differential course of executive control changes during normal aging

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Aging, Neuropsychology, and Cognition, 14: 370393, 2007

http://www.psypress.com/anc

ISSN: 1382-5585/05 print; 1744-4128 online

DOI: 10.1080/13825580600678442

NANC1382-5585/051744-4128Aging,Neuropsychology, and Cognition,Vol. 00, No.0,June 2006: pp.147Aging,Neuropsychology, and Cognition

Differential Course of Executive ControlChanges During Normal AgingExecutive ControlChanges During Normal AgingFriederikeH.Treitz etal.

FRIEDERIKE H. TREITZ, KATRIN HEYDERAND IRENE DAUM

Institute of Cognitive Neuroscience, Ruhr University of Bochum, Germany

ABSTRACT

Normal aging has been associated with executive control deficits, but it is as yet

unclear whether different executive subprocesses are differentially affected during the

course of aging. The present study aimed to investigate age effects on a range of exec-

utive control subcomponents. Four consecutive age groups (2030 years, 3145 years,

4660 years, 6175 years), matched on present state IQ and mood, were compared on

tasks of strategic memory processing, verbal fluency, reasoning, inhibition, task man-

agement, and self-rating of executive abilities. Deficits concerning the suppression of

habitual and experimentally induced prepotent response tendencies and the ability to

efficiently divide attention were observed in subjects over 60 years of age compared to

the younger groups, while memory, verbal fluency, and reasoning were largely unaf-

fected. Results suggest a sharp decline of executive function after age 60 and a differ-

ential course of different executive subcomponents across aging, adding further

support to a multi-dimensional model of executive function.

The cognitive decline associated with normal aging is generally discussed in

relation to mild neurodegenerative changes, including neuronal shrinkage,

loss of dendritic and synaptic density, or alterations in neurotransmitter sys-

tems (Backman et al., 2000; Jernigan et al., 2001; Kaasinen et al., 2000;

Resnick et al., 2003; Tisserand & Jolles, 2003; Wang et al., 1995, 1998).Disproportionate volume loss occurs within the frontal lobes and the hippoc-

ampal region, but changes in the thalamus and the mamillary bodies are also

observed (Guttmann et al., 1998; Jernigan et al., 2001; Raz et al., 1992,

2004; Resnick et al., 2003; Salat et al., 2001; Van Der Werf et al., 2001;

Woodruff-Pak, 1997). With respect to the prefrontal cortex, grey matter vol-

ume loss was reported to reach 8.9% per decade in subjects over 65 years of

Address correspondence to: Friederike H. Treitz, Institute of Cognitive Neuroscience, Department of

Neuropsychology, Ruhr-University of Bochum, 44780 Bochum, Germany. E-mail: [email protected]


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EXECUTIVE CONTROL CHANGES DURING NORMAL AGING 371

age (Van Petten et al., 2004). In addition, functional neuroimaging studies

reported evidence of reduced activations within the prefrontal cortex in older

compared to younger adults (Logan et al., 2002).

Given the frequently reported neuropathological changes in the pre-

frontal cortex (PFC) relative to other cortical areas (Head et al., 2004; Jernigan

et al., 2001; Raz et al., 2004; Resnick et al., 2003), older age has been con-

sidered a model of diffuse mild prefrontal dysfunction and associated execu-

tive control impairments (West, 1996). This view is supported by parallels in

the cognitive profile of healthy elderly subjects and young individuals with

acquired PFC lesions (Daum et al., 1996; Daum & Mayes, 2000; Daum &

Schugens, 1999), characterized by deficits in source memory, memory for

temporal order, or impaired use of cognitive strategies (Daum et al., 1995;

Daum & Schugens, 1999; Glisky et al., 2001; Mayes & Daum, 1997).Executive control has recently been described as a heterogeneous con-

cept, involving five major subcomponents (Smith & Jonides, 1999):

a.focusing attention on relevant information and inhibiting irrelevant

information;

b.task management, including switching attention between tasks;

c.planning a sequence of subtasks to accomplish a goal;

d.updating working memory contents to determine the next step in a

sequential task;

e.coding context of representations in working memory.

Following Shallice and Burgess (1991), the prefrontal cortex is associ-

ated with the supervisory attentional system (SAS), which is responsible for

strategic control of mental processes, such as the use of strategies or rule-

guided retrieval from long-term memory. In novel and unfamiliar situations,

the SAS is responsible for strategy formation, planning, and problem-

solving to achieve goals. Aging has been reported to adversely affect most of

the executive control subcomponents defined by Smith and Jonides (1999),

as well as executive processes describes by Shallice and Burgess (1991).

Meta-analyses by Verhaeghen and Cerella (2002) yielded a consistent age-

related decline of task management abilities beyond the effect of general

slowing. Set-shifting deficits in older adults have been related to mild frontal

dysfunction (Keys & White, 2000; Kramer et al., 1999; Kray & Lindenberger,

2000; Meiran et al., 2001). Age effects have been reported for the use of orga-

nizational strategies in memory, such as self-generated use of semantic catego-

ries (Daum et al., 1996).

Inhibition of habitual responses, as assessed by the Stroop Test (e.g.,

Wecker et al., 2000), as well as inhibition of newly learned prepotentresponses, were also found to be affected by age (McDowd, 1997; Verhaeghen


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372 FRIEDERIKE H. TREITZETAL.

& De Meersman, 1998a). Whether these inhibition deficits can be attributed to

a general slowing of information processing or whether they represent a spe-

cific age-related executive impairment, is a matter of an ongoing debate

(Gamboz et al., 2002; Grant & Dagenbach, 2000; Schelstraete & Hupet, 2002;

Shilling et al., 2002; Uttl & Graf, 1997; Van der Linden, 2000; Verhaeghen &

De Meersman, 1998b; Wecker et al., 2000; West & Alain, 2000).

In spite of the wealth of empirical data, the pattern and course of age

effects on executive function is as yet inconclusive. The available studies

were mainly based on extreme group comparisons where a number of vari-

ables other than age may influence group differences (e.g., Brink &

McDowd, 1999; Grant & Dagenbach, 2000; Shilling et al., 2002; Van der

Linden, 2000). Longitudinal studies often reported an accelerated decline in

later life (e.g. Schaie, 1996), while cross-sectional studies suggest a graduallinear decline across adulthood (Park et al., 1996; Salthouse, 2003). By using

a cross-sectional design with four consecutive age groups, the present study

aimed to assess the course of executive function changes during adulthood.

The investigation of consecutive age groups is useful to economically assess

linear vs. critical threshold changes with respect to different subprocesses of

executive control, even though cohort effects cannot be completely elimi-

nated. The data should contribute to the issue of whether age-related changes

manifest themselves gradually or whether deficits occur once a critical

threshold has been reached, leading to a nonlinear change. In addition, dif-

ferent executive control subprocesses may be associated with independentneuronal substrates (see Heyder et al., 2004; Smith & Jonides, 1999), and

may therefore follow a differential course during aging. It has been sug-

gested that processes related to the dorsolateral PFC are particularly affected

by aging (MacPherson et al., 2002), given that there is evidence of relative

sparing of the orbitofrontal region (Salat et al., 2001), although comparable

effects on dorsolateral and orbital PFC regions have also been reported (Raz

et al., 1997; Tisserand et al., 2002). These conflicting results may relate to

the methods of volume measurement (Tisserand et al., 2002).

In summary, the present study aimed to further elucidate the effect of

normal aging on the course of executive control changes, focusing on task

management and inhibition as the most elementary executive processes (see

Smith & Jonides, 1999).

METHODS

Subjects

Sixty-two healthy subjects (34 men and 28 women) were selected from

a large subject pool to form four consecutive age groups matched on generalintellectual abilities. The first group comprised the 2030 year age range, the


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second group the 3145 year range, the third group the 4660 year range,

and the oldest group the 6175 year range. The four groups were closely

matched on present state IQ, as assessed by the Similarities and Picture

Completion subtests of the short German version from the Wechsler Adult

Intelligence Scale (Dahl, 1972). The groups were also matched on sex ratio

and present-state mood (Visual Analogue Scales; Bond & Lader, 1974). For

an overview of background data see Table 1.

Subjects were recruited by advertisements in the local press. They were

screened in an interview for health problems. Based on the interview, sub-

jects were excluded from participation if they had suffered from psychiatric

or neurological disease in the past or present or from diseases potentially

effecting the central nervous system. Subjects gave written informed consent

and received a 20 euro reimbursement.Neuropsychological Assessment

Strategic Memory Processing

To assess the self-generated use of memory strategies, verbal memory

was assessed by word list recall (Daum & Mayes, 2000). Three lists consist-

ing of 16 items each were read to the subject at a one word per second pre-

sentation rate. The first list consisted of four items of each of four categories

(metals, animals, landscape formations, vegetables), which were presented

in order of category membership (consecutive categories list; CC). The sec-

ond list also contained four items of each of four categories (items of cloth-

ing, fruit, furniture, and weather conditions), which were presented in

randomized order (randomized categories list; RC). Encoding and retrieval

of the RC list can be improved by self-generated categorization of the list

according to semantic categories. The third list was uncategorized (RR), i.e.,

the 16 items were unrelated. List order was randomized across subjects. Sub-

jects were asked to reproduce each list immediately after presentation.

Delayed free recall was assessed after a 30-min delay. The number of cor-

rectly reproduced items per list during both delays (immediate, delayed) as

well as retention rates (correct items delayed recall/immediate recall) wereanalyzed.

TABLE 1. Group Size, Mean (SDs) Age, IQ Estimates, and Scores in the VAS (Visual Analogue

Scales) for the Consecutive Age Groups

2030 yrs. 3145 yrs. 4660 yrs. 6175 yrs. p

Mean age in yrs. 25.4 (3.3) 38.8 (4.3) 52.4 (3.9) 67.5 (4.4)

N 16 16 13 17

IQ estimate 111.4 (4.8) 114.4 (4.6) 112.0 (5.7) 112.0 (7.4) 0.45

VAS 29.9 (11.9) 24.4 (9.3) 25.4 (11.4) 20.7 (13.3) 0.17


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Verbal Fluency/Cognitive Flexibility

To assess efficiency of rule-guided retrieval from long-term memory, a

verbal fluency task with three conditions was administered (see Daum et al.,1994). This procedure involved rule-guided search strategies, strategic

retrieval from semantic long-term memory, as well as cognitive flexibility.

The number of correctly produced exemplars was assessed in three fluency

conditions: a semantic category condition (countries), a phonemic cate-

gory condition (nouns beginning with B), and a switching condition (alter-

nate vegetables and male first names). The switching condition was

added as a measure of task management, which requires the switching of

attention between tasks. Subjects were asked to produce as many exemplars

as possible within 1 minute for each condition.

Reasoning

A German adaptation of the Cognitive Estimates Test (CET; see Shallice

& Evans (1978) and Daum & Mayes (2000) was administered to assess the

ability to draw plausible conclusions and to give realistic estimates based on

the subjects knowledge and reasoning. Subjects were asked to give esti-

mates for 10 problems such as How tall is the Cologne Cathedral? Based

on criteria described by Hodges (1996), the scoring ranged from 0 (response

within the normal range) to 3 (large deviation from the normal range).

Everyday Behavioral Correlates of Executive Impairment

To evaluate the everyday consequences of reduced executive control,

the Dysexecutive Questionnaire (DEX) from the Behavioral Assessment of

the Dysexecutive Syndrome (BADS) Test battery (Wilson et al., 1996) was

administered. The 20-item questionnaire addresses a range of problems

associated with the Dysexecutive Syndrome including emotional or person-

ality changes, motivational changes, or behavioral problems. Each item was

scored on a 5-point scale ranging from never to very often. Sum scores

were obtained for the self-rating and the independent rating, respectively. In

the independent rating, the DEX was also completed by a close relative orfriend.

Inhibition

To assess the ability to focus attention on relevant information and to

inhibit irrelevant responses, an adaptation of the Stroop Test (Bumler,

1985) and a version of the AX-Continuous Performance Test (AX-CPT) pre-

viously described by Braver et al. (1999) were administered.

In the Stroop Test, subjects had to read out the names of colors printed

in black (reading color words (RCW)), name the color of colored patches(NCP), and they also had to name the print color of color words, with print


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color and color names being incongruent (interference (INT)). To account for

reading speed and color-naming speed, reaction times (RTs) in the interfer-

ence condition were analyzed using RTs in the RCW and NCP conditions as

covariates. The number of errors in the interference condition was also recorded.

In the AX-CPT task, letters were presented sequentially on a computer

screen. Letter size was 3.8 3.8 visual angle at a distance of 40 cm. In

70% of the trials, the letter X followed the letter A and subjects had to

press a target key to the A-X sequence. The remaining trial types were A

followed by the letter Y (the inhibition trial type), B-X and B-Y trial types

(Y represents all letters except X, and B represents all letters except

A) with a frequency of 10% each. To these combinations, subjects had to

press a nontarget key on the keyboard. Subjects used the index and middle

finger of their dominant hand. The high frequency of the A-X combinationinduced a prepotent response tendency to the target key after presentation of

the letter, A which had to be suppressed for the rare A-Y sequence (which

requires the nontarget key).

The task was administered in a short- and a long-delay condition (100

trials each), with order being randomized across subjects. Each trial started

with a hyphen presented for 200 ms in the center of the screen for fixation,

followed by the first letter (200 ms). The interstimulus interval (ISI) was

1000 ms (short-delay condition) or 4000 ms (long-delay condition), thereby

varying working memory load. Then the second letter appeared for 200 ms,

to which the subject had to respond.Median RTs and errors were recorded for each trial type. Based on the

procedure used by Braver et al. (1999) and Carter et al. (1998), difference

scores representing inhibition and use of context entered analysis:

1. Context cost: RTs A-Y minus RTs B-Y: Degree of response slow-

ing in nontarget trials where the prepotent response tendency has to

be suppressed.

2. Inhibition cost: RTs A-Y minus RTs A-X. Index of inhibition,

where the prepotent response to the target key induced by A needs

to be inhibited if Y occurs.

3. Context use: RTs B-X minus RTs B-Y. Degree of response slowing

in nontarget trials on the ambiguous X stimulus. Small differences

between both trial types indicate benefit from use of context.

Task Management

Task management in stimulus processing was assessed by two tasks,

the Divided Attention subtest of a German Attention Test Battery

(Zimmermann & Fimm, 1993) and by a dual-task paradigm described byStablum et al. (2000). In the Divided Attention task, subjects had to


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process two sensory channels in parallel, while motor output did not differ.

In the Dual Task, subjects had to coordinate between information from

two sensory channels and two motor responses.

In the Divided Attention task, subjects were asked to press a key as

fast as possible to particular stimulus configurations presented in two sen-

sory domains. In the visual domain, subjects had to respond if crosses

appearing randomly on a computer screen formed a square. Subjects had to

simultaneously attend to a sequence of high and low pitched tones. If two

consecutive tones were of the same pitch, the response key had to be

pressed. Reaction times and errors were recorded.

The Dual Task procedure comprised a single- and a dual-task part

and assessed the ability to process two stimulus features and to coordinate

two motor responses. During both subtests, two letters (3.8 9.5

visual

angle at a distance of 40 cm) appeared in a vertical arrangement on a com-

puter screen (17), either to the right or to the left of a central fixation point.

The letters were either the same or different (50% each) and were presented

for 150 msec. In the initial single task part, subjects were instructed to press

a left or a right key, depending on the location relative to the central fixation

point. In the dual task part, subjects also had to make the location decision

by pressing the respective keys, but they additionally had to indicate ver-

bally whether the letters were the same or different. Eighty trials were pre-

sented in each condition; RTs and errors were recorded. In addition, dual

task costs were assessed by calculating a dual task variable (DTC: (RTs dualtask RTs single task)/RTs single task).

General Procedure

All subjects completed the test battery in one session. Each session

lasted 1.5 to 2 hours. Tasks were administered in the following order: Verbal

Fluency, Cognitive Estimation Test, Word List Recall (immediately),

Divided Attention, AX-CPT, Word List Recall (delayed), Stroop-Test, Dual-

Task, DEX. AX-CPT test administration was followed by a rest interval.

The list order for word list recall was randomized between subjects.

Data Analysis

Statistical analysis was performed using SPSS version 11.0 (SPSS

Inc.). Group differences were evaluated by nonparametric Kruskal-Wallis-H or

Analysis of Variance (ANOVA), where appropriate. If significant group dif-

ferences were found, pairwise comparisons were performed (Mann-Whitney-U

and Bonferroni, respectively). For nonparametric post-hoc comparisons, the

significance level was set to p < .008. Repeated measure ANOVAs were

performed with Bonferroni adjusted post-hoc testing. When Analysis of

Covariance (ANCOVA) was performed, the significance level for post-hocpairwise ANCOVAS was set to p < .008. For Pearson correlations, the


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significance level was set top < .05. In addition, eta2 effect sizes were calcu-

lated in case the demands for ANOVA were met.

RESULTS


The results for word list recall are presented in Table 2. Repeated mea-

sures ANOVA with Group, List (CC, RC, RR) and Delay (immediate vs.

delayed free recall) as factors did not yield a significant group effect (p =

.19) or significant interactions involving the group factor (allp > .59). Effect

size was eta2 = 0.3. A significant delay effect [F1,58 = 450.808; p < .001]

indicated better recall at immediate relative to delayed recall. A significant

list effect [F2,116 = 51.146; p < .001], followed by a paired t-test indicated

best recall of the consecutive categories and randomized categories list and

poorest recall of the uncategorized list (allp < .001).

Analysis of retention rates with group and list as factors indicated a

significant main effect for List [F2,116 = .511; p < .001], but no significant

effects involving the group factor (allp > .79). For the CC-list, effect size

was eta2 = 0.04, for the RC list it was eta2 = 0.01 and eta2 = 0.06 for the

RR-list.

Verbal Fluency/Cognitive Flexibility

The results for the three fluency subtests are presented in Table 2. The

four age groups did not differ significantly with respect to the number of

correctly produced items in the semantic (p = .18; eta2 = 0.08), phonemic

(p = .60; eta2 = 0.03), or switching condition (p = .25; eta2 = 0.07).

Reasoning

The results for the Cognitive Estimation Test are presented in Table 2.

Nonparametric group comparisons did not reveal significant age group

differences (Hdf = 3.58 = 2.265;p = .52).

Everyday Consequences of Executive Impairment

The results for the DEX questionnaire are presented in Table 2.

Repeated measure ANOVA with group and condition (self-rating vs. inde-

pendent rating of dysexecutive behavior) as factors did not yield a signifi-

cant group effect (p = .56, eta2 = 0.04) or a significant interaction (p = .12,

eta2 = 0.11).

Separate group comparisons of subjects DEX self-rating and relatives

independent rating of dysexecutive behavior did not reveal significant group

differences [(F3,58 = .241; p = .87; eta2

= 0.01) and (F5,50 = 1.392; p = .26;eta2 = 0.07), respectively].


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Inhibition

Stroop Test

Results for the Stroop Test are presented in Figure 1. ANCOVA with

RTs in the RCW and NCP conditions as covariates indicated a significantgroup effect for RTs in the INT condition (F3,56 = 6.302;p = .001, eta

2 = 0.25).

TABLE 2. Means (SDs) for Cognitive Performance in the Four Age Groups

2030 Yrs. 3145 Yrs. 4660 Yrs. 6175 Yrs.


Word list recall immediately (no. correct)

consecutive categories 10.6 (2.0) 10.9 (2.6) 9.5 (3.6) 9.7 (2.1)

randomized categories 9.1 (2.3) 8.4 (3.5) 7.5 (2.3) 8.2 (2.3)

uncategorized 7.6 (2.2) 7.7 (2.6) 6.3 (1.8) 6.4 (1.7)

Word list recall delayed (no. correct)

consecutive categories 6.8 (2.2) 6.4 (2.8) 5.7 (2.9) 5.1 (2.4)

randomized categories 6.1 (3.2) 5.3 (3.8) 4.9 (2.6) 5.3 (2.5)

uncategorized 2.6 (1.6) 3.4 (2.8) 2.6 (2.0) 1.8 (1.5)

Verbal Fluency/Cognitive Flexibility (no. correct)

semantic 29.9 (9.5) 28.1 (6.5) 25.7 (7.4) 24.5 (5.9)

phonemic 13.1 (3.8) 12.9 (2.6) 13.2 (3.2) 11.8 (3.8)switching 16.3 (4.0) 17.1 (2.5) 14.7 (2.7) 15.9 (3.0)

Reasoning

Cognitive Estimation Test 4.5 (2.5) 3.7 (3.1) 3.4 (2.5) 3.5 (2.4)

Subjective Rating

DEX self-rating 22.1 (6.0) 22.7 (11.7) 22.3 (7.6) 20.4 (7.0)

DEX independent rating 14.9 (8.0) 17.4 (9.7) 21.7 (10.7) 23.7 (17.8)

Inhibition

Stroop Test

RTs RCW 25.9 (2.1) 28.6 (5.0) 30.0 (5.8) 33.8 (3.8)

RTs NCP 44.1 (6.9) 45.0 (8.4) 43.2 (9.5) 48.7 (6.7)

RTs INT 67.4 (10.2) 70.9 (14.6) 80.2 (18.8) 91.5 (14.6)

INT no. corrected errors 2.9 (2.3) 3.0 (2.7) 2.5 (2.9) 4.0 (4.1)

INT no. uncorrected errors 0.8 (0.7) 0.6 (1.0) 2.6 (2.8) 2.5 (2.4)

AX-CPT

context use 10.8 (35.7) 6.4 (34.8) 16.4 (60.3) 13.5 (38.7)

context costs 220 (53) 189 (85) 200 (57) 341 (117)

inhibition costs 145 (43) 147 (59) 137 (64) 211 (93)

Multi-tasking

Divided Attention

RTs in msec 597 (67) 641 (55) 694 (51) 707 (74)

no. errors 2.4 (2.5) 2.4 (2.2) 5.2 (3.4) 4.4 (3.4)

Dual task

RTs single condition 298 (33) 345 (45) 364 (65) 397 (44)

single condition no. errors 0.7 (0.9) 0.4 (0.6) 0.4 (0.7) 0.4 (0.7)

RTs dual condition 386 (48) 431 (116) 454 (66) 675 (138)dual condition no. errors 2.2 (2.0) 1.3 (1.5) 2.1 (3.2) 5.6 (4.2)

DEX-Dysexecutive Questionnaire; AX-CPT-AX-Continuos Performance Test.


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After Bonferroni correction, subsequent paired group ANCOVAs revealed

significant group effects for comparisons of the oldest and the two youngest

groups (allp < .008), indicating a significant slowing of the 61+ group in the

interference condition. The results for errors in the interference condition are

presented in Table 2. The four age groups did not differ with respect to cor-

rected errors (Hdf = 3 = 1.344;p = .72), but there was a significant group dif-

ference for uncorrected errors (Hdf = 3 = 12.092; p < .01), with the oldest

group making more errors than the youngest group (p = .001).

Continuous Performance Task

Because repeated measures ANOVA of RTs with Group, Condition,

and Delay as factors did not yield any significant interactions involving the

delay factor (allp > .38), further analyses were performed with RTs pooled

for the two delays. Analysis of inhibition costs (RTs A-Y minus A-X, see

Figure 2) and context costs (RTs A-Y minus B-Y, see Figure 3) yielded sig-

nificant age group differences [(F3.58 = 4.159,p < .01, eta2 = 0.18) and (F3,58 =

11.418,p < .001, eta2 = 0.37), respectively], with higher inhibition cost for

the oldest compared to the youngest and second oldest group (all p < .05)and higher context cost (all p .001) for the oldest group compared to all

FIGURE 1. Mean interference scores and SDs of the four age groups in the Stroop Test.

INT-interference; RCW-read color words; NCP-name color pictures.


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other groups. Analysis of context use (RTs B-X minus B-Y, see Table 2) did

not yield significant group differences (F3,58 = 1.594,p = .20, eta2 = 0.08).

In further analyses, group differences were explored using RTs in theB-Y condition as covariate. ANCOVA yielded highly significant age group

FIGURE 2. Means and SDs of inhibition costs (RTs A-Y minus A-X) in the AX-CPT task for the

four age groups.

FIGURE 3. Means and SDs of context costs (RTs A-Y minus B-Y) in the AX-CPT task for the

four age groups.


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differences for inhibition cost (p < .01) and context cost (p < .001), and the

respective paired age group comparisons also remained significant for

context costs (all p .001). For inhibition costs, paired group compari-

sons revealed significant differences between the oldest and youngest

group (p < .004).

Task Management

Divided Attention

Results for the Divided Attention task are presented in Figure 4. There

was a significant age group difference for RTs (F3,58 = 10.193;p < .001, eta2 =

0.35), with the oldest group showing longer RTs than the youngest (p < .001)

and the second youngest group (p < .02). The second oldest group was also

slower than the youngest group (p = .001). There was a significant age group

difference for errors (Hdf = 3 = 10.655;p < .01). Post-hoc comparisons did not

yield significant differences for error rates (allp > .008).

Dual Task

The results for the dual task are illustrated in Table 2 and Figures 5 and 6.

RT analysis with Group and Task as factors yielded a significant interaction

(F3,57 = 24.482; p < .001, eta2 = 0.55). Post-hoc paired group comparisons

revealed significant Group Task interactions (allp < .001) in all compari-

sons involving the oldest group, suggesting that the oldest group showedlarger dual task costs than any of the other groups.

FIGURE 4. Means and SDs of RTs and number of errors in the divided attention task for the

four age groups.


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FIGURE 5. Scattergram of mean RTs in the single and dual condition of the dual task.

FIGURE 6. Mean RTs and SDs in the single and dual condition of the dual task for the four age groups.


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Comparable results emerged for the dual task costs variable (DTC:

(RTs dual task RTs single task)/RTs single task). The groups differed sig-

nificantly on DTC [F3,57

= 18.112;p < .001, eta2 = 0.49]. Post-hoc compari-

sons revealed significant differences between the oldest group and all other

groups (allp < .001).

Repeated measures ANOVA of errors with Group and Task as factors

yielded a significant interaction (F3,57 = 7.205;p < .001, eta2 = 0.28). Post-

hoc analyses yielded significant Group Task interactions (all p < .003)

when the oldest group was compared to the two youngest groups. Compari-

sons including the oldest group, indicating a disproportional increase in

errors from the single to the dual task condition in the oldest group.

After statistical control of response speed by using RTs of the single

task as covariate, ANCOVA yielded a highly significant age group differ-ence for RTs in the dual task (F3,56 = 14.823; p < .001, eta

2 = 0.44), which

was due to slower RTs in the oldest compared to the second youngest and

second oldest group (all p .001).

Correlations

To supplement the group comparisons, cognitive test performance was

correlated with age. Significant correlations were found for found for a

range of executive function variables of the Stroop Test (INT-RCW, r = .50,

p < .001; INT-NCP, r = .63,p < .001; uncorrected errors, r = .45,p < .001),

the AX-CPT (inhibition cost, r = .34, p < .01; context cost, r = .32, p = .01;context benefit, r = .26, p = .05), divided attention (RTs, r = .58, p < .001;

errors, r = .36,p = .004) and the dual task (RTs: single condition, r = .65,p

differential course of executive control changes during normal aging

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