielding age differences in immediate recall and whether there are age differences in rate of forgetting which are independent of registration differences. One aspect of the present procedure and that of Inman and Parkinson (1983) which might have led to poorer registration on the part of the elderly was the rate of presentation of to-be-remembered letters. In both studies memory letters were presented at the relatively fast rate of 2/set and as processing rate is slower in old adults than young adults (Birren 1974; Lindholm and Parkinson 1983) the presentation rate might have differentially affected registration in the elderly. That the rapid presentation rate, per se, was not responsible for age group differences in immediate recall is suggested by another study from our laboratory. In that study, thirty-six young (mean age = 22.9 years) and eighteen old (mean age = 69.5 years) adults received sets of five memory letters presented at the rate of 1 letter per 2 sec. The same interpolated tasks used in experiment 1 were examined and retention was tested following 2, 4, 7 and 13 digit pairs. While there was no immediate recall test in that study, retention was measured following 7 interpolated digit pairs, a data point in common with both Inman and Parkinson (1983) and experiment 1 of the present research. Whereas it is difficult to compare results across different experiments, participants in these experiments were drawn from the same subject populations and were tested with similar procedures and the same equipment. Included in fig. 2 are the recall data for five memory letters from all three experiments. As can be seen, the asymptotic levels achieved by participants of both age groups are quite consistent across experiments. Given the comparable recall values found following 7 interpolated
S.R. Parkinson et al. / Examinatton
of agedifferences
91
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13
7
SUM
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- --O
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INTERPOLATED
,
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DIGIT PAIRS
Fig. 2. Percent correct recall of 5-letter memory sets for young and old adults as a function of interpolated task and retention interval. Data shown are from three experiments. Performances of old adults are indexed by circles and young adults by squares. Solid lines are from experiment 1, Lines shown as combinations of dots and dashes are from Inman and Parkinson (1983). Dashed lines are from the study described in the text with a presentation rate of 1 letter/2 sec.
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S. R. Parkinson et al. / Examination
of age differences
items in the three studies, it would appear that any age differences in registration in the previous experiments were not ameliorated by decreasing presentation rate. Another possible factor mediating age group differences in both immediate and delayed recall is storage capacity. In a series of studies we have found reliable differences between young and old adults in immediate memory span (Inman and Parkinson 1983; Parkinson et al. 1983). The span difference is on the order of one item (digit, word or letter) and the span distributions of the elderly are truncated at the high end. The letter spans of old and young adults who participated in experiment 1 are combined with span data from Inman and Parkinson and are displayed in fig. 3. Again, while it is difficult to compare results across experiments, participants were sampled from the same sources and identical procedures were used to assess span. The data were combined to increase the reliability of the distributions. The span difference between young and old adults has important consequences for delayed recall as Melton (1963) hypothesized that rate of forgetting varies as a function of the number of ‘chunks’ in store. Melton predicted a family of functions with gradual forgetting rates for sub-span lists and rapid decline for span-length sequences. Data generated in his laboratory were consistent with predictions. The relationship between number of items and forgetting rate has also been confirmed in a probe recall task (Carey and Okada 1973). As memory spans of old adults are shorter than those of young adults, old adults would be expected to exhibit more rapid forgetting by virtue of the fact that a given number of items would occupy a greater proportion of the available storage space among members of that group. In order to estimate the extent to which letter span can predict recall following retention intervals of interpolated activity, interpolated task, letter span, and age group were stepwise regressed on the arcsin transform of delayed serial recall probability, averaged over 4- and Netter memory sets. As can be seen in table 1, letter span provides considerable prediction of delayed recall performance, F(1, 57) = 35.94, p -e 0.0001. After the variance attributable to task and span is accounted for, age group remains a significant predictor, F(1, 56) = 8.29, p < 0.01, though its additional contribution to prediction is small. These findings are in accord with the assertion that individual differences in rates of forgetting are a function of the proportion of storage capacity required. As delayed recall is related to memory span and as span is negatively
S.R. Parkinson et al. / Examination
93
of age differences
YOUNG 40
N=76 x=5.9 Y a w
20-
E
lo-
4
30-
5
6
MEAN
I
7
LETTER
9
SPAN
OLD
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5
8
9
MEAN Fig. 3. Distributions of mean letter spans for old and young adults. Individual based on five estimates.
letter spans are each
correlated with age, comparisons such as those in experiment 1 are not optimal for examining age differences in rate of forgetting. A better test of rate differences could be achieved if old and young adults showed equivalent accuracy in immediate recall. In the next experiment we attempted to equate immediate recall performances of young and old adults to determine if age differences in delayed recall are independent of, or dependent on, differences in immediate recall. The method used to equate participants was to vary the number of stimulus repetitions. In previous studies of repetition effects in the Brown-Peterson task, repetition of to-be-remembered
Table 1 Stepwise regression
analysis
Variables
Correlations
for delayed
Proportion
Interpolated task Letter span Age group
serial recall of 4 and 5 letters.
corrt’ct
Interpolated task
- 0.608 0.478 - 0.410
0.027 0.000
Increase , in RL
Letter span
-0.419
Adjusted
.’ ,I i 0.0001;
R R’ R’
0.369’ 0.244 ” 0.050 h 0.814 “ 0.663 0.645
h p
material has been shown to enhance immediate recall and retard the rate of forgetting (Hellyer 1962). Melton (1963) hypothesized that the effect of repetition was to reduce the number of ‘chunks’ in the to-be-remembered unit. By varying the number of repetitions on an individual basis we hoped to equate age groups on immediate recall and observe the influence on delayed recall performance. Each participant was pre-tested prior to engaging in the Brown-Peterson task. The pre-test yielded an estimate of the number of times it was necessary to present a letter sequence in order for the participant to achieve a minimum of 83% accuracy in immediate recall. This number of presentations was then used in a Brown-Peterson task in which letters were presented for either immediate recall or recall following three interpolated digit pairs. Only the more difficult (sum) interpolated task instruction was used. Initially, we attempted to equate young and old adults with five-letter memory sets. Twelve old adults and 12 young adults were tested. However, as in experiment 1, young adults recalled with greater accuracy than old adults in immediate recall, F(1, 22) = 4.58, p < 0.05,and following a delay of three interpolated digit pairs, F(1, 22) = 5.38, p < 0.04. The failure to equate recall performances of old and young adults in immediate recall apparently resulted from the size of the memory set. Ten of 12 young adults attained criterion in pre-testing with only 1 presentation. and the other two young participants needed only 2 presentations. Thus, recall accuracy of young participants was
S.R. Parkinson et al. / Examination ofagedifferences
exceptionally high in the immediate experiment 2 we increased the length sequences to six letters.
95
condition (95%). Therefore in of the to-be-remembered letter
Experiment 2 Method Participants Thirty-six female adults participated in the second experiment. The decision to test only female adults stemmed from the consistent difficulties we have experienced in attempting to recruit elderly males for participation and from the lack of sex effects in Brown-Peterson performance found in the first experiment. The 26 young participants (mean age = 21 years; range 17 to 28) were undergraduates at Arizona State University and their participation served as partial fulfillment of introductory psychology course requirements. Ten old adults (mean age = 68 years; range 62 to 80) were recruited from the same source as in experiment 1. Procedure Testing was conducted individually and 22 participants (12 young adults and 10 old adults) were given the acquisition level test prior to the administration of the Brown-Peterson task. The remaining 14 young participants were given the standard Brown-Peterson task with only a single presentation of to-be-remembered sequences. The performance of this group served as a reference for assessing the effect of repetitions. Acquisition level test. Letters were presented sequentially in the same manner as in experiment 1, except that the rate of presentation was 1.8 letters/set with each letter on the display for 340 msec. The letter presentations were from sets of 6-letter lists; one set for each repetition level, 1 through 8. The lists were generated randomly from the alphabet, excluding the letters E and V’, with the following constraints: (a) no letter was repeated within a list, and (b) no letter was repeated in adjacent lists. The instructions were to read each letter aloud as it was presented and to recall the letters aloud when the word ‘RECALL’ appeared. A guess or the word ‘blank’ was to be substituted for forgotten letters. At least three practice trials preceded testing at each repetition level. Acquisition testing began with a single stimulus presentation and testing continued until the participant recalled ten or more letters incorrectly, or until ten trials had been completed. If the ten trials had been completed with fewer than ten errors (i.e., greater than 83 percent accuracy) testing proceeded to the Brown-Peterson task. If ten or more errors occurred, acquisition testing continued with each letter sequence presented twice with a 0.93 set pause between the offset of the last letter in the sequence and the repetition of the first letter. Acquisition testing continued in a similar manner with additional presentations, if necessary, until fewer than ten errors occurred at a repetition level. Testing then proceeded to the Brown-Peterson task. Trials on which reading errors occurred were discarded and replaced.
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SERIAL RECALL
NUMBER OF INTERPOLATED Fig. 4. Percent correct of retention interval.
FREE RECALL
PAIRS
NUMBER OF INTERPOLATED
PAIRS
serial (left panel) and free (right panel) recall in experiment 2 as a function
Brow,n- Peterson
task. A minimum of 6 practice trials followed by 40 test trials was run with each participant receiving the number of repetitions used for the last set presented in acquisition testing. Recall was written and the procedure was identical to that for experiment 1 except as noted. The letters E and V were excluded from the stimulus set and only sequences of 6 letters were presented. if the letter sequences were repeated a 0.93-set inter-seqence interval was used. To-be-remembered letters were followed by either zero or three pairs of digits and only the difficult (sum) interpolated task instruction was used. As can be seen in fig. 2. stable age differences were obtained with three interpolated pairs in previous studies. Three pairs rather than seven were used in the present study to reduce session time. Participants were instructed to say each letter aloud as it was presented. Trials on which letter reading or interpolated task errors occurred were discarded and replaced.
Young adults required 2.25 repetitions (range 1 to 3) to reach acquisition criterion whereas old adults averaged 2.80 repetitions (range I to 6). and this difference was significant, t(20) = 2.32. p < 0.05. Results of the BrownPeterson task are displayed in fig. 4. Data shown in the left panel of the figure are based on serial recall scoring whereas those in the right panel are based on free recall scoring. Seriul recull
Statistical analyses were conducted on the arcsin transform of the proportion of letters recalled correctly. averaged over serial positions. Two a priori contrasts were
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included in the ANOVA; recall of young adults with repetitions was contrasted with recall of old adults with repetitions and repetition groups together were contrasted with the no-repetition group. As can be seen, the equating procedure produced very similar performances in immediate recall for old (mean = 87.67%) and young (mean = 89.99%) adults and the difference remained very small after the delay. Neither the difference between age groups nor the age group by retention interval interaction was significant when only the repetition groups were considered (both Fs(l, 33) < 1.0). The contrast between the repetition groups and the no-repetition group indicated that repetitions yielded greater recall accuracy, F(1, 33) = 11.19. p < 0.003, wz = 0.077. and that the difference between groups with and without repetitions was greater following the delay, F(1, 33) = 6.46, p i 0.016, w* = 0.009. Immediate recall was. of course, superior to delayed recall F(1, 33) = 377.69, p < 0.0001, CJ*= 0.596. Free recall
The free recall analysis was conducted on the arcsin transform of the proportion of letters recalled correctly without regard to serial order. Again there was no significant difference between the age groups which received repetitions in either immediate recall, F(1, 33) = 1.28, p > 0.26, CJ*= 0.002, or delayed recall, F(1. 33) < 1.0. Those receiving repetitions recalled with greater accuracy than those who did not, F(1, 33) = 8.41, p < 0.007, w* = 0.004, and the difference between these groups was greater following the delay, F(1, 33) = 12.72, p < 0.002, w2 = 0.023.
Discussion Two main results were found in the present experiments. First, age comparisons in a Brown-Peterson task with immediate and delayed recall conditions revealed a main effect of age and an age by retention interval interaction. A subsequent test showed that young adults were significantly more accurate than old adults in the immediate recall of five-letter memory sets. Second, under conditions where immediate recall differences between young and old were minimized by repetition of to-be-remembered sequences, no significant effects of age were found in delayed recall. These results suggest that age deficits in Brown-Peterson recall are not the result of increased susceptibility to interference from distractor items or more rapid decay during the retention interval; rather, age deficits apparently stem from problems at input, either in encoding or storage or both. It is not possible to define a clear separation between the processes of encoding and storage. In experiment 1 and in the research reported by Inman and Parkinson, however, old participants committed more reading errors than young participants suggesting that the elderly
experienced greater difficulty encoding to-be-remembered letters. In that light it is also interesting that a reduction in presentation rate from 2 letters/set to 1 letter/2 set failed to alter the pattern of age differences. If the results of Inman and Parkinson and experiment 1 derived from the use of a presentation speed which prevented elderly participants from forming adequate representations, there should have been some convergence of performances of old and young adults when rate was reduced by a factor of four. These results indicate that the difficulties elderly individuals experience with registration are not eliminated by simply giving them more time. In previous research (e.g., Parkinson 1982; Parkinson et al. 1980; Parkinson et al. 1982) we pursued the working hypothesis that aging is associated with a reduction in storage capacity. We have viewed age differences in memory span as an index of the storage deficit. The present results fit nicely within this framework. By definition, the reduction in storage capacity hypothesis accounts for the difference in immediate recall, and when combined with Melton’s (1963) hypothesized relationship between storage and rate of forgetting. a reduction in storage capacity leads to expected differences in retention following a delay. Moreover, the metaphor of reduced storage capacity is consistent with the observation in the present research that age differences are invariant over large differences in presentation rate. In the present study and in Inman and Parkinson, manipulations of the processing capacity requirements of the interpolated task failed to yield interactions with age. Whereas interpolated task performance was monitored for compliance with instructions in both studies, accuracy of performance was not recorded. Failure of the two groups to achieve equal accuracy in the interpolated transformations could be a reason for the failure to obtain an interaction, although monitoring of a sample of the trials revealed *few errors by participants of either age group. Failure to obtain an interaction could also have stemmed from combinations of ceiling and floor effects so the null results do not provide a stringent test of processing capacity differences between old and young adults. The question of whether processing capacity or storage capacity decrements are responsible for age group differences in recall performance can be well framed by invoking the working memory model of Baddeley and Hitch (1974). That model includes a memory buffer of limited storage capacity and a central executive of limited processing
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capacity. The central executive is responsible for coordinating ongoing mental processes including control processes regulating the contents of the memory buffer. When memory load is low, few demands are placed on the central processor. When buffer capacity is exceeded, some central processing capacity may be allocated to storage of the buffer overflow. Up to the beginning of the retention interval task, age group differences in either buffer storage capacity or processing capacity could account for lower recall performance by the older adults in experiment 1, and the need for more stimulus repetitions by the older adults in experiment 2. If buffer storage capacity were reduced with age, then fewer items could be stored before heavy demands were placed on processing capacity. If processing capacity were reduced with age then approaching or exceeding buffer capacity would be more costly for older adults. The finding from experiment 1 and Inman and Parkinson that increasing the amount of information reduction required during the retention interval increased the rate of forgetting, suggests that processing demands during retention do affect rate of information loss. This assumes that our retention interval tasks required little storage capacity, and can be viewed primarily as demands for processing capacity. Within the working memory model the forgetting rate difference for the two tasks can be ascribed either to dismption of control processes which maintain information in the buffer, or to a decrement in the capacity of the central executive to handle buffer overflow. If reducing the amount of processing capacity available increases the rate of forgetting then reductions in processing capacity as a result of aging should also produce an increase in the rate of forgetting. This increase was not observed in experiment 1, and no differences in rate of forgetting, were observed when age groups were equated for initial levels of retention in experiment 2. Thus processing demands during the retention interval do not appear critical to age differences in Brown-Peterson recall performance. It is possible that the effect of stimulus repetition is to reduce the demand on processing capacity during retention through the formation of more easily maintained traces, and thus age differences in processing may still explain the observed age differences in performance. It is also possible that the effect of repetition is to decrease, perhaps through chunking, the demands on limited buffer storage capacity and that the observed age differences were the result of storage capacity differences.
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In either case our results suggest that the question of whether storage capacity, processing capacity, or both decline with age will best be addressed by closer examination of memory item registration rather than retention. In summary, the results of the present research indicated that differences in rate of forgetting between old and young adults were dependent on age differences in immediate recall. These results suggest that age comparisons in encoding and storage offer a promising direction for delineating information processing mechanisms which decline with age.
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