Caffeine intake modulates the functioning of the attentional networks depending on consumption habits and acute exercise demands

Consume of stimulants (as caffeine) is very usual in different contexts where the performers have to take quick and accurate decisions during physical effort. Decision-making processes are

mediated by the attentional networks. An experiment was carried out to examine the effect of caffeine intake on attention (alerting, orienting, and executive control) as a function of

consumption habit under two physical exertion conditions (rest vs. aerobic exercise). Two groups of participants with different caffeine consumption profiles (moderate consumers vs. low

consumers) performed the Attention Network Test–Interactions under four different conditions regarding activity (rest vs. exercise) and intake (caffeine vs. placebo). Results showed that

whereas exercise led to faster reaction times (RT) in all cases, caffeine intake accelerated RT but only at rest and in moderate caffeine consumers. More importantly, caffeine intake reduced

the alertness effect in moderate consumers only at the rest condition. No interactions between Intake and Activity were observed in the other attentional networks, with exercise reducing

orienting independently of caffeine intake, which suggests that physical exercise and caffeine are different modulators of attention but can interact. Caffeine intake had differential

effects on reaction speed at rest and during physical exercise depending on the individual consumption habit. On the basis of these finding it seems that mainly alertness is modulated

differently by internal and external “arousing” conditions.

At different environmental situations (as practicing sports, driving vehicles, surgery interventions…) people must often perform physical activities and respond quickly to external

challenges. Decision-making processes are mediated by the three attentional networks (alerting, orienting, and executive control) defined by Petersen & Posner1.

The executive control network is involved in situations that require planning, decision making, error detection, execution of novel responses, or overcoming habitual actions2. The orienting

network is responsible for allocating attention to a particular object or region of space in a voluntary-endogenous or reflexive-exogenous way, enhancing its processing while ignoring

irrelevant objects/locations3. Finally, the alerting network participates in the general activation of cortical and thalamic areas, thus preparing the perceptual-motor system for fast

reactions through changes in the norepinephrine system.

During different situations of daily living like working and sport tasks, these attentional networks have to work in cooperation to maintain an efficient psychomotor performance during the

physical-cognitive dual task challenges inherent to those activities.

The relationship between exercise and two of the main core components of executive control (inhibitory processes and cognitive flexibility) has been widely studied by researchers. Inhibitory

control refers to the ability to attend to a relevant stimulus while ignoring the others that are not relevant to the goal or task at hand. Cognitive flexibility refers to the ability to

rapidly switch between different tasks. Previous research reported that the above- mentioned component of executive control were improved (enhancing inhibitory processes4 and reducing switch

cost5) when participants were exercising under an aerobic workload. Yet, and according to the hypofrontality hypothesis6, neuroelectric7 and behavioral8 findings showed impaired conflict

resolution during exercise conditions as compared to rest conditions. In addition, other studies failed to show any differential effects of acute aerobic exercise on participants’

performance9, thus adding more contradictory findings regarding the nature and direction of this relationship. More importantly for the purposes of the present study, scarce studies have

analyzed the effect of concomitant physical effort on the three attentional networks assessed at the same time. Faster RT during the exercise condition were reported by Chang and

colleagues10 using the Attentional Network test (ANT)11, but only on trials with incongruent flankers, thus leading to reduced interference10. Another study, closer to the present one, using

a version of the ANT named Attentional Network Test Interaction (ANT-I)12 showed no effect of exercise on executive control13.

Regarding spatial attentional orienting, in a series of studies Pesce and colleagues found that aerobic exercise enhanced the flexibility in modulating the spatial extent of visual

attentional focus at different ages and sport modalities14,15,16,17,18. Regarding the shifting of spatial attention in the visual space, the ability we will analyze in our study, previous

findings have shown that the effect of exercise on cueing effects in visuospatial attention is modulated by gender19 and sport expertise20. However, to the best of our knowledge, few studies

have explored the effects of acute bouts of aerobic exercise on the deployment of exogenous visual spatial attention. Previous studies found that an acute bout of aerobic exercise performed

during or even immediately before a spatial orienting task eliminated the typical spatial cueing effect21. Later studies established that spatial orienting in overt attentional capture was

influenced by high intensity exercise, and depended on participants’ fitness level showing more reduced attentional effects in low-fit participants than in high-fit participants22. However,

contradictory results were found when attentional networks were assessed simultaneously, with no effects of exercise on exogenous spatial orienting10,13.

On the other hand, the modulation of alertness by acute exercise has been widely studied but also shows mixed results. There are two types of alertness. Phasic extrinsic alertness is

associated with an abrupt increase of nonspecific activation when a warning cue is presented preceding the target (this is related to the orienting response23). By contrast, tonic alertness

or vigilance refers to sustained activation over a period of time24. Regarding phasic alertness, previous studies reported that participants’ accuracy was lower when the task was performed

under an intense physical exertion condition than at rest25. However, contradictory results emerge again when attentional networks are assessed simultaneously using the ANT task.

Neuroelectric findings have showed larger P3 amplitude (a positive-going component associated to the amount of attentional resources allocated to environmental events), on alerting trials in

the exercise condition10. However, another behavioral study has found a reduction of the size of the phasic alerting effect during aerobic exercise as compared to the rest condition13.

Regarding the tonic alertness (sustained attention), there is a broad consensus that moderate to intense exercising enhance the state of tonic vigilance26, though poorer vigilance

performance was observed under heavier exercising conditions27.

Previous research has shown that the effect of acute bout of exercise on attentional networks seems to be moderated by different individual and contextual constraints (e.g., participant

physical fitness, sport modality, exercise intensity, or others as consume of ergogenic or stimulant substances). Therefore these variables need to be considered in the study of the acute

exercise-attention relationship.

During challenging environmental conditions, people often decide to consume stimulants to improve performance. Caffeine (1,3,7-trimethylxanthine) has become one of the most popularly used

ergogenic substances to improve both physical and cognitive performance28,29. However, though improvement in general attentional performance induced by caffeine intake is well accepted,

there is scarce and controversial evidence about its effect on each attentional network and their interactions.

Regarding the effect of caffeine intake on executive control, caffeine has been shown to have a positive influence on the ability to switch attention between tasks (task switching) and

anticipatory control processes30,31. However caffeine consumption has failed to modulate other facets of inhibitory function like the ability to control interference from distracting stimuli

(using Ericksen Flanker Test) and the ability to suppress responses selectively (e.g., by using a Stroop task)32. Previous studies exploring the interactions between exercise and caffeine

on executive control reported a positive effect of caffeine intake after exercise33, during aerobic34 and intermittent exercises35.

Few studies have been nevertheless conducted on the effect of caffeine on attentional orienting36,37. Such studies, performed under resting conditions, have only shown marginal effects of

caffeine intake on orienting when low caffeine consumers were tested. However, controversial findings have been reported when interaction between caffeine intake and orienting was studied

manipulating exercise conditions. While some studies did not find modulations of caffeine on object tracking or covert spatial orienting respectively after aerobic acute exercise38,39,

others showed positive effects on shifting attention after longer medium to high intensity aerobic exercises33.

Considering alertness, systematic reviews have reported significant enhancements of various doses of caffeine on vigilance40,41. Concerning the interaction between caffeine and exercise on

alertness, sustained vigilance appears to be improved by caffeine intake after33 and during34 an acute exercise bout. However, caffeine specific effects on phasic alertness during exercise

hasn’t been studied yet.

Scarce studies have investigated the effects of caffeine on the functioning of the three attentional networks concurrently. To our knowledge, Brunyé and colleagues are the only ones who have

investigated the effects of caffeine intake on the three attentional networks simultaneously by means of the ANT task36,37. They have reported that caffeine-induced physiological arousal

amplifies global spatial processing, vigilance and executive control, but these effects are at least partially driven by the administered dose and habitual caffeine consumption. These

findings are in line with those showing that caffeine increases arousal on every type (low and habitual) of caffeine consumers42. However, most studies in this topic have involved

caffeine–deprived habitual caffeine consumers, which makes it difficult to know whether the observed findings are due to the effects of caffeine intake or the alleviation of caffeine

withdrawal. On this vein, another study investigated acute effects of caffeine in both habitual and non-habitual caffeine consumers observing a more positive effect of caffeine intake on

consumers’ mood state while improving cognitive performance more in the non-consumers43. Thus, previous findings suggest that the effects of caffeine intake on attentional performance could

be explained, not only by a withdrawal reduction model, but also by other mechanisms involving the brain biochemistry and the effect of caffeine on dopamine-rich areas of the brain.

Considering that in many daily life activities caffeine intake is combined with a physical effort, we have to take into account the evidence from previous literature reviewing the effects of

acute exercise on cognition from a neurochemical perspective. A recent review by McMorris44 has shown that long duration, moderate intensity, and heavy exercise generate excessive

concentrations of catecholamines and cortisol inhibiting working memory. Nevertheless, heavy exercise has a positive effect on long-term memory due to activation of β-adrenoreceptors and

increased exercise-induced brain-derived neurotrophic factor (BDNF) levels. Since it is widely known that attentional functioning is also modulated by the exercise- induced physiological

response facilitating or inhibiting attentional performance, it would be worth to study the joint effects of physical exercise and caffeine as both take part in similar mechanism underlying

the brain biochemistry.

However, and importantly for our research, the exercise- and caffeine-induced changes on the attentional networks are somewhat equivocal44, as described above, and therefore future research

is necessary. Furthermore, understanding the interplay between exercise, caffeine intake and consumption habits on the functioning of the three attentional networks is crucial for adapting

caffeine consumption to optimize its expected benefits. Nevertheless, to the best of our knowledge, no study so far has examined this interaction. In order to fill this gap, the goal of the

present study was to explore the individual and interactive effects of caffeine intake and acute physical exercise on the functioning of the attentional networks in the context of a

multi-functional task (ANT-I task12) under four different conditions resulting from the combination of activity (i.e., rest vs. sustained moderate aerobic exercise) and intake (caffeine vs.

placebo).

Following the above reviewed literature, behavioral and neurophysiological findings lead to argue that the efficiency of the central nervous and cognitive function (reaction time and

attention) is modulated by exercise-induced44 and caffeine-induced37 neurobiochemical mechanisms regulating arousal. It is relevant to highlight that caffeine consumption41 and exercise45

modulate the baseline arousal level (lower under rest and placebo conditions), mainly in participants more habituated to caffeine intake. Based on this evidence we expected to observe an

interaction between caffeine intake, consumption habit and exercise condition on cognitive performance. This interaction was expected for overall RT and alertness, as these variables seems

to be affected by opposite manipulations reducing arousal46,47,48. In our case, we expected faster RT (tonic alertness) and reduced phasic alertness effects in the conditions in which the

combination of exercise and caffeine intake would lead to stronger activation. We also wanted to explore whether caffeine intake and exercise also affect the functioning of the other

attentional networks (orienting and executive control). Although some effects have been previously reported in the above reviewed literature, no clear predictions were anticipated.

Twenty-four male undergraduate sport sciences students (age range: 21–25 years; M = 22 years) were selected to participate in this study. We used the usual sample size with the ANTI

task12,49, without using a priori power analyses. Therefore, a sensitivity analysis was conducted using G*power50 which showed that with our sample size (N = 24) and 48 repeated measures,

the minimum effect size that could have been detected for α = 0.5, and 1 − β = 0.80, for 2 groups, is f = 0.11 (minimum detectable effect).

Participants were assigned to one of two groups according to their habitual caffeine consumption based on their responses to a self-report food survey. Following previous recommendations51

we used a between-group design, allocating participants to one of two groups depending on their caffeine consumption habit and dispensing an individual caffeine dosage depending on

participants’ body weight. According to participants’ estimated daily caffeine intake52, they were assigned to either the low or moderate caffeine consumers group using a cut-off point

criterion (Low consumers ≤ 50 mg/day vs. Moderate consumers > 50 mg/day)43. Half of participants were categorized as low caffeine consumers (M = 14 mg/day, range = 0–28 mg/day) and half were

considered moderate caffeine consumers (M = 101 mg/day, range = 57–228 mg/day). All participants were non-smokers who were in good health and were not taking any medication. They all

completed both an ethical clearance form and a statement regarding informed consent.

Present research followed accepted ethical, scientific and medical standards and was conducted in compliance with recognized international standards, in accordance with the revised version

(2013) of the Helsinki Declaration. Informed consent was obtained from all study participants. The study protocol was approved by the Ethics Committee of the Catholic University of Valencia,

Spain (UCV/2016-2017/02).

Participants visited the laboratory on five separate days with a minimum interval of 48 hours and a maximum interval of 96 hours, approximately at the same time of the day. They were

instructed to abstain from consuming caffeine or any stimulant substance and to avoid physical exercise for 24 h prior to each session. On the first visit, participants performed a graded

submaximal test in order to determine their Lactate Threshold (LT). The next four visits were experimental sessions in which participants had to complete the attentional tasks corresponding

to the Intake (caffeine vs. placebo) and Activity (rest vs. exercise) conditions, whose order was counterbalanced across participants using a Latin square design. The experimental timeline

is depicted in Fig. 1. At the end of the last experimental session, participants were debriefed on the purposes of the study and received a description of their physical and cardiovascular

fitness and attentional performance.

Schematic representation of the experimental protocol timing.

During cycling parameter estimation test and attentional evaluation we have employed the same attentional tasks, and similar resources and procedures previously described by the authors in a

similar research about the effect of acute bouts of exercise on functioning of the attentional networks13.

On the first session, participants were informed about the details of the study and gave written informed consent. After that, participants practiced a reduced version of the ANT-I. Next,

participants were fitted with a Polar RS800 heart rate monitor (Polar Electro Ltd., Kempele, Finland). A magnetically braked cycle ergometer (Cardgirus Medical, G&G Innovación, La Bastida,

Alava, Spain) was adjusted to accommodate them to perform a graded submaximal test according to previous recommendations53. After a 7-min warm-up, the test started at an initial workload of

100 W, with increments of 30 W every 4 min. The test was terminated when participants acknowledged voluntary exhaustion, could not maintain the minimum cadence of 60 revolutions per minute

(rev · min−1) or when their heart rate (HR) reached 95% of their maximum HR (HRmax) obtained from the age-predicted equation54: HRmax = 208 − 0.7 · Age. Finally, participants pedaled until

their HR was under 120 beats per minute (bpm) before getting off the cycle ergometer. Earlobe capillary blood samples were collected in the last 15 s of each stage and were analyzed by a

blood lactate test meter (Lactate Pro LT-1710, Arkay KDK, Japan). Power output and HR were continuously monitored using a sampling rate of 1 Hz. Determination of the LT was based on the

criteria established by previous studies55. Workload at LT was defined as the power output elicited at the stage before LT. HR at LT was estimated as the statistical mode value of HR at the

stage before LT. These results were used to set the individual exercise workloads in the exercising experimental conditions.

Prior to each experimental session and upon their arrival in the lab, participants were weighed and given a capsule containing their previously assigned intake substance. We used a

double-blind design to manipulate this variable. Caffeine (4 mg · kg −1 pure anhydrous powder) and placebo (pregelatinized starch powder) were administered in identical color, size, weight,

and shape capsules with ad libitum water. Dosage was selected based on a previous review51 which suggested doses from 3 to 6 mg · kg−1, and considering the recommendations by European Food

Safety Authority56. Next, participants were asked to take a 45-minute break to allow for sufficient plasma concentrations of caffeine after consumption57. Participants completed the ANT-I

task at approximately 60 cm from the computer monitor in a dimly-lit laboratory. A headphone set was used to deliver the acoustic alerting signal. Two computers were used simultaneously in

the experimental set-up. One was used to run E-Prime software58 presenting stimuli and collecting the participants’ responses during the attentional task in all experimental conditions. The

other computer was used to run Cardgirus® software adjusting and collecting power output and HR data during exercise conditions. In the rest condition, participants completed ANT-I while

sitting on a chair.

Previous findings have shown that the efficiency of some brain areas and systems involved in cognitive arousal and cognitive resources is related with physiological exercise workload-induced

changes44,45. During the exercise conditions, participants performed the attentional task while cycling on the ergometer at 80% of their workload at LT. This intensity has been used in

previous studies investigating the effect of acute exercise on attentional networks13, in which physiological variables (e.g., HR blood flow and lactate) were maintained in a moderate

aerobic steady-state zone during the whole session. With the aim of keeping physiological response (HR) in a steady-state zone, a 10-min incremental warm-up was used to reach the initial

predetermined exercise workload. Workload could be regulated lightly by the experimenter manually throughout the exercise sessions to maintain each individuals’ HR in the target steady-state

zone. To confirm that participants performed the cognitive task under a steady state metabolic during both rest and exercise sessions, blood was drawn from their earlobes for lactate

analysis at the beginning and immediately after finishing the attentional task. Moreover, HR was monitored during the whole sessions.

The participants’ task was to respond as quickly and accurately as possible by pressing the left or right key placed on the handlebar. Participants responded to the direction of the target

stimulus (a central target arrow 0.55° long pointing either left or right), which was flanked by two identical to the target irrelevant arrows on each side (0.06° away from each other). On

each trial, an acoustic alerting tone (2000 Hz and 50 ms) and/or spatial orienting visual cue (an asterisk 0.6° long and 50 ms) preceded the target arrow. Participants were strongly

encouraged to keep their eyes fixed on the fixation point (variable duration of 400–1600 ms) throughout the trial. The sequence of events for each trial is shown in Fig. 2. The interference

variable was defined according to the congruency of the direction of the flankers and target arrows: congruent trials (50% of trials), when the target was flanked by arrows pointing in the

same direction, and incongruent trials (the other 50% of trials), when the flanking arrows and the target pointed in opposite directions. The orienting signal was presented in two thirds of

the trials above or below the fixation point. Three orienting conditions were thus established according to the presence of the cue; cued location trials, when the cue was presented at the

same location as the target; uncued location trials, when the cue was presented at the opposite location to the target, or absence of cue, no-cue trials, when the cue was not presented. The

alerting signal was presented before the onset of the target in only half of the trials. The alerting variable was established according to the presence (tone) or absence (no tone) of the

alerting sound. The target was presented until participants responded or for 1800 ms. After the response, or after the maximum time had elapsed, the fixation point was presented for a

variable duration (depending on the RT of the preceding trial and the duration of the initial display for that trial) so that all trials were equally long. Initially, participants completed

a practice block of 48 trials, followed by five experimental blocks of 48 trials each, with resting intervals of about 1 min between them, but maintaining constant cycling parameters in the

exercise sessions.

Thus, participants performed the ANT-I for approximately 25 min in each experimental session. The exercise sessions required longer time (about 35 min) because the warm-up stage was

necessary to reach the physiological steady state to perform the 25 min of continuous cycling at the pre-established HR and exercise workload. Both the workload intensities and the duration

made it possible to obtain the required amount of data from the attentional task during exercise, maintaining a physiological steady state while avoiding fatigue in the participants. Results

regarding physiological response to each Activity and Intake condition confirm the high level of stability of the variables used for measuring physiological response to exercise.

We conducted a mixed repeated-measures MANOVA on the data of each dependent variable (HR, RT, and accuracy), with caffeine Consumption Habit (low, moderate) as a between-group factor, and

the other independent variables (Intake: caffeine, placebo; Activity: rest, exercise; Alerting: tone, no tone; Orienting: uncued, no cue, cued; and Congruency: congruent, incongruent) as

within- participant factors. Subsequent ANOVAs were performed for mean RT and overall error percentages to analyze the general effects of Consumption Habit, Intake, and Activity on each

attentional function. Post hoc analyses (paired t-tests) were conducted to further explore significant interactions.

Descriptive statistics of anthropometric and physiological characteristics of the whole sample and separately for low and moderate caffeine consumers are displayed in Table 1. No significant

differences were observed between groups in none of these variables (all ps > 0.2).

The Mean (M) and Standard Deviation (SD) of HR and lactate values per experimental session are displayed in Table 2.

In order to confirm whether the applied exercise workload and caffeine intake induced different changes in physiological state in each intake group of participants, a mixed 2 (Habit) × 2

(Intake) × 2 (Activity) MANOVA was performed on participants’ average HR values, with Habit as a between-group factor. Results showed a significant main effect of Activity, F(1, 22) = 

4158.19, p