The impact of distractor congruency on stimulus processing in retinotopic visual cortex

The brain is frequently confronted with sensory information that elicits conflicting response choices. While much research has addressed the top down control mechanisms associated with detection and resolution of response competition, the effects of response competition on sensory processing in the primary visual cortex remain unclear. To address this question we modified a typical 'flanker task' (Eriksen and Eriksen, 1974) so that the effects of response competition on human early retinotopic visual cortex could be assessed. Healthy human participants were scanned using fMRI while making a speeded choice response that classified a target object image into one of two categories (e.g. fruits, animals). An irrelevant distractor image that was either congruent (same image as target), incongruent (image from opposite category as target), or neutral (image from task-irrelevant category, e.g. household items) was also present on each trial, but in a different quadrant of the visual field relative to the target. Retinotopic V1 areas responding to the target stimuli showed increased response to targets in the presence of response-incongruent (compared to response-neutral) distractors. A negative correlation with behavioral response competition effects indicated that an increased primary visual cortical response to targets in the incongruent (vs. neutral) trials is associated with a reduced response competition effect on behavior. These results suggest a novel conflict resolution mechanism in the primary visual cortex

The role of the parietal cortex in salience-based selection using a cognitive neuropsychological approach

It has been known that the parietal cortex is important for directing attention in order to fulfil search task goals, although how exactly this is done is unclear. Saliency has been identified as important in parietal selection of targets and suppression of distractors. This thesis attempted to explore the factors underlying salience-based selection in the parietal cortex using a cognitive neuropsychological approach. Chapter 1 explored the literature underlying saliency and the parietal cortex. Chapter 2 addressed the question of salience-based selection in a global-local task using a voxel-based morphometric approach in a wide range of patients, finding parietal and occipital regions as important regions for congruency interference and suppressing salient distractors. Chapter 3 inhibited the right precuneus using repetitive transcranial magnetic stimulation in neurotypical adults on the same task finding reduced congruency interference. Chapter 4 revealed the importance of saliency in mediating level selection in simultanagnosia. Chapters 5 and 6 examined in a bilateral parietal patient using spatial and non-spatial paradigms the importance of stimulus relevance as a means of guiding salience-based selection. The thesis concluded in Chapter 7 that the parietal cortex is important for salience-based selection and suppression mediated by the relevance of the stimulus being made salient