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Do eye movement abnormalities in schizophrenia cause Praecox Gefühl?

‘Praecox Gefühl’ is a classical concept defined by Dutch psychiatrist H. C. Rumke (1893–1967) referring to a unique phenomenological experience or feeling of bizarreness or unease by a psychiatrist while interviewing a patient with schizophrenia. Historically, Praecox Gefühl was considered as a core symptom of schizophrenia, though it is not widely used today as it is susceptible to the subjectivity of clinicians and lacks reliability. Nevertheless, a patient’s facial expression and unique eye movements are very important factors in making a diagnosis in patients with schizophrenia. In this regard, eye movement research may lead to a more objective quantification of what Rumke pointed out. Detecting the critical biomarker and establishing optimized novel treatments 3 in psychiatric disorders is an urgent challenge. Combining a data-driven approach and a theory-driven approach may uncover the biological basis of psychiatric disorders. Among the theory-driven approaches, the unconscious predictive processing theory has provided deeper mechanistic insights into the perceptual and cognitive deficits in several psychiatric conditions. In this regard, eye movement is the very first window of unconscious predictive processing. Eye-trackers noninvasively measure one’s unconscious eye behavior towards presented stimuli. With modern eye-tracking instrumentation, scientists aim to reflect the viewer’s attention and consequently disclose a part of the viewer’s thought processes. In particular, recorded and analyzed gaze parameters help determine which elements attract the observer’s eyes, in what order, and how often. Despite an increasing focus on the eye behavior within cognitive neuroscience and an active import of eye-tracking paradigms in other fields, vision science lags behind in clinical research. However, a promising trend has started to emerge: the study of visual information processing by re-examining paradigms with modern eye-tracking devices. The patterns of gaze parameter abnormalities found in patients with bipolar disorder, attention-deficit/hyperactivity disorder, anxiety disorder, and autism spectrum disorder differ from those found in patients with schizophrenia, which supports the notion that schizophrenia patients have a unique visual processing property. Moreover, a vast majority of modern studies corroborate the fact that a reduced integration of visual information contributes to altered visual experiences in patients with schizophrenia. To this end, Wolf et al. provide a state-of-the-art overview of the potential and scope of eyemovement abnormalities in patients with schizophrenia, ranging from its history, to the recent developments in methodologies, and to promising current findings on visual disturbances in schizophrenia. Since perception deficits seem to persist selectively in schizophrenia, this detailed scoping review by Wolf et al. aims to gather scientific evidence exclusively on saccadic dysfunctions and exploratory eye-movement deficits reported in the context of schizophrenia. Furthermore, this work systematically maps the findings on various gaze metrics in attention-demanding tasks and their application as potential biomarkers for schizophrenia. One may argue that there is considerable variability in the experimental tests (e.g., smooth pursuit, free viewing, fixation stability), procedural parameters (e.g., eye fixations, scanning path length, and exploratory eye movements), and types of stimuli (e.g., landscape, fractal, and face images) that are being used in clinical research in order to encode visual information processing; nevertheless, essentially such variety in performed experiments may provide more valuable insights regarding cognitive impairments and deficits linked to the social-skill domain. Following the literature on gaze metrics in schizophrenia research, it has been strongly underlined that a single test may not serve as a reliable diagnostic tool. Instead, a combination of gaze metrics obtained by multiple tasks may increase the classification accuracy to distinguish schizophrenia patients from healthy subjects and characterize particular clinical dimensions of schizophrenia. Given that eye-movement assessments are noninvasive, a promising future clinical research area is shaping up to evaluate potential relationships between disease characteristics and social functioning in patients with schizophrenia. A growing number of researchers aim to understand the relationship between eye-movement characteristics, intellectual functioning, and differences in brain structures across patients with schizophrenia. Finally, gathered and exemplified reports mark a new beginning of promising research on visual disturbances in schizophrenia with eyetracking technology, which can revolutionize mental health screening in the future.