Borderline personality disorder (BPD) is a severe mental disorder with a prevalence of between 0.7 and 2.7 % in the general population (Eaton and Greene, 2018). There is a significant difference in the prevalence of BPD between the sexes, with a notable female predominance (75 % vs. 25 %) (Newton-Howes et al., 2021) . The prevalence is much higher in the clinical population, reaching up to 22 % among hospitalized patients (Ellison et al., 2018). The core symptoms of BPD include emotional dysregulation, impulsivity, instability in self-image, interpersonal relationships, and life decisions (Leichsenring et al., 2023). In BPD group, non-suicidal self-injury (NSSI) and suicide attempts are much more common than in the general population. Approximately 90 % of BPD people commit NSSI, while about 75 % attempt suicide (Goodman et al., 2017). BPD causes many difficulties in social and professional functioning. People with BPD present pervasive pattern of unstable relationships, instable sense of self identity, intense emotional response for rejection and long latency of emotional rage. This causes feelings of strong suffering and distress.

The etiopathogenesis of BPD is not yet fully understood. In classical understanding, personality disorders are attributed to environmental factors typically acting in early childhood, subsequently shaping atypical personality development. The interaction of biological vulnerabilities and biopsychosocial factors may underlie BPD (Crowell et al., 2009). The factors that may determine the development of personality towards BPD are adverse and traumatic experiences (TE) in childhood. The TE are associated with a variety of mental disorders including affective disorders (S. Zhang et al., 2020), schizophrenia (Bailey et al., 2018), substance use disorders (Debell et al., 2014), and personality disorders (Silva et al., 2024). People with BPD are almost 14 times more likely to experience childhood adversity than the non-clinical population(Porter et al., 2020). The most common childhood TE among people with BPD include emotional abuse, physical abuse, sexual abuse, emotional neglect, and physical neglect (Stepp et al., 2016). However, it is unclear how different types of trauma affect the development of people with BPD (Tyrka et al., 2009). In a study on a Chinese population, BPD was mainly associated with emotional maltreatment, whereas sexual abuse was a predictor of NSSI and suicide risk (Wu et al., 2022). However, the available data remain insufficient to determine how specific types of trauma influence the development of BPD psychopathology.

In recent years, there have been more and more reports that may suggest the participation of biological factors in the development of BPD (Leichsenring et al., 2023). In studies conducted on the entire Swedish population, heritability for BPD was estimated at 46 %, confirming that genetics contributes significantly to the disorder's development (Skoglund et al., 2021). A genome-wide association study by Witt et al. identified genetic overlaps with bipolar disorder, major depressive disorder (MDD), and schizophrenia. However, no single polymorphism has been identified as a definitive marker for BPD, likely due to complex interactions between genetic and environmental factors (Witt et al., 2017). Advances in neuroimaging techniques have enabled the identification of structural and functional brain changes in BPD. Alterations have been observed in gray matter within the limbic system, including the hippocampus and amygdala, as well as regions of the frontal, parietal, and temporal lobes (Cao et al., 2022; de-Almeida et al., 2012; Nunes et al., 2009; Rodrigues et al., 2011; Ruocco et al., 2012). These findings support the hypothesis of reduced inhibitory control by frontal areas over the limbic system, which may exhibit impulsiveness in individuals with BPD. The quality of white matter (WM) plays a critical role in communication between cortical areas. Recent developments in diffusion tensor imaging (DTI) have allowed researchers to study the microstructure of WM nerve pathways. In highly organized WM tissue, water molecules diffuse in a directed manner, a phenomenon measured by fractional anisotropy (FA). The FA index ranges from 1, indicating completely ordered diffusion, to 0, indicating isotropic diffusion, where water molecules move equally in all directions (Podwalski et al., 2021).

The findings from WM imaging studies in patients with BPD suggest potential alterations in the integrity of brain emotional circuits. The most commonly reported FA changes were observed in the corpus callosum (CC) (Carrasco et al., 2012; Gan et al., 2016; Kelleher-Unger et al., 2021; Lischke et al., 2017; Vandekerckhove et al., 2020), cingulum (Lei et al., 2019; Ninomiya et al., 2018; Whalley et al., 2015), inferior longitudinal fasciculus (ILF) (New et al., 2013; Ninomiya et al., 2018; Vandekerckhove et al., 2020).

The superior longitudinal fasciculus (SLF) is the longest association tract, connecting the prefrontal cortex with other ipsilateral brain regions. These regions are located within the occipital, parietal, and temporal cortex (Janelle et al., 2022). Its functions are primarily associated with emotion regulation, language processing, visuospatial functions, memory functions, control over complex motor activities, and body perception (Fox et al., 2014; Kargar and Jalilian, 2024; Koshiyama et al., 2020). The structure of SLF was first identified in the early 18th century by Reil J.C. Subsequently, it was confirmed in 1895 by Dejerine, who studied the white matter fibers running through the peri‑Sylvian region. He used the term superior longitudinal fasciculus interchangeably with arcuate fasciculus due to its arch-like shape (Catani and Mesulam, 2008). Later, in the second half of the 20th century, studies on rhesus monkeys led to the subdivision of the superior longitudinal fasciculus into distinct subcomponents (Petrides and Pandya, 1984) . It appears that due to its broad range of functions, it may play a significant role in the neural basis of psychiatric disorders. Given its role in the regulation of emotional and cognitive processes, which are frequently disrupted in individuals with BPD, this structure may contribute to the pathogenesis of BPD symptoms (Stevens et al., 2004). There are various methods of dividing this white matter fiber, with differences arising primarily from different research methodologies (Nakajima et al., 2020). In this study, we adopt a division of the SLF into three components: SLF I (encompassing the dorsal part of the fiber), SLF II (encompassing the middle part), and SLF III (encompassing the ventral part of the fiber). These fibers are part of the fronto-parietal horizontal connection. The tractography of the SLF bundle is presented in Fig. 1. In our analysis, we exclude the arcuate fasciculus due to its structural and functional differences (Oliveira et al., 2021). Changes in the integrity of the superior longitudinal fasciculus (SLF) have been analyzed in various psychiatric disorders, including schizophrenia (Podwalski et al., 2022), affective disorder (Zhao et al., 2024), and attention deficit hyperactivity disorder (ADHD) (Parlatini et al., 2024). Alterations within the SLF are among the most frequently reported WM changes in post-traumatic stress disorder (PTSD), alongside changes in the corpus callosum and cingulum (Siehl et al., 2018). Differences in fractional anisotropy (FA) values have also been identified in the patient population with BPD within the studied group (Vandekerckhove et al., 2020). Although there are reports of altered integrity within the SLF, the relationship between its specific subdivisions and symptoms in the BPD population has not yet been explored. Given that this fasciculus is a large white matter structure involved in numerous neurobehavioral functions, we chose to conduct a detailed analysis in this study. We believe that such a methodology will allow us to identify differences that may be overlooked in standard procedures. It is possible that TE during developmental stages of the central nervous system lead to disruption of WM structure. These alterations may be associated with the integration of contextual information, the processing of emotionally significant stimuli, and the suppression of unpleasant memories. However, there are currently no studies that thoroughly examine the effects of various types of trauma at different stages of individual development on specific white matter structures.

Drawing on this information, we propose the hypothesis that: (1) Reductions in FA values within specific SLF subdivisions, are more expressed in patients with BPD than the HC group, (2) patients with BPD would experience trauma more frequently than the HC group, (3) some subtypes of trauma would contribute to the structural quality of subdivisions SLF, (4) changes in SLF structure may be related to symptomatology in the BPD group. Based on these hypotheses, the study objectives were defined as follows: (1) to compare the microstructure of specific subdivisions of the SLF between individuals with BPD and HC, (2) to assess the intensity of the TE in the study population, considering factors such as the age of occurrence, type of trauma, and relationship with the perpetrator, and to compare these findings with the HC group, (3) to investigate the relationship between specific types of trauma and the structural integrity of the SLF, (4) to evaluate the association between alterations in SLF integrity and the severity of psychopathological symptoms in BPD, and (5) to explore the potential mediating role of SLF integrity in the relationship between trauma and BPD psychopathology.