Improvement on Attention Networks Among Military Personnel: The Right IFG and tDCS Matter

ABSTRACT

Attention is crucial for military personnel to recover from mental dysfunction and maintain superior cognitive abilities. Transcranial electrical stimulation (tES) is a promising training method for enhancing attention; however, the optimal parameters for tES interventions remain unclear. This study aims to identify the most responsive cortical area and the most effective tES type for attention enhancement. In Experiment 1, 62 healthy male soldiers were examined to determine the most effective stimulation target for attention improvement after 4 (cathodal electrodes) × 1 (anodal electrodes) high‐definition transcranial direct current stimulation (tDCS) on the right inferior frontal gyrus (IFG) or the left dorsolateral prefrontal cortex (DLPFC). Experiment 2, involving 75 participants, focused on modulating the previously identified appropriate cortex using both high‐definition transcranial alternating current stimulation (tACS) and tDCS to ascertain the most effective tES method based on behavioral and neural activity changes. Both experiments were double‐blind and sham‐controlled. Executive control of attention networks was significantly improved after tDCS modulation of both the right IFG and the left DLPFC. Notably, only the modulation of the right IFG effectively decreased the Stroop effect. While both tACS and tDCS on the right IFG induced lower neural activity related to the Stroop effect, only tDCS significantly reduced the behavioral performance of the Stroop effect. Consequently, the right IFG emerges as a key targeted cortex for tES modulation in enhancing attention, with tDCS proving more effective than tACS in regulating the right IFG to improve executive control. These findings lay the groundwork for applying tES interventions in the training of attention abilities among military personnel.

tDCS targeting the right IFG optimally enhances soldiers' executive attention, uniquely reducing Stroop effect behavior by improving neural efficiency. While both tDCS and tACS lowered Stroop‐related neural activity, only tDCS yielded behavioral gains. This establishes tDCS as the superior method for efficient cognitive enhancement in military training.

1. Background

Attention is fundamental to cognitive abilities, enabling concentration on specific events without distractions (Amso and Scerif 2015). This skill is crucial in some occupations, such as the military, where maintaining a high level of attention across diverse activities—including physical training, marksmanship practice, equipment operation, and theoretical learning—is essential (Jamro et al. 2021, 2022; Li et al. 2016). However, challenges such as fatigue, sleep deprivation, and even mental disorders from sustained military operations or complex battlefield environments can impair vigilance and reaction speed (Chen et al. 2021; Heaton et al. 2014; Vrijkotte et al. 2016). Inadequate attention can lead to training injuries, accidents, and decision‐making errors, potentially resulting in health impairments or tactical and strategic losses. In addition, post‐traumatic stress disorder (PTSD) has been linked to decreased attention, impaired psychomotor performance, and low quality of life in U.S. military veterans (Lawrence et al. 2023)—highlighting why maintaining and enhancing attention is critical for military personnel, even in nonclinical samples. Thus, enhancing soldiers' attention is therefore crucial for maintaining mental health and training safety.

Traditional methods, like behavioral exercises or mindfulness, require significant time and professional guidance (Tang and Posner 2009). For instance, a study showed that military populations only observed notable improvements in attention control and social abilities after about 4 weeks of regular behavioral training (Metcalf et al. 2022). As an effective, painless, and noninvasive technique to enhance cognition, transcranial electrical stimulation (tES) can improve attention by delivering electric current to targeted brain areas with lower time investment and cognitive resource demands, even after just one session (Frings et al. 2018; Lin et al. 2021). This technique holds promise for enhancing the cognitive and survival capabilities of soldiers in future battlefields (Davis and Smith 2019). Furthermore, tES may support mental health and in‐flight performance among astronauts during long‐term space exploration (Romanella et al. 2020).

Attention is divided into three core components according to the attention networks theory: alerting (maintaining optimal vigilance over time), orienting (selecting information from sensory input), and executive control (suppressing interference among responses) (Petersen and Posner 2012; Posner 2004). The prefrontal cortex (PFC), a vital area for higher cognitive functions, is closely associated with attention (Bari et al. 2020; Squire et al. 2013). The left dorsolateral prefrontal cortex (DLPFC) is a well‐established hub for sustained attention and executive control—prior tES studies show its modulation enhances attention network function in healthy adults (Laguë‐Beauvais et al. 2013; Liu et al. 2023; Ortuño et al. 2002; Weinberg et al. 2023). In addition, the right inferior frontal gyrus (IFG) plays a critical role in inhibitory control (a core subset of executive attention): damage to this region impairs interference suppression (Yang et al. 2015), and its modulation improves performance on tasks requiring inhibition (Lu, Zhang, et al. 2023). The present study focused on left DLPFC (not right DLPFC) because most executive control research targets the left hemisphere for consistent effects (Lu et al. 2021, 2020), and on right IFG (not left IFG) because right‐lateralized IFG activity dominates inhibitory processing (Lu, Wang, et al. 2023). This contrast allowed us to isolate which region is more effective for enhancing executive control—a key attention component for military tasks like ignoring distractions during training. However, establishing causality from the simultaneous occurrence of task performance and neural activity is challenging. This relationship can be explored by directly modulating cortical activation through tES to influence behavior (Shin et al. 2015). Although previous studies have proved that stimulating the right IFG or the left DLPFC has the potential to improve attention or attention control ability (Liu et al. 2023; Weinberg et al. 2023), the differences in the behavioral benefit of stimulating these two targeted cortices were not clarified. Finding the appropriate cortex as the stimulation target is the main premise to effectively improve the attention of military personnel through tES. Therefore, the Objective 1 of the current study is to determine which cortex (the right IFG or the left DLPFC) is more effective as a stimulation target for attention enhancement during modulation.

tES is currently recognized for its potential to modulate cognitive abilities (including attention) in healthy people and patients, with a well‐established safety profile: it is safe at intensities up to 2 mA for single or repeated sessions (Antal et al. 2022; Bikson et al. 2016). However, the efficacy of tES for attention enhancement varies by factors like stimulation target, population, and task (Brunyé et al. 2019; Hsu et al. 2021), highlighting the need to optimize parameters for military personnel. There are two primary types of tES used for cognitive enhancement: transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS). These methods differ in their neurophysiological mechanisms: tACS delivers frequency‐matched alternating currents (e.g., 20 Hz β‐band) to entrain endogenous brain oscillations—a process linked to improved attention inhibition (Emmons et al. 2019; Leunissen et al. 2022). In contrast, tDCS delivers constant direct current to alter cortical excitability: anodal stimulation enhances neuronal excitability in targeted regions, while cathodal stimulation reduces it—a mechanism shown to improve executive functions in healthy adults (Frings et al. 2018; Lin et al. 2021; Lu et al. 2020). Understanding these distinct impacts is crucial for developing effective attention enhancement strategies for soldiers. Thus, the Objective 2 of this study is to determine whether tACS or tDCS is more effective for the targeted cortex identified in Objective 1.

This study aims to identify the critical cortex for optimizing attention performance in military personnel through tES and to determine the most efficient tES type by examining the differential neural and behavioral benefits during tDCS/tACS modulation. The findings will help to inform the precise parameters of tES needed to effectively improve the attention of military personnel.

2. Methods

2.1. Participants and Ethics

This study recruited 137 healthy male soldiers by posters from a military university (average age 21.99 ± 2.38 years), including 62 participants in Experiment 1 and 75 participants in Experiment 2. All participants had 3–5 years of formal military training, including physical training, marksmanship practice, basic tactical operations, and equipment operation—experiences that require consistent high‐level attention, minimizing baseline variability in attention abilities. Participants were randomly assigned to one of three groups for each experiment (Experiment 1: left DLPFC group, right IFG group, sham group; Experiment 2: tDCS group, tACS group, sham group). All were right‐handed (Oldfield 1971) with normal or corrected‐to‐normal vision and hearing, and without a history of neurological or psychiatric disorders, head injuries, or use of medication affecting neural activity. The study adhered to the 1964 Declaration of Helsinki and its amendments, gaining approval from the Ethics Committee of Xijing Hospital (Approval no. KY20202063‐F‐2). All participants signed informed consent before the experiments and were paid after participation, and they were aware of their right to withdraw at any time. Sample size estimation, using G*Power 3.1.9.6, indicated a minimum of 54 participants per experiment, assuming a medium effect size (0.25), a power of 0.90, and α = 0.05 (Cohen 1992; Faul et al. 2007).

2.2. Cognitive Tasks

2.2.1. Attention Networks Task

The attention networks task (ANT), a cued‐reaction time (RT) flanker task, assesses attention networks by measuring RTs. To avoid the mental fatigue and ensure the quality of ANT data recorded from individuals, a shorter version of ANT was used in the current study (Fan et al. 2005). This version involves five horizontal black arrows, cued by center, spatial, or no cues, with participants identifying the direction of the central arrow (Figure 1A,B). The task consists of 12 runs and 72 trials, and all the trials are presented randomly. Cues appear for 200 ms, with intervals between trials ranging from 3000 to 15,000 ms to reduce expectation effects (Figure 1C). A practice session of 24 trials precedes the main task.

FIGURE 1.

The design details of the ANT and CW‐Stroop task. (A) The four kinds of stimuli in ANT. (B) The three kinds of cue conditions in ANT. (C) The sample of the task procedure in ANT (approximately 15 min). (D) The three conditions of the CW‐Stroop task. (E) A representative block design of the CW‐Stroop task (approximately 11 min); both in Experiment 1 and Experiment 2, this block sequence was pseudorandomized for all participants.

2.2.2. Color‐Word Stroop Task

The Color‐Word (CW)‐Stroop task is usually used to evaluate the Stroop effect, which could represent attention inhibition (executive control). The symbol (“XX”) or Chinese character (“红” “绿” “黄” meaning “red” “green” “yellow”) in a random color (red, yellow, green) appeared in the middle of the screen (Figure 1D). The participants were asked to determine the color of the symbol or characters by pressing the relevant button as soon as possible. The CW‐Stroop task is block‐designed with three conditions (three blocks in each condition, 18 trials in each block) in both Experiment 1 and Experiment 2, and the conditions include incongruent (the color does not match the meaning of the word), congruent (the color matches the meaning of the word), and neutral (colored “XX”) (Figure 1E). Blocks of different conditions were pseudorandomized: each condition (neutral, congruent, incongruent) appeared exactly three times per participant, but the order of blocks was shuffled across participants to avoid sequence effects. Figure 1E shows a representative block sequence (neutral → congruent → incongruent, repeated three times) to illustrate task timing and structure—this sequence was not identical for all participants. A practice session of 36 trials precedes the main task.

2.3. tES Protocol

A Soterix Medical MXN‐33 High‐Definition Transcranial Electrical Stimulator (Soterix Medical Inc., New York, USA) was used to deliver the tES in the present study. Four returned electrodes (cathodal) and one central electrode (anodal) were used to stimulate the target cortical areas of each participant. According to the international 10–10 EEG montage, the central electrode was placed at F3 with four returned electrodes placed at AF3, F1, F5, and FC3 to cover the left DLPFC (Lu et al. 2020); the central electrode was placed at FC6 with four returned electrodes placed at F6, FT8, C6, and FC4 to cover the right IFG (Holland et al. 2011). In addition, we used HD‐explore software (Soterix Medical Inc.) to simulate the theoretical electric field and current intensity to ensure the efficiency of the electrode montage (Figure 2A). For tDCS, a 1.5 mA current was selected based on our team's prior studies and pre‐experiment verification (Lu et al. 2021, 2020), and applied to all participants for 10 min. For the tACS, the current intensity and duration were the same as those in the tDCS, and its frequency was set to the β band (20 Hz). The sham group received sham stimulation at 1.5 mA for 60 s, comprising 30 s for ramping up to 1.5 mA and then 30 s for ramping down. During the 10‐min tES session, participants performed a low‐cognitive‐load visual fixation task: they fixated on a static white cross (500 × 500 pixels) displayed on a black screen, with no additional cognitive demands. This task minimized task‐related neural activity during stimulation, ensuring consistent effects across participants. To ensure blinding efficacy, other colleagues operated the intervention under a blinded operation schema with code names of three tES paradigms, but not the details of parameters. Each participant was briefed before the study that they could receive active or sham stimulation, with no differences in procedure or potential side effects (e.g., mild tingling). After stimulation, they were asked to complete a two‐item questionnaire: (1) “Which type of stimulation do you think you received?” (options: active, sham, uncertain); (2) “How confident are you in your answer?” (1 = not confident at all, 5 = extremely confident).

FIGURE 2.

(A) The arrangement of electrodes, which were placed at F3 (center electrode), AF3, F1, F5, and FC3 to cover the left DLPFC, and which were placed at FC6, F6, FT8, C6, and FC4 to cover the right IFG. (B) The arrangement of optodes and 4 channels were placed around FC6 to cover the right IFG, and 10 channels were placed around Fpz to cover the PFC. (C) The design details of two experiments.

2.4. Raw fNIRS Data Acquisition

A 14‐channel LABNIRS fNIRS system (Shimadzu Co., Kyoto, Japan) emitting light at 780, 805, and 830 nm was used to record the concentration variations of oxygenated hemoglobin (oxy‐Hb) and deoxygenated hemoglobin (deoxy‐Hb) during the CW‐Stroop task in each participant (sample rate of 27.78 Hz). According to the international 10–10 EEG montage, Fpz and FC6 were used as the frontal pole region and the right IFG, respectively. For each participant, a 2 (emitters) × 2 (detectors) optode patch including four channels with FC6 set as the reference point was placed on the right side of the head to record changes in the right IFG, and a 4 (emitters) × 2 (detectors) optode patch including 10 channels with Fpz as the reference point was symmetrically placed on the forehead to record changes in the PFC, as shown in Figure 2B. The distance between the source and the adjacent detector was approximately 30 mm. Furthermore, we used a Fastrak digitizer (Polhemus, USA); we recorded 3D coordinates of all optodes and cranial landmarks (nasion, Cz, bilateral preauricular points). These coordinates were imported into the NIRS‐SPM toolbox (Ye et al. 2009) to map each channel to the MNI brain template, confirming that the 4 right IFG channels localized to the right IFG, and the 10 PFC channels localized to the bilateral PFC.

2.5. Procedure

The study consisted of two experiments. Experiment 1 aimed to identify the cortex effective for attention enhancement by evaluating variations in attention behavior (the ANT and CW‐Stroop task) during two tDCS modulation phases (pre‐stimulation and poststimulation) targeting the left DLPFC and right IFG. Based on Experiment 1 results, the right IFG was selected for modulation in Experiment 2. This experiment focused on identifying the effective type of tES. fNIRS was employed in the pre‐stimulation and poststimulation phases to measure hemodynamic changes associated with the CW‐Stroop task. Detailed designs of the two experiments are presented in Figure 2C.

2.6. Data Analysis

2.6.1. Behavioral Data

Trials with RTs ≤ 150 or ≥ 1500 ms were excluded to ensure response quality. For the ANT, alerting, orienting, and executive control were calculated as follows: Alerting effect = mean RT of no‐cue conditions − mean RT of center‐cue conditions; orienting effect = mean RT of center‐cue conditions − mean RT of spatial cue conditions; executive control effect = mean RT of incongruent conditions − mean RT of congruent conditions (Fan et al. 2005). Higher alerting and orienting scores indicated improved capabilities, while lower executive scores signified enhanced executive control. The Stroop effect in the CW‐Stroop task was calculated as the RT (correct answers) of the incongruent condition minus the RT (correct answers) of the congruent condition, with higher scores indicating lower attention inhibition ability (Treisman and Fearnley 1969).

2.6.2. fNIRS datao Convert the “Txt” Data

Data format conversion: LABNIRS collection and analysis software was used to convert the original “omm” files into “txt” format. In this step, we converted the data format without performing any data preprocessing. Then, the format conversion function (shimaduz2nirs) in the Homer_2 toolbox was applied to the “nirs” format supported by Homer_2. Then, the coefficients of variation (CV) method was employed to eliminate the channels with poor signal quality, and the threshold for CV was set to 7.5% (Hu et al. 2021).

Data preprocessing: First, the original optical intensity was converted to the optical density (OD) by the hmrIntensity2OD function. Subsequently, the motion artifact reduction algorithm (MARA) was utilized. MARA is a correction method that detects artifacts by evaluating whether the amplitude variation or standard deviation within short time windows exceeds user‐specified thresholds. Time windows containing artifacts were widened, and then artifacts were approximated with smoothing splines and subtracted to obtain motion‐corrected signals. Then, the hmrMotionArtifactByChannel function was applied to identify motion artifacts with the MotionCorrectSpline function to correct motion artifacts (tMotion = 0.5, tMask = 2, STDEVthresh = 20, AMPthresh = 0.5, pSpline = 0.99) (Cooper et al. 2012; Reindl et al. 2022). Then, the OD was converted to oxy‐Hb and deoxy‐Hb concentrations by the hmrOD2Conc function (ppf = 6.0 6.0 6.0) based on the modified Beer–Lambert law (Baker et al. 2014). Previous studies have demonstrated that oxy‐Hb concentrations are more sensitive than deoxy‐Hb concentrations in reflecting changes in regional cerebral blood flow (Hoshi 2007); thus, oxy‐Hb concentration data were used as a main indicator in further analyses (the results of deoxy‐Hb concentration data could be found in Supporting Information). Notably, the hmrBandpassFilt function (hpf = 0.01, lpf = 0.1 Hz) was applied to eliminate physiological noise and baseline drift during activation processing.

Activation processing: After preprocessing, the oxy‐Hb data were entered into MATLAB to assess activation of the task state. The hemodynamics of the last 10 s of the interval period (20 s) were used as a baseline to normalize hemodynamic changes during the 36 s task block. Individual HbO data were calculated by averaging three blocks for each condition. The average activation of the four channels surrounding FC6 (Channel 1, Channel 2, Channel 3, and Channel 4) represented the neural activity of the right IFG, and the average activation of the other 10 channels surrounding Fpz represented the neural activity of the PFC. The Stroop effect of activation was calculated by subtracting the activation of congruent conditions from the activation of incongruent conditions.

2.7. Statistical Analysis

The 3 (group) × 2 (phase) mixed‐design ANOVA was used for analyzing behavioral/fNIRS data. An ordinal interaction was expected: active stimulation groups (left DLPFC/right IFG in Experiment 1; tACS/tDCS in Experiment 2) would show greater improvement than the sham group, with potential differences in the magnitude of improvement between active groups (but no reversal of effects). The significance threshold was set at p < 0.05, and significant interactions prompted simple effect analysis with Bonferroni correction for multiple comparisons. Effect sizes were reported as partial eta‐squared (${\eta }_{\mathrm{p}}^2$) values. All analyses were conducted using IBM SPSS software v25.0.

3. Results

3.1. Experiment 1: The Difference of Improvement on Attention Induced by tDCS Modulating Left DLPFC/Right IFG

3.1.1. Characteristics, Blinding Efficacy, and Side Effects of Participation

As shown in Table 1, there were no differences in age or educational level among the three groups at baseline. There was no significant difference among the groups in the number of guesses (χ ² = 3.079, p = 0.545). In addition, the confidence in guesses did not significantly differ (p = 0.361) among the three groups according to a Kruskal–Wallis test. All participants had good tolerance for the discomfort while receiving tDCS, and the side effects reported were slight and transient. All details can be found in Table 1.

TABLE 1.

Basic characteristics, blinding efficacy, and self‐reported side effects of participants in Experiment 1.

	Left DLPFC (n = 21)	Right IFG (n = 21)	Sham (n = 20)	p
	Mean ± SD	Mean ± SD	Mean ± SD
Age (years)	21.71 ± 1.98	21.76 ± 2.91	22.30 ± 2.52	0.710
Education (years)	16.52 ± 2.36	16.43 ± 3.20	16.70 ± 3.11	0.955
Number of guesses (active)	19 (90.48%)	21 (100.00%)	19 (95.00%)
Confidence in guesses (active)	2.95 ± 0.23	3.00 ± 0.00	2.95 ± 0.23
Number of guesses (sham)	1 (4.76%)	0	1 (5.00%)
Confidence in guesses (sham)	2.00	0	3.00
Number of guesses (uncertain)	1 (4.76%)	0	0
Confidence in guesses (uncertain)	3.00	0	0
Itching	3 (14.29%)	3 (14.29%)	2 (10.00%)
Skin redness	0	0	0
Headache	0	0	0
Cervical pain	0	0	0
Distraction	2 (9.52%)	0	1 (5.00%)
Acute mood variation	0	0	0
Sleepiness	1 (4.76%)	0	1 (5.00%)

Note: p values were calculated with one‐way ANOVA; p < 0.05 was considered significant.

3.1.2. Effect on Performance of ANT by Modulating the Left DLPFC/the Right IFG

The 3 (group: the left DLPFC, the right IFG, and sham) × 2 (phase: pre‐stimulation and poststimulation) mixed‐design ANOVAs were separately conducted in three subnetworks of ANT. As shown in Figure 3A,B, the results indicated that the interaction effects of alerting (F _{(2, 59)} = 0.754, p = 0.475, ${\eta }_{\mathrm{p}}^2=0.025$) and orienting (F _{(2, 59)} = 0.706, p = 0.498, ${\eta }_{\mathrm{p}}^2=0.023$) were both not significant. The interaction effect of executive control was significant (F _{(2, 59)} = 4.455, p = 0.016, ${\eta }_{\mathrm{p}}^2=0.131$), and further simple effect analysis indicated significantly lower scores for the poststimulation phase compared to those for the pre‐stimulation phase in the active tDCS groups (ps < 0.05) but not in the sham group (Figure 3C). However, the differences in improvement on executive control did not distinguish the effect of tDCS modulating the left DLPFC or right IFG.

FIGURE 3.

Behavioral performance in ANT and CW‐Stroop task. Variation in the function of alerting (A), orienting (B), and executive control (C) in ANT. Stroop effect of CW‐Stroop task (D). Boxes extend from the 25th to 75th percentiles, with the horizontal line representing the median. Whiskers show the min and max values. Colored dots represent individual data points. Bonferroni‐corrected contrast: *p < 0.05, ***p < 0.001.

3.1.3. Effect on Performance of CW‐Stroop Task by Modulating the Left DLPFC and the Right IFG

As a supplementary indicator, the Stroop effect is analyzed by a 3 (group: the left DLPFC, the right IFG, and sham) × 2 (phase: pre‐stimulation and poststimulation) mixed‐design ANOVA to further assess executive control. The interaction effect of the Stroop effect was significant (F _{(2, 59)} = 3.289, p = 0.044, ${\eta }_{\mathrm{p}}^2=0.100$), and the further simple effect showed significantly lower scores of the poststimulation phase than those of the pre‐stimulation phase in the right IFG group (p < 0.001) but not in the left DLPFC group and the sham group (Figure 3D).

3.2. Experiment 2: The Effect on Hemodynamic Indicator of tACS and tDCS on the Right IFG

3.2.1. Characteristics, Blinding Efficacy, and Side Effects of Participation

As shown in Table 2, there were no differences in age or educational level among the three groups at baseline. There was no significant difference among the groups in the number of guesses (χ ² = 3.083, p = 0.544). In addition, the confidence in guesses did not significantly differ (p = 0.358) among the three groups according to a Kruskal–Wallis test. All participants had good tolerance for the discomfort while receiving tDCS, and the side effects reported were slight and transient. All details can be found in Table 2.

TABLE 2.

Basic characteristics, blinding efficacy, and self‐reported side effects of participants in Experiment 2.

	tACS (n = 25)	tDCS (n = 25)	Sham (n = 25)	p
	Mean ± SD	Mean ± SD	Mean ± SD
Age (years)	22.20 ± 2.80	21.84 ± 2.63	22.12 ± 1.36	0.851
Education (years)	17.08 ± 3.07	16.44 ± 2.65	16.60 ± 1.76	0.654
Number of guesses (active)	23 (92.00%)	24 (96.00%)	25 (100.00%)
Confidence in guesses (active)	2.96 ± 0.21	2.96 ± 0.20	3.00
Number of guesses (sham)	1 (4.00%)	1 (4.00%)	0
Confidence in guesses (sham)	2.00	3.00	0
Number of guesses (uncertain)	1 (4.00%)	0	0
Confidence in guesses (uncertain)	3.00	0	0
Itching	5 (20.00%)	2 (8.00%)	1 (4.00%)
Skin redness	0	0	0
Headache	0	0	1 (4.00%)
Cervical pain	0	0	0
Distraction	1 (4.00%)	0	1 (4.00%)
Acute mood variation	0	0	0
Sleepiness	1 (4.00%)	0	1 (4.00%)

Note: p values were calculated with one‐way ANOVA; p < 0.05 was considered significant.

3.2.2. The CW‐Stroop Task‐Related Activation Variation Induced by tACS/tDCS Modulation

According to the results of Experiment 1, the CW‐Stroop task has been proven sensitive to be used to assess executive control of attention networks by specifically targeting inhibitory control (a subset of executive control that is critical for military tasks). The specific variation of the Stroop effect of oxy‐Hb concentration variation in each channel of fNIRS can be found in Figure 4A. The 3 (group: tACS, tDCS, and sham) × 2 (pre‐stimulation and poststimulation) mixed‐design ANOVAs were separately conducted on the Stroop effect on the average oxy‐Hb concentration of each channel cluster (the PFC cluster: Channels 5, 6, 7, 8, 9, 10; the right IFG cluster: Channels 1, 2, 3, 4). The interaction effect in the PFC cluster was significant (F _{(2, 72)} = 4.254, p = 0.018, ${\eta }_{\mathrm{p}}^2=0.106$), and further simple effect analysis indicated that the Stroop effect of oxy‐Hb concentration in the PFC cluster significantly decreased after stimulation in the tACS and the tDCS groups but not in the sham group (Figure 4B). However, the interaction effect in the right IFG cluster was not significant (p > 0.05).

FIGURE 4.

(A) Stroop effect of oxy‐Hb concentration in each channel, and the channels within the blue circles cover the PFC. (B) Variation on the Stroop effect of oxy‐Hb concentration in the PFC among three groups. (C) Variation in the behavioral performance of the Stroop effect in the CW‐Stroop task. Boxes extend from the 25th to 75th percentiles, with the horizontal line representing the median. Whiskers show the min and max values. Colored dots represent individual data points. Bonferroni‐corrected contrast: *p < 0.05.

3.2.3. Effect on Performance of CW‐Stroop Task by tACS/tDCS Modulation

The Stroop effect is analyzed by a 3 (group: tACS, tDCS, and sham) × 2 (phase: pre‐stimulation and poststimulation) mixed‐design ANOVA. The interaction effect was significant (F _{(2, 72)} = 3.198, p = 0.047, ${\eta }_{\mathrm{p}}^2=0.082$), and further simple effect analysis found a significantly lower score in the poststimulation phase compared with that in the pre‐stimulation phase of the tDCS group (p = 0.013) but not of the other groups (Figure 4C).

4. Discussion

Attention is critical for optimizing training outcomes and supporting cognitive resilience in military personnel. Enhancing this ability is therefore essential for mental health and combat readiness. This study comprised two sequential experiments. In Experiment 1, tDCS was applied to the left DLPFC and the right IFG. We found that the right IFG was more responsive as a target for modulating and improving attention performance, particularly within the executive control component of attention networks. Following these findings, Experiment 2 focused on the right IFG, examining behavioral and neural changes prompted by both tDCS and tACS. The results indicated that tDCS was more effective than tACS in enhancing attention inhibition performance, even though both forms of tES increased neural efficiency in the PFC. A detailed discussion of these outcomes follows.

The left DLPFC is a critical region associated with various cognitive functions, with improvements in attention often linked to increased neural activity in this area (Laguë‐Beauvais et al. 2013; Ortuño et al. 2002). In addition, the right IFG is known to be instrumental in attentional processes, and the damage to this region can lead to deficits (Yang et al. 2015). Our study demonstrated that tDCS applied to the right IFG and the left DLPFC significantly enhanced executive control within the attention networks. This finding establishes a causal link between targeted cortical modulation and improvements in attention‐related behavior. Furthermore, these behavioral improvements bolster the potential of tES as a method for enhancing attention in military personnel. Notably, enhancements were specific to the executive control subnetwork, with no comparable effects in the alerting and orienting subnetworks. This specificity might reflect a ceiling effect, as suggested by previous research [44], particularly since our participants, being military college students, are likely to have already developed high levels of alerting and orienting capabilities (Miler et al. 2018). In previous research, we also observed that repeated tDCS sessions targeting the left DLPFC cumulatively improved executive control in attention networks (Lu et al. 2020). Focusing brain function regulation on a specific targeted cortex tends to be more effective. However, our results did not initially reveal whether the right IFG or the left DLPFC was superior in enhancing the executive control subnetwork. To address this, we introduced the CW‐Stroop task to refine the evaluation of executive control efficiency. The findings were clear that tES modulation of the right IFG significantly attenuated the Stroop effect, indicative of enhanced executive control capabilities. This improvement was not observed in participants who received tES modulation on the left DLPFC. Our recent research has also underscored the significance of the right IFG in enhancing executive control through tES interventions (Lu, Wang, et al. 2023), corroborating the findings of the present study. Hence, the right IFG is more responsive than the left DLPFC to tES in improving attention performance, suggesting its superiority as a targeted area for boosting executive control during tES training.

Both tDCS and tACS, as key types of tES, have shown efficacy in modulating brain function to enhance attention, particularly in executive control. Experiment 1 revealed the higher sensitivity of the IFG to stimulation for attention enhancement than that of the left DLPFC. Yet, the more effective type of tES, critical for regular military personnel intervention to boost attention, remained unclear. Assessing brain function is essential to evaluate the impact of tES modulation, as the cortical mechanisms behind tDCS and tACS may differ. The findings from Experiment 2 indicated that both tDCS and tACS significantly reduced the Stroop effect‐related HbO concentration in the PFC compared to the sham group, suggesting that active tES enhances the PFC's executive control function in conflict situations. Prior research has highlighted the role of the PFC in maintaining attention inhibition to minimize the Stroop effect (Yeung et al. 2020). The transfer of tES benefits was observed through the modulation of the right IFG, which altered the hemodynamics of the PFC. The functional and structural connections between both proximal and distal brain regions are considered potential mechanisms for this effect (Edison 2020; Sporns 2022), a phenomenon also noted in our earlier studies (Lu et al. 2021; Lu, Wang, et al. 2023). While hemodynamic changes did not clarify the differential efficacy of tDCS and tACS in enhancing executive control, further analysis of the behavioral performance of the CW‐Stroop task revealed more findings. The Stroop effect decreased in both the tDCS and tACS groups post‐intervention compared to the sham group, but this change was significantly notable only in the tDCS group. Existing research supports the positive impact of tDCS on the right IFG in bolstering adult executive control (Leite et al. 2018). For instance, a study focusing on language fluency also demonstrated that tDCS modulation of the right IFG significantly improved semantic verbal fluency, which is closely linked to attention inhibition (Nejati et al. 2023). Currently, 20 Hz tACS appears to cause less behavioral enhancement in executive control compared to tDCS. This suggests a need for more refined stimulation parameters and a more precise assessment of attention inhibition to achieve conclusive results in future research.

This study has several limitations that should be addressed in future research. First, a simplified fMRI version of the ANT was used to reduce participant time commitment. This version, while efficient, may not capture the detailed variations across the entire subnetworks due to limited trials. Therefore, a more comprehensive version of ANT or a comprehensive task combining sustained attention and attention‐dependent inhibitory control (sustained attention to response task) (Martínez‐Pérez et al. 2022) is recommended for future studies to gain deeper insights. Second, our analysis was constrained to neural activities in only two cortical areas, the PFC and the right IFG, which was limited by the number of fNIRS channels available. Future studies should consider including more functional brain regions for a broader understanding. Third, Experiment 1 exclusively employed tDCS, which lacks frequency parameter influence, limiting the scope of identifying targeted brain regions, and only offline tES was applied in Experiment 1 or 2 without consideration of online intervention. Incorporating a variety of tES types in subsequent studies could yield more nuanced and specific results. Fourth, individualized tACS (e.g., frequency tailored to each participant's baseline EEG oscillations) was not used due to resource constraints (e.g., additional EEG equipment and data collection time), but future studies will integrate EEG to optimize frequency for individual military personnel.

In summary, this study highlights the crucial role of the right IFG in enhancing executive control, particularly in the aspect of attention inhibition. Increased neural activity in the PFC was found while modulating the right IFG by both tDCS and tACS. However, it was observed that only tDCS led to an improvement in the behavioral performance of executive control. These findings lay a foundational basis for the tES program aimed at augmenting attention abilities in military personnel, offering valuable insights for future applications in this field.

Author Contributions

Y.M., Y.Z., P.H., J.M., and H.L. designed the research. Y.M., Q.N., X.Y., Y.C., W.H., and X.T. performed the research. Y.M., Y.Z., Z.G., X.W., and H.L. analyzed the data. Y.M., Y.Z., Z.G., and H.L. wrote the paper. H.L. revised the manuscript. P.H., J.M., and H.L. provided editorial support for this manuscript.

Ethics Statement

The study adhered to the 1964 Declaration of Helsinki and its amendments, gaining approval from the Ethics Committee of Xijing Hospital (Approval no. KY20202063‐F‐2).

Consent

Informed consent was obtained from all participants for participation in the study. Each participant confirmed their legal adult status (≥ 18 years) and completed hardcopy consent documentation before the experiments. Participants were paid after participation, and they were aware of their right to withdraw at any time. The informed consent forms were then collected and restored after signing in time from October 2–21, 2023 (Experiment 1) and March 4–29, 2024 (Experiment 2).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

FIGURE S1: (A) Stroop effect of deoxy‐Hb concentration in each channel, and the channels within the blue circles cover the PFC. (B) Variation on Stroop effect of deoxy‐Hb concentration in the right IFG among three groups. (C) Variation on Stroop effect of deoxy‐Hb concentration in the PFC among three groups. Boxes extend from the 25th to 75th percentiles, with the horizontal line representing the median. Whiskers show the min and max values. Colored dots represent individual data points.

Lu, H. , Miao Y., Zhang Y., et al. 2025. “Improvement on Attention Networks Among Military Personnel: The Right IFG and tDCS Matter.” Human Brain Mapping 46, no. 14: e70365. 10.1002/hbm.70365.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.