Dimensions of psychosis: Elucidating the subclinical spectrum using neuroimaging markers
Psychosis unifies a collective of disorders characterised by symptom dimensions (Gaebel & Zielasek, 2015). Purposefully delimited clinical descriptors of schizophrenia spectrum and psychotic disorders (American Psychiatric Association, 2013) impose challenges on the identification of aetiologica...
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|Summary:||Psychosis unifies a collective of disorders characterised by symptom dimensions (Gaebel & Zielasek, 2015). Purposefully delimited clinical descriptors of schizophrenia spectrum and psychotic disorders (American Psychiatric Association, 2013) impose challenges on the identification of aetiological and clinically meaningful predictors. The disassembly of psychiatric diagnoses into their elementary symptom dimensions has helped formulate psychosis phenotypes fitted on a psychosis continuum (Verdoux & van Os, 2002). Aetiological models of psychosis may be studied through schizotypy and transient psychotic experiences (Barrantes-Vidal et al., 2015; Nelson, Fusar-Poli, & Yung, 2012), collectively termed subclinical psychosis phenotypes. The dimensional psychometric structures of these phenotypes varying in temporal stability (Linscott & van Os, 2013; Mason et al., 1995; Stefanis et al., 2002), and their implications might be further consolidated when paired with neuroimaging parameters (Siever & Davis, 2004).
Three neuroimaging studies aimed to examine the relationship between subclinical psychotic phenotypes and neurobiology. Surface and volume-based morphometric (VBM) methods were implemented to examine the variety of cortical and subcortical signatures of different phenotype dimensions. Study 1 investigated whether cortical surface gyrification -a maker of genetic and developmental influences on cortical morphology (Docherty et al., 2015; Haukvik et al., 2012)- is associated with dimensional psychosis prone phenomena (Konings, Bak, Hanssen, van Os, & Krabbendam, 2006; Stefanis et al., 2002). Early cortical organisation contributes to cognitive capacities in later life (Gautam et al., 2015; Gregory et al., 2016; Papini et al., 2020). Given that cognitive deficits are present in psychosis prone and clinical samples to varying extents (Hou et al., 2016; Siddi et al., 2017), Study 1 also explored the mediating role of cognition (both as a general measure and intelligence quotient) as a psychosis endophenotype in the relationship between regional gyrification and PLE distress. Study 2 and Study 3 used VBM to investigate structural brain correlates for psychotic-like experiences (PLE) and trait psychosis phenotypes (schizotypy). Different PLE facets (quantity and distress severity) (Hanssen, Bak, et al., 2005; Ising et al., 2012) were used to estimate whole-brain grey matter volume, followed by interaction models in subsequent prefrontal regions of interest (Study 2). The medial temporal lobe includes the hippocampal subfields, which are regions of interest in psychosis pathophysiology (Lieberman et al., 2018; Mathew et al., 2014; Schobel et al., 2013). Based on a previous study in schizoytypy (Sahakyan et al., 2020), Study 3 examined the relationship between schizotypal trait dimensions (Mason et al., 1995) and PLE, and their interactions, and hippocampal subfields and the amygdala.
The results of Study 1 showed that psychometrically assessed PLE were associated with reduced gyrification in parietal and temporal regions, indicating that psychosis proneness correlates with neurodevelopmental factors (Fonville et al., 2019; Liu et al., 2016). A lack of mediating pathways between regional gyrification and PLE suggested that cognition effects may emerge in larger samples (Mollon et al., 2016) and/or increasingly psychosis pone phenotypes. Elaborating on the distinction between PLE quantity versus distress, Study 2 showed that PLE load, but not distress severity, were associated with volume increases in prefrontal and occipitotemporal regions. At increased distress severity for perceptual abnormalities, PLE were associated with regional volume reductions of the superior frontal gyrus. Study 3 showed differential relationships between schizotypy dimensions and volumes of the MTL that are involved in the pathophysiology of schizophrenia. PLE per se did not associate with amygdala or hippocampal subfield volumes, but a positive association between the hippocampal subiculum and PLE was moderated by positive schizotypy. Study 3 underscored the enhanced usefulness of schizotypy as an endophenotype in psychosis research when its multidimensional organisation (Grant, 2015; Vollema & van den Bosch, 1995) is respected.
The results support the use of psychosis symptom dimensions, showing different (positive and negative) neuroanatomical associations. While case-control studies in schizophrenia show consistent volume reductions of the prefrontal and temporal cortices (Haijma et al., 2013; Honea, Crow, Passingham, & Mackay, 2005), these findings contribute to more heterogeneous volumetric relationships in nonclinical individuals. Reduced regional cortical gyrification proposes a continuous distribution of neurodevelopmental impacts. Distress severity and schizotypy occasioned modulatory effects in prefrontal and hippocampal subfield volumes, respectively. Collectively, these three cross-sectional studies extend previous research suggesting that dimensional phenotypes show neuroanatomical variation supportive of a psychosis continuum possibly characterised by an underlying non-linearity (Bartholomeusz et al., 2017; Binbay et al., 2012; Johns & van Os, 2001).|
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