Theory of mind, also known as mentalizing, is a social cognitive ability that allows individuals to comprehend the mental states of others (Premack and Woodruff, 1978). Theory of mind deficits have been well-established in schizophrenia (Bora et al., 2009b; Sprong et al., 2007) and are considered one of the most important determinants of functionality in this disorder (Bora et al., 2006; Fett et al., 2011; Thibaudeau et al., 2021). Meta-analytic evidence has shown impaired theory of mind in schizophrenia, first-episode psychosis, and clinical high-risk for psychosis, with large effect sizes (Bora and Pantelis, 2013, 2016; Bora et al., 2009a). However, it's important to note that theory of mind tasks vary and measure different neurocognitive domains (Bora et al., 2008; Quesque and Rossetti, 2020). Additionally, theory of mind itself is an integrated cognitive function, not a solitary construct. Consequently, some researchers have proposed that theory of mind includes two different mentalizing processes: mental state decoding (social-perceptual) and mental state reasoning (social-cognitive) (Bora, 2009; Mısır et al., 2018; Singer, 2006; Tager-Flusberg and Sullivan, 2000). Mental state decoding (social-perceptual) involves the ability to comprehend other individuals' mental states based on perceptual information, such as facial expressions or body language. On the other hand, mental state reasoning (social-cognitive) requires the integration of contextual and past knowledge to understand others' mental states using representational systems. Both mental state decoding and mental state reasoning are essential and work in harmony to comprehend the mental states of others. However, performances on mental state reasoning and mental state decoding tasks are not always correlated with each other (Njomboro et al., 2008).

Unraveling the neurobiology of theory of mind is still challenging with the current technologies and research methods, but neuroimaging studies have significantly contributed to the field in both health and disease. Theory of mind has been found related to multiple brain regions, mostly the fronto-temporo-parietal areas in healthy people (Blakemore, 2008; Frith and Frith, 2003). More specifically, temporoparietal junction, medial prefrontal cortex, superior temporal gyrus / sulcus, anterior / posterior cingulate cortices, insula, precuneus and amygdala have consistently been reported in structural neuroimaging studies of theory of mind (Kronbichler et al., 2017; Molenberghs et al., 2016; van Veluw and Chance, 2014). These brain regions do not work in isolation; they are connected to each other with white matter tracts. Unfortunately, the number of diffusion tensor imaging (DTI) studies investigating the relationship between white matter tracts and theory of mind is limited compared to cortical thickness/volume and functional MRI studies. Theory of mind has also been found related to multiple white matter tracts, including the cingulum, arcuate fasciculus, superior longitudinal fasciculus and amygdala-related white matter tracts (Wang et al., 2018; Zekelman et al., 2022). Some authors argue that the above-mentioned brain regions and connecting white matter tracts collectively constitute the 'social brain' network (Adolphs, 2009; Brothers, 1990). In addition, some research showed that different theory of mind tasks are related to different brain regions and white matter tracts (Gobbini et al., 2007; Schurz et al., 2014).

Meta-analytic evidence showed widespread decreased cortical thickness, surface area and subcortical volume in schizophrenia, with the greatest effect sizes observed in fronto-temporal regions, the hippocampus and the amygdala (Bora et al., 2011; van Erp et al., 2016; van Erp et al., 2018). The most prominent brain region anomalies in schizophrenia are common with the social brain regions. Theory of mind deficits in schizophrenia have been shown to be related to decreased cortical volumes in the temporoparietal junction, medial prefrontal cortex, orbitofrontal cortex, superior temporal gyrus and temporal pole (Herold et al., 2009; Hirao et al., 2008; Koelkebeck et al., 2013; Shen et al., 2022). Meta-analytic evidences also indicated widespread impairment in white matter in schizophrenia, with the greatest effect sizes observed in the anterior corona radiata and corpus callosum (Kelly et al., 2018). Theory of mind performance has been shown to be related to water diffusion in white matter tissue in the fornix/stria terminalis in schizophrenia (Hegde et al., 2020). Additionally, theory of mind was related to white matter fractional anisotropy in the superior longitudinal fasciculus and left cingulum in first-episode psychosis (Kim et al., 2021). Finally, theory of mind task-based functional MRI studies also showed aberrant activation of numerous brain regions in schizophrenia, including the temporoparietal junction, medial prefrontal cortex, orbitofrontal cortex and inferior parietal cortex (Jáni and Kašpárek, 2018; Kronbichler et al., 2017; Vucurovic et al., 2020).

There is a significant heterogeneity in the outcomes of previous studies. These inconsistent findings are likely partly related to differences in ToM tasks across studies. Different components of theory of mind are related to overlapping, yet different, social brain regions and networks. There are also substantial differences in brain imaging methods used across studies and most studies used a unimodal approach. To have a more nuanced understanding of neural correlates of ToM deficits in schizophrenia, it might be important to investigate correlates of both mental-state decoding and reasoning with multimodal neuroimaging methods. Consequently, the current multimodal neuroimaging study aimed to investigate the associations between T1-weighted, diffusion-weighted brain imaging parameters and distinct components (mental state decoding and mental state reasoning) of theory of mind in patients with schizophrenia and healthy controls (HCs), separately.