Influence of Task-Specific Processing Speed

Copt'right 199J bt The GcftililobRieil

J ournal of Geront olo gy : PSYCHO LOG IC AL SCI ENC ES 1 9 9 3 .V o l . 4 8 . N o . 5 . P 2 4 5 P 2 5 5

Sodtn ol Aneriut

Influenceof Task-Specific Processing Speed on Age Differencesin Memory Timothy A. Salthouseand Vickv E. Coon School of Psychology, Georgia Institute of Technology.

Two studies were conducted to investigate the aspect(s) of processing involved in the hypothesized speed mediation of adult age differences in memory. Both studies involved a serial memory task in which information was to be recalled either in the original order of presentation, or in a reordered sequence. Results from both studies indicated that taskspecificprocessing durations were slower among older adults thon among loung adults, but that the attenuation of the age-related variance in memory was nearly as great after statistical control of a task-independent speed measure as after conlrol of nsk-specffic speed measures. These findings are consistent with the hypothesis that a substantial proportion of the adult age-related differences in memory is associated with a decrease with increased age in the speed of executing many cognitive processes,and not simply the speedof one or two specific processes.

'JHE goal of the studiesdescribedin this report was to r investigatehow processingspeedcontributesto the relationsbetweenageand memory. The relationsof interest havebeendocumented.andsomeinitial evidencerelevantto this issuehasbeenprovided,with datafrom earlierresearch (Salthouse,1983, in press).The memory tasksin the previous studieswere free recall of two lists of, l2 unrelated nounspresented auditorilyat a rateof -2 seconds/word, and paired associatcrecall . 1 5 , i n d i c a t i n gt h a tt h e o v e r a l la g e differenceswere not significantlylargerwhen the processing demands were increasedby requiring that the items be recalled in a different order from that in which they were presented. The significantthree-wayinteractionwas decomposedby conductingAge x Stimuli ANOVAs on the datawith each numberof items.The Stimuluseffectwas significantin each analysis(i.e., Fs > 10.2),but neitherthe Age main effect nor the Ace x Stimulusinteractionwas sisnificantin the

T"bl" r. B".kg.""t Srudy2

Study I Young

old

Young

old

N 3rJ 38 3ri 38 %,Males 5-5.3 44.1 44.7 44.7 A g e ( r a n g e )2 0 . 2 ( 1 8 - 2 36) 9 . 7 ( 5 t l - 8 0 )2 0 . 1( 1 8 - 2 5 )6 8 . 4 ( 5 5 - 7 8 ) Education (sD) 1 4 . 2( 1. 4 ) l s . 2( 2 . 8 ) 1 3 . 9( 1 . 6 ) l s . O( 2 . 6 ) H e a l t(hS D ) | . 7 ( . 9 , l.tl (.8) 1.7 (.U) 1.7 (.7) DigitSymbol Accuracy (sD) 9 5 2 ( 3 . 3 ) 9 4 . 5( 9 . - s ) 9 6 . 2( 2 . 2 ) 9 s . 4( 3 . 2 \ T i m e ( S D )1 . 1 2 6 ( 2 1 5 )l . t t 4 - 5 ( 3 6 3 )I , l 1 1 6 ( 2 2 6 I) . u 9 l ( 4 4 . s ) DigitDigit Accuracy (sD) 9 6 . 6( 2 . 2 ) 9 t t . 4( | . 7 ) 9 1. 4 ( Z . O ) 9 8 . 0( 2 . 7 ) Time(SD/ 523(61) 767(213) 551(tt6) 768(202) Norc. Education ref'ersto number of years of litrmal educationcompleted,and Healthref'ersto sell'-ratingof healthon a 5-point scalewith I : excellentand 5 - poor. Digit Symbol and Digit Digit accuracyvaluesare percentagecorrectresponses,and time valuesare in msec.

Letters

Digits 100

Youno - Orioinal

o o

o0

+

Youno- Re€rdered -aOld -Odginal

o7s

Old - Re0rdered

(t

75

G L

F .* (J

5O

50

o o, (d

5 zs o

25

o

(L

5

7

Numberof ltems

5

7

Numberof ltems

Figure l. Mean percentage c o r r e c ta c r o s so r i g i n a l a n d r e o r d e r e ds e q u e n c e s f o r 3 , 5 , a n d T i t e m s f o r y o u n g a n d o l d a d u l t s , S t u d yl .

P248

SALTHOUSEAND COON

databasedon 3 items, and only the main effect of Age was significantwith 5 items,F(l,74) : 8.31, MS. : 5.00. Both the main effectof Age, F( I ,'/4) : 22.56,MS": 3.06, and the Age x Stimuli interaction,F(\,14) : 8.86, MS" : 1.22, weresignificantwith 7-itemsequences. Inspectionof Figure I revealsthat this latter interactionis attributableto a largeragedifferencewith digits thanwith letters,but the low level of performancewith lettersraisesthe possibility that the interactionis an artifactof a measurementfloor with the letter stimuli. A detailedexaminationwas also conductedof processing times obtainedduring the performanceof the task. Very few subjectswere accurateon many trials involving 7 items, and thus the analyseswere restrictedto 5-item sequences.The analyseswere further limited to subjectswith at least one correcttrial in both the original and the reorderedsequence with eachtype of material, in order to ensurecompletedata from eachsubject.This resultedin subsamplesof 29 young adultsand 25 older adults.The agesof the subjectsin the subsamplewere similar to thosein the entire sample(i.e.,

young 20. I vs 20.2, and old 68.6 vs 69.7), alrhoughboth age groups in the subsamplehad somewhat faster Digit Symbol scoresthanthe total sample.That is, the meanswere 1,079 msec in the subsamplevs 1,126 msec in the total samplefor young adults, and 1,792 msec in the subsample vs 1,845msecin the total samplefor older adults. The meansacrosssubjectsof the mediantime per item in correctmemory trials and correct copy trials for each serial positionaredisplayedin Figure2. SeparateAge x Order x Serial PositionANOVAs conductedon the copy and memory data with digit and letter stimuli revealedthat all main effectsand interactionswere significant(p < .05), exceptfor the Order effect in the Digit Copy condition. The data in Figure 2 indicatethat the first entry is ratherslow, and that the remaining items are enteredquite rapidly except when the items must be reorderedfrom memory, and particularly with letterstimuli. Both young and old adultswere somewhat slowerin the recallof the third item, possiblybecause the items were initially reordered in two-item groups. (Resultsfrom a similar analysisbasedon the datafrom l6

Digits

Letters

MEMORYTIME

MEMORY TIME

o

I o tr

-> (D E

tr

o o c o oa o IE c .g

&-'

sux) 2@O i@o

o

Young - Original + Young - Re-Ordered -O-

Old - Original

Digits

Eooo

COPYTIME

7m

Letters

-F

Old - ReOrdored

COPYTIME

-O-

E (t)

20m 1()oo

1

2

3

4

SerialPosition

s

o

1

2

3

4

SerialPosition

Figure 2. Median responsetime in 5-item copy and memory trials with original and reorderedsequences fbr young and old actults,Stu6y I

TASK-SPECIFICPROCESSING SPEED

young subjectswith at leastone correct trial on the 7-item lists in every condition were consistentwith this interpretation becausethere were two peaks in the recall times of 7letterlists, one at the third item and anotherat the fifth item.) The slower entry on the third item is largely restrictedto memoryprocessing,becauseit is not pronouncedin the copy task. Perhapsthe most importantpoint to be noted from the data in Figure 2 is that, although the older adults were generallyslower than the young adults, the overall pattern appearsvery similarin the two groups. The median entry times in the copy and memory tasks with original and reorderedrecall were usedas predictorsof recall accuracywith 5-item lists. (Data from 3-item and 7item lists were not includedto avoid measurementceilings and floors.) An additionalpredictor was a compositemeaprocessingspeedcreatedby aversure of task-independent aging the z-scoresfor the time measuresfrom the Digit Symboland Digit Digit tasks,which hasa correlationof .71 with each other. The resultsof the hierarchicalregression analysesfor the 54 subjectswith completedata are summarized in Table 3. Regressionanalysesincludingthe taskspecific speedmeasureswere designedsuch that the times for the simplestor most fundamentalprocesses were entered first, followed by times representingmore complex processes.That is, the entry time in the copying condition precededthe entry time in the memory condition in the prediction of both original and reorderedrecall accuracy, and the entry time in original recall precededthe entry time in reordered recall in the prediction of reordered recall accuracy. Analyses were also conductedwith component durations derived by subtractingone time from another (e.g., entrytime with recallin originalorderwas subtracted from entry time in reorderedrecall to yield a measureof reorderingtime). However, the resultsof thoseanalysesare not reported becausethese difference score measureshad low reliability and exhibited very weak relations to the criterion memory performancemeasures. Five major points should be noted about the results in Table 3. First, there is no significantage-relatedvariancein the accuracyof recalling 5 items in their original order of (Equationl). Second,thereis significantagepresentation related variance in the accuracy of recalling items in a different order (Equation5), and it is still significantafter recall accuracyin the original order is controlled(Equation 7). Third, both the age-relatedvariance(Equation6) and the influence of recall in the original order (Equation 8) are reducedwhen the task-independentspeedmeasureis controlled. In fact, examinationof Equations6 and 8 reveals that there was no residualage-relatedvariancein the measureof reorderedrecall after eliminatingthe variationin the task-independent speedmeasure. The fourth point to note from Table 3 is that significant relationswere evidentbetweenthe task-specificspeedmeasuresand memory performance(Equations3, 9, and l0). Furthermore, the age-relatedvariance in reorderedrecall was substantiallyreducedafter controlling the variation in these task-specificmeasures(Equations9 and l0). However, the fifth point to be noted is that the reduction in the age-relatedvariancewas greater,and the relationsbetween memory and the task-specificspeedmeasuresweaker, after

P249

the task-independent speedmeasurewas controlled (Equat i o n sl 1 a n d l 2 ) . The relation of age on the task-specificprocessingdurations before and after control of the task-independent speed index was also examined in regressionanalyses,with the resultssummarizedin Table 4. Only in recall time for letters in their original order (Equation6 with letterstimuli) was the age-relatedvariancesignificantlygreaterthan zero after the task-independentspeedindex was controlled. Even in this case,however, the degreeof attenuationafter control of the varianceassociatedwith the task-independentspeedmea-

Table3. Prediction of Performance With 5-ltemMemory Lists(N : 54),StudyI

Equation Original I

Digits

Letters

Cum. Incr. F for R2 R2 lncr. R2

Cum. Incr. F fbr R2 R2 Incr. R2

Age

.019 .019

2

Speed Age

.052 .064

3

CopyTime-O .096 .096 5.42* MemTime-O .l l5 . 0 1 9 t . t 2 Age . l 1 7 .002 .06

4

Speed CopyTime-O MemTime-O Age

Reordered 5 Age

1.03

.052 2.82 .0t2 .69

.052 .052 2.tt4 .053 .0-s3 2.86 .056 .003 . l8 .099 .099 5.47* .099 .000 .00 .099 .000 .03

.052 .096 .l 16 .123

.052 2.89 .044 2.46 .020 I .16 .007 .39

.053 .099 .099 .099

.124

.053 2.titt .046 2.5| .000 .00 .000 .01

.124 7.39*

.t94 .t94 12.494

6

Speed Age

.150 .150 9.05* . 1 5 2 .002 .08

.221 .227 15.05* .230 .003 .21

7

Original Age

.100 .197

.100 6.32* .091 6. 17+

.t85 .185 13.66x . 3 0 9 . t 2 4 9 . tl *

8

Speed Original Age

. 1 5 0 . 1 5 0 9 . 5 1 + . 2 2 7 . 2 2 7 1 7 |. 3 * . 2 0 5 .055 3.44 . 3 3 6 . 1 0 9 t i .1 9 + .209 .004 .25 .33'7 .001 .l r

9

CopyTime-R . 183 . 1 8 3 I l . 7 l * MemTime-R .205 .022 t.42 Age .219 . 0 1 4 .89

.113 .113 I 1.84* .178 .005 .34 .268 .090 6.09*

l0

MemTime-O .250 .250 t6.74+ M e m T i m e - R . 2 5 2 .002 .09 Age .253 .001 .09

.148 .148 9.46* .148 .000 .00 .218 .070 4.46*

ll

Speed CopyTime-R MemTime-R Age

.150 .199 .214 .219

. 150 9.44* .o49 3.01 .015 .99 .005 .30

.227 .256 .2't4 .28|

.22'7 15.48* .029 1.98 .Oltt t.2l .007 .50

12

Speed MemTime-O MemTime-R Age

.150 .254 .255 .255

.150 9.90* .104 6.83* .001 .06 .000 .01

.227 .248 .258 .258

.227 15.00* .021 1.35 .010 .66 .000 .01

Note. Cum. : cumulative;lncr. : increment;CopyTime-O : median entry time in copy condition with original sequence;CopyTime-R : medianentry timp in copy conditionwith reorderedsequence;MemTime-O : medianentry time in memory condition with original sequence;MemTime-R : median entry time in memory condition with reordered sequence. *p < .05.

P250

SALTHOUSEANDCOON

Table4. Prediction of Recall(or Entry)TimeMeasures With 5-ltemListsfromAge andSpeed (N : 54),SrudyI Digits Cum. R2

Equation Copy-Original I Age 2

Speed Age

Copy-Reordered 3 Age 4

Speed Age

Mernory-0riginal -5 Agc 6

Speed Age

Speed A8e

Letters F lbr lncr. R2

Cum R2

Incr. R2

F fbr lncr. R2

.409

.409 36.06*

.446

.446 4 1 . 8 8 *

.554 .-554

.554 63.28* .000 .00

.465 .490

.465 16.52+ .025 2.46

.2U9

. 2 8 9 2 1. 1 4 *

.254

.254

.487 .503

.487 49.99* .016 1.68

.340 .340

.340 26.28* .000 .00

MprHon

.421

.421 37.86+

.422

.422 37.119*

164 .419

.464 45.441. .01-5 | .43

. 3 1| . 3 tI .422 .l I I

Subjects.- Characteristics of the subjectsamples,38 adultsin eachof two agegroups,aresummarized in Table2. None of theseindividualshad participated in Study I .

Menrory-Reordered 7 Age .200 tt

Incr R2

The present study therefore included manipulationsof stimuluspresentationtime to determinethe relationbetween a measureof encodingtime and recall accuracy.The stimulus durationrequiredto achievea specifiedlevel of accuracy was then usedas the rneasureof the individual'sstimulus encodingtime. In most other respectsthe study was similar to Study l. That is, the numberof subjectswas the same, they were recruited in the same manner, and the Digit Symbol and Digit Digit taskswere again usedto provide an index of task-independent processingspeed.However,becauseof the time neededto examinethe effect of variations in stimuluspresentation time, only the memorytaskwith the 5-itemlettersequences was administered.

.215 .775

.200

| 3.001,

.275 19.35+ .000 .01

17.73+

2l .46r, 9.70*

.214 .2t4

l4.lz+

.lt{l .2| tt

t2.29+ I.96

.luu .030

C u m . : c u m u l a t i v c ;I n c r . : r n c r e m e n t . 'ip < .05.

s u r e w a s c o n s i d e r a b l(ei . e . , l 4 o / o , f i o m a n R ' o f . 4 2 2 t o .I I l ) . To summarize,the resultsof this studv reolicatedthe linding of substantialsharedvarian." urnnngage, perceptual comparisonspeed,and memory perfbrmance.In addition, the resultsindicatedthat althoughsome of the task-specific speedmeasureswere significantlyrelatedto the measuresof memory perfbrmance,only a small proportionof thoserelations was independentof a presumablymore generalspeed indexderivedf'romseparate tasks.Moreover,nearlyall of the age-relatedvariancein the task-specificspeedmeasureswas sharedwith the task-independent speedmeasure. Study 2 Kliegl and colleagues haverecentlyprovidedconvincing evidencethat input processingtime is an importantfactor in the age diff'erencesin serial recall by the finding that older adults need more time to achievethe same level of recall performanceas young adults (Kliegl, Smith, & Baltes, 1989;Thompson & Kliegl, 1991).Furthermore,Thompson and Kliegl (1991)cite an unpublishedstudy by Kliegl and Lindenbergeras having found that the "age diff'erencesin recall accuracywere completelyaccountedfor by rndividual differencesin criterion-referenced encodins times" (o. 5 4 4 ) .W h a ti s n o tc l e a rl r o m t h eK l i e g ls t u d i e s l w s h e t h etrh e influenceof input processingtime is independentof a more general,or task-independent, processing speed,andwhether a similar relationbetweenencodingtime and recall performanceexistsamonguntrainedsubjects(i.e., research participantsin the Kliegl studiesreceivedextensivetrainingwith the method-of-locimnemonicstrategy).

Procedure.- The taskswere perfbrrnedin the fbllowing orderby all subjects:Digit Syrnbol,Digit Digit, and Memi ory Letters.The Digit Symbol and Digit Digit taskswere identicalto thoseof Study I . Mean levelsof accuracywere )95o/oin both tasksand both agegroups(cf. Table 2), and thus mediantime per response servedas the primarydependentvariablein thesetasks. The memory task dif-fbredfrom that in Study I by using only 5-letterstimulussequences and by rcplacingthe fixed stimulus duration of I second/itemwith a duration that variedacrosstrial blocks.A difl'erentrangeof presentation times was used for young and old adults, becausepilot research revealedthatthe olderadultswereseldomsuccessful with presentation durationsof 100 msec.Young adults therefbrereceiveddurationsof 100, 200, 400, 800, and I ,600msec/item,whereasolderadultsreceiveddurationsof 200, 400, 800, 1,600,and 3,200 msec/irem.Blocksof l6 trials each were presentedfiom the slowest to the f'astest durations,and then againin the reverseorder suchthat each presentation durationwas represented by 32 trials.All trials in a given block had the samepresentation time, but on a randomlyselectedhalf of the trials within each block the letterswereto be recalledin theoriginalorder,andon half of the trialsthey were to be recalledin alphabeticorder. Rp,sulrsANDDISCUSSToN The meanpercentageof 5-lettertrials recalledcorrectlyas a functionof stimuluspresentation durationis displayedin Figure 3. An Age x Order X Duration ANOVA was conductedon the accuracyfor trials with presentationdurationscommonto both agegroups(i.e., 200, 400, 800, and 1,600 msec).All threemain effectswere significant(p < . 0 5 ) ;A g e ,F ( 1 , 1 4 ): 4 6 . 1 8 ,M S " : 5 7 . 7 1 O ; rderF , (I,14) : 21.'17,MS" - 7.59;and Duration,F(3,222) : 146.73, MS" : 6.03, but noneof the interactions involvingage was s i g n i f i c a n(ti . e . , a l l F < 1 . 3 ) . Two methods were used to derive estimatesof each individual'sinput processing time or task-specific encoding duration. One method was basedon the fact that the means summarizedin Figure 3 could be accurately(i.e., all R'

TASK-SPECIFICPROCESSINGSPEED

Youno - Orioinal

--'

- R6ordqed - ra- -.

Old - Original Old - Reords€d

--€--.

o o 6 0 (J o 6 c

o 4 0 o o &

1m 'X,"""n,iilon o",,ff-t'l$

32m

Figure 3. Mean percentagecorrcct fbr original and reorderedS_letter sequencesas a function of presentationdurationfor ytrungand old adults. S t u d y2 .

>.99) describedby negativeexponentialfunctionsof the fbrm: P r o p o r t i oC n o r r e c t: C * ( l - e x p ( - ( t i m e * A ) * B ) ) . The C parameterin this equationcorresponds to the asymptotic levelof accuracy,theA parameter to the time at which accuracybeginsto increase,and the B parameterto the rate of increasein accuracywith additionaltime. Analysesbased on the group meansrevealedthat relativeto older adults, young adultshad smallerA parameters (i.e., .086 and .065 for originaland reorderedrecallfbr young adults,and .100 and . l5l for originaland reorderedrecallfor older adults), larger 8 parameters(i.e., 1.19 and 5.28 for original and reorderedrecall fbr young adults, and 3.63 and 4.73 for originaland reorderedrecallfor older adults),and largerC parameters(i.e., .7ll and .689 fbr original and reordered recall for young adults, and .520 and .428 for the correspondingvaluesfbr older adults).Becausethe asymptotic level of accuracywas only .428 with reorderedrecall in the older group, the time correspondingto an accuracyof .25 (i.e., correctrecallof 25o/o ofthe lists)servedas the measure of encodingtime. (SeeAppendix,Note I .) The timesbased on the equationsfrom the group meanswere 142 msec for original recall in young adults, 150msecfbr reorderedrecall in youngadults,28 I msecfor originalrecallin olderadults, and 337 msecfor reorderedrecall in older adults. Becausethe goal was to derive encodingtime estimates from eachsubject,attemptswere madeto fit negativeexponential functionsto the original and reorderedrecall data of eachsubject.Data of somesubjectscould not be fit because of small or unsystematicaccuracy variations, and time estimatescould not be derived in five additional cases becausethe accuracyasymptoteswere lessthanthe criterion valueof .25. Age x Order ANOVAs wereconductedon the parameters,and on the estimatedencoding times correspondingto an accuracyof.25, for the 32 youngadultsand 27 older adults with relevant data. No effects were significant in the analysesof theA, B, or R2parameters,and only the Age effect was significant(p < .05) on the C parameter,

P25r

F(1,57) = 22.83, MS. : .037. All three effecrs were significant(p < .05) in the analysisof estimatedencoding t i m e :A g e , F ( 1 , 5 7 ) : 1 0 . 3 3 ,M S " - . 1 3 3 ;O r d e r ,F ( I , 5 7 ) : 13.52,andAge x Order,F(l ,57) : 9.69, MS, : .O4t. Means of the estimatedtimes were 169 msec for young adultswith recall in the original order, 195 msecfor young adults with recall in alphabeticorder, 355 msec for older adults with recall in the original order, and 512 msec for older adultswith recall in alphabeticorder. The second method used to estimateeach individual's task-specificencodingtime was basedon linear regression equationsrelating log presentationtime to accuracyacross thethreeshortestdurations(i.e., 100,200. and400 msecfor young adults,and 200, 400, and 800 msecfor older adults). These equations were then used to predict the duration requiredto recall 25o/ootithe lists correctly. Data from five subjects,one young and four old, are omitted from these analysesbecausethe encodingduration estimateswere not meaningfuldue to correctrecall of 125o/oof the lists across all threeof the shortestpresentationdurations.Meansof the correlationsrepresentingthe goodness-of-fitof the regression equationswere .92 and.90 for original and reordered trials,respectively, for the 37 youngadults,and .8 I and .80 for the two typesof trials for the 34 older adults.An Age x Order ANOVA on the correlationsrevealedthat only the A g e e f f b c tw a ss i g n i f i c a n F t ,( I , 6 9 ) : 7 . 3 2 ,M S . : . 0 5 1, p < .05, indicatingthatthe relationsbetweenstimuluspresentation time and accuracywere somewhatlessprecisefbr the older adults.The predicteddurationscorrespondin g to 25o/o accuracyaveraged139msecfbr recallin the originalorder and 160 msecfbr recall in alphabeticorder for the 37 young adults,and 412msecand 626 msec,respectively, fbr the 34 olderadults.Only the Age maineffectwas significantin the A g e x O r d e r A N O V A , F ( 1 , 6 9 ) : 2 4 . 0 3 ,M S , : . 4 2 . Although the patternof meanssuggeststhat an interaction might be present,the Age x Order interactionwas not significant,ic'(1,69)< 1.0. Correlationscomputedbetweenthe predictedtimes correspondingto correctrecallof 25o/oof the trials derivedfrom the two methods were very high, with r : .95 for the estimatesin the original order condition and r : .86 for estimatesin the reorderedcondition. Becauseestimateswere available for more subjects when the linear regression methodwas used(i.e., 7l comparedwith 59), subsequent analyseswere basedon the encodingtimes derivedfrom the linear regressionprocedure.It should be noted, however, that the high conelationsbetweenthe two setsof estimates suggestthat very similar resultswould have been obtained with encoding times derived from negative exponential functions. Figure 4 displays the mean of the median recall times (averagedacrosstrials with 1,600and 800 msecpresentation durations)by serial position. (Thesepresentationdurations were selectedbecausethey were the largestdurationscommon to both agegroups.)An Age x Order x Serialposition ANOVA revealedthatall main effectsandinteractionsexcept for the three-wayinteractionweresignificant(F's > 3.9, p < .05). Inspectionof Figure 4 revealsthat the patternis very similar to the top right panel of Figure 2, in that the older adultswere always slower than the young adults, and recall

P252

SALTHOUSEAND COON

8000

Youm - Odoinel

Table5. Prediction of Performance With s-ltemMemoryI_ists From 1,600and800MsecPresentation DurationTrials ( N : 7 l ) . S r u d v2

+--

ao a g

Yourc- --- RFordered a--

7000

Old -gdSinal Old - Reordsed --a--

6(no

o

.E F o a

so@

Original I

5 0 m a

e

Incr. R2

F for lncr. R2

3ooo

Age

.279

.279

26.72*

2

Speed Age

.t67 .280

.t67 .lt3

15.75* 10.69*

3

EncodingTime Age

. 3 0I .4t2

.301 .lll

34.84E I2.88*

4

MemTime-O Age

.326 .365

.326 .039

34.88* 4.25*

5

Speed EncodingTime Age

.t67 .376 .415

.t6t .209 .039

19.102* 24.02* 4.42*

6

Speed MemTime-O Age

.167 .326 .373

.167 .159 .047

17.828 17.06* 4.99*

Speed EncodingTime MemTime-O Age

.t67 .376 .432 .453

.167 .209 .056 .02t

20.124 25.3I * 6.67*

c

.q E

Cum. Equation

20@

1

2

s serial losition

Figure 4. Median responsetime in 5-letter memory trials with original for young and old adults, Study 2. and reorderedsequences

time for reorderedsequenceswas slower than recall in the original sequence,particularlyin the third serialposition. Table 5 containsresultsof the analysesparallelingthose reportedin Table 3 for recall with presentationdurationsof 800 and 1,600msec,which most closelycorrespondto the l-sec presentation durationusedin Study l. Severalpoints shouldbe notedabouttheentriesin this table.First,the agerelatedeffects in letter recall were much larger in this study than in the previous study, both for recall in the original o r d e r ( i . e . , R ' f o r a g e o f . 2 7 9 v s . 0 5 2 ) a n d f o r r e c a l li n a l p h a b e t i co r d e r ( i . e . , R 2 f o r a g e o f . 3 8 1 v s . 1 9 4 ) . T h e reasonsfor the larger age relations in this study compared with thosein Study I are not obviousbecausethe procedure was generallysimilar and the samplesappearcomparablein most respects(seeTable 2). The fact that more trials were administeredin the relevantconditionsin this study compared with the previous one (i.e., 32 vs 8) may have contributedto the differenceacrossstudies,eitherbecauseof differential practice effects or increasedreliability of the memory measure.(See Appendix, Note 2.) Second, although control of the compositeperceptualspeedmeasure substantiallyreducedthe age-relatedvariance(i.e., percentage attenuationsof 597oand,737ofor original and reordered recall, respectively), the residual age-related variance (Equations2 and 9) was significantly > 0. This indicates that, unlike Study l, control of the task-independent speed index is not sufficientto accountfor all of the significantagerelated variance in immediate recall in this study. Third, significantage-relatedvarianceremains in the measureof reorderedrecall after the measureof recall in the original order is controlled (Equation l0), but not after the taskindependentspeedmeasureis also controlled(Equation I l). Both of these findings are consistentwith the results of Study l. Another important finding to note from Table 5 is that statisticalcontrol of the task-specific(i.e., Encoding Time and Recall Time; Equations3, 4, 12, and 13) speedmeasuresresulted in substantialattenuationof the age-related variancein memory performance.That is, the percentage attenuation of the age-relatedvariance was 6O.2Voand

'1

Reordered E

2.58*

Age

. 3 8|

.38|

42.46*

9

Speed Age

.307 .403

.307 .l0l

34.964 10.88*

l0

Original Age

.5t2 . 5 9|

.5t2 .079

8 5 .1 2 * 13.21*

ll

Speed Original Age

.307 .594 .609

.30'7 .287 .015

52.57* 49.t4* 2.4' 7

t2

EncodingTime Age

.590 .661

.590 .011

120.52* t5.88*

I3

MemTime-O MemTime-R Age

.37s . 4t 8 -+t-)

.375 .043 .055

41.65* 5.49* 7.08*

l4

Speed EncodingTime Age

.307 .644 .67|

.30'7 .33'7 .027

62.61* 68.62+ 5.66*

l5

Speed MemTime-O MemTime-R Age

.307 .410 .431 .473

.307 .103 .02t .o42

38.49* 12.89* 2.68 5.27*

l6

Speed EncodingTime MemTime-O MemTime-R Age

.307 .644 .657 .658 .678

.307 .337 .013 .001 .020

61.93* 6'7.87* 2.64 .2'r 3.96

Nole. Cum. : cumulative; Incr. : increment; Encoding Time : predicted stimulus presentation time required to achieve 25Eo accuracy; MemTime-O : median entry time in memory condition with original sequence;MemTime-R : median entry time in memory condition with reordered sequence. *p < .05.

86.07o after control of the encoding time and recall time speedmeasuresfor recall in the original order, and 19.87o and85.6Vo, respectively,after control of thesemeasuresfor

P253

TASK.SP ECIFIC PROCESSINGSPEED

recall in alphabeticorder. The residualage-relatedvariance was significantly greaterthan zero, indicating that factors independentof both thesespeedmeasurescontributedto the age-relateddifferences in memory performance. In this respect,the current resultsdiffer from those of Kliegl and Lindenberger,as reportedin Thompsonand Kliegl (1991). Another noteworthyresult from Table 5 is that the relations betweenthe task-specificspeedmeasuresand memory were smaller after control of the composite task-independent s p e e dm e a s u r (ei . e . , E q u a t i o n3s v s 5 , 4 v s 6 , l 2 v s 1 4 ,a n d l3 vs l5). This suggests that only a portionof the variance sharedbetween memory and the task-specificspeedmeasureswas distinct from the presumablymore general,taskindependentspeedmeasure. A final setof regressionanalyseswere similar to thoseof Table 4 in that they examinedthe extent to which the agerelatedvariancein the task-specificspeedmeasurescould be accounted for by the task-independentspeed measure. Resultsof theseanalysesare presentedin Table 6. Notice that only with recall time for reorderedrecall (Equation8) was therenot significantresidualage-relatedvariancein the task-specificmeasuresafter control of the presumablymore generalmeasuresof perceptualspeed.However, it should also be noted that each of the task-specificspeedmeasures had considerableage-relatedvariancein common with the perceptual speed measure because control of the taskindependent speed measure resulted in attenuationsof 39.2o/o,78.la/o,86.5o/o,ancl94.4o/o,respectively,for the measuresof original encodingtime, reorderedencoding time, originalrecalltime, and reorderedrecalltime.

Times Table6. Prediction of Task-Specific Processing (N : 7l), Study2 FromAgeandSpeed Cum p2

Equation

In c r R2

F fbr lncr. R2

EncodingTime Measure Original I 2

Reordered 3 4

.166

.166

13.74*

Speed Age

.06tt .t69

.068 .l0t

5.58* 8.29*

Age

.237

237

2 1. 4 1 +

Speed Age

.204 .256

204 052

18.64* 4 . 74 *

Age

.443

.443

54.17+

Speed Age

.462

.462 .060

65.12't' tl.'10*

A8e

.33u .437 .456

. 33 8

+ 35.23

.437 . 0l 9

-54.6-5* 2.45i'

Recall Time Measure

Original 5 6 Reordered 7 tJ

Speed Age

.) ll

*p < .05

General Processing Speed

General Discussion The primaryquestionof interestin this projectwas, how does a slower processingspeed contribute to age-related differencesin memory'?The researchstrategyconsistedof trying to measurethe durationsof componentspostulatedto be involvedin a particularmemorytask,andthenexamining how thesetask-specificspeedmeasureswere relatedto age and to performancein the memorytask, and how both setsof relationswere influencedby speedmeasuresderived from independenttasks. Figure 5 illustratesa frameworkwithin which the present researchcan be interpreted.A major purposeof the studies reportedhereinwas to examinethe plausibilityof eachof the relationsrepresentedin this figure. Three measuresof task-specificprocessingspeed were examinedin thesestudies.The speedof initial registration and encodingof the items was representedby the stimulus presentationduration neededto correctly recall a specified percentage of lists,the speedofaccessingor retrievingitems from memory was representedby the additional time required to enter items from memory comparedwith entering t h e m f r o m t h e d i s p l a y( i . e . , E q u a t i o n s3 , 4 , 9 , a n d l l i n Table 3), and the speedof reorderingthe items was representedby the additional time to enter items in a different order relative to entering them in the original order (i.e., Equationsl0 and I 2 in Table 3 , and EquationsI 3 and 15 in Table 4). The measuresof a presumably more general processingspeedwere derived from independenttasks re-

Memory Performance

Task-Specific Processing Speed Figure 5. lllustration of hypothesizedrelations among age, memory performance,and generaland specilicmeasuresof processingspeed.

quiringdecisionsaboutthe physicalidentityor associational equivalenceof a pair of items. Previousresearch(e.g., Salthouse,1992)hassuggestedthat memoryfactorscontribute very little to the age-relatedvariationin thesetasks,and that performanceon the taskscan be interpretedas primarily reflectingthe speedwith which relativelysimple perceptual and cognitiveoperationscan be executed. Although path analytic or structural equation methods were not used in this project becauseof the small sample sizes, results from the curent studiesnonethelessallow a number of inferencesto be drawn concerningthe relations in Figure5. First, considerthe pathslinking age represented to memory performanceeither directly (Path I ) or indirectly by means of a reduction in general, or task-independent, processingspeed (Paths 2 and 5). (Note that a direct influencein this contextmerely meansthat it is independentof

P254

SALTHOUSE AND COON

the other variables being considered, and not that it is unmediatedby any factor.) The substantialreductionin the age-relatedvariance in memory after the variation in the index of general processingspeedwas controlled implies that Paths2 and 5 are involved in the relationsbetweenage and memory. However, the discoverythat significantresidual age-relatedvarianceremainedin Study 2 after control of the index of generalspeedsuggeststhat Path I sometimes contributes to the age-memory relations. Both of these pathways can be viewed as representingunexplainedinfluences,becausethe mechanismsby which increasedage, or a slower speed of performing perceptualcomparison tasks, contributesto lower levels of memory performance cannotyet be specified. Second, similar types of comparisonswith the taskspecificspeedmeasuressuggestthat Paths3 and 6 in Figure 5 arealso involvedin the relationsbetweenageand memory. That is, becausethe age-relatedvariance in memory was reducedwhen the task-specificspeedmeasureswere controlled, it can be inferredthat someof the agedifferencesin memory are mediatedthrough reductionsin the speedof memory-specificprocessing.The causalmechanismslinking these task-specificspeedmeasuresto memory performancemay be relatedto impairedquality of encoding,or to a greaterloss of information.That is, the degreeof elaboration of encodedinformationmight be less,or the proportion of displaced or decayed information greater, when more time is requiredto encode,access,or process(i.e., reorder) to-be-recalledinformation. And third, informationabout the relationsbetweenthe general and specific measuresof speed is available from results of regressionequations predicting specific speed from age and the generalspeedmeasure(Tables4 and 6), and predicting memory perfbrmancefiom age, the general speedindex, and the specificspeedmeasures(Tables3 and 5). The formerequationsarerelevantto the pathslabeled2, 3, and4 because the strengthof the indirectinfluenceof age on the specific speed measures(Paths 2 and 4) can be ihferred to be proportionalto the degreeto which the agerelatedvariancein the task-specificspeedmeasuresis attenuatedafter control of the more generalspeedmeasure.The resultsin Tables4 and 6 indicatethat much of the relation betweenage and the task-specificspeedmeasuresis mediatedthroughgeneralspeed(via Paths2 and 4), althoughthe existenceof significantindependentage-relatedvariancein some measuresindicatesthat Path 3 can also contributeto the relation. Sequentialexaminationof the influence of the general speedindex, the task-specificspeedmeasures,and age on the measuresof memory performanceis informative about the Pathslabeledl, 5, and 6 in Figure5. Althoughthereis some variation acrossthe task-specificspeedmeasures,it appearsthat all of the paths illustrated in Figure 5 can contributeto the relationsbetweenageand memory. That is, the age-relatedvariancein memory performanceis reduced when either the generalor the specific speedmeasuresare controlled, and, at least in some cases, there is still a significant relation between memory performanceand the specificspeedmeasureeven afterthe generalspeedmeasure was controlled. Analyses with the general speedmeasure

enteredin the prediction equation after the specific speed measureswere not reportedin Tables 3 and 5, but results from these analyseswere consistent with the preceding interpretation.That is, in at least severalcombinationsof predictorsand criterion memory measures,the influenceof the generalspeedmeasurewas still significantafter removing the varianceassociated with the speci{icspeedmeasures. This occurredafteralteringthe orderof entry of the variables in Equations1l and 12 with letterstimuliin Table 3, and in E q u a t i o n5s , 1 4 , 1 5 ,a n d l 6 i n T a b l e5 . To summarize.the resultsof thesestudiesconfirm earlier findingsofa large influenceofprocessingspeedon adult age differencesin memory. Earlier researchis also extendedby revealingthat increasedage is associatedwith slower processingof severalcomponentshypothesizedto be involved in the performanceof theseparticularmemory tasks.However, it is important to note that the task-independentand task-specificmeasuresof speedhave substantialage-related variancein common, and thus they are not independent. Indeed,the patternof resultsis consistentwith the interpretation that the age differencesin the specificspeedmeasures, includingthe encodingtime measurebasedon procedures similarto thoseusedby Kliegl andcolleagues (Kliegl et al., 1 9 8 9 ; T h o m p s o&n K l i e g l , l 9 9 l ) , a r ep a r t i a l l yc a u s e db y a moregeneralage-related slowing. Althoughthesestudiesfocusedon encodingtime, memory retrieval or accesstime, and processingor reordering time, it seemsreasonable to expectsimilarresultswith other measures of relevantprocessing durations.For example,the reports that young and old adults differ in both memory performance and in subvocal rehearsal rate (Salthouse, 1980)or articulationrate (Kynette, Kemper, Norman, & Cheung,1990)couldbe viewedas additionalmanif'estations of the interrelations of age, a relativelygeneralprocessing speed,and memoryexaminedin thesestudies.Moreover, the current perspectiveleads to the expectationthat if the durationsofcomponentssuchasassociation, elaboration, or organizationcould be assessed, then they would also be relatedto age and to more generalmeasuresof processing speed, as well as to memory. The causesof age-related differencesin the postulatedgeneralprocessingspeedhave not yet been determined,and the exact manner by which slowerprocessingreducesmemory effectivenessstill cannot be specified.Nevertheless, the resultsof thesestudiessuggest that at least some of the age-relatedimpairments in memory are attributableto declineswith age in the speedof relevantprocessing,and that the speedof memory-specific processingis closely related to the speed with which the individual can perform other relatively simple tasks. AcKNowLEDcMENTS This researchwas supportedby a grant from the National Instituteon Aging (R37-AG06ll26). We thank Philip Allen, Nelson Cowan, Robert Kail, and Reinhold Kliegl for helpful commentson an earlier versionof the manuscript. Addresscorrespondence to Dr. Timothy A. Salthouse,School of Psychology, Georgia Instituteof Technology,Atlanta, GA 30332-0170. REFERENcES Babcock,R. L., & Salthouse,T. A. ( I 990). Effectsof increasedprocessing

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demands on age differences in working memory. Psychologl tuttl Aging,5,421 428. G i c k , M . L . , C r a i k . F . l . M . , & M o r r i s ,R . G . ( 1 9 8 8 ) .T a s kc o m p l e x i t ya n d age differencesin working memory. Memory'& Col1rtition,16, 353361. K l i e g l , R . , S m i t h , J . , & B a l t e s ,P . B . ( 1 9 8 9 ) .T e s t i n g - t h e - l i m i tasn d t h e study of adult age differencesin cognitive plasticity of a mnemonic skill. Devclopmennl Ps1'thoktg,",25, 247 256. K y n e t t e ,D . , K e r n p e r S , . , N o r m a n ,S . , & C h e u n g ,H . ( 1 9 9 0 ) .A d u l t s ' w o r d r e c a l la n dw o r d r e p e t i t i o nE. x p e r i m e n t aAl g i r t gR c s e a r c h1, 6 , l 1 7 - 1 2 1 . M o r r i s , R . G . , G i c k , M . L . , & C r a i k . F . I . M . ( 1 9 u 8 ) .P r o c e s s i nrge s o u r c e s and age dilfbrencesin working memory. Memorv & Cognition, 16, 362-366. Salthousc,T. A. (1980). Age and memory: Stralegiesfirr localizing the l o s s .I n L . W . P o o n .J . L . F o z a r d ,L . S . C e r m a k ,D . A r e n b e r g & , L. W. Thompson (Eds.), Nex, dircctions in memort'and aging (pp. 47-65). Hillsdale. NJ: LawrenceErlbaum Associates. S a l t h o u s eT, . A . ( 1 9 9 0 ) . W o r k i n g m e m o r y a s a p r n c e s s i n gr e s o u r c ei n cognitive aging.DcvclopmcntttlReviev',10. l0l-124. S a l t h o u s cT. . A . ( 1 9 9 2 ) .W h a t d o a d u l ta g ed i f fe r e n c e si n t h e D i g i t S y m b o l SubstitutionTest reflect'l.lournal of Gerontololit,: Ps'"r'hologitalSt'ie n c a s , 1 7P, l 2 l - P l 2 u . S a l t h o u s eT. . A . ( 1 9 9 3 ) .S p e e da n dk n o w l e d g ea sd e t e r m i n a n tosl ' a d u l ta g e dil'lercncesin verbal lasks. Journul ol Cerontologt': Psv-chologital Scicnczs,1ll. P29-P36. S a l t h o u s eT. . A . ( i n p r e s s ) .S p e e dn r e d i a t i o no l - a d u l t a g e d i t l ' e r e n c eisn cognition. Devck4tmcntulPstcholog,t'. S t i n e .E . L . . & W i n g f i c l d ,A . ( l 9 t l 7 ) . P r o c e s sa n d s t r a t e g yi n m e m o r yf i r r

and Aging, 2, speechamong younger and older adults. Ps.r'cftolog1'

272-279. T h o m p s o n ,L . A . . & K l i e g l . R . ( 1 9 9 1 ) .A d u l t a g ee f f e c t so f p l a u s i b i l i t yo n memory: The role of time constraintsduring encoding. Journal of Learning, Memort and Cognition, 17, 542ExperimentulPs,-r'hologl': 555. ReceivedSeptemberI 7, I 992 AccepredApril l, 1993

Appendix Notes l. This particularlevel of accuracyis somewhatarbitrary, but encodingtime estimatesbasedon diff'erentlevelsof accuracywere highly correlatedwith one another.For example, the corelations in the sampleof young subjectsbetweenencodingtime estimates with accuracylevclsof l07o and 25o/owere.92 (original)and .87 (rcordcred),and .94 (original) and .94 (rcordered)betweenaccuracy levelsof 254/aand4OVa. 2. Reliabilityof the criterion memory measurecould not bc determinedin Study I, but the estimatesin Study 2 derivedby boosting the correlationbetweenaccuracyin the 800 and I,600 msec conditions by the Spearman-Brownformula were .83 fbr recall in the orisinal order and .8ll for reorderedrecall.