Age-Related Slowing of Control Processes: Evidence from a Response Coordination Task

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Age-Related Slowing of Control Processes: Evidence from a Response Coordination Task
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  AGE-RELATED SLOWING OF CONTROL PROCESSES:EVIDENCE FROM A RESPONSE COORDINATION TASK Sandro Rubichi, 1,2 Mirco Neri 3 and Roberto Nicoletti 4,1 ( 1 Istituto di Psicologia, Università di Urbino; 2 Dipartimento di Scienze Biomediche,Università di Modena; 3 Dipartimento di Medicina Interna, Università di Modena; 4 Dipartimento di Psicologia dello Sviluppo e della Socializzazione, Università di Padova) A BSTRACT Normal aging is associated with slowing of performance mostly due to a slowedfunctioning of central response-related processes. In this paper we set out to discoverwhether slowing occurs also when the process controlling the coordination of responses isengaged by the task. To this end, we compared the mean-reaction time performance of twogroups of subjects (young vs. elderly) in single- and dual-task experimental paradigm. Theresponse coordination process is required only by the dual-task paradigm. The resultsindicate that, in the dual-task situation, the elderly are markedly slower than young subjects.The eventual relevance of information-processing speed in determining the cognitiveperformance of the elderly is considered in the discussion of the results.Keywords: aging, cognitive slowing, control process, dual-task I NTRODUCTION One of the most amply documented features of aging is the gradual slowingof performance (Birren, 1965; Cerella, 1985; Salthouse, 1985; Vercruyssen,1993; Welford, 1981; for reviews see Craik and Salthouse, 1992, and Salthouse,1991a). More than thirty years ago, Brinley (1965) showed that the responsetime of older adults can be expressed as a simple linear function of the time of response of younger adults, regardless of the specific nature of the task (e.g.,verbal, arithmetic, perceptual, etc.). It was therefore suggested that age-relatedslowing was a general rather than task-specific process.Interestingly, Brinley (1965) found that an important factor in determiningage-related slowing was the complexity of the task (Complexity Hypothesis).This notion was confirmed and further refined in the course of a wide variety of information processing tasks. Variations in the slowing model have beenproposed, such as multilayered slowing (Cerella, 1985), overhead slowing(Cerella, 1990) and information loss (Meyerson, Hale, Wagstaff et al., 1990).An explanation for age-related slowing is that older adults experience areduction in the processing resources that are fundamental for cognitiveprocessing (Salthouse, 1985, 1988). Processing resources can be characterized asattentional capacity, working memory capacity and speed of processing.Age-group differences in attentional capacity have been demonstrated by Cortex, (1999) 35, 573-582  means of the divided-attention paradigm. Older adults have more difficulty thanyoung adults when performing two different tasks simultaneously (Greenwoodand Parasuraman, 1997; Hartley, 1992; for reviews see Kramer and Larish,1996). Evidence of an age-group difference in working memory has been foundin a variety of measures, including digit span (Salthouse, 1988), the sentencespan (Light and Anderson, 1985; Gick, Craik and Morris, 1988), and other typesof computation spans (Dobbs and Rule, 1989; Salthouse, 1990, 1994). Finally, areduction in the processing speed leading to a slowing of all cognitive operationshas been proposed as a general characteristic of adult aging (Salthouse andSomberg, 1982).There is broad general agreement that the elderly perform much more slowlythan the young when the complexity of the task is increased (e.g., Bashore,1994). However, investigators in the field are now differentiating the relevancethat each information processing stage has in determining the general cognitiveslowing. In other words, greater effort is now going into determining process-specific slowing by manipulating the complexity of the task (Fisk and Fisher,1994), and task difficulty has been used to determine the specific cost of eachcognitive operation in order to assess whether slowing is uniform across allcomponents of information processing or not. Task difficulty has beenmanipulated (a) by increasing the number of processing steps involved in thesolution of a given task or (b) by rendering the operations performed by aparticular information processing stage more complex.By increasing the number of steps in a task, it has been demonstrated thatcentral (elaborative) processing speed slows more with age than peripheral(sensory-motor) processing speed. An extensive longitudinal study on reactiontime (RT) performance has shown that even simple RTs increased at a rate of about 0.5 ms/yr. beginning from the age of 20, but the slowing is more evidentfor more demanding RT tasks (e.g., Fozard, Vercruyssen, Reynolds et al., 1994).Thus, both peripheral and central processing speed slows with age, but greaterslowing occurs when central processing is required by the task.By increasing the complexity of the operations to be computed at theprocessing stage, it has been shown that, of the central components of information processing, response selection-related processes slow selectivelywith age (Bashore, 1990; Lorsbach and Simpson, 1988). For example, byreducing the degree of compatibility in a stimulus-response compatibility task(which affects the response selection stage, see Lu and Proctor, 1995), olderadults show increased RT compared with young adults (Simon, 1967). Inaddition, the performance of the elderly slows down more than that of the youngwhen the difficulty of the response selection operations is enhanced byincreasing the number of stimulus-response alternatives (McDowd and Craik,1988).The notion that the response selection stage is the main cause of slowing alsoarises from psychophysiological studies without the manipulation of taskdifficulty. For example, Zeef and Kok (1993), in a study combining RTperformances and psychophysiological parameters such as P3 latency, the onsetof the lateralized readiness potential, and the electromyogram of hands,concluded that, in young subjects, central and peripheral response-related 574 Sandro Rubichi and Others  processes appear to coincide, whereas, in the elderly, response selection precedesresponse execution. This conclusion is in agreement with previous findingssuggesting that in the elderly the onset of response selection is slower than inyoung adults (e.g., Bashore, Osman and Heffley, 1989), that is, even thoughslowing of information processing also occurs in stimulus processing, it isgreater in the response selection stage (Bashore, 1994). It is worth noting thattime restriction in stimulus processing by itself does not account for age-relateddifferences. Given unlimited time, but no specific strategyinstruction, the elderlystill perform substantially less well than their younger counterparts (for reviewsee Craik, Anderson, Kerr et al., 1995).Accordingly, the above-mentioned studies have focused mainly on theefficiency of the information processing stages, while control processes havereceived less attention. By control processes we mean that set of cognitiveoperations that are not directly involved in the representation of cognitive states,but rather with the organization of such states towards attaining a specific goal.In other words, the term “control” refers to those processes by which anindividual optimizes his performance in multicomponent tasks. The basic idea isthat a number of mental representations are normally active simultaneously andin parallel, but, at any given time, only some of them guide action and thought.An essential feature of control processes is that they are functionally separatedfrom a multitude of processes that can be controlled by them. Control processesare therefore considered to be fundamental for top-down control of behavior(Norman and Shallice, 1986). Moreover, it is generally accepted that normalaging implies a decline in frontal cortex functioning, which supports controlprocesses (Moscovitch and Winocur, 1995; Rafal and Henik, 1994).Considering that cognitive slowing in the elderly is mostly attributable to theslowed functioning of the response selection stage, the aim of the present workwas to determine whether older adults exhibit slowing in the control processesinvolved in the coordination of two responses. An experimental paradigmdeveloped to study how control processes operate in the scheduling of behaviorhas been proposed by Umiltà, Nicoletti, Simion et al. (1992). In it, the samestimulus requires either one or two responses, depending on the experimentalcondition. In the first block (single-task), subjects are required to make aspeeded manual response (to right or left) depending on the stimulus position(right or left). In the second block (dual-task), subjects are also instructed to sayaloud, after the manual response has been produced, whether the two letterswhich composed the stimulus are the same or different. For the second taskthere is no time constraint (unspeeded response). Thus, in the dual-taskcondition, the response sequence has to be coordinated. This is a basic mentaloperation that is crucial for efficient performance in cognitive tests and, moreimportant, for everyday activities (De Jong, 1995).Young subjects displayed a longer RT to the stimulus position in the dual-than in the single-task condition. According to the authors, given that in thedual-task condition the two responses are relative to different  features of the same stimulus, RT slowing occurs because the control process of responsecoordination acts as a bottleneck, causing postponement of the speeded responseto the stimulus position when engaged in the decision about the coordination of   Age-related slowing and control processes 575  the two responses (see Pashler, 1984, 1989, 1994, for the decision and response-selection bottleneck model).This paradigm has proved to be a very powerful tool in detecting the residualattentional deficit in both severe and mild closed-head injury (Stablum,Leonardi, Mazzoldi et al., 1994; Stablum, Mogentale and Umiltà, 1996).Interestingly, in the mild closed-head injury group, the response coordinationprocess is impaired only for the patients over the age of 30. This finding leadsus to hypothesize that normal aging per se could produce an impairment in theresponse coordination process.In the present study, we applied the Umiltà et al. (1992) paradigm to young andold adults to find out whether response coordination is particularly slowed in theelderly. Note that in this paradigm task difficulty is not manipulated by increasingthe complexity of the operations to be computed at an information processingstage. Rather, task difficultyis manipulated by the insertion of the responsecoordination process that is required only by the dual-task condition. In this mannerwe are able to determine whether the time cost of this process is greater for theelderly than for the young. Bearing in mind the psychophysiological and behavioraldata reported above, we expected to find longer RTs for both single- and dual-taskconditions in the elderly than in young adults. However, if the responsecoordination process were impaired in older adults, we would expect to find anoveradditive interaction between age and task. M ATERIALS AND  M ETHODS Subjects Two groups of subjects participated in the experiment. One consisted of 10 youngsubjects (5 male and 5 female; mean age = 28.2 years, S.D. = 4.3; mean years of education= 15.5, S.D. = 2.6) working at the Geriatric Department of the University of Modena. Theother group consisted of 10 elderlysubjects (5 male and 5 female; mean age = 72.3 years,S.D. = 2.5; mean years of education = 8.3, S.D. = 3.5) recruited from the center of motorand leisure activity at the same Geriatric Department. Subjects were all right-handed, hadnormal or corrected-to-normal visual acuity (Salthouse, Hancock, Meinz et al., 1996), andwere naive as to the purpose of the experiment.In normal aging there is increasing heterogeneity (Berkman, 1988) due to a set of conditions, including the presence of subtle, subclinical or well-controlled chronic diseasesthat do not interfere functionally with everyday activities (Fried, Storer, King et al., 1991).These conditions are able to impair performance in many physical and mental tasks bycomparison with subjects who age “successfully”, that is, those who are free of suchlimitations (German, 1995).For these reasons, in the case of elderly subjects, both the following inclusion criteriawere taken into account:(a) A score on the Cumulative Illness Rating Scale (CIRS) of 14 (Parmalee, Thuras,Katz et al., 1995). CIRS is a standardized instrument for obtaining the physician’s rating of a patient’s health status according to 14 major organ/system groups. The item score rangesfrom 1 to 5 along with the presence and severity of the disease. A score of 14 means theabsence of either subclinical disease or functional impairment.(b) Mini Mental State Examination (MMSE) score of 27 or more. MMSE is aninterviewer-administered measure out of 30-point of overall cognitive functioning (Folstein,Folstein and McHugh, 1975). A score of 24 has traditionally been considered as the cut-off point for “normality”. However, there is recent evidence to show that a score of 27 or moreis a better guarantee that subjects with incipient dementia, or at risk of developing dementia, 576 Sandro Rubichi and Others  are not recruited into the study (Hanninen, Hallikainen, Koivisto et al., 1995; Braekhus,Laake and Engedal, 1995). Task  The experiment took place in a dimly-lit room. Each participant seated in front of acathode-ray tube screen driven by a Tulip dt 386sx personal computer. The head waspositioned on an adjustable head-and-chin rest, and the distance between the eyes and thescreen was approximately 40 cm. The stimuli (1.5°  × 5.5°) were shown for 2000 ms 10°tothe left or right of a central fixation cross (1°  × 1°). The stimuli consisted of two letters,one above the other; the letters were either the same or different. The responses were madeby pressing one of two keys on the computer keyboard. One key (the character “z”), locatedto the left of the bodymidline, was pressed by the left index finger, whereas the other (thecharacter “\”), located to the right of the bodymidline, was pressed by the right indexfinger. The software was based on MEL Professional (Schneider, 1988). Procedure Each subject took part in a single experimental session comprising 2 blocks of 72 trialseach, with a 5-min block-rest in between. The task of the first block was locationdiscrimination only (i.e., single-task) while that of the second block was for the locationplus identity discriminations (i.e., dual-task) 1 . For each block, the experimental trials werepreceded by some practice trials. The stimuli appeared in a quasi-random sequence, withthe proviso that there should be an equal number of left- or right-side presentations and anequal number of same- and different-letter pairs.In each trial, the central cross was presented for 2000 ms, followed by the stimulus for2000 ms. Participants were instructed to make the speeded keypress for locationdiscrimination the moment the stimulus was presented. The intertrial interval was 2000 ms.The instructions differed according to the experimental condition. In the single-task,subjects were required to press the right-hand key if the stimulus appeared on the right, andto press the left hand key if the stimulus appeared on the left. In the dual-task, subjectswere also instructed to say aloud, after making the manual response, whether the two letterswere the same or different. It must be noted that the subjects were instructed to postponethe same-different decision until after the manual response for the location task had beenmade. The instructions stressed that there was no time constraint for uttering the verbalresponse. R ESULTS The mean level of education of the two groups proved to be different, t = 5.20,p < .001. Even if education was found to be non-predictive of RT performanceover the age of 70 years (Christensen, Korten, Jorm et al., 1997), we preferred totake it as covariate for the following analysis. On the whole, errors in the left-right discrimination task were 0.3% for the young adults group and 1.7% for theelderly adults group.  Age-related slowing and control processes 577 1 Since we did not counterbalance the block order of the experimental conditions, one might argue that fatigue couldhave skewed the results obtained in the elderly subjects. However, in a pilot session, some elderly subjects failed toachieve optimum performance during the practice trials on the dual-task block when it was performed first.Accordingly, we decided to keep the same block order for all subjects (that is, one block for the single-task followedby one block for the dual-task) and to insert a 5-min rest between blocks, thereby avoiding the risk of fatigue in theelderly subjects. Indeed, none of the subjects complained of fatigue; rather, the single-task block served as practice forthe dual-task block in all cases.
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