Browsing by Author "Kersten, D."

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Open Access
Attention modulates neuronal correlates of interhemispheric integration and global motion perception
(Association for Research in Vision and Ophthalmology Inc., 2014) Akin, B.; Ozdem, C.; Eroglu, S.; Keskin, D. T.; Fang, F.; Doerschner, Katja; Kersten, D.; Boyacı, Hüseyin
In early retinotopic areas of the human visual system, information from the left and right visual hemifields (VHFs) is processed contralaterally in two hemispheres. Despite this segregation, we have the perceptual experience of a unified, coherent, and uninterrupted single visual field. How exactly the visual system integrates information from the two VHFs and achieves this perceptual experience still remains largely unknown. In this study using fMRI, we explored candidate areas that are involved in interhemispheric integration and the perceptual experience of a unified, global motion across VHFs. Stimuli were two-dimensional, computergenerated objects with parts in both VHFs. The retinal image in the left VHF always remained stationary, but in the experimental condition, it appeared to have local motion because of the perceived global motion of the object. This perceptual effect could be weakened by directing the attention away from the global motion through a demanding fixation task. Results show that lateral occipital areas, including the medial temporal complex, play an important role in the process of perceptual experience of a unified global motion across VHFs. In early areas, including the lateral geniculate nucleus and V1, we observed correlates of this perceptual experience only when attention is not directed away from the object. These findings reveal effects of attention on interhemispheric integration in motion perception and imply that both the bilateral activity of higher-tier visual areas and feedback mechanisms leading to bilateral activity of early areas play roles in the perceptual experience of a unified visual field.
Open Access
Attention-dependent representation of a size illusion in human V1
(Cell Press, 2008) Fang, F.; Boyacı, Hüseyin; Kersten, D.; Murray, S.O.
One of the most fundamental properties of human primary visual cortex (V1) is its retinotopic organization, which makes it an ideal candidate for encoding spatial properties, such as size, of objects. However, three-dimensional (3D) contextual information can lead to size illusions that are reflected in the spatial pattern of activity in V1 [1]. A critical question is how complex 3D contextual information can influence spatial activity patterns in V1. Here, we assessed whether changes in the spatial distribution of activity in V1 depend on the focus of attention, which would be suggestive of feedback of 3D contextual information from higher visual areas. We presented two 3D rings at close and far apparent depths in a 3D scene. When subjects fixated its center, the far ring appeared to be larger and occupy a more eccentric portion of the visual field, relative to the close ring. Using functional magnetic resonance imaging, we found that the spatial distribution of V1 activity induced by the far ring was also shifted toward a more eccentric representation of the visual field, whereas that induced by the close ring was shifted toward the foveal representation, consistent with their perceptual appearances. This effect was significantly reduced when the focus of spatial attention was narrowed with a demanding central fixation task. We reason that focusing attention on the fixation task resulted in reduced activity in-and therefore reduced feedback from-higher visual areas that process the 3D depth cues.
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
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
Rapid classification of specular and diffuse reflection from image velocities
(Elsevier BV, 2011-09) Doerschner, K.; Kersten, D.; Schrater, P. R.
We propose a method for rapidly classifying surface reflectance directly from the output of spatiotemporal filters applied to an image sequence of rotating objects. Using image data from only a single frame, we compute histograms of image velocities and classify these as being generated by a specular or a diffusely reflecting object. Exploiting characteristics of material-specific image velocities we show that our classification approach can predict the reflectance of novel 3D objects, as well as human perception.
Open Access
Rapid classification of surface reflectance from image velocities
(Springer, Berlin, Heidelberg, 2009) Doerschner, Katja; Kersten, D.; Schrater P.
We propose a method for rapidly classifying surface reflectance directly from the output of spatio-temporal filters applied to an image sequence of rotating objects. Using image data from only a single frame, we compute histograms of image velocities and classify these as being generated by a specular or a diffusely reflecting object. Exploiting characteristics of material-specific image velocities we show that our classification approach can predict the reflectance of novel 3D objects, as well as human perception. © 2009 Springer Berlin Heidelberg.
Open Access
Visual adaptation to reflectance-specific image motion
(Vision Sciences Society, 2012-05) Doerschner, Katja; Kam, T. E.; Kersten, D.; Fleming, R.; Seong-Whan, L.
Recently, we described several surface reflectance-specific motion characteristics that the visual system may use to determine whether a rotating object appears shiny or matte (Doerschner et al., 2011). We used an adaptation paradigm to test whether there exist neuronal populations that are sensitive to such reflectance-specific image motion characteristics. Stimuli were computer-rendered movies of 5 unfamiliar, rotating objects. Eleven stickiness levels for each object were created by morphing between 'sticky' (diffusely reflecting) and 'slipping' (100% specularly reflecting) renderings of a given object with different mixing values, resulting in a total of 55 movies. For familiarization purposes observers were first shown a sequence of movies of an object transitioning from sticky to slipping though all 11 levels. In the pre-test observers rated the apparent shininess for each movie on a scale from 1 (very matte) to 5 (very shiny). The order of presentation was randomized. During adaptation, observers first adapted to a sticky movie for 120 s. This was then followed by a 2 s test in which observers rated shininess. Every fifth trial was preceded by a 24 s top-up adaptation period. Importantly, in order to prevent low-level motion adaptation we randomly selected a new rotation axis (out of 6) for each 2 s interval for the adaptor during adaptation periods. We compared the shininess ratings of all movies in pre-, and post-test. Overall, we found that, across observers and objects, adaptation to a sticky movie significantly affected the perceived shininess of subsequent stimuli (All observers: F(1,1098)=10.4781 p<0.002). Post-hoc analysis revealed that an increase in perceived shininess occurred mainly at higher levels of stickiness. These results support the notion of cortical mechanisms sensitive to reflectance-specific image motion patterns.
Open Access
Visual motion and the perception of surface material
(Cell Press, 2011) Doerschner, K.; Fleming, R. W.; Yilmaz, O.; Schrater, P. R.; Hartung, B.; Kersten, D.
Many critical perceptual judgments, from telling whether fruit is ripe to determining whether the ground is slippery, involve estimating the material properties of surfaces. Very little is known about how the brain recognizes materials, even though the problem is likely as important for survival as navigating or recognizing objects. Though previous research has focused nearly exclusively on the properties of static images [1-16], recent evidence suggests that motion may affect the appearance of surface material [17-19]. However, what kind of information motion conveys and how this information may be used by the brain is still unknown. Here, we identify three motion cues that the brain could rely on to distinguish between matte and shiny surfaces. We show that these motion measurements can override static cues, leading to dramatic changes in perceived material depending on the image motion characteristics. A classifier algorithm based on these cues correctly predicts both successes and some striking failures of human material perception. Together these results reveal a previously unknown use for optic flow in the perception of surface material properties.