Cigarette smoking is a serious public health problem worldwide, with adverse effects on the cardiac, pulmonary, and vascular systems (Ng et al., 2014). Long-term smoking is associated with damages in neuroanatomical and functional organizations, as well as poor cognition, including memory, impulse control, attention, and executive function (Durazzo, Meyerhoff, & Nixon, 2010). Smoking affects not only functional activities in local brain regions, but also functional interactions between brain regions (Fox et al., 2016). A growing number of studies have shown that exploring brain connectivity can provide insights into the pathological mechanisms of smoking addiction (Fedota & Stein, 2015; Fox et al., 2016; Niu et al., 2023; Yip et al., 2022).
Resting-state functional connectivity (FC), statistical correlations between blood oxygen level dependent signals of two distinct brain regions based on resting-state functional magnetic resonance imaging (rs-fMRI), was widely used to explore pathological mechanisms in many psychiatric and neurological disorders (Barch, 2017; Fedota & Stein, 2015; Fox et al., 2016). For example, smokers, but not nonsmokers, show an inverse relationship between the strength of resting-state FC between the right anterior insula seed and the ventromedial prefrontal cortex and higher personality trait alexithymia, which is in turn associated with a deficiency in subjective awareness and regulation of emotions (Sutherland, Carroll, Salmeron, Ross, & Stein, 2013). In addition, the salience network appears to be posited to moderate the dynamics between internal default-mode network and external executive control network information processing which may be related to the alteration of attentional resources to interoceptive states in smokers (Sutherland, McHugh, Pariyadath, & Stein, 2012; Yip et al., 2022). Despite these advances, most studies assumed that FC is temporal stationary (i.e., static FC), without considering the fact that the brain is a dynamic system. Therefore, static FC may ignore spontaneous fluctuations in information interaction of the brain.
Recently, some studies have observed that brain regions interact not in a static manner, but in a dynamic process (Capouskova, Kringelbach, & Deco, 2022; Preti & Van De Ville, 2017; Van De Ville, 2019). Dynamic FC (dFC) aims to evaluate FC fluctuations and assess how functional organization evolves over time (Calhoun, Miller, Pearlson, & Adali, 2014). Compared to static FC, dFC can explore rs-fMRI time series on a much finer scale and capture brain connectivity in more detail, which is crucial for understanding the temporal variability in the intrinsic organization of the brain (Preti, Bolton, & Van De Ville, 2017). Therefore, dFC has attracted attention as an effective tool for revealing the dynamic spatiotemporal organization of the brain. Many studies have demonstrated that dFC is related to a wide range of cognitive and behavioral traits (Beaty, Benedek, Silvia, & Schacter, 2016; Capouskova et al., 2022; Marchitelli et al., 2022). Alterations in FC dynamics have also been observed in many brain disorders, such as schizophrenia, major depression disorder, autism, Alzheimer’s and Parkinson’s diseases (Fu et al., 2021; Gu et al., 2020; Kim et al., 2017; Marchitelli et al., 2022; Xie et al., 2022).
Although dFC has been used to explore spatiotemporal dynamic architectures in brain disorders, only one study reported dFC changes in modular communities and across community boundaries during the acute nicotine abstinence (Fedota et al., 2021). Hence, there is still lack of systematic examinations of FC dynamics in smokers at the whole-brain level. Actually, smoking-related brain abnormalities are not limited to local regions but spread throughout the whole brain, thus, it is important to conduct a whole-brain dFC analysis for smokers. Moreover, whole-brain dFC analysis can obtain a reproducible set of brain states exhibiting distinct patterns of brain connectivity, which are related to different cognitive states (Beaty et al., 2016) as well as psychiatric and neurological diseases (Fornito, Zalesky, & Breakspear, 2015). Therefore, exploring abnormalities of whole-brain dFC in smokers may bring new discoveries in brain pathology of smoking addiction.
In this study, using dynamic functional network connectivity (dFNC) analysis based on rs-fMRI, we investigated the dynamics of whole-brain functional networks as well as their relationship with smoking-related variables in heavy smokers. The group independent component analysis (GICA) was first conducted to identify independent components (ICs) representing intrinsic connectivity networks (ICNs). A sliding-window method and k-means clustering were used to identify dFNC states (i.e., brain states) and their connectivity patterns. For each dFNC state, network-based statistics (NBS)(Zalesky, Fornito, & Bullmore, 2010) was applied to explore group differences of network connectivity between heavy smokers and non-smokers, and correlation analysis was performed to investigate relationships between the temporal proprieties of dFNC states and smoking-related variables.