Browsing by Subject "Surround suppression"

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    Human neurophysiological mechanisms of contextual modulation in primary visual cortex.
    (2010-05) Schumacher, Jennifer Frances
    This dissertation examines visual processing of contextually modulated artificial and natural stimuli in primary visual cortex on a local scale. Understanding how local features are integrated into a global structure or ignored as irrelevant background is a critical step in comprehending human vision. To investigate these mechanisms, first it was necessary to measure the relationship between inferred neural responses, such as those obtained with blood oxygenation level-dependent (BOLD) fMRI, and local stimuli. From this point, orientation-dependent contextual modulation was analyzed locally or with a contour. While focusing on primary visual cortex, these experiments with stimuli of increasing complexity provide a foundation for how local features are grouped into global structures. BOLD fMRI provides a non-invasive method to measure the inferred neural response in humans. Because BOLD fMRI is a result of interaction between neural activity, blood flow, and deoxyhemoglobin concentration, it is not obvious that there is a linear relationship between these mechanisms as well as established functions, like the contrast response function (CRF). Chapter 2 measures the BOLD response to single Gabor patches of increasing contrast with two pulse sequences: Gradient Echo (GE) and Spin Echo (SE). GE measurements include signals from large and small veins while SE measurements eliminate the signal from large veins. Comparing these signals, at ultra-high field strength (7 Tesla) found the relationship between the CRF and BOLD fMRI for local stimuli is not linear with GE measurements. Chapters 3 and 4 focus on orientation-dependent contextual modulation of a single Gabor patch or of a vertical line of Gabor patches. In the periphery, surrounds of parallel orientation suppress the center stimulus while surrounds of orthogonal orientation facilitate the center stimulus. The relationship between the BOLD response and these suppressive or facilitative mechanisms was measured on a local scale (Chapter 3). Then, to compare the mechanisms for orientation-dependent contextual modulation and contour integration, performance in a contour detection task was measured over an extensive parameter space (Chapter 4). These data show that the BOLD response to suppressive stimuli do not behave as predicted by psychophysical results and that orientation-dependent contextual modulation and contour integration operate over different spatial scales, and likely different neural mechanisms. This dissertation provides data on the relationship between the BOLD response and local stimuli as well as data on the neural mechanisms behind orientation-dependent contextual modulation, contour integration, and texture classification. An over-arching theme is that inferred neural responses, such as those measured with BOLD fMRI, behave differently on a local scale than a global scale. However, other non-invasive measures provide details for how local stimuli are processed and further integrated into a global structure. Future work can incorporate computational models of neural activity and the BOLD response to clarify why measured responses differ on a local scale compared to a global scale.
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    Neural mechanisms of visual context processing in healthy adults and those with Schizophrenia
    (2014-12) Schallmo, Michael-Paul
    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.