Theoretical frameworks of schizophrenia (SPR) spectrum disorder propose various mechanisms contributing to its manifestations; however, a comprehensive explanation is yet to be established. The prevailing approach emphasizes the fundamental and essential factors that may subsequently generate a broad array of symptoms. Current early detection strategies for identifying individuals at high risk for psychosis consider both the ultra-high risk (UHR) criteria1 and basic symptoms criteria [2,3]. Individuals with attenuated psychotic symptoms and functional decline are designated as UHR for psychosis [1], reflecting subthreshold psychotic symptoms and observable SPR spectrum features. Whereas, basic symptoms denote subtle, subjectively perceived subclinical disturbances in mental processes, known solely to the individual who experiences them [4]. The integration of basic symptoms criteria and attenuated psychotic symptoms in assessing SPR susceptibility underscores the significance of subjective experiences in identifying the risk of psychosis. Research has shown that high-risk groups often exhibit heightened levels of self-disturbance [5-7], suggesting that anomalous self-experience is a core feature of the prodromal stages of SPR [8]. A wide range of tools and methods are employed to measure these anomalous experiences, providing insights into the nuanced transformations in self-experience linked to SPR and enhancing clinical evaluation and research.

The Examination of Anomalous Self-Experience (EASE; a semi-structured clinical interview designed to assess self-experience) is a psychometric checklist developed by Parnas et al. [9] to evaluate subjective anomalies. The EASE functions as a semi-structured clinical interview, investigating cognition and stream of consciousness, self-awareness and presence, bodily experiences, demarcation of self, and existential reorientation. It provides a comprehensive account of subjective anomalies that are not readily captured by conventional clinical metrics, making it an indispensable tool for understanding the fundamental phenomenological experiences inherent in SPR. Numerous studies using the EASE with cohorts across the SPR spectrum have shown that individuals in this group score significantly higher on the EASE compared to other diagnostic groups and healthy controls (HCs) [10]. Additionally, empirical research in UHR groups has demonstrated the predictive efficacy of the EASE, with elevated scores linked to increased clinical severity and a higher likelihood of transitioning to overt psychosis [6,11].

Another pivotal instrument focusing on subjective experiences is the Frankfurt Complaint Questionnaire (Frankfurter Beschwerde-Fragebogen [FBF]; a self-report questionnaire assessing basic symptoms such as cognitive and perceptual disturbances) [12]. The FBF is a widely used self-assessment tool for identifying basic symptoms, covering a broad range of cognitive complaints, including those related to perception, thinking, memory, language, movement, and emotion [12]. FBF scores have been significantly associated with positive symptoms [13] and strongly correlated with objective neuropsychological test results in patients with SPR [14]. Furthermore, the FBF has been used to assess subjective experiences in first-degree relatives of patients with SPR, revealing higher scores compared to control groups, suggesting a potential genetic predisposition [15]].

Schizotypy is another relevant concept emphasizing the subjective experiences of individuals within the SPR spectrum. Schizotypy is a complex construct that incorporates a range of traits and symptoms associated with an increased susceptibility to psychosis and a subclinical spectrum of psychotic expressions and cognitive-perceptual anomalies [16]. Research on schizotypy often employs psychometric inventories such as the Chapman Psychosis Proneness Scales (CPPS; a set of self-report inventories measuring schizotypy traits, including Perceptual Aberration Scale [PER], Magical Ideation Scale [MAG], and anhedonia) [17-20]. Traits such as unconventional beliefs or perceptual experiences are referred to as positive schizotypy, with the CPPS encompassing scales for PER [18] and MAG [20]. Negative schizotypy is characterized by deficits in social engagement and emotional expression, with Physical Anhedonia Scale (ASP) [17] and Social Anhedonia Scale (ASS) [19] being key to this classification. In contrast to the EASE and FBF, the CPPS reflects the content of subthreshold psychotic experiences rather than the forms or processes of anomalous experiences [21]. Previous studies have shown that both the positive and negative dimensions of the CPPS predict SPR spectrum disorder [22], with negative schizotypy being particularly associated with the transition to psychosis during the prodromal stages of SPR [23-25]. Taken together, examining the EASE, FBF, and CPPS in parallel offers unique insights, as each instrument captures distinct yet overlapping facets of subjective experience, thereby allowing a more comprehensive understanding of vulnerability across the SPR spectrum.

Because subjective experiences are inherently personal, the wide range of experiences individuals report cannot be fully captured using a single concept or assessment tool [21]. To date, few studies have explored the interrelationships among various tools designed to measure these experiences. For instance, the Anhedonia Scale of the CPPS has been found to relate to UHR criteria but not to basic symptom criteria [25]. Another study reported significant associations between schizotypal ideation and self-reported cognitive difficulties [26]. Additionally, an examination of the relationship between the CPPS and FBF revealed that basic symptoms were significantly associated with positive schizotypy but not with negative schizotypy [27]. No comprehensive study has yet integrated diverse psychosis-proneness scales to examine their interrelations across different stages of the SPR spectrum. This study addresses this gap by investigating the relationships among the EASE, FBF, and CPPS across UHR and SPR populations. Specifically, we aimed to explore these dynamics within the UHR and SPR groups. Understanding these relationships may offer valuable insights into the early identification of psychosis risk and the progression of SPR spectrum disorder. We hypothesized that both the UHR and SPR groups would exhibit significantly higher scores on the EASE, FBF, and CPPS compared to HC, reflecting elevated disturbed subjective experiences. Furthermore, we anticipated stronger correlations between these measures in the SPR group than in the UHR group, suggesting more pronounced disruptions in the interplay of subjective experiences as the disorder advances.

This study uniquely examined the interrelation between three psychosis-proneness evaluations centered on subjective experiential symptoms: the EASE interview for anomalous self-experiences, the FBF self-assessment for basic symptoms, and the CPPS for assessing schizotypy. Although each tool is often used independently to measure different aspects of subjective experiences, our findings demonstrate the complementary nature of these scales in capturing overlapping yet distinct facets. By assessing the relationships between the EASE, FBF, and CPPS across the UHR and SPR groups, we highlight how these tools collectively provide a nuanced understanding of subjective experiences within the SPR spectrum.

The study revealed significant differences between the HC group and both the UHR and SPR groups across the EASE, FBF, and CPPS measures, indicating heightened disturbed subjective experiences in the clinical groups, consistent with prior research on self-disorders in SPR spectrum disorder [10,36]. However, the disparity between the UHR and SPR groups was not statistically significant, which contrasts with previous investigations [7,36,37]. This suggests that UHR cohorts demonstrated an attenuation in self-disorder; thus, the findings can be interpreted as a consequence of intrinsic heterogeneity within UHR populations. The limited sample size in the present study may have contributed to the lack of significant results. While ceiling effects could be considered as a potential explanation, the mean and standard deviation values in both groups were well below the maximum ranges of the EASE, FBF, and CPPS, indicating that such effects are unlikely to account for the non-significant differences. Instead, the heterogeneity of the UHR group, encompassing individuals with different levels of symptom severity and diverse trajectories of risk, may have obscured group-level contrasts. In contrast, the relatively homogeneous characteristics of the UHR cohort in this study, composed exclusively of individuals meeting the APPS criteria (n=25), may suggest that, in the prodromal stage of SPR, the awareness of disturbed subjective experiences could be as pronounced as in SPR. Alternatively, subjective symptoms may not follow the same progression pattern as objective clinical symptoms, with some disturbances plateauing early in the illness course while others fluctuating over time. Taken together, these considerations suggest that the absence of significant differences between UHR and SPR should be interpreted with caution.

This outcome suggests that, despite the different stages (prodromal and active) of symptom severity, individuals in the UHR and SPR groups share common features in terms of subjective disturbances. Bleuler [38] characterized SPR by prioritizing certain trait-like, non-psychotic symptoms as fundamental ego disorders, while designating delusions and hallucinations as accessory symptoms and interpreting them as diagnostically non-specific. Despite ongoing debates regarding the heterogeneity of UHR and SPR, our findings suggest that these clinical groups constitute a homogeneous continuum with shared subjective experiences of disturbance. This perspective underscores the necessity of evaluating subjective experiences and the core experiential aspects of selfhood to capture the essence of SPR spectrum disorders, as well as the value of encouraging patients to spontaneously share their experiences [39].

Examination of the interrelations between the EASE, FBF, and CPPS within each group (UHR and SPR) revealed a complex interplay between self-disturbances, basic symptoms, and schizotypal characteristics. Initially, the EASE and FBF were significantly correlated in both the UHR and SPR groups. Prior research [40,41] has shown that basic symptoms assessed through questionnaires often poorly correspond to those evaluated through interviews, as demonstrated in studies using the Eppendorf Schizophrenia Inventory [42] and the Schizophrenia Proneness Instrument (SPI [43]; highlighting methodological diversity. The EASE construct addresses disturbances pertaining to the self across a comprehensive array of fundamental symptoms, thus indicating a correlation with the FBF in this study. The findings also suggest that the EASE and FBF may complement each other as interview-based self-assessment tools for evaluating subjective anomalous experiences. This also indicates the potential of these tools to represent related constructs, warranting further investigation, such as factor analysis to identify correlated items and better understand their interrelationships.

Significant correlations were identified between the FBF and the positive (PER and MAG) CPPS subscales in the UHR group. Furthermore, in the SPR group, both EASE and FBF were associated with the CPPS positive subscales (PER and MAG). No correlation was observed between negative schizotypy and the FBF. In studies examining the relationship between basic symptoms and schizotypy, a cohort characterized by high schizotypy exhibited elevated levels of basic symptoms [44], whereas another investigation revealed that questionnaires assessing positive schizotypy and the FBF demonstrated several overlapping characteristics [27]. Conversely, one study indicated that the PER, MAG, and ASP dimensions of the CPPS had no significant correlation with basic symptoms, which were assessed using the SPI instead of the FBF [25]. Amid these inconclusive findings in previous research, our results offer evidence to support the association between self-disorders, basic symptoms, and positive schizotypy. Additionally, our study suggests that future comprehensive research on this relationship would benefit from a comparative analysis of various assessment tools.

Although our cross-sectional design and small sample size limit definitive conclusions on progression, the more intricate and interlinked correlation patterns in the SPR group may be suggestive of a more cohesive integration of subjective anomalous self-experiences, basic symptoms, and schizotypy measures, potentially contributing to observable psychotic symptom constellations. Previous research [16,24] has indicated that self-disturbance or schizotypy alone does not intrinsically result in clinical significance. Instead, attenuated psychotic symptoms may be triggered by high schizotypy and neurobiological aberrations in information processing, which are articulated as basic symptoms; the accumulation of these symptoms potentially leads to the development of overt psychosis. These findings are consistent with the notion that as the severity of psychosis or related symptoms increases, the interactions between different subjective symptom measures may become stronger, possibly reflecting more generalized or systematic disruptions in cognitive and emotional processing. This interplay underscores the importance of addressing multiple dimensions simultaneously to gain a more holistic understanding of SPR psychopathology. However, as we did not conduct formal interaction analyses, future research with larger samples and interaction analyses is needed to further explore these complex relationships.

Despite these findings, this study had several limitations. First, the cross-sectional design limited our ability to draw causal conclusions regarding the progression of disturbed subjective experiences across the SPR spectrum. Longitudinal studies are required to track the development and interaction of these symptoms over time. Second, the small sample size may have limited the generalizability of our findings. Third, although medication status and chlorpromazine equivalent doses were recorded, we did not conduct subgroup analyses according to medication status. Given the relatively small sample size, such analyses would have lacked sufficient statistical power. Nevertheless, antipsychotic medication may influence subjective symptom reporting, particularly for EASE domains involving self-awareness and cognitive function. While treatment may reduce the intensity of self-disorders, side effects such as sedation or cognitive dulling may alter participants’ responses on certain items. Future studies with larger samples should explicitly address the role of medication in shaping subjective symptom profiles. Fourth, although we observed stronger correlations in the SPR group compared to the UHR group, Fisher’s z-tests did not reveal significant group differences, and several comparisons could not be tested because correlations were available only in one group. The relatively small sample size further limited the statistical power of these analyses. Fifth, while our primary goal was to explore the relationships between these measures, the lack of a formal multiple comparison correction for all statistical tests is a limitation. Future studies with larger and more diverse samples will help validate and expand our results. Additionally, although our study used well-established tools (EASE, FBF, and CPPS), further research should incorporate additional subjective measures to capture a broader range of experiences related to SPR spectrum disorders. Given that our sample was drawn entirely from a Korean population, cultural factors may have influenced the way participants reported subjective symptoms. For example, stigma related to mental illness may reduce willingness to disclose anomalous experiences, and cultural norms emphasizing collectivism and social harmony may shape how self-awareness and interpersonal disturbances are expressed. These factors limit the generalizability of our findings, underscoring the importance of cross-cultural research to validate the applicability of subjective symptom measures across diverse populations.

In summary, our investigation showed that clinical cohorts at different stages of the SPR continuum, including the UHR and SPR groups, exhibited significant deviations in their subjective experiences. Furthermore, the associations identified among EASE, FBF, and CPPS are consistent with the notion that the interrelations among these measures may become more pronounced as the intensity of psychosis or related symptoms increases, potentially due to more pervasive or systematic disturbances in cognitive and emotional processing. However, longitudinal studies are needed to confirm this. Understanding these interrelationships provides valuable insights into the phenomenological structure of SPR and indicates that combining these tools can help mitigate the limitations of each individual scale. Taken together, these findings highlight the importance of assessing subjective experiences when evaluating individuals at clinical high risk for psychosis. Clinicians should routinely evaluate multiple dimensions of subjective experiences in UHR individuals, as their patterns of disturbance may resemble those of SPR more than previously recognized. Incorporating structured instruments such as the EASE, FBF, and CPPS into clinical assessments may facilitate earlier detection of individuals at risk and support more personalized intervention strategies.