Top-Down Signals in Visual Selective Attention

This thesis describes experimental work on the brain mechanisms underlying human visual selective attention, with a focus on top-down activity changes in visual cortex. Using a combination of methods, the experiments addressed related questions concerning the functional significance and putative origins of such activity modulations due to selective attention. More specifically, the experiment described in Chapter 2 shows with TMS-elicited phosphenes that anticipatory selective attention can change excitability of visual cortex in a spatially-specific manner, even when thalamic gating of afferent input is ruled out. The behavioural and fMRI experiments described in Chapter 3 indicate that top-down influences of selective attention are not limited to enhancements of visual target processing, but may also involve anticipatory processes that minimize the impact of visual distractor stimuli. Chapters 4-6 then address questions about potential origins of such top-down activity modulations in visual cortex, using concurrent TMS-fMRI and psychophysics. These experiments show that TMS applied to the right human frontal eye field can causally influence visual cortex activity in a spatially-specific manner (Chapter 4), which has direct functional consequences for visual perception (Chapter 5), and is reliably different from that caused by TMS to the right intra-parietal sulcus (Chapter 6). The data presented in this thesis indicate that visual selective attention may involve top-down signals that bias visual processing towards behaviourally relevant stimuli, at the expense of distracting information present in the scene. Moreover, the experiments provide causal evidence in the human brain that distinct top-down signals can originate in anatomical feedback loops from frontal or parietal areas, and that such regions may have different functional influences on visual processing.