Spatial attention and paracontrast masking
Kafalıgönül, Hacı Hulusi
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Visual masking is a powerful methodological tool to investigate the dynamics of sensory processing associated with object visibility and identity. Previous paracontrast masking studies revealed three distinct components that have been proposed to reflect processes at different stages and to be mediated by the distinct interactions within and/or across pathways [1, 2]. The brief and prolonged inhibition components are mainly observed within short and long stimulus onset asynchronies (SOAs) and they have been interpreted as the reflectance of early lateral inhibition and late recurrent inhibition within the parvo-dominated P-pathway. On the other hand, the facilitation typically becomes dominant at intermediate SOAs and the excitatory modulations of sub-cortical structures on the parvo-dominated pathway have been proposed as the underlying mechanism. An important question to address is how attention modulates these components and associated processes. In this thesis, two experiments were designed to understand the effects of attention on the components involved in paracontrast masking. In the first experiment, using an experimental design  combined with a contour discrimination task, the set-size was varied to manipulate attention in the spatial domain. The paracontrast masking functions indicated robust brief and prolonged inhibitions. Importantly, the set-size differentially altered these components. An increase in set-size (i.e., attentional load in the visual field) decreased brief inhibition while increasing the prolonged inhibition. In a second experiment, a brightness/contrast matching task was used to understand the effects of attention on the facilitation. Although the paracontrast masking functions showed facilitation at intermediate SOAs and the component was higher for increased set-size condition, these observations were not supported by statistical tests. Taken together, these findings revealed differential effects of spatial attention on the inhibitory mechanisms operating at distinct stages of P-pathway. In the last part of the thesis, an elaborated experimental design was also proposed to further understand and reveal possible effects of attention on the facilitatory mechanism. Future neuroimaging studies will be informative to understand the neural correlates of attention and paracontrast interaction, and hence the role of attention in object visibility.