The brain's response to a visual stimulus depends in part on the context in which it appears. For example, objects appearing within similar-looking backgrounds tend to evoke smaller neural responses than those seen in isolation. While it is known that schizophrenia (SZ) may reduce visual context effects, the neural mechanisms involved are not fully understood. This dissertation uses functional magnetic resonance imaging (fMRI) and visual behavioral tasks to examine the role of context during normal visual processing, and how context processing is affected by SZ.Chapter 1 provides an overview of the forms of contextual modulation that will be addressed later, and their impairment in SZ. Chapter 2 describes a series of five experiments probing how factors such as stimulus geometry, presentation timing, and attention affect the fMRI response to small groups of visual stimuli. In primary visual cortex, the relative strength of contextual modulation was found to increase when subjects directed their attention away from the stimuli. Further, fMRI responses to parallel center and surrounding stimuli did not show the predicted sensitivity to center contrast.In Chapters 3 and 4, the effect of spatial context during early visual processing in SZ patients was assessed using behavioral measures. Surround suppression of perceived contrast was examined in Chapter 3 among SZ patients and their unaffected relatives, as well as subjects with bipolar affective disorder (BP), relatives of BP subjects, and healthy controls. Weaker surround suppression was observed in SZ versus control subjects, while BP patients showed an intermediate deficit. These deficits did not depend on the configuration of surrounding stimuli. Normal performance was observed among relatives of SZ and BP subjects, indicating deficits in surround suppression were not associated with a genetic risk for these disorders. Chapter 4 examined how SZ impairs the ability to detect visual contours in cluttered backgrounds. Contours were presented in more- or less-similar backgrounds, in order to assess contextual modulation. While SZ patients performed worse than healthy controls or SZ relatives when detecting contours, performance in SZ was less influenced by background context. These experiments were designed to explore the neural basis of visual context processing in healthy adults, and to help uncover how SZ impairs these processes. The large body of research into the neurophysiology of human vision provides powerful tools with which to study how SZ may disrupt neural processing. Studying visual context processing may ultimately help to uncover computational principles conserved across many neural systems, and aid in identifying new targets for the treatment of mental disorders.
University of Minnesota Ph.D. dissertation. December 2014. Major: Neuroscience. Advisor: Cheryl A. Olman. 1 computer file (PDF); viii, 111 pages.
Neural mechanisms of visual context processing in healthy adults and those with Schizophrenia.
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