The purpose of the present investigation was to examine how well a switching version of the Stroop task could capture attentional control deficits that presumably accompany aging and Alzheimer’s dementia. The results indicated the Stroop switch task was sensitive to group differences in age and importantly, exceeded the performance of current psychometric tests in discriminating healthy aging from early stage DAT.
As expected, when cued to respond to color, individuals with very mild DAT produced larger Stroop interference in error rates than healthy older adults. This finding replicates past studies by Spieler et al. (1996)
and Castel et al. (2007)
in which very mild DAT individuals showed increased errors in the incongruent Stroop and Simon conditions, respectively. This pattern of errors on incongruent trials has become a signature of early stage DAT. In the current study, it is likely that the added requirement of switching between naming words and colors weakened participants’ ability to consistently ignore words in favor of color naming. This is especially true for healthy older adults and early stage DAT individuals, who showed considerable increases in errors during color naming trials relative to young adults (see ). In contrast to the Spieler et al. (1996)
and Castel et al. (2007)
results, we found that Stroop interference errors on color naming trials were also greater for older adults than young adults. However, as previously mentioned, it is likely that the addition of the switch component to the Stroop task increased the attentional demands of the task and thus disrupted older adults’ ability to maintain the appropriate task set
In contrast to the robust group differences in Stroop interference observed in the current study, there was little-to-no difference in cross-trial switch costs across groups. All three groups showed larger costs when switching to word naming than when switching to color naming. This pattern replicates the interesting switch cost asymmetry first observed by Allport et al. (1994)
and lends support to involuntary retrieval accounts of residual switch costs (Allport & Wylie 1999
; Waszak et al., 2003
; Wylie & Allport, 2000
) in which stimuli involuntarily invoke previous S–R mappings, rather than to a persisting controlled inhibition of the previously irrelevant (but currently relevant) task (Mayr & Keele, 2000
; Gilbert & Shallice, 2002
; Yeung & Monsell, 2003
). If the latter were true, then one would have expected diminished switch costs among DAT individuals, in whom controlled inhibitory processes are presumably deficient.
Our regression analysis further bolstered the claim that Stroop interference errors are particularly sensitive to early stage DAT. The ability of incongruent errors in the Stroop Switch task to predict DAT status was examined using logistic regression and this discrimination ability was compared to that of the 18 psychometric measures. Of particular interest, incongruent error rates were better at discriminating early stage DAT from healthy controls than each of the standard 18 psychometric tests available on these participants. Moreover, only one of those tasks (the selective reminding task) significantly predicted DAT status when entered into the logistic regression analysis after incongruent errors.
As discussed previously, frontal brain areas such as the prefrontal cortex and anterior cingulate show considerable pathology early in DAT (Killiany et al., 2000
; Mintun et al., 2007
; van der Flier et al., 2002
; Yanaguchi et al., 2001). Given that both the Stroop task and Task Switch paradigms have been shown to activate such frontal regions (Brass et al., 2005
; Braver, Reynolds, & Donaldson, 2003
; De Pisapia and Braver, 2006
) it is not surprising that a task combining these paradigms is sensitive to early impairments accompanying DAT. An abbreviated version of this task may therefore be useful as part of the diagnostic tools available to clinicians, as suggested by Fine et al. (2008)
In addition to the Stroop Switch task, it is of interest that average performance across the 5 Memory
tasks also discriminated very mild DAT from controls. Of course, this finding is also not surprising, since many studies have found evidence for memory measures as early discriminators between healthy aging and DAT (see Bäckman, et al., 2005
, for a review) and medial temporal brain areas show some of the earliest neuropathology linked to DAT (Braak & Braak, 1991). In a recent meta-analysis, Bäckman, et al. (2005)
reported that the largest effect sizes in discriminating DAT from healthy aging came from tasks measuring episodic memory (e.g., California Verbal Learning Test, Logical Memory tests), executive functioning (e.g., Stroop task, Trails B), and Perceptual Speed (e.g., Digit-Symbol task, Letter cancellation task). The current evidence for the importance of the Stroop Switch task and Memory measures for early discrimination are therefore in line with the meta-analysis of Bäckman et al.
Within the memory domain, it is of interest that the selective reminding task was especially effective at discriminating DAT. As with the current study, past research has demonstrated that this task can accurately discriminate healthy aging from DAT (Grober et al., 1988
; Grober et al., 2000
). In the selective reminding task (Grober et al., 1988
), participants are presented 16 stimuli at study and then are queried for the immediate cued recall of each stimulus item. If the participant fails to retrieve the item, they are then given the cue and item together. After a short delay, the participants are given a free recall test for all 16 items and then a cued recall test for those items not recalled during the free recall period. Importantly, this entire procedure is then repeated two more times (with 16 new items each time). Each participant receives both a free recall score (how many items they recalled during the 3 free recall attempts, 0 – 48) and a total recall score (free recall score plus how many items recalled when given cue, 0–48). Only the free recall score was used in the present study (the total recall score did not aid in discriminating DAT).
One reason why this task is especially effective at discriminating DAT may lie in the requirement to repeat the study-test sequences across 3 separate blocks. This repetition requires subjects on later blocks to recall currently relevant items while suppressing potentially interfering items from earlier blocks. Indeed, an examination of Grober’s (1988)
initial data shows that the free recall of healthy older adults improved over trials whereas the recall of DAT participants did not, increasing the difference between groups across blocks. It is therefore possible that any practice effects among DAT participants are counteracted by the buildup of such proactive interference across blocks (see Tse et al., 2009, for recent evidence of strong proactive and retroactive interference effects in early stage DAT).
Neuroimaging researchers have identified the critical importance of prefrontal cortices to memory monitoring processes such as those needed to avoid proactive interference from earlier trials (Feredoes, Tononi, Postle, & Smith, 2006
; Hedden & Yoon, 2006
; Postle, Brush, & Nick, 2004
). Hence, part of attentional control involves the ability to exclude no-longer-relevant information from entering into the current search set (Unsworth & Engle, 2007
). Indeed, tasks designed to measure working memory (e.g., Ospan or Reading Span tasks) are often influenced to a large extent by participants’ ability to overcome proactive interference from previous trials (Bunting, 2006
; Lustig, May, & Hasher, 2001
Given that the selective reminding task likely involves attentional control in the form of memory monitoring (presumably located in the PFC), it is not surprising that it surpasses other memory tasks in its discrimination of DAT. It is also not surprising, therefore, that this task correlates more strongly with Stroop Interference errors in the current study than any other memory task. Sommers and Huff (2003)
also obtained a correlation between Stroop performance and memory in a memory task (the Deese, Roediger & McDermott, DRM false memory paradigm) that required memory monitoring between studied and highly similar, but unstudied, items. Indeed, this is precisely the account afforded by Balota et al. (1999)
in explaining the relative increased false memory in early stage DAT, compared to healthy control individuals, with the DRM stimuli.
In a similar vein, because attentional control requires the ability to maintain a goal across time to modulate competition between relevant and irrelevant information, this ability should rely partially on memory processes. Thus, like the selective reminding task, the Stroop task is not process pure. However, the fact that the Stroop Switch and selective reminding tasks function so well in discriminating DAT suggests perhaps that it is the process of coordinating attention and memory that is most sensitive to DAT, rather than any one particular cognitive domain or system.