Browsing by Subject "Illusions"

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    Altered Visuospatial Context Processing in Psychotic Psychopathology
    (2024-05) Pokorny, Victor
    The present dissertation examines atypical use of visuospatial context in psychotic psychopathology. There is a large literature suggesting individuals with schizophrenia, and to a lesser extent bipolar disorder, perceive visuospatial illusions differently than healthy controls. Such work has the potential to identify behavioral and neural markers of psychotic disorders. The first chapter is a meta-analysis that aims to quantify and summarize more than 50 years of research on atypical use of visuospatial context in psychotic psychopathology. When pooling across all task types, we found weak evidence of reduced use of visuospatial context in schizophrenia (Hedges’ g=0.25), and bipolar disorder (g=0.25). The strongest evidence was observed for altered contrast perception paradigms in schizophrenia (g=0.72). We propose altered feedback to the primary visual cortex as a potential neural mechanism underlying this effect. The second chapter is an original study of atypical orientation-dependent surround suppression of perceived contrast in people with schizophrenia (SCZ, n=31) and bipolar disorder (BP, n=29), first-degree biological relatives of these patient groups (SREL, n=28; BPREL, n=21) and healthy controls (CON, n=29). Individuals with schizophrenia exhibited reduced surround suppression across orientations, consistent with weakened untuned gain control. Group differences in orientation-dependent surround suppression magnitude were moderated by visual acuity. These findings clarify the mechanisms of weakened contextual modulation of perceived contrast in psychosis and suggest that visual acuity is an important driver of atypical orientation-dependent suppression in schizophrenia. The third and final chapter is an fMRI examination of brain responses associated with atypical orientation-dependent surround suppression of contrast in the same sample as Chapter 2. We examined BOLD responses to center-surround gratings in individuals with schizophrenia (SCZ, n= 34), bipolar disorder (BP, n = 25), unaffected first-degree relatives of SCZ (SREL, n = 20), unaffected first-degree relatives of BP (BPREL, n = 13) and healthy controls (CON, n = 23). We observed orientation-dependent modulation of V1 BOLD activation to near surrounds across groups. In particular, the SCZ and CON groups exhibited roughly equivalent orientation-dependent contextual modulation (Cohen’s dz SCZ= .56; CON = .63). Surprisingly, the direction of the contextual modulation for near surrounds was opposite of predicted: greater BOLD activation for the condition that was expected to produce suppression. Our unexpected results suggest that spatial attention and figure-ground modulation are important to consider when studying orientation-dependent surround suppression. By disentangling these effects, clinical studies may become more informative with respect to the neural pathophysiology of mental health disorders. Taken together, the findings described in the present dissertation suggest that atypical suppression of perceived contrast holds great potential for clarifying the neural pathophysiology of schizophrenia. In particular, our pattern of results are most suggestive of altered feedback mechanisms driving this effect rather than V1-intrinsic mechanisms. Future research will seek to further disentangle the degree to which spatial attention, figure-ground modulation and attentional lapses contribute to altered surround suppression of contrast in psychotic psychopathology.