The oxygen scarcity and low-pressure conditions of high-altitude regions present unique physiological challenges to the inhabitants living in these regions. Among them, erythrocytes play a crucial role in adapting to the high-altitude environment. The body improves the efficiency of oxygen carrying and delivery by compensatory increase in the number of erythrocytes to maintain the normal function of tissues (Wood and Johansen, 1972; Raberin et al., 2022; Villafuerte et al., 2022). However, in recent years, it has been shown that such compensatory changes in red blood cells (RBCs) in immigrant populations living at high-altitude may not only involve the physiological aspects of oxygen transport, but also relate to cognitive functions such as attention (Xue et al., 2022).

The brain is the most oxygen-consuming organ in the human body (Raichle and Gusnard, 2002; Reichle, 2010; Yan, 2014), and hypoxia is a stressor of human beings (Virués-Ortega et al., 2004); thus, high-altitude hypoxia exposure would induce extensive effects on brain function (Chen et al., 2017, 2021; Zhang et al., 2018a,b; Xue et al., 2022). Previous studies have demonstrated that functional adaptations to high-altitude include multidimensional physiological and neurocognitive changes, such as efficiency of oxygen usage (Yan et al., 2010; Yan, 2014; Xue et al., 2022), inflammatory response (Caris and Santos, 2019; Nguyen et al., 2021), physiological homeostasis (Beall, 2007; Frisancho, 2013), brain morphology (Virués-Ortega et al., 2004; Yan et al., 2010), and cognitive performance (Ma et al., 2015; Taylor et al., 2015; McMorris et al., 2017; Zhang et al., 2018a, 2021; Chen et al., 2021). In human cognition, attention systems have been reported to be sensitive to hypoxia (Zhang et al., 2018a,b, 2021; Chen et al., 2021; Xue et al., 2022). High-altitude exposure leads to a generalized decrease in executive attention in immigrants. Longitudinal and controlled studies comparing high- and low-altitude adults have demonstrated effects on a variety of attentional domains, including alerting, orienting, and executive control. A 2-year follow-up study showed changes in executive control efficiency in high-elevation immigrants, but no significant changes in alerting or orienting networks (Xin et al., 2017). Similarly, an assessment of a 3-year cohort of immigrants showed a decrease in executive control efficiency relative to adults at lower altitudes (Zhang et al., 2021). Another study utilizing the Stroop task found a significant reduction in migrant accuracy and delayed response times after 2 years of exposure (Chen et al., 2021). However, it remains unclear how human attention changes with hypoxia-induced neurophysiological changes during prolonged hypoxic exposure. However, it remains unclear how human attention changes in response to hypoxia-induced neurophysiological activity during long-term hypoxia exposure.

Human behavior and cognition in high-altitude environments are tightly related to oxygen transport, the inflammatory response, and the autonomic nervous system (ANS) (Goodman et al., 2011; Trimmel, 2011; Omrani et al., 2017; Pun et al., 2019; Li et al., 2020). Exposure to high-altitude could increase inflammation, such as neutrophil count and the neutrophil/lymphocyte ratio (NLR), a reliable marker of chronic low-grade inflammation (Bhat et al., 2013; Lynall et al., 2019), which is associated with decreased executive attention function (Hou et al., 2022). The inflammatory response to oxygen metabolism is regulated by the ANS (Williams et al., 2019) and the properties of RBCs. The ANS plays an important role in the neurophysiological pathway responsible for adaptively regulating inflammatory processes (Williams et al., 2019), in which HRV is usually used as an index to depict the ANS (Zhang et al., 2014). Of note, HRV is generally considered a biomarker of top-down self-regulation (Holzman and Bridgett, 2017), reflecting the capacity of regulating cognitive activity, mood, and behavior in response to changing environmental demands (Porges, 2007; Thayer and Lane, 2009). The low frequency/high frequency (LF/HF) ratio is a sensitive indicator of HRV, and an increased LF/HF ratio indicates a lower HRV (Rombold-Bruehl et al., 2019). Many studies have shown that immediate exposure to high-altitude results in increased sympathetic activity followed by a significant increase in the LF/HF ratio (Bernardi et al., 1998; Kanai et al., 2001; Chen et al., 2008; Qian et al., 2020). In addition to the HRV change from high-altitude exposure, RBCs were compensatory increased in a hypoxic environment (Wood and Johansen, 1972; Raberin et al., 2022; Villafuerte et al., 2022), and RBCs are critical players in inflammation distinct from their function in oxygen transport (Lam et al., 2021; Xu et al., 2022). Together, HRV and RBCs are closely related to oxygen transport capacity, and both are associated with inflammatory responses; the complex interactions together determine the individual's physiological homeostasis (Taylor et al., 2010). Furthermore, the changed HRV and RBCs properties were observed to be associated with poorer external attention performance (Duschek et al., 2009; Williams et al., 2016).

To examine how the hypoxia-induced neurophysiological response (oxygen transport, inflammation and ANS) connects with human attention function during the long-term exposure, the current cross-sectional study employed ANT recording EEG and ECG simultaneously, also combining specific blood routine markers to examine the three distinct attentional processes (alerting, orienting, and executive control) in 116 immigrants who had been living in Tibet (3680 m) more than 3 years. Our primary analysis examined three neural markers of attention networks, and then we used mediation analyses to explore whether physiological indicators mediated the relationship between exposure and the attention network through a series of mediation analyses. Finally, we used a path model to explore the neurophysiological pathway by which erythrocytes influence the attention network through inflammation and HRV. To exclude the possible biases due to demographic, cultural, and socioeconomic differences, we controlled for sex, BMI, and age in assessing neuropsychological/attention differences. Studying this relationship in depth, we are expected to reveal the potential effects of erythrocyte changes on cognitive functions in high-altitude environments, providing new perspectives for understanding high-altitude adaptation. This will not only help to expand our knowledge of erythrocyte physiology in high-altitude environments but may also provide important clues for the prevention and treatment of related diseases and cognitive disorders.

In this study, ANT was used in combination with EEG, ECG, and blood routine indicators to explore the relationship between hypoxic-induced neurophysiological response and attentional function of Tibetan immigrants. We first identify, a priori, key neural markers that underlie the attention network in Tibetan immigrants. Furthermore, we explored the effect of oxygen carrying on attention networks, and pathway analysis showed that the RBCs as oxygen carrying cells affected the three attention networks through inflammation and HRV. All these observations provide experimental evidence for the existence of a neuroimmune pathway rooted in RBCs, which represents oxygen transport capacity and is the basic physiological mechanism that determining human attention performance in a high-altitude environment.

RBCs, as oxygen-carrying cells, indirectly affected the three attention networks. Inflammation and HRV mediate this relationship. Given that HRV and inflammatory markers are important indicators of center-peripheral integration and homeostasis (Taylor et al., 2010), we believe that the key to the influence of RBCs on the attention network of immigrants may be to improve the oxygen transport capacity and maintain homeostasis by increasing RBCs within the normal range. Blood consists of 80% RBCs, especially hemoglobin, which carries oxygen to maintain brain functions. RBCs also play an important role in inflammation and the immune system (Lam et al., 2021; Xu et al., 2022). Our previous study also found that RBCs not only triggered internal-sensory representation of the insula association of orienting but also induced immune responses corresponding to executive control (Xue et al., 2022). Interoceptive inference (that is, the approximate Bayesian inference about the state of the internal) can be used to explain how organisms regulate homeostasis in hypoxic environments by making decisions about external perception and proprioception, in which the insular cortex plays a key role (Seth, 2013; Gu and FitzGerald, 2014). In a hypoxic environment, the brain senses hypoxia externally, increasing the oxygen-carrying capacity of RBCs as compensation (Wood and Johansen, 1972; Raberin et al., 2022; Villafuerte et al., 2022), and then the internal environment of the body is homogenized and controlled by autonomic reaction. When the brain perceives the gap between external perception and proprioception, it makes decisions through interoceptive inference, which is the basis of behavioral response (Gu and FitzGerald, 2014).

HRV may be the key to regulating the relationship between oxygen transport capacity and attention networks. The response of the autonomic nervous system (ANS) is the key to acclimation during high-altitude exposure, which were often evaluated by HRV (Zhang et al., 2014; Temme et al., 2023). High-altitude exposure is an effective activator of the ANS (Roche et al., 2002; Taralov et al., 2018; Temme et al., 2023), resulting in decreased parasympathetic tone and increased sympathetic tone, and decreased HRV. HRV as one of the robust resilience markers (Walker et al., 2017) indicates the activity of the vagus nerve (i.e., primary parasympathetic nerve), which reflects the influence of the parasympathetic nervous system (PNS) on cardiac regulation (Laborde et al., 2017). Higher HRV physiologically indicates lower stress, higher ability to cope with stress, appropriate adaptation and flexibility to environmental changes, and more effective self-regulation (Shaffer and Ginsberg, 2017; Christodoulou et al., 2020). HRV can also predict the outcome of cognitive tests representing executive function. The neurovisceral integration model (NIM) view suggests that the subfrontal inhibitory circuit is connected to the heart via the vagus nerve, which provides inhibitory input to the heart, and that the prefrontal cortex (PFC) is also associated with the inhibitory capacity of executive function (Thayer et al., 2009). Furthermore, based on consistent evidence from many studies, a decrease in cardiac parasympathetic activity has been reported in both rat and human studies because of high-altitude exposure (Siebenmann et al., 2017; Beltrán et al., 2020).

Furthermore, the complex combination of physiological, behavioral, emotional, and cognitive processes involved in self-regulation and adaptation may have a common basis in that HRV will be linked to all these various forms of regulation (Holzman and Bridgett, 2017). HRV plays a key role in homeostasis, and there is evidence that the parasympathetic branch of the autonomic nervous system (ANS) plays a particularly critical role in modulating inflammation (Borovikova et al., 2000). Specifically, the parasympathetic nervous system (PNS) is involved in monitoring current levels of inflammation and reducing the production of inflammatory cytokines through rapid afferent and efferent signals from the vagus nerve (Tracey, 2002). LF/HF ratio is a sensitive index of HRV at high-altitude (Zhang et al., 2014), is an indicator of sympathetic vagus nerve balance or a reflection of sympathetic nerve regulation (Heathers, 2014). Elevated LF/HF ratios may indicate ANS dysfunction, leading to sympathetic vagus imbalance and sympathetic overweight (Kuriyama et al., 2010; Chen et al., 2011). HRV reflects the regulatory function of ANS and the brain's control over behavior and peripheral physiology and is a good biomarker of individual adaptation (Zhang et al., 2014). The degree of inflammatory response is important because an extreme or inadequate response can cause varying degrees of harm to an individual (Williams et al., 2019). In our results, the overall NLR range was < 3. Here, it should be carefully explained that the increase of RBCs is accompanied by the increase of NLR. Some studies have shown that NLR < 3 belongs to the normal range (Bahrami et al., 2019).

We also observed the effects of demographic variables on the attention network and immune system of immigrants. Specifically, we observed that gender was significant in the prediction of LF/HF ratios and NLR, while BMI also showed significance in the prediction of Executive RT. The effect of gender may reflect biological differences in neuropsychological functioning and immune system regulation. Previous studies have suggested that gender may play a key role in the interactions between the autonomic nervous system and the immune system (Ferguson et al., 2013; Fransen et al., 2017), which in turn affects the LF/HF ratio and the NLR. Further studies could explore the underlying mechanisms of these gender differences to better understand the effects of these variables on attentional networks and immune indexes. And the significance of BMI in the prediction of Executive RT may reflect the complex relationship between BMI and executive function. Several studies have suggested that changes in BMI may be associated with adjustments in executive function (Stanek et al., 2013; Mac Giollabhui et al., 2020), and our findings emphasize the importance of this association. Overall, the significant effects of these demographic variables provide the impetus for further research to delve into the physiological basis of the relationship between gender and BMI and the attentional network and immune system. It also highlights the importance of the need to consider these factors in more detail in future studies to better understand the role of individual differences in the neuroimmune regulation of immigrants.

Our results confirm the theory of the neurovisceral integration model (NIM) in high-altitude. According to Thayer and Lane (2000) on neuro-visceral integration (Thayer et al., 2009; Park and Thayer, 2014), the strong vagal regulation of the heart is related to the effective function of self-regulating neural circuits, which enable the body to respond quickly and flexibly to various environmental demands. Our results suggest that HRV (LF/HF ratio) can predict performance of executive control behavior, as well as brain activity during alerting and orienting. It is worth noting that the theta power represents the cognitive resources related to attention (Thayer and Lane, 2000; Thayer et al., 2009; Park and Thayer, 2014), alpha ITC represents the selective inhibition of target information in sensory input (inhibition of irrelevant information and discrete cognitive processes) (Hanslmayr et al., 2005; Busch et al., 2009; Gutteling et al., 2022), P3 may be a neurophysiological marker sensitive to high-altitude exposure (Wesensten et al., 1993; Singh et al., 2004; Wang et al., 2021). P3 amplitude reflects the allocation of attentional resources (Polich, 2007; Fogarty et al., 2018), and a decrease in amplitude indicates a reduction in the remaining effective indicator of mental resources (Magliero et al., 1984; Clayson and Larson, 2011; Schmidt-Kassow et al., 2013; Fogarty et al., 2018). These also indicate that immigrants with better oxygen transport ability have more cognitive resources in attention activities and better selective inhibition of target information, which is realized through the regulation of the autonomic nervous system on the homeostasis of the organism. Previous studies also found that cardiac vagus nerve (CVN) activity is associated with early attention orienting and promotes flexible absorption of effective orientation information (Sørensen et al., 2019). These results suggest that RBCs in high-altitude immigrants is associated with better cardiac vagal tone, which is related to the effective function of attentional self-regulating neural circuits, increasing cognitive resources in the process of selective attention, and improving the selective attention ability to target information.

Limitations of this study and several problems need to be considered in the future work. First, the results of this study need to be further verified and extended, for example (1) to test other aspects of cognitive function; (2) Study individual differences in cognitive function caused by exposure; Secondly, the present study only identified a neuroimmune pathway of RBCs affecting attention function in high-altitude immigrants, and we do not know whether this pathway is also present in low-altitude populations or in age- and gender-matched indigenous Tibetan residents, and future studies could give the results of differences in the study variables between the indigenous Tibetan residents and immigrant populations and enrich the results of the possible adaptive differences. (3) We did not consider the factors that influence HRV, such as smoking, alcohol consumption, and exercise. These may have affected our overall understanding of how HRV changes under different lifestyle conditions and environmental influences. Therefore, future studies should consider these factors in more detail and explore their relationship with HRV to provide a more comprehensive understanding of the mechanisms of HRV variation in high-altitude immigrants. However, these do not affect the goal of the whole study, which is to seek intervention measures for high-altitude migration. And, in the future, we can also seek personalized interventions from the perspective of individual differences. The mechanism revealed in this study may provide reference for future interventions in high-altitude immigrants from the point of view of erythrocyte function, for example, by increasing the RBCs in high-altitude immigrants within the normal range through dietary therapy or other interventions.

In summary, we have identified a neuroimmune pathway rooted in erythrocytes, representing oxygen transport capacity as the basic physiological mechanism that determines human attention performance at high-altitude. Inflammation and HRV mediate the relationship between erythrocytes and attentional function. Our results also confirm the theory of neurovisceral integration model (NIM) at high-altitude. Overall, these findings provide valuable insights into solving the health problems of immigrants who are chronically exposed to high-altitude environments and provide a scientific basis for further research in this area.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Ethics Committee of Tibet University. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

N-NW: Formal analysis, Investigation, Methodology, Resources, Software, Visualization, Writing – original draft, Writing – review & editing. S-FY: Methodology, Writing – original draft. PD: Methodology, Writing – review & editing. RS: Investigation, Writing – review & editing. HL: Investigation, Writing – review & editing. H-LM: Methodology, Writing – review & editing, Conceptualization, Project administration. ML: Conceptualization, Writing – review & editing, Supervision. D-LZ: Conceptualization, Supervision, Writing – review & editing, Project administration.