Subjective cognitive decline (SCD), also known as subjective cognitive/memory impairment/complaints, refers to a self-experienced decline in cognition among individuals with normal performance on standardized neuropsychological tests [1]. It is important to note that interventions during the SCD period may reduce the risk of development into objective cognitive decline or dementia [2]. Acupuncture, a promising non-pharmacological therapy, has the potential to treat multiple types or stages of cognitive decline [3, 4], with a better treatment prognosis especially in earlier SCD stages. For instance, 8 weeks of acupuncture or electroacupuncture treatment has shown promising effects on amnesic mild cognitive impairment (MCI) [5] and vascular cognitive impairment with no dementia [4]. Moreover, our previous research revealed that a 12-week course of acupuncture treatment improved the global cognitive function of individuals with SCD [6]. However, the neural basis for acupuncture intervention in SCD has yet to be fully investigated.

The neurobiological basis of multiple cognitive abilities is governed by two fundamental principles of human brain function: integration (i.e., the formation of large-scale brain networks through long-range functional connectivity) and segregation (i.e., the specialization of distinct regions through local differentiation) [7, 8]. Successful reconfiguration underlying better memory relies on the functional segregation, integration, and balance of large-scale resting brain networks [7]. Cognitive complaints may involve similar vulnerabilities in segregation and integration in large-scale brain networks [9]. A longitudinal study suggested that individuals with SCD with a greater decline in subjective cognitive function had lower functional connectivity within networks, although functional connections (FCs) between networks increased [10]. Patients with MCI have also been reported to exhibit abnormal FCs in multiple brain networks, including the sensorimotor network (SMN) and executive control networks [11]. Specifically, interruption of FCs within the SMN of MCI patients has been associated with decreased cognitive function [12] and deterioration of working memory and attention [11].

Brain connectivity and network topological analyses provide a powerful approach to quantify information communication and network efficiency, offering in-depth insights into the system-level mechanisms of brain network integration and segregation [13, 14]. Acupuncture has multi-target therapeutic effects [15] and network-based analysis has been widely adopted to uncover the complex mechanisms of acupuncture [14]. The regulatory effects of acupuncture on large-scale brain networks have been demonstrated in objective cognitive disorders [16]. A systematic review has summarized that the FCs of the default mode network (DMN), central executive network, and salience network can be regulated by acupuncture in patients with MCI [16]. However, whether and how acupuncture alleviates SCD symptoms by modulating the integration and segregation of large-scale brain networks remains an unexamined, crucial question.

Memory impairment is one of the major symptoms of SCD [17]. In the present study, we sought to examine changes in the large-scale brain network associated with memory improvements attributed to acupuncture, using resting-state functional magnetic resonance imaging (fMRI) data from a previous randomized controlled trial (RCT) of acupuncture treatment in individuals with SCD [6]. Unlike previous seed-based functional connectivity analysis [6], in the present study we extracted time series from all brain regions using the Dosenbach atlas, constructed a functional connectivity matrix of the brain network, and examined alterations in FCs at the large-scale brain network level. Graph theory was subsequently applied to compare the topological properties of regions showing significant differences. Finally, correlations between these network changes and memory performance were evaluated to elucidate the central mechanisms of acupuncture in SCD. We hypothesized that acupuncture may alter memory in SCD by modulating large-scale resting-state brain network FCs and topological properties.

Our secondary analysis investigated the FCs and topological reorganization of large-scale brain networks after 12 weeks of acupuncture treatment in individuals with SCD. First, the benefit of acupuncture treatment on AVLT-H total scores was identified. For brain network properties, compared with the SG, acupuncture treatment significantly modulated five FCs within and between the SMN and the CON in individuals with SCD. Additionally, enhanced nodal efficiency of the left parietal lobe and decreased betweenness degree of the right parietal lobe, which belongs to the SMN, were observed according to graph-theoretical analysis. Furthermore, the correlation between functional connectivity changes in the left middle insular and left PCC and improvements in AVLT-H was significant. These findings shed light on how acupuncture improves the memory of individuals with SCD by modulating large-scale resting-state brain network properties.

We found that the AVLT-H total scores in individuals with SCD showed a significant group $\times$ time interaction effect, indicating more improvement in objective memory in the AG than in the SG. This finding aligns with previous studies that have demonstrated cognitive function (including memory) improvements after non-pharmacological therapies [31, 32]. In particular, a double-blinded RCT has found that the AVLT-H scores of MCI patients were noticeably improved after 24 weeks of transcutaneous auricular vagus nerve stimulation [31]. Considering previous results [6], which showed a significant within-group effect but no significant group $\times$ time interaction effect in SCD-Q9 scores between groups, we speculated that acupuncture may significantly enhance memory function, and the improvement of objective memory performance may precede self-reported memory amelioration.

Shreds of evidence have indicated that a mutual influence exists between the subjective cognition complaint and the objective cognitive function. SCD may reflect concurrent objective impairment, which in turn influences subjective complaints [23, 33]. A recent longitudinal systematic review found that SCD patients exhibited poorer overall cognitive and objective memory performance compared with non-individuals with SCD [34]. Better objective memory performance in individuals with SCD is associated with fewer subjective complaints [23]. Objective memory impairment in individuals with SCD suggests possible Alzheimer’s disease (AD) [23]. Our study indicated that acupuncture could improve objective memory, potentially offering a new intervention strategy against dementia progression in SCD patients.

Regarding brain network properties, two positive FCs within the SMN were increased by acupuncture, with a significant group $\times$ time interaction effect, but these FCs were decreased by sham acupuncture. The SMN is not only responsible for motor control and somatosensation but is also involved in neurocognitive processing [35]. Numerous analyses have identified an association between abnormal FCs of the SMN and cognitive function in participants with SCD [36, 37]. For instance, evidence indicates that the mean values of FCs within the SMN are positively related to cognitive assessments for SCD [36]. The disruption of FCs in the SMN can be potential biomarkers of cognitive decline in older adults [36]. In another study, the FCs of SMN-related regions in individuals with SCD were shown to be increased, but those of healthy people and AD patients were not [37], suggesting that the increase of intrinsic SMN connections in SCD is an early compensatory adaptation, whereas the decrease of intrinsic SMN connections in AD patients may be a sign of decompensation. In the present study, we postulated that the mechanism of acupuncture’s therapeutic effect may enhance vulnerable FCs in the SMN and promote compensatory adaptation of SMN-related regions in SCD. Furthermore, significant enhancement in the nodal efficiency of the left parietal SMN region and decreased betweenness of the right parietal SMN region were observed after acupuncture, which suggests that acupuncture can improve the information transfer capability of the left parietal region and reduce the importance of the right parietal region in the brain network, consistent with the functional lateralization of the SMN [38].

The CON, one of the executive control networks [39], is closely related to cognitive complaints, especially in the executive, attention, and episodic memory domains [40]. Greater intrinsic FCs in the CON have been associated with better episodic memory [41]. Decreased inter-network FCs, combined with increased within-network FCs, suggest the successful functional segregation of the SMN and the CON after acupuncture treatment. The decline in brain network FCs associated with cognitive impairment has previously been shown to predominantly occur between the DMN and the executive control networks (including the CON) [42]. The correlation between the DMN and executive control networks significantly increases with task intensity, especially during working-memory tasks [43]. However, there has been relatively little research examining the relationship between FCs between the SMN and CON and cognitive decline. Our findings fill this gap and show that acupuncture may modulate functional connections between the CON and SMN networks in individuals with SCD, thereby improving cognitive function.

Decreased SMN-CON connectivity is significantly associated with improvements in objective memory. Specifically, a decrease in the middle insular cortex-PCC connectivity negatively correlates with an improvement in AVLT-H scores. The insula is one of the most susceptible regions to SCD and a critical integration hub in human brain networks [37, 44]. Alterations in FCs between insular subregions and regions of the executive control network are essential to the neural mechanisms underlying cognitive decline in SCD [37]. Notably, the left posterior insula shows significant negative connectivity with the PCC in healthy older adults [37]. Our results suggest that acupuncture may improve objective memory in SCD by enhancing negative connectivity of the middle and posterior insula, which are both considered to be components of the SMN [26, 45].

The enhancement of negative connectivity signifies functional segregation and mutual inhibition between brain regions, as activation in one region diminishes activity in another [46]. Research has shown that both the insula and the PCC support memory encoding and retrieval [47]. A common pattern of deactivation has been observed in the PCC during attentionally demanding tasks [48], whereas the insula is significantly activated in human-face- or tactile-recognition-memory tests [49]. Hence, negative insula-PCC functional connectivity enhanced by acupuncture may indicate successful segregation or inhibition between the insula and the PCC, thus prompting memory consolidation and attention in individuals with SCD. Our findings suggest different regulatory directions of acupuncture and sham-acupuncture in the middle insula-PCC functional connectivity. Although significant group differences remained after controlling for baseline values (F = 8.227, p = 0.006, Partial $\eta$² = 0.115) using ANCOVA, the phenomenon of opposite directions might be caused by the differences in FCs among individuals and should be confirmed by further studies.

Our study has some limitations. First, the p values reported were obtained using 5000 random sampling tests (without correction); validation is required in future studies that include more samples, which will provide greater statistical power. Second, our study also found that changes in FCs between brain regions associated with emotions, such as the insular cortex and the PCC, showed an interaction effect after the intervention. Since previous research has suggested that older adults with emotional burdens are more likely to perceive cognitive decline [17], future research could delve deeper into the relationship between emotions and cognition in SCD. Third, we focused primarily on the large-scale brain networks related to memory function in this study. The cerebellum potentially contributes to language, attention, and episodic memory encoding; the exact mechanisms by which the cerebellum contributes to these cognitive processes need further elucidation.

The present study investigated the brain network basis of acupuncture in improving memory. Acupuncture intervention (12 weeks) enhanced objective memory performance in individuals with SCD by reducing FCs between the SMN and CON and modulating SMN network topology, enhancing the functional segregation of these networks as potential targets for acupuncture treatment.

fMRI, functional magnetic resonance imaging; TR, Repitation Time; TE, Echo Time; FWHM, full width at half maxima; ROI, regions of interest; MNI, Montreal Neurological Institute; AUC, area under the curve; SMN, sensorimotor network; CON, cingulo-opercular network; AVLT-H, Auditory Verbal Learning Test-Huashan version; SG, sham acupuncture group; AG, acupuncture group; SE, standard error of the mean; FC, Functional connectivity; vFC, ventral frontal cortex; PCC, post cingulate cortex; L, left; R, right.

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Study concept and design: CZL, XW, CQY, GXS, LW. Acquisition, analysis, and interpretation of data: XW, HZ, XYW, CKL, ZYW, CQY. Drafting of the manuscript: HZ, LW, GXS, XW. Revising the manuscript critically for important intellectual content: CZL, XW, HZ, GXS, LW, XYW, CKL, ZYW. Statistical analysis: XW, HZ. Study supervision: GXS, XW. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was conducted in accordance with the Declaration of Helsinki. The research protocol was approved by the ethics committee of the Beijing Hospital of Traditional Chinese Medicine affiliated with Capital Medical University (Ethic Approval Number: 2017BL-061-03), and all of the participants provided signed informed consent.

This work was partially funded by the National Natural Science Foundation of China (Grant No. 81674055) and the National High-Level Traditional Chinese Medicine Hospital Clinical Research Funding (DZMG-QNHB0006, DZMG-QNZX-24002).

The authors declare no conflict of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/JIN45003.