Acute incremental exercise, performance of a central executive task, and sympathoadrenal system and hypothalamic-pituitary-adrenal axis activity
Introduction
The primary purposes of this study were to examine the effect of acute incremental exercise on the performance of a central executive task; the responses of the sympathoadrenal system (SAS) and hypothalamic-pituitary-adrenal axis (HPAA) during exercise, while simultaneously carrying out the central executive task; and the ability of changes from baseline (Δ) plasma concentrations of epinephrine, norepinephrine, adrenocorticotropin hormone (ACTH) and cortisol to predict Δ performance on the central executive task. The need of sports performers and the military to make quick and accurate decisions, while simultaneously undertaking exercise of various intensities, has led to a large amount of research in this area (see Tomporowski, 2003, McMorris, 2006, for reviews). Moreover, as exercise is a stressor, there may be implications for the effect of other physiological stressors on cognition.
The majority of the research examining the effect of exercise on cognition has tended to use simple cognitive tasks, e.g. choice reaction time, visual search, which do not activate the same areas of the brain that are used during decision making in sports and military situations (Tomporowski, 2003, McMorris, 2006). Recently a number of researchers have examined the effects of exercise on central executive tasks, however in the majority of studies, cognitive testing has taken place post-exercise (Hillman et al., 2003, Themanson and Hillman, 2006, Kamijo et al., 2007, Coles and Tomporowski, 2008). The problems with testing post-exercise have long been documented (Tomporowski and Ellis, 1986). Individuals, especially fit ones as most subjects were in these experiments, recover very quickly from exercise of the types used by the researchers (Kjaer, 1989). Thus the psychophysiological status of the subjects during cognitive testing that is undertaken post-exercise is not the same as it was during the exercise. Also Tomporowski (2003) highlighted the fact that exercise intensity may be a key factor affecting results. Therefore, we decided to examine the effect of exercising at 50% and 80% maximum aerobic power (MAP) on performance of a central executive task, Eriksen's flanker task (Eriksen and Eriksen, 1974). Thus the problem of recovery would be controlled by testing during exercise, and the protocol would also allow comparison of the effects of exercising at differing intensities.
No other study has attempted to examine the effect of incremental exercise on performance of the flanker task. However, Pontifex and Hillman (2007) examined the effect of undertaking the task while exercising at 60% maximum heart rate (HRMAX), a moderate intensity. They found no effect on response time (RT) and showed a decrease in accuracy of response but only on incongruent trials, i.e. where the target and noise stimuli elicit conflicting responses. In the present experiment, we also tested during moderate intensity exercise, albeit higher than that used by Pontifex and Hillman, and during heavy exercise. Pontifex and Hillman found that N2 amplitude, which is indicative of response inhibition (Ridderinkhof, 2002), was greater in incongruent trials compared to congruent, i.e. when the target and noise stimuli elicit the same response. They also found that N2 amplitude decreased during exercise.
Given that response inhibition is of greater importance during incongruent compared to congruent trials and that Pontifex and Hillman (2007) found a decrease in N2 amplitude during exercise, we hypothesized that incongruent and congruent trials would be affected differently by exercise. For incongruent trials, we hypothesized an increase in errors and RT at 80% MAP. Exercise at this intensity is far more stressful than that used by Pontifex and Hillman (Kjaer, 1989, Deuster et al., 1989) and several authors have claimed that central executive tasks will be disrupted at high exercise intensities only (e.g., Dietrich, 2003, Dietrich, 2009, Tomporowski, 2003). These claims are based on cognitive-energetic/arousal–performance interaction theories, which state that performance at high levels of stress will be disrupted (e.g., Sanders, 1983), and cognitive neuroscientific claims that, during high levels of stress, there is competition for resources between different centers of the brain resulting in weaker cognitive functioning (Miller and Cohen, 2001). Given that Pontifex and Hillman (2007) found a significant increase in errors at 60% HRMAX, we hypothesized such an increase at 50% MAP. Although Pontifex and Hillman showed no significant effect on RT at 60% HRMAX, we hypothesized an increase in RT at 50% MAP on incongruent trials as exercise at this intensity constitutes a significant increase in perceived stress from rest (Borg, 1998). We also hypothesized a significant increase in errors and RT from 50% to 80% MAP, as the latter is considerably more stressful than the former (Borg, 1998).
Pontifex and Hillman's (2007) results would suggest no effect of exercise on performance on the congruent trials. However, as there is little response inhibition involved in these trials they are very similar to choice reaction time tasks. Research examining the effect of exercise on choice reaction time tests has tended to show a linear improvement in performance during exercise at the same intensities as those used in this study (see Tomporowski, 2003, McMorris, 2006, for reviews). McMorris (2006) claimed that the exercise-induced increases in arousal, even at an intensity as high as 80% MAP, have a beneficial effect on these comparatively simple tasks. Therefore, we hypothesized a linear improvement in performance from rest to 80% MAP for congruent trials. We also examined the effect of exercise on responses following an error. Gehring et al. (1993) claimed that, when an individual perceives an error, they compensate by allocating more resources to higher centers of the brain, particularly the prefrontal cortex (PFC), thus slowing RT on the next response. Based on Dietrich's (2003) and Miller and Cohen's (2001) claims concerning competition for resources between different centers of the brain during stress, we expected that post-error responses would show a linear increase from rest to 80% MAP. However, we expected that post-correct responses would not be significantly effected as there would be no increase in PFC activity.
The second major aim of this study was to examine the interaction between exercise, cognitive performance, and SAS and HPAA activity, as measured by plasma concentrations of epinephrine and norepinephrine, and cortisol and ACTH, respectively. Animal studies have shown increased turnover of the neurotransmitters norepinephrine and dopamine in the brain during exercise (Gerin and Privat, 1998). As a result, researchers have argued that there is an interaction between SAS activity, exercise and cognitive performance (Cooper, 1973, Chmura et al., 1994, McMorris et al., 2008). Evidence for such an interaction is, however, somewhat equivocal (Winter et al., 2007, McMorris et al., 2008). This may be because, during exercise, the HPAA is also active, resulting in moderate concentrations of plasma cortisol during moderate intensity exercise and very high concentrations during heavy exercise (Deuster et al., 1989). Vedhara et al. (2000) found that high concentrations of cortisol had a negative effect on cognition. We hypothesized that plasma concentrations of norepinephrine, epinephrine, ACTH and cortisol would be greater during exercise than at rest and concentrations at 80% MAP would be higher than at 50%.
The third purpose of this study was to examine the ability of Δ plasma concentrations of epinephrine, norepinephrine, ACTH and cortisol to predict Δ performance on the central executive task. If plasma concentrations are indicative of the effects of SAS and HPAA activity in the brain, one would expect that Δ plasma concentrations would predict Δ performance on the central executive task.
Section snippets
Subjects
Subjects (N = 24) were paid male students, mean (SD) age 24.32 (7.10) years. All participated regularly in recreational sports. HRMAX (184 (9) bpm) was determined during a MAP test. MAP was 305.63 (50.4) W, which is classified as low to moderate (Shvartz and Reibold, 1990). They signed written consent forms, were fully informed about the protocol and completed medical questionnaires prior to testing. They were told that they could withdraw from the study at any time and would still be paid. The
Results
Mean (SD) percentage of HRMAX was 35.83% (5.64) at rest, and 77.08% (5.00) and 89.83% (3.59) at 50% and 80% MAP, respectively. Mean (SD) power output at 50% MAP was 141.58 W (19.21) and 226.33 W (30.47) at 80% MAP.
Fig. 1 shows the mean (SD) RT for congruent and incongruent trials on the flanker task. A congruency × exercise intensity ANOVA showed that RT in the congruent trials was significantly faster than in the incongruent trials (F(1,23) = 158.68, p < 0.001, ηp2 = 0.87). There was also a main
Discussion
The results of this experiment provide partial support for our hypotheses. For the RT variable, results of the congruency × exercise intensity ANOVA showed main effects for congruency and exercise intensity but no interaction effect. Post hoc Tukey LSD tests showed that RT at 80% MAP was significantly slower than at 50% MAP. RT at 80% MAP was also slower than at rest and the difference was approaching significance (p = 0.08). Observation of Fig. 1 suggests a difference between RT at rest and during
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