Caffeine is a central nervous system stimulant and is likely the most widely used psychoactive substance worldwide [1] . A review article suggested that caffeine is the world’s most frequently consumed psychostimulant [2] . Its usage is increasing worldwide, primarily driven by its benefit in improving concentration, memory, and physical performance [1] . In addition to its primary use as an additive in numerous beverages, it also has clinical applications, such as in treating apnea of prematurity, serving as an adjuvant in pain therapy, and a regulatory-approved use for short-term fatigue treatment [2] . Caffeine has been shown to reduce health risks such as diabetes, obesity, and vascular diseases, providing health benefits when consumed in moderation, typically three to four cups per day. Caffeine improves alertness, attention, concentration, and reaction time [3] [4] , particularly in sleep-deprived individuals. Moreover, it may improve endurance performance and muscle strength. Intoxications with caffeine are relatively rare but can be fatal. Within typical doses, caffeine use is generally considered safe [2] . Coffee and caffeine-containing products affect the cardiovascular system through positive inotropic and chronotropic effects and the central nervous system by stimulating locomotor activity and producing anxiogenic-like effects. Thus, it is of interest to examine whether these effects could be detrimental to health [1] . Low-dose caffeine intake selectively enhances cognition and brain activity compared to medium- and high-dose caffeine, improving executive function and prefrontal cortex activity [5] .

On the other hand, it has been suggested that caffeine may have harmful effects depending on the dose. Caffeine abuse and dependence are becoming more and more common and can lead to caffeine intoxication, which puts individuals at risk for premature and unnatural death [1] . Acute caffeine intake can attenuate homeostatic sleep pressure and worsen sleep quality [6] . It also exhibits characteristics of addictive substances, including withdrawal symptoms and tolerance [2] . Interestingly, a randomized, double-blind, placebo-controlled trial reported that compared to placebo, participants showed a higher error rate and a longer reaction time on working memory (WM) tasks in the caffeine condition. Building on the earlier evidence highlighting increasing cerebral metabolic demands for WM function after acute caffeine intake, this finding suggests that such demands might be impeded over daily intake and may worsen performance [7] .

As mentioned above, although the health benefits and disadvantages of caffeine intake have been studied, no substantial attention has been paid to the effects of caffeine’s aroma. Furthermore, a recent review article stated that future studies must provide continued evidence on the effect of caffeine on various types of cognitive functions [8] . Therefore, the purpose of this study was to examine the effect of caffeine scent on brain activity during cognitive tasks using electroencephalography (EEG). Furthermore, the results of this study will provide new insight into whether it can serve as a complementary substance to promote rehabilitation for patients with central nervous system disorders, such as dementia.

Table 1 shows the comparison of outcomes between conditions. No significant main effect was found, although significant interactions were found at six spots (†). After the Kruskal-Wallis test, there were significant differences observed during the post-task period (P < 0.05) on the (i) left-frontal θ (coffee < smell and water) and (ii) right-frontal α (coffee < smell). However, no significant differences were observed during the pre-task between conditions. Also, Wilcoxon’s signed rank test showed a significant difference in left-occipital θ between pre- and post-task during water conditions (pre < post). Moreover, there was a significant trend toward differences observed in correct reactions of performance, although there was no significant difference was found.

Significant differences are indicated (†P < 0.05 by Linear mixed model).

This study examined the effect of caffeine scent on brain activity during cognitive tasks using EEG. Post-task analysis revealed the following: the left-frontal θ showed a significantly reduced power level with the coffee condition than smell and water conditions, and the right-frontal α showed a significantly reduced power level with the coffee condition than smell, although there was no significant outcome difference during pre-task between conditions. Also, the power level of left-occipital θ significantly increased during post- than pre-task with water condition. Moreover, there was a notable trend toward condition timing interaction difference on correct performance reactions, although no significant differences were found. Notably, drinking coffee reduces power levels of θ and α waves on the frontal lobe, and drinking water increases θ wave power on the occipital lobe than drinking or smelling coffee. However, no specific EEG features were identified with the smell condition. These EEG changes might affect cognitive task performances and concentration.

A randomized controlled trial suggested that a 50 mg caffeinated drink significantly reduced the percentage changes from baseline of α wave activity over the midline electrodes, i.e., frontal, central, and occipital areas after drinking during a 5 min eyes-closed resting state. Cognitive function test results, such as the trail-making test and digit span forwards, significantly improved after drinking. A significant inverse correlation between the diminished α wave activity over the midline central and occipital cortical regions and the Trails B positive scores was observed [14] . Previous studies have suggested that a reduced α power may correlate with increased cognitive flexibility and arousal, while an increased θ power correlates with greater cognitive control and decision-making under pressure [15] . In the speed game, caffeine had a significantly greater number of target shots post-dose than pre-dose. However, it also exhibited slower reaction times post-dose compared to pre-dose. EEG data collected concomitantly with game playing showed that better performance in the speed game resulted in significantly reduced α power and increased θ activity, consistent with enhanced cognitive processing [15] . Moreover, a previous study reported that the EEG showed that power density in the θ during waking was increased for at least two days after a single dose of caffeine, revealing long-term effects of a single dose of caffeine on vigilance states, EEG, and neuronal activity in the lateral hypothalamus [16] .

Consistent with previous studies, drinking coffee containing caffeine induces lower α and higher θ power. Specifically, reduced α activity in the frontal lobe relates to improving cognitive functions such as executive function, attentional function, cognitive flexibility, and arousal. Higher θ relates to greater cognitive control and vigilance states. Therefore, these two phenomena are important to improve cognitive performance. In the present study, lower α was observed after drinking coffee. However, higher θ was observed after smelling coffee or drinking, without significant cognitive performance difference. Hence, drinking coffee affects concentration performance, but smelling coffee and drinking water may also affect cerebral activities and induce better concentration performance. To seek the best way to induce the best cognitive performance, it is imperative to conduct further experiments incorporating various condition settings such as caffeine content control, amount of drinking, or time of smelling.

This study has some limitations. First, it focused solely on healthy young adults; therefore, it is unclear whether our results can be generalized to older patients or those with neurological disabilities. Second, the concentration task was limited to the CPT; therefore, whether Cerebral activities during other concentration tasks are comparable to those observed during the CPT remains unclear. Third, the amount participants drank was a cup of coffee/water. Thus, the effects of different amounts of coffee/water or various beverages remain unclear. Finally, our study included only a small number of participants. To address these limitations, future studies should include more participants under varying conditions, diverse concentration tasks, and investigate cerebral activity.

In conclusion, a reduced α on the frontal lobe and an increased θ are associated with improving arousal and cognitive functions. Drinking a cup of coffee reduces α power, whereas smelling a cup of coffee or drinking water increases θ power. Not only does drinking coffee affect cognitive performance, but smelling coffee and drinking water may also affect cerebral activities and induce better cognitive performance.

The authors extend their gratitude to all the study participants.

MM: Conceptualization, methodology, investigation, software, resources, data curation, visualization, supervision, project administration—original draft preparation, and writing—review and editing.

TI, SH, and RT: Conceptualization, methodology, investigation, writing, and editing.

TH: Validation, formal analysis, and review.

All the authors have read and agreed to the published version of the manuscript.

The study was approved by the Ethics Committee of Nishikyushu University (approval no. 23XQB18).

Informed consent was obtained from all subjects involved in the study.

The datasets generated and analyzed in the current study are available from the corresponding author upon reasonable request.