Browsing by Subject "Masking"
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Item Open Access Cortical processes underlying attentional modulations of dynamic vision(Bilkent University, 2022-09) Çatak, Esra NurVisual attention is one of the most fundamental cognitive functions guiding and influencing a various number of processes. However, how different neural mechanisms are modulated by selective attention to process information is still subject to debate. Utilizing electroencephalography (EEG), the current thesis focused on understanding the time course of visual information processing and its neural underpinnings with paradigms that operate in different attentional modes, such as visual masking, attentional load, and transparent motion design. First, we aimed to understand the role of spatial attention in information processing and its possible interactions with metacontrast masking mechanisms. The behavioral results revealed an interaction effect that suggests differential effects of spatial attention on metacontrast masking. The following EEG analyses revealed significant activation due to masking and attentional load on early negative components located over occipital and parieto-occipital scalp sites, followed by a late positive component centered over centro-parietal electrodes. These findings suggest that the effect of spatial attention may have distinct characteristics at different stages of sensory and perceptual processing regarding its relationship with metacontrast masking. Secondly, by employing a novel variant of transparent motion design with color and motion swapping, we aimed to isolate the object-based cueing effect from a possible feature-based explanation in both psychophysical measures and neural activities. Our results demonstrate that the behavioral effects of attentional cueing survived feature swaps, providing evidence for an object-based attention mechanism. We also observed event-related potential correlates of these object-based selection effects in the late N1 component range, over occipital and parieto-occipital scalp sites, significantly associated with the variation in behavioral performance. Our findings provide the first evidence of the role of the N1 component in object-based attention in this transparent-motion design under conditions that rule out possible feature-based explanations. Taken together, the present results highlight the substantial effects of selective attention on the processing of visual information after the initial entry of information into the visual system and before the completion of its processing.Item Open Access Multifrequency invisibility and masking of cylindrical dielectric objects using double-positive and double-negative metamaterials(Institute of Physics Publishing Ltd., 2009) Serebryannikov, A. E.; Özbay, EkmelWe demonstrate that a circular dielectric cylinder can be nearly invisible at multiple frequencies when being coated with a ring shell, which is made of an isotropic material simultaneously showing large positive or large negative values of permittivity and permeability. The suggested cloaking mechanism is based on the use of radial resonances, which are similar to those in conventional Fabry-Perot resonators. It can be used for cylindrical objects for a wide range of variation of the diameter-to-wavelength ratio, which includes the values corresponding to subwavelength to resonant-sized objects. The presence of frequency dispersion of the shell material positively affects the possibility of multifrequency operation.Item Open Access Spatial attention and paracontrast masking(Bilkent University, 2021-01) Konyalı, AfifeVisual 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 [3] 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.