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Journal of Clinical and Experimental Neuropsychology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ncen20
Cognitive reserve moderates relation between global cognition and functional status in older adults a
a
Bryant Duda , Antonio N. Puente & Lloyd Stephen Miller
a
a
Department of Psychology, University of Georgia, Athens, GA, USA Published online: 10 Mar 2014.
Click for updates To cite this article: Bryant Duda, Antonio N. Puente & Lloyd Stephen Miller (2014) Cognitive reserve moderates relation between global cognition and functional status in older adults, Journal of Clinical and Experimental Neuropsychology, 36:4, 368-378, DOI: 10.1080/13803395.2014.892916 To link to this article: http://dx.doi.org/10.1080/13803395.2014.892916
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Journal of Clinical and Experimental Neuropsychology, 2014 Vol. 36, No. 4, 368–378, http://dx.doi.org/10.1080/13803395.2014.892916
Cognitive reserve moderates relation between global cognition and functional status in older adults Bryant Duda, Antonio N. Puente, and Lloyd Stephen Miller Department of Psychology, University of Georgia, Athens, GA, USA
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
(Received 7 October 2013; accepted 5 February 2014) The ability to perform instrumental activities of daily living (IADLs) is necessary for independent living. Research suggests that community-dwelling older adults are at risk for experiencing subtle decrements in the performance of IADLs. Neuropsychological tests have been used to account for differences in IADL status. Studies of the relationship between cognitive ability and functional status have produced variable results, however, and cognitive ability appears to be only a moderate predictor. Several studies of normal aging have revealed cognitive and functional benefits of higher cognitive reserve (CR) in healthy, nondemented older adults. The purposes of the present study were to: (a) examine the relationship between global cognitive ability and IADL performance among 53 community-dwelling older adults, and (b) determine whether formal education, as a proxy of CR, significantly moderates this relationship. Consistent with previous findings, global cognitive ability accounted for a considerable portion of variance in IADL performance [ΔR2 = .54; ΔF(2, 53) = 67.96; p < .001]. Additionally, CR modestly but significantly attenuated this relationship [ΔR2 = .044; ΔF(4, 53) = 5.98; p = .018; total R2 = .65]. This finding suggests that community-dwelling older adults with lower levels of formal education may be at greater risk for functional decrements associated with age-related cognitive decline. Keywords: Cognitive reserve; Activities of daily living; Aging; Cognition; Education.
Advancements in health care are increasing the lifespan of individuals worldwide. The number of people aged 65 or older is projected to grow from an estimated 524 million in 2010 to nearly 1.5 billion in 2050 (National Institute on Aging & World Health Organization, 2011). This will be accompanied by new and potentially daunting challenges. Providing care for older persons will undoubtedly be costly, in large part due to a parallel increase in the prevalence of Alzheimer’s disease (AD). Alzheimer’s Disease International (2010) estimated that the total worldwide cost of dementia exceeded $600 billion in 2010. Notably, over and above the direct costs of medical and social care provided by community professionals, a considerable portion of this estimate (i.e., 42%) was attributed to the need for informal assistance
(e.g., unpaid care by families) with activities of daily living (ADLs; e.g., eating, dressing, bathing, shopping, preparing food, managing finances) due to cognitive and functional decline. Costs of informal care will likely extend beyond provisions made for individuals affected by AD or dementia. Empirical evidence suggests that communitydwelling older adults are at risk for experiencing subtle decrements in the performance of instrumental activities of daily living (IADLs)—that is, abilities necessary for independent living, such as preparing meals, managing money, and taking medications (Burton, Strauss, Hultsch, & Hunter, 2006; Grigsby, Kaye, Baxter, Shetterly, & Hamman, 1998; Mariani et al., 2008). IADLs are often assessed via self- or informant reports of functionality in various domains, or less
Acknowledgements: The authors would like to thank Anisha Patel for her assistance with data acquisition and management. Disclosure statement: The study was approved by the University of Georgia institutional review board (IRB). The authors had no conflict of interest when conducting this research or reporting the results. Address correspondence to: Bryant Duda, University of Georgia, Department of Psychology, Athens, GA 30602-3001, USA (E-mail: [email protected]).
© 2014 Taylor & Francis
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
RESERVE, COGNITION, AND FUNCTIONAL STATUS
often by performance-based measures (i.e., clinicianrated, laboratory assessments). Recent findings suggest that performance-based measures of functional status have provided the strongest evidence of IADL variability among community-dwelling older adults. For example, performance-based measures have demonstrated sensitivity to the healthy aging process (Chapman, Duberstein, & Lyness 2007; Mitchell & Miller, 2008; Schmitter-Edgecombe, Parsey, & Cook, 2011: Suchy, Kraybill, & Franchow, 2011) and to be effective at delineating deficits among patients with mild cognitive impairment (MCI), AD, and healthy older adults (Pereira et al., 2010; Tabert et al., 2002; Teng, Becker, Woo, Cummings, & Lu, 2010). The degree of functional variability among community-dwelling older adults, however, is unclear. One reason is due to the discrepancy between subjective (i.e., self- and informant report) and objective (i.e., performance-based) measures of functional status. Recent findings suggest that this discrepancy may be influenced by multiple factors, such as lack of insight or self-awareness (Mitchell & Miller, 2008; Suchy et al., 2011), personality factors (Suchy, Williams, Kraybill, Franchow, & Butner, 2010), and reporter biases (Loewenstein et al., 2001; Royall et al., 2007). Additionally, although performance-based tools may improve upon the accuracy and prediction of functional status, researchers and clinicians often find them somewhat cumbersome and time consuming and thus better suited to academic rather than clinical applications (Cahn-weiner et al., 2010; Mitchell & Miller, 2008; Mitchell et al., 2010; Rosenthal et al., 2013; Royall et al., 2007; Suchy et al., 2011). As a result, performance-based measures have not been utilized as the “gold standard” for functional assessment (Mitchell & Miller, 2008). Nonetheless, clinicians are increasingly being asked to answer questions regarding everyday functioning of older adults (Marcotte, Scott, Kamat, & Heaton, 2010). Clinicians have used neuropsychological measures to account for differences in functional status among older adults. However, neuropsychological tests are designed to evaluate specific cognitive abilities rather than functional abilities, and empirical evidence to support such test use is scant (Chaytor & SchmitterEdgecombe, 2003). Evidence does suggest that measures of global cognition (e.g., the Mini-Mental State Examination; Folstein, Folstein, & McHugh, 1975, Repeatable Battery for Assessment of Neuropsychological Status Total Score; Randolph, 1998, Telephone Interview for Cognitive Status Total Score; Brandt & Folstein, 2003), as well as measures of executive functioning (e.g., DelisKaplan Executive Function System; Delis, Kaplan,
369
& Kramer, 2001, Wisconsin Card Sorting Test; Robinson, Heaton, Lehman, & Stilson, 1980) account for the greatest proportion of variance in functional ability (Bell-McGinty, Podell, Franzen, Baird, & Williams, 2002; Farias, Harrell, Neumann, & Houtz, 2003; Inzarti & Basile, 2003; Mitchell & Miller, 2008; Mitchell et al., 2010; Royall et al., 2007; Schmitter-Edgecombe et al., 2011). However, comparisons across studies are difficult due to differences in how and which cognitive abilities are measured, age groups and populations studied (e.g., cognitively intact versus cognitively impaired), and covariates controlled. These cognitive variables, furthermore, are only modest predictors of functional status in older adults. A review by the Committee on Research of the American Neuropsychiatry Association, for example, reported that measures of cognitive functions explained, on average, only 21% of variance in functional outcomes (Royall et al., 2007). Of note, this review included self- and informant-report measures of basic activities of daily living (BADL; e.g., eating, dressing, grooming, toileting) and IADL capacities. Given that cognitive variables appear to account for a greater proportion of variance in performancebased measures of ADLs, therefore, this finding may underrepresent the strength of the relationship between cognition and functional ability. Nonetheless, it is clear that a considerable proportion of IADL variance among older adults remains unaccounted for by cognitive measures. One variable that may help in the understanding of the relationship between cognitive ability and functional status is cognitive reserve (CR). The term “reserve” has been cited as a theoretical framework for explaining individual differences in functional/behavioral responses to neuronal disease or injury (Satz, 1993; Stern, 2002). Specifically the concept of CR has traditionally been defined on the basis of a hypothetical construct that may buffer the effects of brain pathology on clinical outcomes (Satz, Cole, Hardy, & Rassovsky, 2011). The vast majority of research on CR has concerned AD or nonspecific cognitive impairment and decline (Sachdev & Valenzuela, 2009). The protective effects of education in incident dementia, for example, have been well documented (Stern, 2009; Valenzuela & Sachdev, 2005). Further, it has been well documented that higher levels of education and occupational attainment can serve as buffers against the clinical manifestations of dementia (Hall et al., 2007; Stern, 2002). Multiple proxies of reserve have been used in research, including premorbid IQ (e.g., Wechsler Test of Adult Reading; Psychological Corporation, 2001; National Adult Reading Test–Revised; Nelson
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
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& Willison, 1991; Vocabulary subtest of the Wechsler Adult Intelligence Scale–Fourth Edition; Wechsler, 2008), occupational complexity, educational attainment, and mental activities. Pertinent to the current investigation of healthy, community-dwelling older adults, several studies of normal aging have reported slower cognitive and functional decline in nondemented individuals with higher educational attainment (Albert et al., 1995; Butler, Ashford, & Snowdon, 1996; Chodosh, Reuben, Albert, & Seeman, 2002; Christensen et al., 1997; Colsher & Wallace, 1991; Farmer, Kittner, Rae, Bartko, & Regier, 1995; Lyketsos, Chen, & Anthony, 1999; Snowdon, Ostwald, & Kane, 1989). From a theoretical standpoint, therefore, CR may protect against functional decline that is associated with cognitive change, such that individuals with lower levels of CR are at greater risk for earlier functional decrements and/or decline. As explained by Frazier, Tix, and Barron (2004), the identification of moderator variables (Baron & Kenny, 1986; Holmbeck, 1997) permits researchers to more closely explore “when” or “for whom” a predictor is more strongly related to an outcome. Given the indirect nature of the relationship between cognition and functioning, identification of a moderator effect may improve our understanding of the relationship between cognitive ability and IADL performance. In the present study, we investigated whether global cognitive ability predicts independent functional ability in community-dwelling older adults and whether this relationship is moderated by CR. In support of previous findings (Farias et al., 2003; Miller, Brown, Mitchell, & Williamson, 2013; Mitchell et al., 2010), we first hypothesized that global cognition, over and above age, would account for a significant amount of IADL variance. More importantly, in an effort to expand upon our current understanding of the relationship between global cognitive ability and IADL performance in community-dwelling older adults, we hypothesized that years of formal education, as a proxy of CR, would moderate the relationship between cognitive ability and IADL performance. Specifically, we expected that global cognition would be less predictive of IADL performance in healthy older adults with higher CR.
METHOD Participants Participants were 53 (64% female) right-handed, community-dwelling healthy adults, ranging in age from 65 to 84 years (M = 74.17, SD = 5.64)
and education from 9 to 21 years (M = 15.93, SD = 2.77). Participants were recruited from a host of local resources and events, including local retirement communities, caregiver support groups, and those attending one of a number of community talks by the investigators on memory functioning and aging in the Northeast GA area. Exclusion criteria included absence of a reliable informant, self-reported history of neurological disorder, or dementia (clinical dementia rating; CDR > 0.5). Of the 53 participants, 32 individuals were classified with a CDR of 0, and 21 were classified with a CDR of 0.5. Participants were compensated with a small honorarium for their involvement in the study. Upon request, the participant or collateral were provided with contact information for referral sources (e.g., memory clinic, local neurologist). Procedure After a telephone screening, testing commenced in a neuropsychological laboratory. All participants underwent standard informed consent procedures. Each participant and informant participated in a semistructured interview from which the participant’s CDR (Morris, 1993) was derived by a CDR-certified administrator. As part of a larger study, participants completed several self-report measures, were administered a battery of cognitive measures, and completed the IADL assessment. The testing session lasted approximately 90 minutes. This protocol was reviewed and approved by the university Institutional Review Board. Measures Clinical dementia rating The clinical dementia rating (CDR; Morris, 1993) is a semistructured interview designed to rate severity of dementia through the assessment of six domains of cognitive and functional abilities. Domains assessed on the CDR include memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care. Each domain is rated and entered into an algorithm to generate an overall summary score on a 5-point scale. Administrators were trained in the CDR interview and scoring techniques by the CDR author via an online training course (The Brief Training and Reliability Protocol, 2011). The Brief Training and Reliability Protocol (BTRP) includes an introduction to the CDR, three videotaped patient interviews for training purposes, and six videotaped interviews for
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RESERVE, COGNITION, AND FUNCTIONAL STATUS
reliability certification. Each of the CDR administrators achieved reliability certification by successfully completing agreement with a “gold standard” on at least five out of the six reliability tapes. The CDR has been well validated as a classification tool for dementia severity (Morris, 1993), with demonstrated interrater reliability of 80% (Burke et al., 1988; McCulla et al., 1989). The CDR was used to rule out participants with dementia (i.e., participants with CDR scores >0.5 were excluded). Developers of the CDR determined a score of 0.5 to evidence some cognitive change that is not severe enough to classify an individual with “mild dementia” (Morris, 1993). While there is some evidence to suggest that very mild dementia may be overlooked by a CDR of 0.5 (Chang et al., 2011), our use of the CDR as exclusionary criteria was to reduce invalid data reporting by participants. Cognitive assessment Cognitive ability was assessed with the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS; Randolph, 1998). The RBANS is a 30-min neuropsychological test designed to assess cognitive decline in older adults and serve as a screening tool for cognitive functioning in younger adults (Randolph, Tierney, Mohr, & Chase, 1998). The RBANS consists of 12 subtests that generate five index scores: Visuospatial/Constructional, Attention, Language, Immediate Memory, and Delayed Memory. The RBANS additionally yields a global score (the total scale score, TS). The RBANS TS raw scores were used in all analyses. RBANS scores have demonstrated split-half reliability coefficients in the .80s for the index scores and an average reliability for the total scale score ranging from .86 to .94 across age groups (Randolph et al., 1998). Cognitive reserve Formal years of education served as our proxy measure of CR and were acquired from each participant via self-report. Education was treated as a continuous variable in the analyses. Following the procedures of Siedlecki et al. (2009), formal years of educational attainment were considered, with 20 the highest level representing a doctoral degree, 18 for a master’s degree, 16 for a bachelor’s degree, 14 for an associate’s degree, and 12 for a high-school degree or GED (General Educational Development tests). Additionally, each year of formal education, regardless of whether it was or was not a degree-granting year, was counted as a
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formal year of education. For example, a highschool graduate who completed one year of college would be considered to have completed 13 years of formal education. The maximum number of years of education was limited to 20. Assessment of functional status Functional status was assessed with the Direct Assessment of Functional Status–Revised (DAFS– R; Loewenstein et al., 1989). The DAFS–R is a clinician-rated, laboratory assessment of activities of daily living (ADLs). ADLs have been classified into two domains (BADLs and IADLs) based on the complexity of tasks related to time orientation, communication, transportation, preparing for grocery shopping, financial skills, dressing, grooming, and eating. Scores from the individual scales combine to create a total score ranging from 0–133. In order to focus on IADLs, we used criteria specified by Mitchell and Miller (2008) to separate the different domains into IADL or BADL categories. Specifically, the BADL domain was computed based on their raw scores on the dressing/grooming and eating items of the DAFS–R. The IADL domain then included functional abilities that recruit higher order cognitive processes, including communication (i.e., using the telephone, preparing a letter for mailing), financial skills (i.e., identifying currency, counting change, writing a check, balancing a checkbook), shopping (i.e., memory for grocery items, selecting grocery items with a list, making correct change), driving (i.e., correct identification of road signs), meal preparation (i.e., following cooking instructions), and taking a telephone message (see Table 1). The DAFS–R IADL raw scores were used in all analyses. Interrater reliability has demonstrated to be at least 85% on TABLE 1 DAFS–R IADL categories and tasks Domain Communication Financial skills
Shopping
Driving Meal preparation Taking a phone message
IADL task Using a telephone Preparing a letter for mailing Identifying currency Counting change Writing a check Balancing a checkbook Memory for grocery items Selecting items with a list Making correct change Identifying road signs Following cooking instructions
Note. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living.
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each item of the scale (Loewenstein et al., 1989). Test–retest reliability coefficients range from .55 to .91 for the various subscales.
TABLE 3 Correlations among demographics, global cognition, and IADL performance Variable
Age
Education
RBANS TS
— .12 .25 –.02 .04
— .23 .13 –.24
— .29* .19
— .70**
Gender Age Education RBANS TS DAFS–R IADL
RESULTS Preliminary analyses
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Gender
Data were analyzed using the Statistical Package for Social Sciences (SPSS 20.0 for Windows, SPSS, Chicago, IL) and RStudio (RStudio 0.96.122 for Windows, Boston, MA). Demographic, cognitive, and functional characteristics of the sample are displayed in (Table 2). Consistent with previous studies (Chapman et al., 2007; Suchy et al., 2011), performance on IADL assessment among communitydwelling older adults was variable, with scores ranging from 54 to 91 (M = 75.76, SD = 8.40; DAFS– R IADL maximum raw score = 91). Typical errors produced on these tasks were making incorrect change, difficulty grocery shopping from memory, and errors in taking a telephone message. Initial bivariate correlation analyses were conducted to examine zero-order correlations among demographic, cognitive, and functional characteristics of the sample (see Table 3). As expected, global cognitive and IADL performance scores were significantly correlated (r = .70; p < .01), as were years of education and global cognitive scores TABLE 2 Demographic data and mean summary data for the sample Variable
Mean
SD
Age (years) Education (years) RBANS TS DAFS–R IADL
74.17 15.93 96.04 75.76
5.64 2.77 17.63 8.40
Range 65–84 9–20 55–145 54–91
(19) (11) (90) (37)
Note. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living (max score = 91); RBANS TS = Repeatable Battery for the Assessment of Neuropsychological Status total score of performance on five subtests; education = years of formal education attained.
Notes. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living (max score = 91); RBANS TS = Repeatable Battery for the Assessment of Neuropsychological Status total score of performance on five subtests; education = years of formal education attained. *p < .05. **p
Journal of Clinical and Experimental Neuropsychology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ncen20
Cognitive reserve moderates relation between global cognition and functional status in older adults a
a
Bryant Duda , Antonio N. Puente & Lloyd Stephen Miller
a
a
Department of Psychology, University of Georgia, Athens, GA, USA Published online: 10 Mar 2014.
Click for updates To cite this article: Bryant Duda, Antonio N. Puente & Lloyd Stephen Miller (2014) Cognitive reserve moderates relation between global cognition and functional status in older adults, Journal of Clinical and Experimental Neuropsychology, 36:4, 368-378, DOI: 10.1080/13803395.2014.892916 To link to this article: http://dx.doi.org/10.1080/13803395.2014.892916
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Journal of Clinical and Experimental Neuropsychology, 2014 Vol. 36, No. 4, 368–378, http://dx.doi.org/10.1080/13803395.2014.892916
Cognitive reserve moderates relation between global cognition and functional status in older adults Bryant Duda, Antonio N. Puente, and Lloyd Stephen Miller Department of Psychology, University of Georgia, Athens, GA, USA
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
(Received 7 October 2013; accepted 5 February 2014) The ability to perform instrumental activities of daily living (IADLs) is necessary for independent living. Research suggests that community-dwelling older adults are at risk for experiencing subtle decrements in the performance of IADLs. Neuropsychological tests have been used to account for differences in IADL status. Studies of the relationship between cognitive ability and functional status have produced variable results, however, and cognitive ability appears to be only a moderate predictor. Several studies of normal aging have revealed cognitive and functional benefits of higher cognitive reserve (CR) in healthy, nondemented older adults. The purposes of the present study were to: (a) examine the relationship between global cognitive ability and IADL performance among 53 community-dwelling older adults, and (b) determine whether formal education, as a proxy of CR, significantly moderates this relationship. Consistent with previous findings, global cognitive ability accounted for a considerable portion of variance in IADL performance [ΔR2 = .54; ΔF(2, 53) = 67.96; p < .001]. Additionally, CR modestly but significantly attenuated this relationship [ΔR2 = .044; ΔF(4, 53) = 5.98; p = .018; total R2 = .65]. This finding suggests that community-dwelling older adults with lower levels of formal education may be at greater risk for functional decrements associated with age-related cognitive decline. Keywords: Cognitive reserve; Activities of daily living; Aging; Cognition; Education.
Advancements in health care are increasing the lifespan of individuals worldwide. The number of people aged 65 or older is projected to grow from an estimated 524 million in 2010 to nearly 1.5 billion in 2050 (National Institute on Aging & World Health Organization, 2011). This will be accompanied by new and potentially daunting challenges. Providing care for older persons will undoubtedly be costly, in large part due to a parallel increase in the prevalence of Alzheimer’s disease (AD). Alzheimer’s Disease International (2010) estimated that the total worldwide cost of dementia exceeded $600 billion in 2010. Notably, over and above the direct costs of medical and social care provided by community professionals, a considerable portion of this estimate (i.e., 42%) was attributed to the need for informal assistance
(e.g., unpaid care by families) with activities of daily living (ADLs; e.g., eating, dressing, bathing, shopping, preparing food, managing finances) due to cognitive and functional decline. Costs of informal care will likely extend beyond provisions made for individuals affected by AD or dementia. Empirical evidence suggests that communitydwelling older adults are at risk for experiencing subtle decrements in the performance of instrumental activities of daily living (IADLs)—that is, abilities necessary for independent living, such as preparing meals, managing money, and taking medications (Burton, Strauss, Hultsch, & Hunter, 2006; Grigsby, Kaye, Baxter, Shetterly, & Hamman, 1998; Mariani et al., 2008). IADLs are often assessed via self- or informant reports of functionality in various domains, or less
Acknowledgements: The authors would like to thank Anisha Patel for her assistance with data acquisition and management. Disclosure statement: The study was approved by the University of Georgia institutional review board (IRB). The authors had no conflict of interest when conducting this research or reporting the results. Address correspondence to: Bryant Duda, University of Georgia, Department of Psychology, Athens, GA 30602-3001, USA (E-mail: [email protected]).
© 2014 Taylor & Francis
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
RESERVE, COGNITION, AND FUNCTIONAL STATUS
often by performance-based measures (i.e., clinicianrated, laboratory assessments). Recent findings suggest that performance-based measures of functional status have provided the strongest evidence of IADL variability among community-dwelling older adults. For example, performance-based measures have demonstrated sensitivity to the healthy aging process (Chapman, Duberstein, & Lyness 2007; Mitchell & Miller, 2008; Schmitter-Edgecombe, Parsey, & Cook, 2011: Suchy, Kraybill, & Franchow, 2011) and to be effective at delineating deficits among patients with mild cognitive impairment (MCI), AD, and healthy older adults (Pereira et al., 2010; Tabert et al., 2002; Teng, Becker, Woo, Cummings, & Lu, 2010). The degree of functional variability among community-dwelling older adults, however, is unclear. One reason is due to the discrepancy between subjective (i.e., self- and informant report) and objective (i.e., performance-based) measures of functional status. Recent findings suggest that this discrepancy may be influenced by multiple factors, such as lack of insight or self-awareness (Mitchell & Miller, 2008; Suchy et al., 2011), personality factors (Suchy, Williams, Kraybill, Franchow, & Butner, 2010), and reporter biases (Loewenstein et al., 2001; Royall et al., 2007). Additionally, although performance-based tools may improve upon the accuracy and prediction of functional status, researchers and clinicians often find them somewhat cumbersome and time consuming and thus better suited to academic rather than clinical applications (Cahn-weiner et al., 2010; Mitchell & Miller, 2008; Mitchell et al., 2010; Rosenthal et al., 2013; Royall et al., 2007; Suchy et al., 2011). As a result, performance-based measures have not been utilized as the “gold standard” for functional assessment (Mitchell & Miller, 2008). Nonetheless, clinicians are increasingly being asked to answer questions regarding everyday functioning of older adults (Marcotte, Scott, Kamat, & Heaton, 2010). Clinicians have used neuropsychological measures to account for differences in functional status among older adults. However, neuropsychological tests are designed to evaluate specific cognitive abilities rather than functional abilities, and empirical evidence to support such test use is scant (Chaytor & SchmitterEdgecombe, 2003). Evidence does suggest that measures of global cognition (e.g., the Mini-Mental State Examination; Folstein, Folstein, & McHugh, 1975, Repeatable Battery for Assessment of Neuropsychological Status Total Score; Randolph, 1998, Telephone Interview for Cognitive Status Total Score; Brandt & Folstein, 2003), as well as measures of executive functioning (e.g., DelisKaplan Executive Function System; Delis, Kaplan,
369
& Kramer, 2001, Wisconsin Card Sorting Test; Robinson, Heaton, Lehman, & Stilson, 1980) account for the greatest proportion of variance in functional ability (Bell-McGinty, Podell, Franzen, Baird, & Williams, 2002; Farias, Harrell, Neumann, & Houtz, 2003; Inzarti & Basile, 2003; Mitchell & Miller, 2008; Mitchell et al., 2010; Royall et al., 2007; Schmitter-Edgecombe et al., 2011). However, comparisons across studies are difficult due to differences in how and which cognitive abilities are measured, age groups and populations studied (e.g., cognitively intact versus cognitively impaired), and covariates controlled. These cognitive variables, furthermore, are only modest predictors of functional status in older adults. A review by the Committee on Research of the American Neuropsychiatry Association, for example, reported that measures of cognitive functions explained, on average, only 21% of variance in functional outcomes (Royall et al., 2007). Of note, this review included self- and informant-report measures of basic activities of daily living (BADL; e.g., eating, dressing, grooming, toileting) and IADL capacities. Given that cognitive variables appear to account for a greater proportion of variance in performancebased measures of ADLs, therefore, this finding may underrepresent the strength of the relationship between cognition and functional ability. Nonetheless, it is clear that a considerable proportion of IADL variance among older adults remains unaccounted for by cognitive measures. One variable that may help in the understanding of the relationship between cognitive ability and functional status is cognitive reserve (CR). The term “reserve” has been cited as a theoretical framework for explaining individual differences in functional/behavioral responses to neuronal disease or injury (Satz, 1993; Stern, 2002). Specifically the concept of CR has traditionally been defined on the basis of a hypothetical construct that may buffer the effects of brain pathology on clinical outcomes (Satz, Cole, Hardy, & Rassovsky, 2011). The vast majority of research on CR has concerned AD or nonspecific cognitive impairment and decline (Sachdev & Valenzuela, 2009). The protective effects of education in incident dementia, for example, have been well documented (Stern, 2009; Valenzuela & Sachdev, 2005). Further, it has been well documented that higher levels of education and occupational attainment can serve as buffers against the clinical manifestations of dementia (Hall et al., 2007; Stern, 2002). Multiple proxies of reserve have been used in research, including premorbid IQ (e.g., Wechsler Test of Adult Reading; Psychological Corporation, 2001; National Adult Reading Test–Revised; Nelson
Downloaded by [SDSU San Diego State University] at 08:58 28 December 2014
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& Willison, 1991; Vocabulary subtest of the Wechsler Adult Intelligence Scale–Fourth Edition; Wechsler, 2008), occupational complexity, educational attainment, and mental activities. Pertinent to the current investigation of healthy, community-dwelling older adults, several studies of normal aging have reported slower cognitive and functional decline in nondemented individuals with higher educational attainment (Albert et al., 1995; Butler, Ashford, & Snowdon, 1996; Chodosh, Reuben, Albert, & Seeman, 2002; Christensen et al., 1997; Colsher & Wallace, 1991; Farmer, Kittner, Rae, Bartko, & Regier, 1995; Lyketsos, Chen, & Anthony, 1999; Snowdon, Ostwald, & Kane, 1989). From a theoretical standpoint, therefore, CR may protect against functional decline that is associated with cognitive change, such that individuals with lower levels of CR are at greater risk for earlier functional decrements and/or decline. As explained by Frazier, Tix, and Barron (2004), the identification of moderator variables (Baron & Kenny, 1986; Holmbeck, 1997) permits researchers to more closely explore “when” or “for whom” a predictor is more strongly related to an outcome. Given the indirect nature of the relationship between cognition and functioning, identification of a moderator effect may improve our understanding of the relationship between cognitive ability and IADL performance. In the present study, we investigated whether global cognitive ability predicts independent functional ability in community-dwelling older adults and whether this relationship is moderated by CR. In support of previous findings (Farias et al., 2003; Miller, Brown, Mitchell, & Williamson, 2013; Mitchell et al., 2010), we first hypothesized that global cognition, over and above age, would account for a significant amount of IADL variance. More importantly, in an effort to expand upon our current understanding of the relationship between global cognitive ability and IADL performance in community-dwelling older adults, we hypothesized that years of formal education, as a proxy of CR, would moderate the relationship between cognitive ability and IADL performance. Specifically, we expected that global cognition would be less predictive of IADL performance in healthy older adults with higher CR.
METHOD Participants Participants were 53 (64% female) right-handed, community-dwelling healthy adults, ranging in age from 65 to 84 years (M = 74.17, SD = 5.64)
and education from 9 to 21 years (M = 15.93, SD = 2.77). Participants were recruited from a host of local resources and events, including local retirement communities, caregiver support groups, and those attending one of a number of community talks by the investigators on memory functioning and aging in the Northeast GA area. Exclusion criteria included absence of a reliable informant, self-reported history of neurological disorder, or dementia (clinical dementia rating; CDR > 0.5). Of the 53 participants, 32 individuals were classified with a CDR of 0, and 21 were classified with a CDR of 0.5. Participants were compensated with a small honorarium for their involvement in the study. Upon request, the participant or collateral were provided with contact information for referral sources (e.g., memory clinic, local neurologist). Procedure After a telephone screening, testing commenced in a neuropsychological laboratory. All participants underwent standard informed consent procedures. Each participant and informant participated in a semistructured interview from which the participant’s CDR (Morris, 1993) was derived by a CDR-certified administrator. As part of a larger study, participants completed several self-report measures, were administered a battery of cognitive measures, and completed the IADL assessment. The testing session lasted approximately 90 minutes. This protocol was reviewed and approved by the university Institutional Review Board. Measures Clinical dementia rating The clinical dementia rating (CDR; Morris, 1993) is a semistructured interview designed to rate severity of dementia through the assessment of six domains of cognitive and functional abilities. Domains assessed on the CDR include memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care. Each domain is rated and entered into an algorithm to generate an overall summary score on a 5-point scale. Administrators were trained in the CDR interview and scoring techniques by the CDR author via an online training course (The Brief Training and Reliability Protocol, 2011). The Brief Training and Reliability Protocol (BTRP) includes an introduction to the CDR, three videotaped patient interviews for training purposes, and six videotaped interviews for
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RESERVE, COGNITION, AND FUNCTIONAL STATUS
reliability certification. Each of the CDR administrators achieved reliability certification by successfully completing agreement with a “gold standard” on at least five out of the six reliability tapes. The CDR has been well validated as a classification tool for dementia severity (Morris, 1993), with demonstrated interrater reliability of 80% (Burke et al., 1988; McCulla et al., 1989). The CDR was used to rule out participants with dementia (i.e., participants with CDR scores >0.5 were excluded). Developers of the CDR determined a score of 0.5 to evidence some cognitive change that is not severe enough to classify an individual with “mild dementia” (Morris, 1993). While there is some evidence to suggest that very mild dementia may be overlooked by a CDR of 0.5 (Chang et al., 2011), our use of the CDR as exclusionary criteria was to reduce invalid data reporting by participants. Cognitive assessment Cognitive ability was assessed with the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS; Randolph, 1998). The RBANS is a 30-min neuropsychological test designed to assess cognitive decline in older adults and serve as a screening tool for cognitive functioning in younger adults (Randolph, Tierney, Mohr, & Chase, 1998). The RBANS consists of 12 subtests that generate five index scores: Visuospatial/Constructional, Attention, Language, Immediate Memory, and Delayed Memory. The RBANS additionally yields a global score (the total scale score, TS). The RBANS TS raw scores were used in all analyses. RBANS scores have demonstrated split-half reliability coefficients in the .80s for the index scores and an average reliability for the total scale score ranging from .86 to .94 across age groups (Randolph et al., 1998). Cognitive reserve Formal years of education served as our proxy measure of CR and were acquired from each participant via self-report. Education was treated as a continuous variable in the analyses. Following the procedures of Siedlecki et al. (2009), formal years of educational attainment were considered, with 20 the highest level representing a doctoral degree, 18 for a master’s degree, 16 for a bachelor’s degree, 14 for an associate’s degree, and 12 for a high-school degree or GED (General Educational Development tests). Additionally, each year of formal education, regardless of whether it was or was not a degree-granting year, was counted as a
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formal year of education. For example, a highschool graduate who completed one year of college would be considered to have completed 13 years of formal education. The maximum number of years of education was limited to 20. Assessment of functional status Functional status was assessed with the Direct Assessment of Functional Status–Revised (DAFS– R; Loewenstein et al., 1989). The DAFS–R is a clinician-rated, laboratory assessment of activities of daily living (ADLs). ADLs have been classified into two domains (BADLs and IADLs) based on the complexity of tasks related to time orientation, communication, transportation, preparing for grocery shopping, financial skills, dressing, grooming, and eating. Scores from the individual scales combine to create a total score ranging from 0–133. In order to focus on IADLs, we used criteria specified by Mitchell and Miller (2008) to separate the different domains into IADL or BADL categories. Specifically, the BADL domain was computed based on their raw scores on the dressing/grooming and eating items of the DAFS–R. The IADL domain then included functional abilities that recruit higher order cognitive processes, including communication (i.e., using the telephone, preparing a letter for mailing), financial skills (i.e., identifying currency, counting change, writing a check, balancing a checkbook), shopping (i.e., memory for grocery items, selecting grocery items with a list, making correct change), driving (i.e., correct identification of road signs), meal preparation (i.e., following cooking instructions), and taking a telephone message (see Table 1). The DAFS–R IADL raw scores were used in all analyses. Interrater reliability has demonstrated to be at least 85% on TABLE 1 DAFS–R IADL categories and tasks Domain Communication Financial skills
Shopping
Driving Meal preparation Taking a phone message
IADL task Using a telephone Preparing a letter for mailing Identifying currency Counting change Writing a check Balancing a checkbook Memory for grocery items Selecting items with a list Making correct change Identifying road signs Following cooking instructions
Note. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living.
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each item of the scale (Loewenstein et al., 1989). Test–retest reliability coefficients range from .55 to .91 for the various subscales.
TABLE 3 Correlations among demographics, global cognition, and IADL performance Variable
Age
Education
RBANS TS
— .12 .25 –.02 .04
— .23 .13 –.24
— .29* .19
— .70**
Gender Age Education RBANS TS DAFS–R IADL
RESULTS Preliminary analyses
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Gender
Data were analyzed using the Statistical Package for Social Sciences (SPSS 20.0 for Windows, SPSS, Chicago, IL) and RStudio (RStudio 0.96.122 for Windows, Boston, MA). Demographic, cognitive, and functional characteristics of the sample are displayed in (Table 2). Consistent with previous studies (Chapman et al., 2007; Suchy et al., 2011), performance on IADL assessment among communitydwelling older adults was variable, with scores ranging from 54 to 91 (M = 75.76, SD = 8.40; DAFS– R IADL maximum raw score = 91). Typical errors produced on these tasks were making incorrect change, difficulty grocery shopping from memory, and errors in taking a telephone message. Initial bivariate correlation analyses were conducted to examine zero-order correlations among demographic, cognitive, and functional characteristics of the sample (see Table 3). As expected, global cognitive and IADL performance scores were significantly correlated (r = .70; p < .01), as were years of education and global cognitive scores TABLE 2 Demographic data and mean summary data for the sample Variable
Mean
SD
Age (years) Education (years) RBANS TS DAFS–R IADL
74.17 15.93 96.04 75.76
5.64 2.77 17.63 8.40
Range 65–84 9–20 55–145 54–91
(19) (11) (90) (37)
Note. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living (max score = 91); RBANS TS = Repeatable Battery for the Assessment of Neuropsychological Status total score of performance on five subtests; education = years of formal education attained.
Notes. DAFS–R = Direct Assessment of Functional Status– Revised; IADLs = instrumental activities of daily living (max score = 91); RBANS TS = Repeatable Battery for the Assessment of Neuropsychological Status total score of performance on five subtests; education = years of formal education attained. *p < .05. **p
