A defining characteristic of attention-deficit/hyperactivity disorder (ADHD) is a difficulty in controlling, maintaining, and directing attention (1
). However, ADHD affects executive function more widely, compromising processes such as response inhibition and planning (2
). Such deficits may account for the negative outcomes of individuals with ADHD in academic, social, and workplace settings (3
Some theories suggest that reduced WM ability in individuals with ADHD contributes to their deficits in attentional control and executive function (5
). Indeed, lower WM ability in non-ADHD individuals is associated with poorer cognitive ability (7
) and increased mind-wandering away from challenging activities in real-life settings (8
). Furthermore, poorer WM ability has been correlated with poorer performance on the Stroop color-word task (9
), a classic measure of attentional control (11
). Computational models of the Stroop task have suggested that maintenance of a task set provides support for top-down bias toward task-relevant processing (12
), and other studies suggest that better WM ability improves the active maintenance of task-relevant information (13
Attentional control and WM rely on a common set of neural structures, many of which have been implicated as dysfunctional in ADHD. Meta-analyses of both the Stroop task (14
) and WM tasks (16
) have implicated bilateral dorsolateral prefrontal cortex (DLPFC), bilateral inferior frontal gyrus (IFG), dorsal anterior cingulate cortex (dACC), inferior parietal lobule, and precuneus. These same regions show atypical functioning in individuals with ADHD (18
). Despite these similarities, no studies have investigated directly how the neural substrates that support both WM ability and attentional control, namely DLPFC, might be altered in young adults with ADHD.
The current study investigated whether WM ability in adults with ADHD and controls could explain differences in performance and attentional-control activity during the Stroop color-word task. We used a data set from our laboratory (see reference 19 for details) that demonstrated significant effects of ADHD on activation of attentional-control regions. Of note, relative to controls, the ADHD group showed significant yet reduced DLPFC activation during three Stroop conditions (incongruent, congruent, neutral), suggesting that individuals with ADHD have reduced maintenance of the attentional set to the task-relevant ink color. Furthermore, the ADHD group showed increased activation in linguistic regions such as the left temporal gyrus relative to controls, which suggests increased processing of the task-irrelevant word dimension.
We expected WM differences between ADHD and control groups to relate to attentional control in two different ways. First, WM ability may capture group differences in performance and brain activation associated with attentional control, suggesting that attentional control deficits in ADHD arise, in part, from WM deficits. Second, the relationship between WM ability and brain activation related to attentional control might differ between the groups, suggesting that WM ability affects different pathways and/or processes in the ADHD group compared to controls. For example, in individuals with ADHD, high WM ability might promote activation of brain regions associated with compensatory attentional processes, while in controls, recruitment of those same neural processes may be unnecessary due to effective task-set maintenance. Importantly, the two groups can show different associations between WM ability and activity in some brain region whether or not differ in average activation. Consequently, we may observe relationships with WM ability in brain regions that show group differences in average activity, as well as those that do not.
The relationship of WM ability with performance and brain activation associated with attentional control may depend on the type of attentional demand. The Stroop task contains at least three levels of attentional demand: selection and maintenance of a task set for task-relevant information (i.e., attend to chromatic color), selection of the task-relevant stimulus dimension (i.e. the chromatic color blue, not the semantic meaning “RED”), and response selection (the response associated with blue versus with red). We propose that WM networks may maintain representations of task sets, resulting in a relationship between WM ability and task-set maintenance.
Others have argued that the demands of task set maintenance vary across different Stroop task conditions (9
). On incongruent trials, the inherent conflict between the ink color and the word provides a subtle reminder that only one of those task sets is correct (i.e., ink color identification). Hence, the conflict present during incongruent trials may serve as a reminder to reactivate the correct task set of identifying the ink color (20
). In contrast, on congruent and neutral trials, the word name does not conflict with the ink color, resulting in no reminder of the need to identify the ink color (i.e., “goal neglect”; 9
). Consequently, task set maintenance may be more contingent upon intrinsic WM ability during the congruent and neutral conditions compared to the incongruent condition. Consistent with such an idea, Kane & Engle (2003) demonstrated that low WM ability results in greater interference on Stroop accuracy when blocks are mostly congruent compared to when blocks are mostly incongruent. Hence, we expect the effects of WM ability on group differences in DLPFC activation to be stronger during congruent and neutral blocks and weaker during incongruent blocks.
According to our Cascade-of-Control model (20
), posterior regions of DLPFC implement a task set to exert top-down attentional control (24
). When top-down control from posterior DLPFC is inadequate, other brain regions must resolve difficulties at later stages of processing (26
). In particular, we have posited that anterior regions of DLPFC are involved in selecting the task-relevant dimension of the stimulus, and that dACC is involved in selecting the response (23
). For selecting the task-relevant stimulus dimension, attentional demands will be less for neutral trials than for incongruent and congruent trials, when two sources of color information must be disambiguated (i.e., the ink color and the word identity; 23
). For response selection, attentional demands will be greater for incongruent than congruent and neutral trials, because the two sources of color information in incongruent trials relate to potential responses that conflict. We hypothesize that individuals with ADHD may compensate for poorer task-set maintenance using these later-stage attentional processes.
Of note, we examined these issues in high-functioning (i.e. college-enrolled) young adults with ADHD combined type versus a control group consistent in age, IQ, and gender. The ADHD group was not comorbid for other psychiatric disorders or learning disabilities. This means that between-group differences likely result from ADHD rather than other conditions that might influence WM and attentional control.