Browsing by Subject "Visual cortex"

Now showing 1 - 5 of 5

Open Access
Border ownership selectivity in human early visual cortex and its modulation by attention
(Society for Neuroscience, 2009) Fang, F.; Boyacı, Hüseyin; Kersten, D.
Natural images are usually cluttered because objects occlude one another. A critical aspect of recognizing these visual objects is to identify the borders between image regions that belong to different objects. However, the neural coding of border ownership in human visual cortex is largely unknown. In this study, we designed two simple but compelling stimuli in which a slight change of contextual information could induce a dramatic change of border ownership. Using functional MRI adaptation, we found that border ownership selectivity in V2 was robust and reliable across subjects, and it was largely dependent on attention. Our study provides the first human evidence that V2 is a critical area for the processing of border ownership and that this processing depends on the modulation from higher-level cortical areas.
Open Access
Contrast affects fMRI activity in middle temporal cortex related to center-surround interaction in motion perception
(Frontiers Research Foundation, 2016) Türkozer, Halide B.; Pamir, Zahide; Boyacı, Hüseyin
As the size of a high contrast drifting Gabor patch increases, perceiving its direction of motion becomes harder. However, the same behavioral effect is not observed for a low contrast Gabor patch. Neuronal mechanisms underlying this size-contrast interaction are not well understood. Here using psychophysical methods and functional magnetic resonance imaging (fMRI), we investigated the neural correlates of this behavioral effect. In the behavioral experiments, motion direction discrimination thresholds were assessed for drifting Gabor patches with different sizes and contrasts. Thresholds increased significantly as the size of the stimulus increased for high contrast (65%) but did not change for low contrast (2%) stimuli. In the fMRI experiment, cortical activity was recorded while observers viewed drifting Gabor patches with different contrasts and sizes. We found that the activity in middle temporal (MT) area increased with size at low contrast, but did not change at high contrast. Taken together, our results show that MT activity reflects the size-contrast interaction in motion perception. © 2016 Turkozer, Pamir and Boyaci.
Open Access
Perceptual grouping-dependent lightness processing in human early visual cortex
(Association for Research in Vision and Ophthalmology, 2010) Boyaci, H.; Fang, F.; Murray, S. O.; Kersten, D.
Lightness, the perceived relative achromatic reectance of a surface, depends strongly on the context within which the surface is viewed. Modest changes in the two-dimensional conguration or three-dimensional scene geometry may lead to profound variations in lightness even though the surface luminance remains constant. Despite recent progress, we are far from a complete understanding of how various aspects of spatial context affect lightness processing in the cortex. Here we use a novel stimulus to show that perceptual grouping through occluders can affect lightness. We first report behavioral results showing how lightness across occlusion depends on spatially distant image features, including luminance and contrast. Next using functional magnetic resonance imaging (fMRI) we show that human early visual cortex responds strongly to occlusion-dependent lightness variations with little or no attention. These results suggest that elements of three-dimensional scene interpretation play a role in early cortical processing of lightness.
Open Access
The visual cortices of congenitally and non-congenitally blind individuals become a part of cognitive control network
(2023-12) Duymuş, Hasan
Neural plasticity is crucial for understanding the extent to which a biological structure determines its function. As such, the fate of visual and auditory cortices when deprived of their standard inputs has immense significance in neuroscientific research. Empirical findings suggest that the visual cortex of the congenital and early blind activates across a very wide range of tasks in auditory, tactile and olfactory modalities. We hypothesized that these regions may transform into task positive multiple demand (MD) regions. A key feature of these regions is that they activate to all kinds of tasks in all kinds of modalities. In this study, we investigate whether deprived cortices in blind exhibit key characteristic inherent to fronto-parietal MD regions. We had congenitally and non-congenitally blind participants done four fMRI cognitive control tasks in tactile and auditory modalities. The visual cortices of the blind group showed (1) intense activity during more demanding conditions of the four diverse tasks along with the fronto-parietal control regions, (2) the same set of occipital voxels in blind participants activated across diverse modalities under increased cognitive demands of the four diverse tasks. Our findings suggest that deprived visual cortices of the blind, in fact, become a part of cognitive control network.
Open Access
Vision: a systems neuroscience perspective
(Uskudar University, 2014) Kafalıgönül, Hulusi
The visual system is the most studied part of the cortex, providing a basis for understanding not only visual processing per se but also the fundamental operations of the brain in general. Significant progress has been made in understanding neural structures sensitive to different visual attributes such as form, surface brightness, color and motion. Here, the basic neural structures and processing pathways for these visual features are reviewed. Dysfunctions in these processing pathways lead to deficits in the perception of different aspects of a visual object. In recent years, there is a growing interest in applying accumulated knowledge in vision science to investigate altered neural structures and abnormal perceptual processing observed in neurological disorders. Key issues and clinical studies are also discussed within the context of visual feature processing.