A growing body of research suggests that older adults show impaired performance in tasks that require a high degree of cognitive control, such as when information must be maintained within working memory (e.g., Craik et al. 1990
; Salthouse 1990
; Daigneault and Braun 1993
), when attention must be endogenously focused particularly in the face of distraction or interference (e.g., Duchek et al. 1998
), or when inappropriate response tendencies must be inhibited (e.g., Spieler et al. 1996
; Zacks et al. 1996
; West and Bell 1997
; May et al. 1999
). Theorists have put forth a range of ideas as to the mechanisms that lead to such age-related declines in cognitive controls tasks. These include generalized slowing (Cerella 1985
; Myerson et al. 1990
; Salthouse 1996
), reduced processing resources (Craik and Byrd 1982
), reduced working memory capacity (Salthouse 1990
; Park 2000
), inhibitory deficits (Hasher and Zacks 1988
), and disturbed attentional control (Balota et al. 2000
). In our own work, we have put forth the hypothesis that one of the mechanisms underlying age-related cognitive changes is a deficit in the ability to actively represent and maintain task goals. This goal maintenance deficit (which we have also previously referred to as a context processing impairment) has been found to account for a wide range of findings related to older adult behavioral performance (Braver et al. 2001
; Braver and Barch 2002
; Braver et al. 2005
; Rush et al. 2006
; Braver and West, forthcoming
). The current study directly tested predictions from this goal maintenance account that relate to age-related changes in brain activation. Specifically, we examined whether deficits in task-goal maintenance among older adults are associated with disturbances in the functional activation of the prefrontal cortex (PFC).
We have argued that goal maintenance is a critical component of cognitive control that is required for successful performance in a wide variety of cognitive situations (Cohen et al. 1996
; Braver and Cohen 2000
; Braver et al. 2002
). Goal representations contain information regarding the actions needed to bring about specific outcomes, which can help guide planning and behavior. However, these representations may also exert influence over perceptual or attentional processes. For example, in the Stroop task, task goals must be actively represented and maintained in a form that can bias attention allocation and response selection toward the ink color rather than the word. Goal representations are particularly important in situations with a strong competition for response selection. These situations arise when the appropriate response is one that is relatively infrequent (e.g., withholding a response to an infrequent “no-go” stimulus) or when the inappropriate response is dominant and must be inhibited (e.g., the word in a Stroop task). We argue that goal representations are maintained in an active online state and are continually available to influence processing. Consequently, goal representations can be thought of as the subset of representations within working memory that govern how other representations are used. Thus, goal representations can subserve both storage and control functions. This aspect of the theory differentiates it from classic accounts of working memory (e.g., Baddeley 1992
), which postulate a stricter separation of representations for storage versus control.
Although there are a number of behavioral paradigms that can tap goal maintenance, we have frequently used a version of the classic continuous performance test (Rosvold et al. 1956
) that operationalizes representation, maintenance, and updating of task goals in terms of the effects of contextual cues on task performance (Barch 1993
; Servan-Schreiber et al. 1996
; Braver and Cohen 1999
; Cohen et al. 1999
; Braver and Cohen 2000
; Barch et al. 2001
; Braver et al. 2001
; Barch et al. 2003
; Braver et al. 2005
). In this AX version of the continuous performance task (AX-CPT), participants are presented with cue--probe pairs and told to make a target response to an X-probe, but only when it follows an A-cue. Nontarget responses are required on all other trials. Because target (AX) trials occur with high frequency (70%), 2 types of biases are present that assess the utilization of contextual cues to update task goals in different ways. First, goal-related utilization of contextual cues is critical for inhibiting a target response bias that occurs when an X-probe follows a non--A-cue (“BX” trials). Second, goal-related utilization of contextual cues produces an expectancy bias that primes and facilitates target responses following an A-cue. However, this expectancy bias can also impair performance when the A-cue is followed by a non--X-probe (“AY” trials). These biases can be measured behaviorally by contrasting performance in AY and BX trial types against a third type of nontarget trial, BY, which serves as a control condition because neither type of bias is present. Thus, intact representation and utilization of task goals should lead to impaired AY performance but enhanced BX performance. Conversely, individuals with impaired goal maintenance should show poorer performance on BX compared with AY trials. Importantly, these predictions distinguish a deficit in goal representation from a deficit in inhibitory control. Specifically, a deficit in inhibitory control would predict poor performance in both AY and BX trials because both trial types require the ability to overcome a strong tendency to execute a primed, but incorrect response (i.e., primed by the cue in AY trials and by the probe in BX trials). In contrast, a goal maintenance deficit should lead to reduced cue priming and thus not impair performance on AY trials.
The AX-CPT also provides a way to investigate active maintenance of task goals by manipulating the delay between the cue and probe. When the cue--probe delay is lengthened (e.g., 5--10 s as compared with 1 s), the ability to maintain access to goal-related information is challenged. A prediction of the goal maintenance theory is that the effect of delay will interact with performance on AY and BX trials. When task-goal information is actively maintained, then the strength of goal representations should stay the same or increase with delay (i.e., BX performance should stay the same or improve at longer delays, whereas AY performance should stay the same or worsen). In contrast, if goal maintenance is impaired, then BX performance should worsen with delay, but AY performance should actually improve.
In previous behavioral studies, we have observed a pattern of performance in older adults that was indicative of a selective deficit in goal representation and maintenance. Healthy older adults performed more poorly than young adults in both accuracy and reaction times on BX trials, which is the nontarget trial type in which goal representations are needed to prevent errors (Braver et al. 2001
; Braver et al. 2005
; Paxton et al. 2006
). At the same time, older adults performed better on AY trials, which is the nontarget trial type in which intact goal representations lead to worse performance. Moreover, in the oldest adults, these effects were amplified at long cue--probe delays, suggesting a further impairment in goal maintenance (Braver et al. 2005
). The AY trial effects are particularly interesting as they provide a unique example of a task situation in which older adults actually performed both more accurately and as quickly as young adults, though this “better” performance was theoretically predicted by the presence of a deficit in goal maintenance.
These prior findings of abnormal lateral PFC activity in older adults are consistent with the hypothesis that goal maintenance deficits in older adults are due in part to altered function in this brain region. The goal of the current studies was to examine the hypothesis that healthy older adults would show a specific impairment in the ability to activate lateral PFC regions in response to the need to represent and maintain task-goal information. In particular, our hypothesis suggests a possible explanation for the mixed findings in neuroimaging studies with older adults regarding overrecruitment versus under-recruitment in PFC regions. We suggest that older adults will show a general overrecruitment pattern in task-related activity in both PFC and other brain regions as an attempt to compensate for underrecruitment in the more focal PFC regions that are required to meet specific demands on goal maintenance and cognitive control tasks. Thus, a comparison of task conditions that isolates these specific demands on goal maintenance and cognitive control will reveal underrecruitment within focal regions of lateral PFC, whereas overrecruitment will be found more broadly in PFC and other cortical regions in task condition comparisons that isolate more general task-related processing demands.
Study 1 tested these hypotheses in a sample of healthy young and older adults performing a variant of the AX-CPT with standard blocked design functional magnetic resonance imaging (fMRI) methods. We predicted that an examination of general task-related activation (i.e., task vs. fixation) would reveal an enhanced pattern of overall PFC as well as posterior cortical and subcortical activation in older adults. In contrast, the goal maintenance account predicts that contrasting conditions with high versus low goal maintenance demands (long vs. short delay conditions) would reveal reduced activation within lateral PFC among older adults.