## 1 Introduction  
Pain is a complex physiological and psychological feeling involving sensory-discriminative, cognitive-evaluative, affective-motivational and motor networks in the human body [1]. Several mental disorders, such as anxiety, depression and sleep disorders, are induced by long-term painful perception. The emotional state also has an enormous influence on pain: a negative emotion increases painful perception, whereas positive emotion reduces pain [2]. Many studies found that placebo analgesia was associated with functional activity within painful-sensitive brain regions [3, 4]. These findings provide evident that placebos can alter the experience of pain. Moreover, [PERSON] and colleagues indicated that the brain network could regulate attention, expectation and reappraisal related to painful cognition through the descending pain modulatory system [5]. In addition, functional tissue of white matter (WM) is also associated with pain processing [6]. This suggests that the WM functional network is feasibly involved during the modulation of pain. The effects of cognition on the descending pain modulatory system have been examined extensively in neuroimaging studies. The periaqueductal grey (PAG) plays a key role in the modulation of pain processing [7]. It receives direct (and indirect) nociceptive input from the dorsal horn and sends projections to the inferior olive as well. Based on these connections, PAG will deliver sensory and motor inputs to the cerebellum [8; 9]. Some researchers documented that pain could be modulated by the cerebellum and found that chemical stimulation applied to the cerebellar fastigial nucleus decreases visceral nociceptive reflexes [10]. The pain's unpleasantness was positively correlated with an increased activation of the posterior cerebellum, suggesting that the posterior cerebellum may be a key region in the cognitive assessment of pain [11]. In summary, the cognitive region in the cerebellum may be related to the encoding of pain, possibly as a cognitive modulator. In addition, [PERSON] and colleague found that an interruption of a connection during pain processing may be associated with abnormal function of the WM bundle [12]. However, the relationship between the WM functional network and the top-down pain modulatory network is unclear. Previous studies mainly used diffusion tensor imaging (DTI) to study the relationship between the WM structure and pain. The function of WM was rarely assessed during the painful stimulation. In patients with chronic musculoskeletal pain, changes of axial diffusivity (AD) in WM were directly related to its symptom severity [13]. The study of patients with acute thermal pain has been found that a decreased fractional anisotropy (FA) of the WM region can affect the functional response to pain [14]. The WM integrity within and between regions of the descending pain modulatory network is critically linked with the individual ability for endogenous pain control [15]. Therefore, WM changes may play a crucial role during pain processing. In a study of orthodontic pain patients, researchers found that WM network could mediate emotional and cognitive networks during pain stimulation [6]. This finding provides a basis for studying the changes of the WM functional network in the process of pain. Although there is little research on the WM function network of pain, its significance may be great and important. There are mainly two different experimental pain models. One model uses electrical stimulation to induce pain perception [16]. Another model induces pain through cold stimuli (CPT: cold pressor test) [17; 18]. Cold pressor pain has been shown to be opioid sensitive [19]. Moreover, the descending pain modulatory system is implicated in opioid analgesia [3]. Thus, in this study, subjects were given cold pain stimulation to induce the transmission of pain information by the descending pain modulatory system. We hypothesized that the WM functional network played a conduction role in the painful regulatory system.