Given the evidence supporting exercise-related benefits in brain structure and mechanical function, it is not surprising that benefits in cognitive function have also been reported, particularly in older adults. Using composite scores of executive functioning, researchers have shown that aerobic fitness is linked to efficacy of the overall cognitive construct in older adulthood (Brown et al., 2010; Netz, Dwolatzky, Zinker, Argov, & Agmon, 2010). Other studies have also reported positive links between aerobic fitness or exercise and older adults’ performance on standard clinical tests of executive functioning, including the Wisconsin card sorting task (Albinet, Boucard, Bouquet, & Audiffren, 2010; Gordon et al., 2008), the REY Auditory Verbal Learning Test (Trials 7 and 10; see Kramer et al., 2001), the Trails B test, the Digit Symbol test, and the verbal fluency test (Trails B, Digit Symbol, verbal fluency, Barnes, Yaffe, Satariano, & Tager, 2003; Trails B, Digit Symbol, Blumenthal et al., 1991). In addition, academic achievement in children has been positively linked to both chronic engagement in physical activity (reviewed in Tomporowski et al., 2011) and aerobic fitness (Castelli, Hillman, Buck, & Erwin, 2007); and a recent study linked successful street crossing, particularly when distracted, to aerobic fitness (Chaddock, Neider, Lutz, Hillman, & Kramer, 2012). Together, these data support the idea that regular exercise can aid performance with respect to the broad construct of executive functioning. However, the multifaceted nature of these tasks makes it difficult to discern which cognitive functions underlie the reported links. Below, we review the evidence to date from exercise–cognition research using simple cognitive tests in an effort to determine which components of executive functioning benefit from regular exercise (see Table 1). Three specific components of executive functioning will be reviewed, since they have been tested most extensively in the exercise–cognition literature and they have also been linked to effective performance of daily activities: (1) task switching, (2) selective attention and inhibitory control, and (3) working memory.

Table 1 Aerobic exercise and executive functioning studies Full size table

Task switching

The typical task-switching paradigm involves responding to similar target stimuli, but on the basis of one of two different rules. For example, a participant might be instructed as follows: If the target digit is green, indicate whether the digit is greater or less than 5; if the target digit is red, indicate whether the digit is even or odd. On nonswitch trials, the response rule remains the same as on the previous trial; on switch trials, the response rule changes. The difference in reaction times between rule switch and nonswitch trials is referred to as the switching cost, because it reflects the degree of slowing arising from the need to mentally change task goals before responding. Such mental-set-shifting requires executive control, including volitional inhibition, working memory, and mental flexibility; thus, smaller switching costs can be interpreted as reflecting more efficient executive functioning (Banich, 2009; Monsell, 2003). Consistent with this interpretation, larger switching costs have been demonstrated in populations widely believed to have impaired executive function, including older adults (e.g., Cepeda, Kramer, & Gonzalez de Sather, 2001).

Exercise–cognition researchers who utilized task-switching paradigms have reported some promising results, particularly in older adults. While behavioral data from cross-sectional studies involving young and older adults indicate that the magnitude of switching costs does not depend on the amount of self-reported physical activity per week in either young or older adults (Hillman, Kramer, et al., 2006; Themanson, Hillman, & Curtin, 2006), event-related potential (ERP) data collapsed across young and older adults from the Hillman, Kramer, et al. study supported shorter P3 latencies during task switching in more active participants, and data from the Themanson et al. (2006) study supported a positive association between physical activity and error-related negativity (which is thought to reflect efficiency of error detection and ability to deal with the conflict arising during switch trials). Furthermore, a large randomized control trial on which sedentary older adults engaged in either aerobic exercise (brisk walking) or strength and flexibility training for 6 months indicated that aerobic exercisers showed a significantly greater reduction in the magnitude of the switching cost, as compared with those in the strength and flexibility group, at the end of the intervention (see also Hawkins, Kramer, & Capaldi, 1992; Kramer et al., 2001). In summary, it seems that older adults can benefit from regular physical activity in terms of task-switching performance; however, it is not clear that young adults reap such benefits (regarding fitness, see Scisco, Leynes, & Kang, 2008). A more recent young adult study found that task-switching performance does depend on physical activity levels (Kamijo & Takeda, 2010). However, in this study, task switching was predictable because a switch occurred every two trials; thus the exercise-related benefits in performance may have reflected better preparatory processes or working memory, rather than switching capabilities specifically (note that the same argument can be made regarding Hawkins et al., 1992). Regarding children, a lack of published studies utilizing task-switching paradigms prevents comment, but given the lack of improvement in switching costs seen throughout childhood (Kray, Karbach, & Blaye, 2012), exercise-related benefits may be unlikely in healthy children.

Selective attention and inhibitory control

In this review, tasks that tap selective attention and inhibitory control capabilities are grouped together because their designs typically prevent separate assessment of the two constructs (e.g., Sanders & Lamers, 2002; Shiu & Kornblum, 1996). In other words, successful performance of these tasks depends on both selective attention and inhibitory control, although it could be argued that some tasks rely more on one construct than on the other (Schulte et al., 2009). For example, tasks that involve suppression of stimuli with prepotent response links presumably rely more on motor inhibition than on selective attention (Eimer, Hommel, & Prinz, 1995), whereas tasks that involve suppression of stimuli with relatively arbitrary response links presumably rely more on selective attention than on motor inhibition (discussed in Machado, Devine, & Wyatt, 2009). In the present section, we review studies that investigated the relationships between regular exercise and performance on tasks that entail suppressing more prepotent responses (Stroop, go/no-go, stop signal, flanker arrows), as well as those that involve stimuli with more arbitrary response links (flanker letters, flanker colors).

The most common version of the Stroop (1935) task involves asking participants to indicate the color of the ink that a word appears in by either saying it aloud or pressing a specific button. The key condition in this task is when the ink color does not match the identity of the color word (e.g., “red”)—that is, when there is interference between the distracting word (“red”) and the target ink color (black). Performance during this interference condition is considered an effective measure of executive functioning because, in order to respond correctly, participants have to selectively attend to the color of the ink and inhibit the prepotent response of reading the word (Miyake et al., 2000). The task is particularly difficult because the distracting information appears in the same spatial location as the target information; as a result, participants cannot divert their attention away from the location of the distracting information to aid performance. Children and older adults frequently show impaired performance on Stroop tasks, as compared with young adults (regarding development, see Ikeda, Okuzumi, Kokubun, & Haishi, 2011; regarding aging, see West & Alain, 2000).

A recent cross-sectional study involving older adults performing a Stroop task indicated that higher aerobic fitness was associated with smaller amounts of interference (based on reaction times in the interference condition), greater accuracy, and greater task-relevant activation in the prefrontal cortex (based on fMRI data; Prakash et al., 2011). Importantly, findings from two randomized control trials in older adults investigating the effects of aerobic exercise on Stroop task performance support these results. In the first study, sedentary older adults were assigned to one of three conditions: (1) aerobic exercise (fast walking), (2) strength and flexibility training, or (3) no exercise (sedentary controls; Dustman et al., 1984). Those in the two exercise groups met with the researchers and trained for three 1-h sessions per week for 4 months. Performance on a Stroop task (as well as other nonexecutive control tasks) was assessed before and after the intervention. Comparisons of participants’ pre- and posttraining reaction times revealed that only the aerobic exercise group showed a reliable reduction in Stroop interference after the intervention. In the second study, sedentary older adults were assigned to either aerobic exercise or strength and flexibility training (Smiley-Oyen, Lowry, Francois, Kohut, & Ekkekakis, 2008). Participants exercised under supervision for 30 min a day, three times a week for 10 months. A Stroop test (along with other cognitive tests) was administered before, during, and at the end of the exercise program. After about 5 months of exercise, response accuracy in the interference condition improved only in the aerobic exercise group. Moreover, after 10 months of exercise, postintervention analyses revealed robust improvements in response speed and accuracy in the interference condition only in the aerobic exercisers. Taken together, the results of these studies support the idea that the ability of older adults to selectively attend and to inhibit prepotent responses may benefit from regular engagement in aerobic exercise.

Studies in children utilizing Stroop tasks have yielded far less promising results (Buck, Hillman, & Castelli, 2008; Castelli, Hillman, Hirsch, Hirsch, & Drollette, 2011). Although these studies provided some indication of an association between aerobic fitness and faster performance, there was no indication that executive function was influenced (i.e., the effects did not differentially involve the interference condition). It may be the case that the lack of an association in children in part reflects word reading being less automatized in this group due to limited experience, and thus there should be less of a need to recruit controlled processes to avoid being influenced by the distracting word. Alternatively, Buck et al. suggested that, in contrast to older adults, functions requiring higher levels of executive control in children may not be preferentially benefited by increased fitness.

Research investigating inhibitory control over prepotent responses, using tasks that do not involve selective attention, has produced inconsistent results. For example, in Kramer and colleagues’ (2001) randomized control trial, older adults in an aerobic exercise group showed a significantly greater reduction in stop signal reaction times than did those in a stretching and toning group at the end of a 6-month intervention. In contrast, in Smiley-Oyen and colleagues’ (2008) randomized control trial referred to above, the older adults in the aerobic exercise group showed no improvement in go/no-go reaction times after the 10-month intervention. Thus, it appears from these data in older adults, that regular engagement in aerobic exercise may benefit only select inhibitory control processes, presumably due to process-specific reliance on select brain regions that benefit most from aerobic exercise.

Several studies investigating links between exercise and selective attention or inhibitory control have employed variations of the Eriksen flanker task (Eriksen & Eriksen, 1974). In a typical flanker paradigm, participants indicate the identity of a centralized stimulus while ignoring distracting stimuli appearing in the periphery. There are two main trial types: compatible and incompatible. On compatible trials, the distractors are associated with the same response as the target; on incompatible trials, the distractors are associated with a response different from that required by the target. Successful performance on flanker tasks involves selectively attending and responding to the target stimulus, while simultaneously inhibiting distractor-related activity (Machado, Wyatt, Devine, & Knight, 2007). Despite efforts to selectively process the target, the distracting stimuli tend to influence a person’s ability to respond effectively to the target, resulting in worse performance (longer reaction times and/or more errors) on incompatible than on compatible trials. This difference in performance is referred to as the flanker effect, with smaller flanker effects reflecting more efficient executive control (Callejas, Lupiáñez, & Tudela, 2004). Although flanker and Stroop tasks tap similar cognitive functions (i.e., selective attention and inhibition), a key point of difference is that in flanker tasks, the distracting information appears in the periphery, away from the focus of attention. Furthermore, the distracting information is not necessarily associated with a prepotent response; indeed, in the seminal flanker task, stimulus–response associations were assigned arbitrarily during the testing session.

The most common version of the flanker task employed by exercise–cognition researchers involves presenting participants with five arrowheads and asking them to indicate the direction of the central arrowhead by pressing the button on the left- or right-hand side (e.g., left button for <). On compatible trials, all arrowheads face in the same direction (e.g., > > > > >); on incompatible trails, the distractor arrowheads face in the opposite direction to the central target arrowhead (e.g., > > < > >). Because indicating the direction of an arrowhead is a relatively automatic response, the incompatible trials on this task are thought to require strong inhibitory control over motor activity (Eimer et al., 1995). As with switching costs and Stroop interference, flanker effects caused by arrows increase with advancing age (Zhu, Zacks, & Slade, 2010), and inflated effects have also been reported in children (see van Meel, Heslenfeld, Rommelse, Oosterlaan, & Sergeant, 2012; see also Voss, Chaddock, et al., 2011).

Exercise-related research utilizing arrow versions of the flanker task suggests that exercise attenuates age-related decline. In one cross-sectional study involving 241 healthy adults 15–71 years of age, the frequency with which the participants over 40 reported engaging in physical activity sufficient to induce sweating was associated with smaller flanker effects (based on accuracy rates; Hillman, Motl, et al., 2006), which suggests that higher levels of physical activity may lead to better attentional and/or motor control in relatively older adults. Consistent with this possibility, in another study involving 41 healthy older adults, those with high scores on an aerobic fitness test had smaller flanker effects than did those who scored poorly on the fitness test (Colcombe et al., 2004, Study 1). Furthermore, fMRI data from that study indicated that, during performance of the flanker task, fit older adults exhibited greater activation in areas of the brain associated with regulating attention (middle and superior frontal gyri, superior parietal lobule) and less activation in areas related to response interference (anterior cingulate cortex). Importantly, similar results were reported in a follow-up randomized control study involving 29 older adults, with those who had been aerobically exercising for 6 months replicating the reduced flanker effects and the changes in the pattern of fMRI activation, whereas those in the strength and flexibility control group showed no improvement and no changes in activation patterns (Colcombe et al., 2004, Study 2). Together, these studies provide evidence that regular aerobic exercise benefits control over responses during selective attention in older adults.

Evidence in developing children is much weaker due to a lack of studies addressing engagement in aerobic exercise, but cross-sectional fitness studies provide some support for superior performance on flanker arrow tasks in fit (above the 70th percentile), as compared with unfit (below the 30th percentile), children, evidenced by smaller proportional flanker effects (Chaddock, Erickson, Prakash, VanPatter, et al., 2010), higher accuracy rates (Pontifex et al., 2011;see also Voss, Chaddock, et al., 2011), maintenance of accuracy rates across test blocks (Chaddock, Erickson, et al., 2012a), and less variable response latencies (Wu et al., 2011). Furthermore, fMRI data from 28 of the children in the Voss, Chaddock, et al. study indicated that the unfit children showed greater differential activation in association with incompatible versus compatible trials in brain areas linked to cognitive control, including the prefrontal, supplementary motor, and anterior cingulate cortices (note that the task in this study involved fish directed toward the left or right, instead of arrows). In the Chaddock, Neider, et al. (2012) study, fMRI data showed that fit, as compared with unfit, children exhibited an initial activity increase, followed by a decrease in activity in the later test block in the frontal (middle frontal gyrus and supplementary motor area) and superior parietal cortex. ERP data from the Pontifex et al. study showed that fit, as compared with unfit, children exhibited shorter latency and smaller amplitude N2, shorter latency and larger amplitude P3, and smaller amplitude error-related negativity. Together, these functional imaging and neuroelectric data are consistent with superior cognitive control in fit children. Additional evidence supporting superior cognitive control in fit, as compared with unfit, children arose in the Pontifex et al. study when participants were asked to respond on the side opposite the direction of the arrow; fit children exhibited higher accuracy rates as well as stronger modulation of P3 and error-related negativity in the anti than in the pro task. Developmental studies assessing engagement in aerobic exercise are now needed to determine the extent of its contribution to these fitness findings.

Although healthy young adults generally outperform older adults and children on tasks that depend on inhibitory control, there is some evidence in young adults to suggest that aerobic fitness might still confer a benefit for top-down control in the context of arrow versions of the flanker task. In 2008, Themanson and colleagues had 72 young adults perform an arrow flanker task before undergoing an aerobic fitness test. In one condition of the flanker task, participants were instructed to focus on responding as quickly as possible on each trial; in another condition, participants were instructed to focus on responding accurately. Regression analyses revealed that aerobic fitness was not associated with reaction time on either compatible or incompatible trials, in either the speed or the accuracy condition. However, in the accuracy condition, fit young adults tended to be more accurate on trials that immediately followed an error. Furthermore, ERP data collected during task performance showed that fit young adults had greater error-related negativity during the accuracy condition. The authors interpreted these converging findings as evidence for better top-down modulation of motor responses in fit young adults.

Another version of the Eriksen flanker task employed by exercise–cognition researchers involves stimuli that have been arbitrarily assigned to responses. For example, participants are asked to indicate the identity of a central letter by pressing one of two buttons (e.g., left button if “F,” right button if “X”). On compatible trials, distractor letters are the same as the target (e.g., F F F); on incompatible trials, distractor letters differ from the target and are associated with the incorrect response (e.g., F X F). Because the association of left/right buttonpresses to arbitrary letters is less prepotent than that to left-/right-pointing arrowheads, letter versions of the flanker task presumably depend more on effective selective attention than on strong response inhibition. As with flanker effects based on prepotent stimulus–response mappings, flanker effects based on arbitrary stimulus–response mappings increase with advancing age (Machado et al., 2009). Whether children also show inflated effects for stimuli with arbitrary response links has not yet been reported to our knowledge.

A small cross-sectional study that used a letter version of the flanker task found that low, moderate, and highly active older adults did not differ in performance, but it should be noted that each group included just 8 participants, so the lack of significant differences could reflect a lack of power (Hillman, Belopolsky, Snook, Kramer, & McAuley, 2004). ERP recordings taken while participants performed the task indicated that older adults who reported regularly engaging in moderate to high levels of physical activity exhibited on the incompatible trials of the task better attentional focus (as indicated by larger P3 amplitude), than did 7 young adults, but no differences emerged in comparison with their less active peers. In addition, in contrast to the less active older adult groups, highly active older adults did not exhibit the typical age-related increase in cognitive processing speed (measured by P3 latency), but note that P3 latency did not differ between the older adult groups. Another cross-sectional study using a larger sample size of older adults and measuring aerobic fitness yielded evidence that fitness predicts at least some aspects of performance in a flanker task involving arbitrary response associations (Voelcker-Rehage, Godde, & Staudinger, 2010). Specifically, the authors of that study found a small but reliable association between older adults’ physical fitness and their reaction times on the incompatible trials of a color version of the flanker task, such that fit older adults responded faster than their unfit peers. Finally, a 6-month intervention study in older adults revealed significantly greater reductions in the size of the flanker effect (letter version) for aerobic exercisers, as compared with stretchers and toners (Kramer et al., 2001). Overall, these findings suggest that regular aerobic exercise in older adulthood could be beneficial for performance on an executive control task that requires resolution of the interference arising from competing distractors with arbitrary response links.

Although data in young adults are sparse, one study suggests that there may be some benefits of aerobic fitness on participants’ reaction times in a letter version of the flanker task (Themanson & Hillman, 2006). In that study, 28 young adults were categorized as either highly fit or of lower fitness on the basis of whether their score on an aerobic fitness test was above or below the 80th percentile for their age. The authors reported that there were no differences in the size of the flanker effect between the two fitness groups. However, they also reported that fit individuals exhibited significantly more slowing on trials following commission of an error. This posterror slowing effect was interpreted as evidence for better top-down control of attentional processes because it reflects effortful modulation of responses in order to avoid making another error. Furthermore, ERP data from that study suggested that, as compared with their less fit peers, highly fit individuals exhibited less neural conflict on incompatible trials and greater attentional focus after receiving error feedback (as indicated by error-related negativity and error positivity). Thus, the authors concluded that aerobically fit young adults are better able to use executive control processes to modulate their neural responses in a selective attention task.

Data in children are also sparse, but again one cross-sectional fitness study provided some evidence of superior performance in fitter children on a letter version of the flanker task (Hillman, Buck, Themanson, Pontifex, & Castelli, 2009). In that study, 38 children were selected from a pool of 592 on the basis of ranking in the top or bottom 10 % on an aerobic fitness test (Progressive Aerobic Cardiovascular Endurance Run). Although the size of the flanker effect did not differ between the two fitness groups, overall accuracy rates were higher in the fit group. Furthermore, ERP data suggested that fit participants exhibited superior cognitive control, including better allocation of attention (evidenced by larger amplitude P3) and posterror modulation (evidenced by smaller amplitude error-related negativity and larger amplitude error positivity).

Taken together, the research presented here provides compelling evidence for exercise-related benefits on selective attention and inhibitory control in older adults. To summarize, studies in older adults indicate that aerobic fitness is a good predictor of performance on tasks that rely relatively heavily on inhibitory control over prepotent responses (e.g., Colcombe et al., 2004, Study 1; Prakash et al., 2011) and also that regular aerobic exercise improves performance on such tasks (e.g., Colcombe et al., 2004, Study 2; Dustman et al., 1984). In addition, there is some evidence that regular aerobic exercise improves older adults’ performance on tasks that rely more on selective attention than on inhibitory motor control (e.g., Kramer et al., 2001). In contrast, current research in young adults and children provides only limited support for exercise-related benefits in selective attention and inhibitory control via associations with fitness levels, with the bulk of the supportive evidence relating to accuracy rates and indices of posterror behavior (e.g., Hillman et al., 2009; Themanson & Hillman, 2006; Themanson et al., 2008; Voss, Chaddock, et al., 2011). Clearly, intervention studies are needed in children and young adult populations to determine whether the reported associations with fitness relate specifically to regular engagement in aerobic exercise.

Working memory

Working memory tasks involve holding information in mind for a short period of time and rapidly updating that information in order to respond correctly (Baddeley & Hitch, 1974). One working memory task commonly used in the exercise–cognition literature is the two-back task, in which participants are asked to press a button whenever the currently displayed stimulus matches the stimulus that appeared two stimuli back in a sequential presentation (Cohen et al., 1994). Since this task requires participants to keep a relatively small amount of information in mind, it is presumed to rely heavily on the updating component of working memory (E. E. Smith & Jonides, 1997), and performance depends on the lateral prefrontal cortex (Muller, Machado, & Knight, 2002). In older adults, cross-sectional research has shown that physical fitness is a significant predictor of accuracy on the two-back task (Voelcker-Rehage et al., 2010). In relatively young adults, training studies have shown that aerobic exercise programs can increase two-back accuracy (Hansen et al., 2004) and reduce reaction times (Stroth et al., 2010). Together, these findings suggest that both aerobic fitness (in older adulthood) and regular engagement in aerobic exercise (in young adulthood) are beneficial for the updating component of working memory.

Other paradigms used to assess the effects of regular exercise on working memory include digit span and Sternberg tasks. Forward span tests involve presenting numbers to participants one at a time and then asking them to report the whole sequence to the experimenter in the correct order. Backward span tests are identical, except that participants are asked to report the sequence in the reverse order. In both tasks, the sequence length is gradually increased until participants can no longer correctly recall all of the digits. While span tests rely on mental updating to some degree (particularly backward versions), they mainly test the amount of information that a person can hold in mind at one time. Although an early study indicated no effect of regular exercise on older adults’ forward or backward digit spans (Blumenthal et al., 1991), more recent research suggests that exercise can be beneficial for this type of working memory. For example, data from a large-scale cross-sectional study showed that higher levels of physical activity in older women are associated with longer backward digit spans (Weuve et al., 2004). Furthermore, a 12-month intervention study involving 187 older women revealed that aerobic exercise can significantly increase older adults’ forward digit spans (Williams & Lord, 1997).

In contrast to the span tasks, the modified Sternberg tasks used by exercise–cognition researchers involved simultaneous presentation of a memory set of variable size (e.g., three, five, or seven letters) followed, after a short delay, by a single probe stimulus; participants indicated whether the probe was present in the memory set. Given the relatively low requirement for updating in this task, performance presumably depends primarily on memory capacity. One cross-sectional fitness study in 64 young adults that used a median split to categorize participants into higher and lower fitness groups found no differences in Sternberg task performance between the fitness groups; however, ERP data showed that fitter participants exhibited smaller amplitude frontal contingent negative variation when instructed to maximize speed, which could be interpreted as an indication of more efficient preparation processes (Kamijo, O'Leary, Pontifex, Themanson, & Hillman, 2010). By comparison, a 9-month intervention study in 20 children did find performance differences, with accuracy improved after aerobic training, and this was accompanied by an increase in the amplitude of initial frontal contingent negative variation, which was interpreted as an indication of superior cognitive control, given past findings regarding this ERP subcomponent (Kamijo et al., 2011). Overall, the results from the span and Sternberg tasks suggest that regular exercise can also confer benefits for the volume of information that children and older adults can hold in mind at one time.